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Ducting

A. Ducting Introduction

This information focuses on what we need to do to fix our tools, tool hoods, tool ports, ducting, and ducting layout to provide good fine dust collection.

Most do not realize how much tool ports, tool hoods, duct size, ducting type, ducting layout, and leaks impact dust collection! In my testing every shop badly failed their air quality testing regardless of how big their cyclone or dust collector if they did not have a table saw blade hood that blocked the sawdust blasting off the blade and had a good airflow from either a 4” dust collection port or a 2.5” port connected to strong vacuum that generated at least a 90” of pressure. Also, not one single shop with typical 4” ducting passed again because that sized ducting prevents most dust collection blowers from moving ample air to provide good fine dust collection at our larger tools. Likewise, those shops that had large enough ducting but had poor layouts and leaky fittings also had excessive dust levels. Sadly, most of these problems are created by small shop vendors. Most sell nice 4” ducting solutions that just will not move enough air for good fine dust collection at our larger tools. The remaining few mostly sell commercial ducting systems and graduated ducting designs intended for shops where collection runs to all machines running at once. In our small shops that only run one machine at a time, these wonderful looking designs end up with too little airflow in the mains resulting in plugging, dangerous dust piles, and potential fire risks. This mix of information leads to considerable unneeded ducting confusion and controversy.

Fortunately we each already understand airflow and ducting layout far better than most realize! At normal dust collection pressures air behaves like water and is almost incompressible. Think of our blowers as sucking water through a series of pipes. Most small shops use tiny blowers with barely enough suction pressure to collect from one tool at a time. Blast gates act as dams to divert that suction to a single machine. Just like with a water hose, any restriction, sharp bend or small hose will limit how much air will flow. Our ducting from start to finish must be just big enough to carry the air needed to collect from our largest tool. With a one tool at a time collection system we really need a main with branches and down drops all sized nearly the same, otherwise smaller pipes will act just like a water valve and kill our flow. If that flow drops too low the chips will build up piles in the mains that further hurt our airflow and pose a fire hazard. Likewise our tool ports and hoods must not limit how much air will flow. Just like with water, nice straight runs of the shortest length with long sweeping curves will move the most air. Also leaks can kill our limited flow.

1.      Getting Started

Before staring we need to first decide on what level of dust collection we want because our level of protection defines what we need to do. With all hobbyists and six out of seven professional woodworkers working in small shops that are not covered by any government air quality standards, our only protection is what we choose for ourselves. We need to choose between two levels of “chip collection”, OSHA air quality, ACGIH air quality, and European Union medical recommended air quality. Each level of protection requires a different approach, more work, and higher cost. Most of our tools and dust collection equipment is configured for “chip collection” which totally ignores fine airborne dust. In commercial facilities subject to fire marshal and building inspections “chip collection” means picking up the same sawdust and chips we would otherwise sweep up with a broom amply to avoid personal injury accidents, plus properly configuring and locating the dust collection equipment. “Chip collection” poses known fire and explosion risks with larger dust volumes so must comply with National Fire Protection Association (NFPA) regulations. Woodworking shops located in commercial buildings subject to fire marshal inspection must comply with local codes which mostly require use of all metal ducting and either use of dust collection equipment certified for indoor use or equipment properly secured outside. With most small professional and hobbyist shops located in or next to homes not subject to fire marshal inspection, “chip collection” is unregulated.

Starting in the early sixties insurance data and medical studies showed a strong causal relationship between airborne wood dust exposure and health problems with woodworkers in commercial facilities that complied with NFPA dust collection guidelines. The insurance data and medical testing showed almost all full time workers in large woodworking facilities developed significant lost of respiratory function with about one in eight developing such serious wood dust triggered permanent health problems that they were forced into early retirement. The medical was clear there is no safe level of wood dust exposure. Every wood dust exposure causes some measurable loss of respiratory function and exposure over time creates permanent damage. The U.S. Department of Labor, Office of Safety and Health Administration (OSHA) is charged with protecting the health and safety of workers. With most facilities already blowing most of their fine dust away outside woodworking facility owners complained strongly saying cleaning up the air further to meet medical recommendations was cost prohibitive. Studies commissioned by facility owners soon appeared. Some claimed woodworking makes no fine dust. Others showed most facilities already blew the majority of the fine unhealthy away outside. Some showed there are no health risks from wood dust. Others showed commercial woodworkers received so little exposure there were no health risks. A long heated debate over the many conflicting study results delayed OSHA issuing air quality standards for woodworkers.

Air engineering firms that make dust collection equipment for larger woodworking facilities had years to prepare for long expected OSHA standards. These firms needed to ensure their large woodworking facility dust collection systems subject to proposed OSHA air quality inspections met the new standards or their customers could be cited, fined and even closed down. These firms did considerable study, engineering and testing with most reaching the same basic conclusion and developing similar solutions. They found fine dust spreads so rapidly even a big air cleaner or exhaust fan needs hours to bring the dust level down enough to be safe. During this time that dust will harm workers and shops will fail air quality checks. Fine airborne dust spreads so quickly that good air quality requires collecting the fine dust collected at the source as it is made before it can spread. Air engineers went through years of testing and decades of refinement to work through the details to meet proposed OSHA air quality regulations. These firms found the most existing tools came with no dust collection or at best poor “chip collection” built in, so worked poorly for good fine dust collection. With costs to replace existing tools prohibitive, air engineers found they could get good fine dust collection at most older tools by moving enough air and upgrading tool hoods to better contain, control, and deliver the fine dust for collection. They developed new CFM requirement tables for each size and type of tool that with hood upgrades moved enough air to ensure capturing the fine dust. They found the only effective way to get good airborne dust control at each tool source is to upgrade tool hoods to contain and control the fine dust, upgrade tool ports to not restrict airflow, move far more air to ensure collecting over a large area to capture the fine dust before it can escape, use larger diameter ducting able to carry ample airflow, separate off the heavier sawdust and chips, then get rid of the fine dust. It was most cost efficient to exhaust the fine dust outside. When climate or local laws preclude exhausting outside, far more work and cost is required to amply filter the air for returning it safely into our shops. In 1989 when OSHA finally declared wood dust a nuisance and set air quality standards, most large commercial shops were already running OSHA compliant dust collection.

Many experts called OSHA air quality requirements too lax. Medical studies and insurance records already showed most workers in OSHA compliant facilities eventually developed dust related medical problems. The Australian Ministry of Health says this exposure level eventually leaves all ill with roughly one in fourteen forced into an early medical retirement due to dust related health problems. The American Conference of Industrial Hygienists (ACGIH) responded recommending five times lower airborne dust levels and these recommendations were supported by the Environmental Protection Agency (EPA). Many large commercial woodworking firms now voluntarily follow ACGIH recommendations. Although that upgrade helps, wood dust has since been classified as a cancer causing agent so medical experts recommended a fifty times higher than OSHA air quality standard which has already been adopted by the European Union. Meeting these higher air quality standards requires the same hood, port, and tool upgrades, but more airflow. In spite of ACGIH, EPA and the European Union activity, the 1989 OSHA standard remains the only U.S. standard. Moreover, only the largest woodworking facilities receive regular OSHA air quality testing, so most professional woodworkers and hobbyists remain on their own.

