Wood shops harbor many perils. Obviously, there are the tools themselves, electrical wiring, heavy objects, trip hazards, poisonous and flammable fumes and fluids, etc. It turns out that the wood dust created by power machinery is, itself, a dangerous and often-ignored carcinogen. Our "dust redistribution" system is not the best, but I describe it here in the event that the information might be useful to others.
First, our bodies are able to deal with larger, common dust particles quite reliably, and common air is filled with particles that we are constantly filtering. However, the smallest particles, those under about 10 microns in size, tend to go right past our body's filtering system and can end up deep in the lungs where they irritate the tissue to differing degrees depending upon the material, concentration and our sensitivity. Long term irritation of this kind might result in any number of chronic diseases, including those that accelerate one's otherwise distant demise; most people like to avoid this, myself included.
The little paper masks sold everywhere, though fine for preventing large particle entry -- particles the body can deal with anyway -- appear practically useless when it comes to filtering these small particles. Unfortunately, most of the dust collection systems sold are also a joke. Shop vacuums and most air filters weed out the largest particles -- the visible chips -- but pump the small ones right back into the air where they can circulate