2.      Airflow

Fortunately, the better air engineering firms share most of what they learned on the Internet covering what we need to do to capture the fine dust before it can escape. They established in tables like the
Cincinnati Fan Engineering Data the air speeds measured in feet per minute (FPM) we need to collect various sized chips and transport those chips through ducting without plugging or building up dust piles. Plugging and dust piles pose a potential fire hazard and when these piles break loose over time they will destroy blower impellers, bearings and filters. This reference is also one of the best to understand more about airflow and blowers. It gives the airspeed we need to pickup and transport the dust. We need the same airspeed to pickup the chips as we do to keep it moving in vertical ducting runs, so collection and vertical transport speed are often interchanged. We can get by with a little less airspeed to keep the dust moving in horizontal runs because horizontal runs do not have gravity working against us. These material handling tables show we must maintain airspeed inside our ducts of at least 3700 FPM to collect (and vertical transport) most normal sawdust but larger chips need up to 4500 FPM airspeed for collection. About 2800 FPM is needed to keep the horizontal runs clear. Air engineers established most woodworking operations that do not make heavy big chips are well covered if they target their blowers and ducting to move 4000 FPM airspeed. If you try to pickup larger chips and small blocks this is not enough airspeed.

They did considerable experimenting and testing, followed by decades of refinement to establish the similar to the one linked from AAF that show what airflow measured in cubic feet per minute (CFM) we need to collect at each type and size of woodworking tool before it can escape. They also found these CFM tables are worthless unless we also modify our older tool designs that spray fine dust all over with better hoods, ports, and internal ducting to protect, control, and deliver the fine dust for collection as it is made. Also with permission below shares some of the vendor recommended hood and collection designs for most major tools.

Air engineers also came up with the formulas and tables to ensure we have the right ducting and blower. They came up with simple formulas and approaches to help design our ducting to move the needed air with ample volume and speed at each tool. They built like the one linked here that estimate the resistance created by our ducting, flex hose, duct fittings, hoods, filters, cyclones, separators, etc. They provide that let us use our airflow requirement and resistance level to look up what sized blower with the right size impeller and motor to move the amounts of air we need. They also formalized this process into something that most can follow without special training or expertise to come up with a good dust collection system. Since we use the same size and types of tools as smaller commercial tools, most of what we need to do for good fine dust collection is already laid out in detail. Each of us can go through this process to determine our needs, pick a blower large and strong enough, configure our ducting, separate off the heavier dust, and either exhaust the fine stuff outside or appropriately filter that dust.

This engineering information shares most of the minimums we need for each type of dust collection. Roughly 350 CFM proved ample air volume to do good “chip collection at most of our larger small shop stationary tools and dustier operations. Their ducting formula FPM=CFM/Area shows that for our 4000 FPM and 350 CFM we need almost exactly 4” diameter duct. We can use smaller pipe, but then have to have a much bigger blower to generate extra pressure. Likewise, if we use bigger pipe, we then have to use a bigger blower to keep the airspeed up enough to avoid our pipes plugging and building up dust piles. Unfortunately the 350 CFM needed for good “chip collection” falls far short of the airflow needed for good fine dust collection. They found that meeting OSHA air quality requires just over 795 CFM and larger better hoods. Instead of just having to cover a 4” diameter area they found for good fine dust collection our hoods needed to cover roughly an 18” diameter collection area. The same simple ducting formula feeing in our 50 FPM needed to collect the fine dust over that 18” sphere shows we need. Rounding that to 800 CFM and keeping our 4000 FPM airspeed in our ducts to keep them clear says we need 6” diameter duct to move the 800 CFM needed to meet OSHA air quality standards. Their testing found we need about 900 CFM at our larger tools to cover a little more area and roughly 1000 CFM to meet medical air quality recommendations that are now the European standard. The same formulas show we need to use 7” diameter ducting to move the air for both the ACGIH and medical standards already adopted in Europe.

3.      Resistance

Because air will compress, we assume we will get nearly the same flow on both sides of a small obstruction because the air will speed up to get around a small obstruction. Based on that knowledge, I assumed that we could minimize the length of that obstruction by using tapered adapters right at our machines and still use their built in 4″ ports. I then received an email that asked me to further clarify, because in many instances this will not work. I pulled out my gauges and found out I was dead wrong.

Air at the low pressures we use in dust collection, air is more like water, and does not compress. Any obstruction, small pipe, or tight turn will kill our airflow dramatically just like closing a water valve. This means any obstruction, small port, undersized hood, restrictive internal air pathway in a tool, small section of hose, or restrictive duct fitting will act just like a water valve and seriously reduce flow. This also means our tapered and smaller adapters from our ducting to our tools are all but useless because they also kill our needed airflow.

When we have high resistance from an obstruction, that resistance will control the flow. To test this for yourself, try a simple experiment I learned in engineering classes long ago. Get a few feet of 1/8″ interior diameter clear hose. Now cut off about a 1/2″ of this hose. Without choking on that tiny piece, try to breathe through it. Most can but find it difficult. Now try to breathe through the longer piece. Most cannot because it has too much resistance. Now go back to the small piece and try to breathe through it quickly. You cannot because our lungs cannot create the pressure needed to overcome the high resistance of that small pipe. The same is true of our ducting. The resistance of the duct will define the airflow. If we use too small of ports, duct, fittings, or outlets, we kill the airflow needed for fine dust collection. (my thanks to Dave for reminding me that under typical dust collector pressures air is virtually incompressible).

Because air is near incompressible at the low pressures we use in dust collection, we end up with the pipe diameter controlling our air volume. This is just like water. We open and close a valve to regulate how much water comes out of our faucets. To support the 800 CFM we need for good dust collection, we need to upgrade those 4″ port connections to 6″ and the ducting inside our machines to 6″. For machines like my table saw with an interior duct, I had to use a larger port and a larger interior hose. Another serious resistance issue is the use of standard flex hose. Much of that hose is poorly made with ribs sticking into the airflow adding up to nine times the resistance of smooth pipe. Always buy and use a minimum of smooth interior walled flex hose as it that only adds about three times the resistance of smooth pipe.

4.      Ducting Diameter

Ducting Diameter affects the amount of airflow just like the size of the pipe and water valve size affects water flow. Unlike a shop vacuum that generates roughly ten times the pressure, air from our blowers will not do a good job of squeezing around obstructions or through small openings. As a result size has everything to do with how much air you can move at a given pressure. We must size our ducting correctly. If made too small it kills our airflow needed for good fine dust collection. If made too large it does not maintain the airspeed in the ducting to avoid plugging and a build up of dust piles. This makes sense as a garden hose would empty a city water tank far slower than a 6″ diameter pipe because the garden hose is too small. We control water flow by opening and closing our faucet, meaning add just one constriction in the line and it can kill our flow. Opening the faucet wide gets you a flow limited by the size of the pipe. Getting a larger central storage tank has little effect unless you put it higher in the air where it can generate more pressure. As with water, getting a bigger blower with more horsepower does little good if the airflow is too restricted by the size of the ducting, tool ports, hoods, and duct openings.

For instance a 1.5 hp dust collector that can move a maximum of 1100 CFM moves far less air than that maximum depending upon what sized ducting we use. This typical small shop dust collector blower only generates 4″ to 6″ of pressure when working. With the added overhead of our filter and minimum ducting, that pressure is only ample to move about 800 CFM when hooked up with a short piece of 6” flex hose. That pressure will only pull about 550 CFM when connected with 5” flex hose and only about 450 CFM when hooked up with 4″ flex hose.

Whether you have a modest 1/2 HP 600 CFM blower to a roaring 5 HP maximum 2300 CFM blower you need to balance the ducting size. We constantly trade off our ducting size to move the right air volume at ample speed with minimum resistance. To get the needed 800 CFM that larger tools need for good fine dust collection through a 4″ duct or hose you need about 9,000 FPM that takes a monster impeller and huge motor. That’s why knowledgeable woodworkers use 6″ ducts and 6″ flex right to their larger machines even with portable dust collectors. Without a monster blower, if your duct is smaller than 6″ to your larger machines, then it will not move enough air to capture the fine, most unhealthy dust. The best you can hope to do is make your system a little more efficient.

5.      Leaks

Leaks kill system performance and can cause all kinds of other problems. A leak between the collection barrel and cyclone will quickly clog your filters just like when the barrel gets too full killing any ability to collect the dust. Likewise, many small leaks in the ducting quickly add up to the equivalent of having an extra open blast gate that will severely cut the airflow at your machines killing performance and potentially leading to plugged ducting. I don’t want to waste my electricity or limited capacity blower on leaks.

6.      Airflow Requirements

Airflow Requirements for good dust collection are a paradox. It takes very little airflow to move really fine dust, yet we need far more airflow to capture that same fine dust than we need to pick up the same dust we get with a broom. To make sense of this on my other pages I share a simple experimental game with a balloon and two straws. One person is only allowed to blow and the other to only suck. The one who blows always wins because sucking pulls air from all directions and requires a lot more air movement to make any effect even a tiny distance away. Any fine dust that does not get trapped by the dust hood gets launched by almost any airflow from our tools, belts, cutters, motors, etc. The only way to prevent this is to have good hoods that keep the fine dust controlled and move enough volume of air to capture it before it gets launched. We measure that volume of airflow in cubic feet per minute or CFM for short.

7.      Power requirements

Power requirements to move more air also differ from water greatly. Because air is compressible to double the amount of air you move measured in cubic feet per minute (CFM) you must double the fan speed or double the surface area of the fan. This will require a four-fold increase in static pressure that will cost you a NINE-fold increase in horsepower! This fan law says adding 1/2 horsepower and a bigger impeller to a 1.5 HP 1100 CFM blower nets you only 100 CFM in additional airflow! The wrong pipe size, leaks, fittings that change the air to abruptly, overly long runs, an inefficient separator, and a poorly designed cyclone can each cost far more CFM loss. Every time you force air to make a sharp direction change you lose lots of efficiency. Between use of the very inefficient flex hose, too small of a diameter pipe, and too many sharp angled fittings in their ducting, most hobbyist woodworkers severely kill the potential airflow needed for good dust collection. These practical suggestions can help you to first address the inefficiencies before having to pay for expensive motors, bigger blowers, and more electricity.

B. Layout & Ducting Design

Although we all enjoy having bragging rights and a shop full of all different sizes of ducting looks incredible, the reality is with our small shop blowers we only move enough air to power a single machine at a time. With just one ducting run at a time open, we need a totally different ducting design than the traditional larger woodworking shops that run dust collection to every tool working at once. If we did that, we would need monster sized blowers that would cost a fortune to buy, install, maintain, and run. As a result, we end up with simple systems that should all be the same size duct from the blower right to the machine. At the machine the duct may split into two equal diameter collection pipes for two collection points, but otherwise our ducting remains very simple. Sadly, a number of small shop vendors offer commercial ducting design programs that offer a wide range of ducting sizes with each down drop sized just right for each machine. These designs can cause fires and even explosions in our ducting because the small pipes kill the airflow needed to keep the larger mains clear.

To design your ducting lay all out to make the shortest straightest runs possible with minimal sharp bends or joints. No short 90 degrees joints should ever be used. Either use curved duct with a big radius or break up the 90-degree joints with a straight run between two 45-degree joints. All should be long sweeping curves or Y type takeoffs with narrow angles. In addition, most want to put the dust collector off in a corner. It really goes right in the middle of the wall closest to your tools that require the most CFM for good dust collection. Your main duct manifold should make a straight run down the center of your shop, not around the perimeter! Also, for maximum efficiency, particularly with a cyclone, you want a straight run for at least 4′ or longer going into your inlet. This keeps the incoming air very smooth making for far better material separation and improved efficiency as turbulence kills efficiency.

1.      Fire Safety

You need to setup your dust collection in a way that will protect against fires! If you have a unit where material can directly hit the impeller, then rocks and pieces of metal can hit that impeller, cause small sparks and those sparks fall into your sawdust and slowly grow into a disaster, often many hours after you leave your shop. If you have this kind of setup, you should never clean your shop floor or other areas that might have metal or stone bits with your dust collector. For this reason, most agree that dust collection bins should be metal. I used a 30-gallon metal trashcan on one unit and a 40-gallon metal trash can on the other, both with metal lids and metal flex duct going to them from my cyclone. That makes the dust more awkward to empty, but safer and the metal flex duct is much less expensive. With a two-stage unit using a separator or a cyclone before the blower this becomes less critical.

2.      Pipe Size

Always use the largest diameter duct that your blower can use with the least number of restrictions. If your ducting is too small, then it instead of your blower defines the CFM that your system can provide at your machines to pick up the dust. If your ducting is too large you might not maintain the airspeed needed to keep sawdust and chips from building up and blocking your ducting. You need about 3000 feet per minute (FPM) airspeed to keep light sawdust moving horizontally and about 3700 FPM to move it vertically. Air engineers target their designs to maintain about 4000 FPM to keep the dust entrained (moving). FPM is simply CFM divided by the area of the duct in square feet instead of inches (144/sq. in.).

3.      Ducting Resistance

Ducting resistance is known as static pressure. Even a short run of duct that is too small for a blower will cut the airflow down to the highest speed that pipe can sustain. The impact on most hobbyist blowers is terrible. A 3/4 HP blower with a maximum airflow of about 600 CFM will rarely provide more than about 300 CFM real air flow when connected to a 4″ pipe. On that same 4″ ducting a 1 HP unit that gives 650 CFM maximum rarely will maintain 350 CFM. A 1.5 HP rated at 1100 CFM barely gives 450 CFM. And a 2 HP capable of 1200 CFM is lucky to provide 500 CFM. Bumping up to 5″ pipe adds about 100 CFM to each of these configurations. Bumping up to 6″ pipe causes problems for the under 1.5 HP units because the air speed (FPM) can fall too far and make dust block the pipes, but with this bigger pipe, the bigger units end up going to 800 and 900 CFM. As a result, you need to use at least 5″ duct for any hobbyist blower rated up to 1100 CFM and 6″ duct for blowers rated 1100 CFM to 1800 CFM.

4.      Ducting Reductions

Unlike big industrial sites, most hobbyists should run the same sized ducting, fittings and hose right up to their machines. Don’t do like many and run a 6″ trunk line then come off with smaller duct or flex hose. With only one port open at a time because that is all most hobbyist sized blowers can support the smaller pipe will kill the airflow needed to keep the mains from plugging. You have to keep three 3″ ports open at once or two 4″ ports to avoid the plugging of your mains. Having so much open often kills the airflow needed to collect the fine dust. If you use standard hoods you should still convert over to 6″ ports. Even reducing ducting size right at the machine for the shortest possible distance to a small 4″ port will still kill system performance unless you have a very short run and open up the airflow right after that small hood. The smaller ducting, flex hose, and small ports limit the maximum airflow just like we proved in that little air test sucking experiment! If you do change size ducting, use a big enough blower to support opening multiple blast gates and appropriate connections for enlarging and reducing:

5.      Ducting Material

The next most important aspect of building a good ducting system is which type of ducting material you will use. We need to be careful when buying either dust collection ducting or HVAC metal pipe. We can easily spend a small fortune and still not end up with a nice leak free system. Also, most “official” dust collection pipe uses proprietary connectors and sizes that often limit your expansion to working with just that firm. With special tools required to make junctions, you also end up generally paying for a number of custom made parts. Likewise, HVAC metal ducting thickness and size can vary considerably, but most recommend getting at least 26 gauge pipe. Most “snap lock” ducting in the larger home centers comes in light 30-gauge or heavier 26 gauge galvanized steel. Unfortunately, if you make your duct pipes or cyclone out of the thinner metal, you could collapse your cyclone and long metal duct runs if you have a large blower and should leave all the blast gates closed. The fittings even in the light metal have enough support they are not at risk of collapsing. Another inexpensive option is building square or rectangular ducts from Melamine coated particleboard. Particleboard ducts can be quite inexpensive and are very efficient. Unfortunately, many alternative ducting materials are flammable so are totally inappropriate for use anywhere except in plain sight. Putting a flammable duct material in a ceiling area or enclosed attic can create an unacceptable risk of fire.

Although I am a fan of and personally use the higher end steel snap lock type ducting that is very pricey, I started off using HVAC pipe which worked well, then shifted to use of the thinner PVC pipe that is readily available in my area due to all the agricultural irrigation. Today with the price of oil driving PVC pipe costs crazy, if cost was a major concern I would use HVAC pipe. I suspect most of my pipe and fittings would come from one of the large box stores with my wyes and longer radius bends coming from a local ducting supplier. Use of two 45 degree fittings with a short segment would work in place of these long radius bends almost as well if those are not available locally.

Ideally we need the smoothest pipe we can get to minimize resistance and still ensure the level of safety in our shops. To minimize resistance, the interior of the pipe needs to be as smooth as possible and you need long smooth sweeping smooth joints on all your fittings.

There are lots of ducting choices and often we slip into a mode where we think if something costs less it is not as good. The smoothest walls that make for the least resistance come with using plastic coated pipe and fittings such as PVC pipe or plastic coated metal or wood duct. The next best is smooth walled laser welded steel pipe followed by a tossup between top quality metal spiral dust collection ducting and fittings and HVAC metal ducting, followed at a far distance by corrugated metal pipe and flex hose. The low cost S&D PVC (plastic) pipe (see if you want to do “magic” with fitting PVC into your ducting.) is generally one of the best small shop dust collection material choices because it is smooth, far stronger than most HVAC metal pipe or spiral pipe, costs less, and fittings are a fraction of the price. Airflow depends upon ducting friction. Here are the Hazen/Williams friction factors for various duct types (a higher C number is better).

  1. Corrugated steel duct= 60
  2. Spiral Duct = 90-100
  3. Laser Welded Steel Duct = 110-125
  4. 3-PVC Duct = 146

Notice that corrugated duct is so restrictive it should never be used! The same is also true of flex hose that has a rough interior. Also notice that in spite of some vendor claims, these values show that PVC moves more air with less friction than even equal sized spiral ducting. I recommend the PVC ASTM 2729 “Sewer and Drain (S&D)” pipe for most small shop woodworkers. The 2729 PVC pipe and fittings cost far less than metal or standard schedule 40 PVC, are much stronger, don’t leak like metal pipe, and have a much lower coefficient of friction than even spiral metal ductwork. The 2729 PVC is rated for many times the pressure that even large industrial blowers generate. I’ve seen no flex with my PVC pipe even when doing testing on 10 hp blowers driving a 16″ impeller. Alternatively, you can use either the more expensive but less efficient heavier spiral pipe or the 26 gauge snap lock HVAC pipe.

6.      Ducting Cost

It is best to buy your dust collection pipe, fittings and flex hose from either a firm with free shipping or from a local supplier, otherwise you can easily pay nearly as much for shipping as for the cost of the ducting.

7.      Ducting Connections

Making your dust collection connections can be done with glue, screws, special duct sealant, and many other things, but most find that slipping all together works best. This is especially true if you buy the S&D PVC for your main runs with the built in seals. They only cost a little more and make tight joints that normally don’t need anything more to hold them in place. Most agree that not gluing is by far the preferred way to go, as you may change your mind on your shop arrangement, and you will for sure eventually catch something that clogs requiring you to take one or more joints apart. The best way to seal leaking temporary joints is with standard 2″ aluminum duct tape found in home and HVAC stores. Be careful when handling his tape as it is very sharp! Permanent joints are better sealed with either duct sealing compound or polyurethane caulk.

John Koster reminded me that a good solution for those who need to connect dissimilar pipe materials or who need to deal with noise or vibration should consider using standard plumbing neoprene “no-hub” connectors. These units come sized to mate Cast Iron/PVC, Steel/PVC, and PVC/PVC. These connectors are easily placed and removed for access, provide a good seal on an adjustable range of diameters, and act as “dampeners” for vibration and noise transmission.

8.      Duct Hanging

There are lots of ways to mount your ducting to the ceiling or wall.

a.                  Nylon Cable Ties

I found at an electronics outlet some nice screw-in mounts designed to be used in electronic cabinets. These mounts work well to hold heavy nylon cable ties right to the ceiling or wall. I snapped a chalk line then put in one of these every other stud held in place with a 2″ deck screw. To use just thread a nylon tie (took me using two linked together) and leave in a loop. Insert the 6″ duct and then pull the ties up tight. A good metal cable tie gun will cinch these up tight enough even heavy PVC pipe will not slip down a wall.

b.                  Band Clamps

Another approach is to mount blocks with regular stainless steel band clamps. The clamp threads through a thin slot I cut in the side of the block. I then secure that block to a stud with a deck screw. That screw also secures the open edge of the block and squeezes down on the clamp to hold the clamp in place. This same approach works equally well with the nylon cable ties.

c.                  Wire

The traditional approach to have hanging ducts from a high ceiling simply uses heavy galvanized steel wire or plumbers strapping screwed to the ceiling or looped around a pipe. This works well, but is difficult to adjust without a lot of practice.

d.                  Wire Rope

Another solution for hanging duct is to use stranded metal cable (called wire rope) with special clamps that let you adjust the length. A number of firms make and sell different types of cable connectors. The most simple are the “8” shaped metal ferrules that you squeeze onto the cable with a crimping tool. Most boat supply stores sell both the cable and ferrules, or you can get them from a cable supply store. I bought a large spool and a pound of ferrules for what it would have cost me to buy just a few pieces from a boat store.

e.                  Suspension System

A far more elegant and expensive solution for hanging duct is to use wire rope with special adjustable fittings that allow you to quickly do the installs and set the pipe height. Just about all HVAC supply stores and ducting supply firms offer one or more of these types of systems. One of the better known that many woodworkers have used successfully is the

9.      Pipe Under Floor

Although most prefer to hang their ducting from the ceiling, there are also lots of ways to put your dust collection pipe under your floor.

 .                    Subfloor

For those who have a shop that sits on a heavy subfloor, it can be very convenient to run your ducting under the floor. I like a wooden sub floor in a shop because it is much easier on the legs. Both of my two larger shops had nice sub floors and I ran most of the ducting under those floors. For my first, I carefully designed and put every hole just where it needed to be. That worked for about two weeks until I bought a few more machines and ended up having to crawl back under the floor and redo all. For my second shop I put a fitting every six feet along the walls and down the center of the shop. Each had a plug and was covered by a piece of tile, so when I needed to make a change, I just had to lift the tile, remove the plug and put in a flex hose to the machine. That worked pretty well.

a.                  Concrete Slab

Cost and convenience often end up with many shops having a concrete slab floor. If you are fortunate enough to be building your shop from scratch, you can build in some trenches. I’ve helped build one of these type shops and did most right, but also learned a few lessons the hard way.

  1. Because I included both a sink and small shower, I ran water and drains in dug trenches, plus another trench to bring in power. Living right on the edge of an industrial area, I was fortunate enough to be able to bring in three-phase power. The floor I built was put over those utilities on well-tamped ground covered in 4″ of crushed rock then a vapor barrier.
  2. I also tapered each of my trenches with just a little grade and put drains at the end of each trench. The mistake I made with those drains was sharing them with the sewer drain without a water trap to kill any odors. I ended up having to reconnect them to my yard drains instead.
  3. I built my trench with a lip that will hold a 2×8 flush with the rest of the floor. That let me cover up the trench and not create problems moving around my equipment and tables that were on rollers. If I were to do this again today, I would make the trench sized with a lip to use the surplus aluminum raised computer room floor tiles. These are much stronger than the 2×6 and much easier to install and remove for access.
  4. Into my trench went the ducting, compressed air, and power. I protected the power by putting it in well-sealed PVC coax. Had I used that raised computer room flooring my trench would have been big enough to also include lines for my shop vacuum, drainage, and possibly water if permitted by my building codes.
  5. I carefully framed in the trench to make a nice rectangular pour. Even with rebar sides, my straight walled trench cracked. Those cracks made for a moisture problem that was not compatible with my galvanized ducting. A far better way to go is to make a nice smooth pour as shown in the picture.

b.                  Computer Floor

We are in an interesting time right now where the computer world is changing so fast that this leaves an opportunity for woodworkers. There are huge quantities of surplus very well made raised computer floor that provides a raised floor of 8″ to 12″ to permit running electrical, communications, cables, etc. under the floor using easily removed tiles mounted on stands. These surplus computer floor squares and uprights are readily available for little cost. They create a floor that is nothing short of incredible for woodworking. You can put in your ducting, power, water, and whatever else you need or want to run under that floor, then have easy access to make changes.

C. Ducting Components

1.      Ducting Accessories

There are many nice accessories that you can use with your dust collection, but realize that even with these accessories a dust collection system does some functions poorly.

.                    My first accessory mistake was buying a transition that mated my 4” dust collection hoses to the tiny 1.25” port on my band saw. So little air was moved I went back to using my powerful shop vacuum on all tools with small ports. I also bought the 5” Delta replacement port for my band saw lower port, plus used Lockline adjustable pickup duct with hood pickup above by the table attached to my strong shop vacuum. The rule is never hook a dust collector or cyclone to any size pipe less than 3” in diameter.

  1. My dust collector came with an upper and lower filter bag that worked poorly and required constant emptying because I made lots of dust from preparing rough stock. My next mistake was buying a metal trashcan and trashcan separator lid. The trashcan separator reduced the fire risks and saved on emptying the dust collector. When I bought a set of air gauges and did some testing, I realized that my trashcan separator was not good news at all. It pulled the real 1100 CFM I got from my dust collector through a 6” test pipe to under 450 CFM.
  2. My next mistake was replacing the dust collector bags with an easy to empty lower plastic bag and fine oversized fine upper filter bag. but I still constantly struggled with the new upper filter bag getting so full of dust it felt like cement and would barely pass air. Every filter cleaning left me and my shop covered in the fine dust I bought that fine bag to avoid. I later learned during the medical air quality testing done on my shop that my so called filter was a sieve that freely passed most of the finest unhealthiest invisible dust.
  3. I made my next mistake buying a “best” hobbyist cyclone system. It moved less air than my dust collector, just about half of what the vendor claimed in the advertizing. That cyclone killed the airflow below about 450 CFM, plus did not move enough air to keep the ducting clear. After digging into the engineering behind cyclones, I realized that almost all small shop cyclones were copies of the same original design. That design was never made to be used inside with fine filters, so I designed my own more efficient cyclone. It works well to power my downdraft table, collect chips from my router table and larger tools, and pick up the leaves that constantly get blown into my garage shop. It also is a huge help in cleaning my shop when used with a ShopSmith vacuum attachment. Do not use a dust collector as a vacuum because any steel screw or nail picked up hits the impeller and can put a spark in the collection bin. Also, most dust collectors make poor vacuums because the airspeed is too low for good pickup. My cyclone moved more air and also separated off the material before it got to the impeller so worked well with my new floor sweep. I hot melt glued in big neodymium super magnets on the front of that sweep to collect nails and screws before they get sucked up.
  4. Even with a good cyclone and vacuum, I found some tools still spray dust. When using them I wear my mask, open up the side door and garage doors and then run a big fan in the side door to keep that dust blowing outside. For quick cleanup after using these dusty tools I wear my mask, open all up with the fan running, and use my big compressor or leaf blower to quickly blow out the whole shop then turn on and leave on my ceiling mounted air cleaner.

2.      Blast Gates

You close the airflow to a machine with a blast gate. There are many different types of blast gates including many that can be opened automatically through electric motors, air pressure, and even hydraulics. The best place to put your blast gates is next to the wyes off your main line up high. The more open pipe or hose you leave exposed between the main run and the blast gate, the more resistance it causes. Also, if that pipe fills too much, opening the gate will cause all that material in the down drop to slam into the impeller potentially ruining motor bearings and even breaking impellers. It then goes into the filters potentially poking holes and greatly reducing filter life by requiring far more cleanings.

Many buy or make poorly designed blast gates and get very frustrated. Many of these gates, including expensive metal commercial gates, leak badly and get clogged with sawdust that prevents them from closing fully. Most used a slide that slips out of the way leaving an open track that will pick up sawdust even if the gate is set with the slide pointing down. Every time you close a gate like this it jams sawdust into that track. Eventually you pack in the dust so tightly that the gate will not fully close, will jam, and can even split. A much better design is shown here where the portion that closes goes all the way through ensuring there is no place for the sawdust to clog.

 

The only retail carrier I’ve found who that sells these types of “self cleaning” blast gates in 6” for an affordable price is Lee Valley Tools (search on blast gate). Otherwise buying this type of blast gate is difficult without a commercial license to buy from a wholesaler.

Because there was no available source when I needed my gates, I found provides an excellent set of well pictured directions on how to make these types of gates. Making your own similar gate is fairly easy. The only changes to Phil’s design that I recommend are using a split female PVC connector instead of pipe on the gates. The couplings let me connect directly and tightly to my PVC and metal ducting without leaks or having to use another often expensive and for sure bulky fitting. On a few of my gates that end up next to my machines and get connected with flex hose I instead use a short length of PVC pipe for the lower portion of the gate outlet. I prepare that pipe before gluing by first cutting six 2” long slits in the end made by my band saw in three cuts then wrapping that pipe with a heavy wire in a spiral that I hold in place with glue. This creates a threaded taper that lets me screw on my flex hose onto the pipe for an easy tight fit and no need for another expensive clamp. The result is tight enough that a piece of tape is ample to make a good seal.

This type of blast gate will work great if placed between your tools and cyclone, but for making the gates to go on a wye coming off of your blower outlet, you need a different kind of gate or valve. These gates work because the suction pulls the flat portion down tight to make a good air seal. With a gate on the other side where air is blowing on the gate, then the air pressure opens the gap in the gate. For these it is better to make a Wye with a diverter valve as pictured below that will swing to make the air close tightly on one side or the other.

One other important note is don’t go with the cutesy HVAC metal flanges on your blast gates that I see popping up on woodworking forums as a recommended way to make these gates. They do look pretty but greatly increase the cost of your gates, don’t seal well, are razor sharp so will cut with even a small touch, and unless modified don’t fit either standard dust collection pipe or even 2729 S&D PVC.

3.      Flex Hose & Hose Clamps

The internal ridges on rough or poorly made flex hose can create as much as nine times more resistance than smooth walled pipe of the same diameter. Even good smooth walled flex will increase resistance three or more times over straight duct. This resistance kills airflow, so when you use flex hose, always use minimum lengths and only use flex hose with smooth interior walls to get the best possible airflow from your blower. Additionally, there are flex hoses available with plastic reinforcement ribs, but these plastic ribs provide poor crush resistance and make this type of hose generate so much static electricity that it is not permitted in commercial shops subject to fire marshal inspections. Finding 6″ smooth walled flex hose with metal reinforced ribs that can be grounded can be a price shock. Good flex hose costs considerably more than the bulk rough interior walled 4″ stuff that many buy only to eventually learn that small diameter will not support the airflow required. I found Amazon.com, Wynn Environmental, and Northern Tool sell very good quality larger diameter smooth walled flex hose. Wynn continues to have the best pricing on hose if you buy 25’ lengths. Lesser quality hose with rougher ribs can be purchased through Grizzly.

Although most buy stainless metal band screw type clamps for securing their flex hose, standard stainless steel band clamps work poorly. When we use normal clamps the hose reinforcement ribs create a gap which keeps the hoses from sealing tightly all the way around, so the joints leak air and fine dust. A bridge or wire hose clamp provides a much better seal as these clamps bridge over the ducting reinforcement rib to create a good seal all the way around. Amazon.com sells expensive good wire clamp that bridge over the ribs. Wynn Environmental and LeeValley sell the best stainless steel bridged band clamps, but you need to buy these in either left or right handed versions to match the spiral on your hose.

4.      Fittings

Types: Many choose to run to their local box store and buy HVAC (heating ventilation and air conditioning) fittings. For the most part these are a poor choice just as they are for ducting. Unlike the thinner 30 gauge ducting pipes these fittings will not collapse, but if you look closely these fittings are all made for the air to flow away from the blower. The result is every joint is backward where the air ends up jamming against an interior lip. Most fittings can just be turned around, but some like wyes require changing the gender on each of the ports to keep from trapping sawdust and strings. I use a roller to flatten all the male ends and a crimper to convert the female ends to male. Moreover, few of these fittings seal real well or are that smooth inside. Many also have very tight angles and bends. The result in terms of efficiency is not too bad for each joint, but unless you are careful the total can add up to terrible performance. Many choose to avoid the work needed to clean up the HVAC joints by buying either PVC fittings or using actual dust collection fittings that are smooth walled without the joint reversal problems.

 .                   Wyes

You come off your main run with Wyes. There are many different types of wyes, but the best are going to be those that are smooth and either the most gentle angle or longest radius. Many choose to use the inexpensive sheet metal HVAC wyes found in home centers, but these have far more resistance and potential to cause plugging than what is appropriate for dust collection. As shown some configurations are far better than others.

a.                 Elbows

You change ducting direction with elbows. As with wyes, there are many different types and the best are going to be those that are smooth and have the longest radius. As with wyes the inexpensive sheet metal HVAC units found in home centers have far more resistance and potential to plug. As shown some configurations are far better than others.

Homemade Fittings: With the cost of both HVAC and PVC fittings so high, many choose to make their own fittings. I made quite a few of my own both in sheet metal and in PVC. At first I used a free sheet metal transition program I found on the Internet that let me print out full sized patterns and then cut out my own patterns in either PVC or metal remembering to add for the PVC extra thickness. Lots of work with my metal sheers and soldering torch or heat gun and PVC cement created whatever custom fitting I wanted. Eventually I changed to two different approaches after realizing I was mostly making 45 degree wye fittings in 6” PVC. I setup my lathe with a 6” piece of PVC made into a sanding drum by slotting then wrapping tightly in heavy sandpaper. The slot keeps the final drum diameter with sandpaper the same as the rest of the pipes. I use a protractor to set the angle at 45 degrees with a jig to move pipe into that piece. I also have a stubby precut pipe piece that I lay on other pipe where I want to mark a hole that gets cut with a saber saw. Just a little hand sanding on the hole leaves a near perfect joint. The other nicer approach I use is a little more work and takes far more skill, but makes better joints and allows using longer pipes. For this one I also mark where I am going to cut my hole in the pipe, but make the cut 1” all the way around too small. I then heat the pipe with a pair of heat guns and use a custom made wooden mandrel that I slip into the hole from the inside pushing out a perfect female fitting that when cool can be used to join a pipe.

Stan Harder developed a resource to help woodworkers save money when building their dust collection transitions. His free online software program creates templates to assist in cutting PVC pipes that can be joined together without the use of expensive pipe fittings. He has had good success using the thicker CA glues used by wood turners to make the joints. Thanks for sharing Stan!

5.      Transitions

You need a way to go from your 6″ round ducting to your filters, your blower, your cyclone, and some tools that have square or rectangular duct. These pieces are called transitions. There are three relatively straightforward approaches to getting a transition. The best is to build a transition that makes a perfect fit. Next best is to buy an HVAC fitting that is close then modify it to work. It turns out a 4″ x 10″ to 6″ round HVAC heating register fitting easily changes to be a 4.5″ x 9″ to 6″ transition. Likewise, I found by measuring the perimeter of the blower outlet that it just so happens that each of my designs is a perfect fit with one of the various sized round transitions when you compute the circumference. Least best is to change your tool or other unit to fit a standard transition. You can follow the below information to make your own transition. If you would like to learn more on making a transition and have a spreadsheet to calculate all of the specific distances, please see .

There is a fairly simple but lengthy way to build a transition. Sheet metal workers call this an evolution. Because my plans are scalable, your evolutions will change based on cyclone and blower sizes. I’ve done one by hand for a 13.5″ and another for an 18″ cyclone.

Ronald Scalise shared an easier jig technique he learned from a friend to quickly layout a transition for metalworking. Ronald said he is more used to working in thousandths of an inch and was amazed at this technique because it comes out just right every time. Ronald says, ”You need three things, a wood dowel a round disc the size of your incoming duct and a wooden rectangle the size of your inlet. Find the center of both the disc and rectangle then drill a hole through each and connect them using a dowel. The dowel needs to be exactly the length of the desired transition. Place the jig on the sheet metal and carefully roll it around using the jig as a ruler to mark your lines on the sheet metal. The result after marking all four sides of the rectangle is a perfect layout that only needs cut and formed to be done.

My friend who has done this for years made this look too easy. I ended up with way too many lines on the sheet metal my first try. Although it turns out to be easy, I do recommend starting with some practice paper to get the technique down. It only takes a few tries to get it down pat. I learned to start with the longest side and draw a line then follow the circle around as I turn the disc.”

The traditional way to build a transition is fairly simple and also works well, but takes more time. By drawing a view of that fitting looking down at the circular inlet you can see a circle and the rectangle that it joins.

Add to that drawing the fold lines used to transition from the circle to the rectangle. The fold lines in this view are red. Looking at these fold lines from the circle inlet gives us the actual length of a base of right triangle whose height will be the length of the fitting.

The diagonal for each of these right triangles is the actual length of the line used to layout the metal cutting/folding diagram. You can either use math to calculate the lengths of these fold lines or you can get them by drawing a right angle. This angle forms a right triangle as tall as the height of the fitting and base taken with a set of dividers from that view picture. You can then use dividers to go from the top to the base to get the diagonal. The more fold lines, the smoother your circle. Most metal workers find it is easiest to divide a circle into 24 parts making for 24 fold lines. Draw a fold line from each corner to the seven closest circle divisions. This means the first and seventh line each end up going to two corners. Once you have the lengths of the seven fold lines that go from each corner, you have all it takes to layout your cutting diagram.

This is all the information you need to actually draw your layout for your cutting diagram. Start by drawing with a horizontal line that is the length of the longest side of the fitting rectangle. Use a compass to set the length of line one. Swing arcs from either end of that horizontal line to set the top of that triangle. I used a second compass that is set to the length of an arc that is 1/24th of a circle. To get that length I drew a full sized circle, then divided the circle into sixths, then twelfths, then twenty-fourths. If you don’t know how to do this, you have to look it up yourself. (Radius will give you sixths, splitting any angle will give you 12ths, then one more split for twenty-fourths.)

 

Carefully in order add the lines that go to that corner joining the first two to make a triangle. Each successive line makes another triangle whose base equals the length of a segment that is 24th of a circle.

After drawing seven lines, the eighth line starts from where the seventh ended and creates a triangle whose base is the shorter rectangle size.

Anyhow when it is all said and done, the result plus 3/8″ of soldering tabs gives the transition duct. I only showed the soldering tabs for the sides. You also need them for both the circular part and the rectangular part. I used those extra tabs to solder a 2″ ring onto the circular part to mate with my PVC.

D. Tool Ducting

Each machine needs the dust collection to protect, control and deliver good fine dust collection. Each machine requires appropriate hoods to keep the airflow from our blades, bits, cutters, belts, motor fans, etc. from launching the fine dust before it can be collected. These hoods need to control the dust and then deliver it right to the dust port for collection. Sadly, most small shop tools come with internal ducting, hoods, and ports far too small and too restrictive to effectively protect, control, and deliver the fine dust. That means we almost always have to do extensive tool modifications. In some cases we cannot amply fix our tools, so we must use them with a powerful downdraft table. Some even a downdraft table cannot help, so these either need used outside or replaced.

1.      Dust Ports

The connection from your machine to your ducting is often a problem with hobbyist equipment. In looking at the CFM requirements table it is clear that most stationary small shop tools with a single port connection need that port enlarged to a 6″ port. Most with the need for two collection ports should have a larger 5’ port and a smaller 3.5” port. Often we cannot get the 3.5” ducting, but using a 5” with a 4” on two port machines still works well. The total area of the ports should closely match the area of our ducting. If that total area is too small we kill our airflow needed for good fine dust collection and can create dangerous dust piles in our ducting. If the combined area is too large we lose the airspeed needed to pull in the dust and keep all moving in our ducting. Because the small shop industry remains in the “dark ages” of chip collection in terms of only going after the same dust we otherwise sweep up with a broom, most small shop users will have to upgrade their machine dust ports themselves. It is a real shame to buy a top notch piece of equipment and suddenly have to make a big 5” or 6″ hole in it. Although many use HVAC connections it is far better to use actual dust collection ports that at least leave a good looking smooth strong port. I like the Lindab and Nordfab smooth walled laser welded steel take off flanges plus they and have a built in gasket.

I heard from Steven Thompson that a good source for these parts and other DC ducting components is Mechanical Equipment in Georgia at (770) 963-6226. He said they have great prices and are willing to ship UPS.

2.      Dust Hoods

Dust collection hoods and pickups are one of the least explored areas in small shop dust collection, yet one of the most important. Although many vendors would like us to believe good fine dust collection is all brand new and about as complicated as rocket science, the reality is we have decades of air engineering experience that show just what we need in terms of our hoods and airflow. A good hood must do the following things:

.                    Safety – A good hood needs to provide a reasonable measure of safety. This means it helps us keep away from our blades, bits and cutters while also protecting us from kickback and flying debris. For saw blades I strongly recommend use of hoods that incorporate at least a splitter or riving knives plus use anti-kickback pawls. Our hoods also need to be made of something strong enough that if things go flying we get good protection. I strongly recommend hoods be made of either metal or the polycarbonate plastic used in safety glasses, bulletproof windows, etc. I made the side of a blower from this type of material to see when it was time to clean the impeller. One of my test impellers exploded inside my blower. That 1/8″ thick polycarbonate plastic held up better than the metal parts.

  1. Visibility – A good hood also in my opinion needs to let us see what we are doing. Strangely, some saw blade and router guards are smoky or solid steel like the one that came with my European saw. I admit my saw guard soon got removed and ignored because taking it on and off to ensure my cuts were aligned correctly was too much trouble. I strongly prefer a clear polycarbonate plastic saw guard because it gives excellent visibility and minimizes a buildup of static adhered dust so we don’t have to do a lot of cleaning.
  2. Chip Collection – A hood also must provide good chip collection meaning collect the same sawdust and chips that we would otherwise sweep up with a broom. It is real simple. Our saw blade tips along with many of our other blades, bits and cutters launch dust at over 100 miles an hour. Even a powerful cyclone like my design only moves air at about 60 miles an hour. Unless our hoods block the fast moving airflows, we lose and are going to have sawdust and chips all over.
  3. Fine Dust Collection – For those who want good fine dust collection, a good hood must also move enough air in the right places. If our tools are made from the ground up to totally trap all the sawdust and chips we make then a good shop vacuum will provide excellent dust collection. Because most of us use do not use tools built from the ground up for good fine dust collection we need to instead use a different approach. What decades of air engineering found works with our older tool designs is having hoods that block the fast moving streams. These hoods then also must move a large volume of air around the working areas of our tools. We know that the slightest breath can move the fine airborne particles, so to keep the fine dust from being blown all over our shops, we need to surround the working areas of our tools with air moving fast enough to overcome normal room air currents. Again the research followed by decades of refinement show what we need in terms of hoods and what is needed in terms of total airflow. It only takes about 50 feet per minute airspeed to overcome normal room air currents, but we have to provide that airspeed over such a large area that we need about three times more total airflow for good fine dust collection as we do for good “chip collection”. Because air at dust collection pressures will barely compress at all, any undersized hose, duct, or port will restrict our flow. This means almost all of our typical 4” tool ports and ports on our 4” hoods must be need increased to 6” to carry ample air. For tools with two ports, we generally need a 6” down drop that ideally splits into a 5” and 3.5” separate hoods. Since the 3.5” ports and ducting is near impossible to find, we still can go fairly well using a 4” instead. I originally thought I could get by with using my existing 2.5″ port on my splitter mounted blade guard on my table saw. The CFM Requirements chart showed I needed at least 350 CFM, but testing the airflow with that smaller port showed only 183 CFM. In short, for good fine dust collection we almost always have to rebuild our hoods and use larger ports.

I recently did air quality testing all over the State of California. One of these tests was at an engineer friend’s shop with the testing coming out worse than ugly. He carefully followed my instructions from these pages building his own cyclone from my plans, upgraded to the big 16” blower impeller from Clear Vue Cyclones and built one of the prettiest carefully laid out metal ducting systems I’ve seen. Without doing any woodworking at all, our just moving around in his very clean looking shop stirred up enough of the fine invisible dust that his shop failed the EPA, medical and European air quality standards. I saw the problem but rather than tell him what was wrong we then did a standard test. We started using his table saw to cut 54 linear feet of ¾” thick MDF. Within seconds his shop air quality went well over fifty times worse to exceed the OSHA air quality maximum. I showed him what was wrong while his big exhaust fan and cyclone rapidly made the air quality safe.

The problem was the normal. Decades of air engineering show that if we do not capture the fine dust as it is made, we are going to fail the air quality test. The only way to capture the fine dust as it is made is to start with hoods that block the fast moving air streams. His dust problem was simple. His big new over arm blade guard was wide open in front so launched the dust off the tip of the saw blade right out under his guard. A 3450 RPM blade speed with a 10” diameter blade creates a 102 miles an hour air stream. His saw pulleys increase the blade speed closer to 4000 RPM so the blade tips launched the dust even faster. A typical dust collector or cyclone only moves air at about 40 miles an hour. With the hot rod oversized impeller on his cyclone, he was got 60 mile per hour air speed which had zero chance of capturing the well over 100 mile an hour dust stream. To effectively control the fine dust as it is made we must have hoods that mechanically block all the fast moving air streams or there is zero chance of effectively capturing the fine dust before it escapes collection.

Shark Guard – My friend suffers from a common engineer disease meaning he is a touch obsessive sort of like a mule is a touch stubborn. He was really upset at that air quality test and soon called me back ordering me to give his shop a retest. In place of the blade guard that came with his over arm blade guard system my friend added a pretty amazing product. He replaced his over arm saw guard with a new pictured here made by Lee Styron. That guard rested on the wood and blocked the fast moving airstream that was spraying the fine dust all over. The bottom line is we again tested his shop and with that guard in place he got none of the dust that sprayed out all over in front of his saw and his air continued to test clean. Frankly, I was so impressed that I contacted Lee and ordered up the works for myself including his 4” dust port, tail gate, and special splitter with anti-kickback pawls that fits my European saw plus picked chose to have the metal parts in a bight easy to see safety red.

In addition to having good collection over the saw blade, it is even more important to have good collection below the blade because most of the dust from table saw use gets sent downward. Cabinet saws enclose the lower portion of the saw and generally come with a 4″ port in the cabinet. A 4” port will only support about 350 CFM, but the airflow charts say we need 550 CFM for good fine dust collection. This means we need to open the port below our saws to a full 5″.

Dustroyer – Contractor saws also need good collection below the blade, but most lack the enclosures needed to control the dust sprayed below the blade. This makes collecting the dust from cabinet saws one of the more difficult dust collection challenges. Eric Fuller, a fellow woodworker decided to address this problem and made some clear plastic covers held in place by magnets to do the job. These worked so well that he shared and suddenly found himself the expert on cabinet saw dust collection. He has the needed covers to provide good under the table collection for a wide range of contractor saws. Check out his covers and his video on his Dustroyer web page.

The same is true for all our tools. If we don’t have good hoods that control the fast moving airflows and move enough air, there is no chance of getting good fine dust collection. Fortunately, the better air engineering firms who certify that their customer shops will pass the air quality tests have been kind enough along with the EPA to share different hood designs that we know work well. Each of these hoods does a good job catching the fast moving heavier sawdust and chips, plus mechanically blocks the fast moving air streams. Even with good hoods, that still leaves most small shop dust collector and cyclone users in trouble because our vendors sell us equipment that moves about half of the maximum airflows they advertise. These lower airflows leave us with a bad false sense of security, meaning clean looking shops that tend to build up dangerously high amounts of fine invisible dust. AAF was kind enough to let me share one of their publications with their designs for dust collection hoods. I also have slowly been adding other hood designs that work well:

 

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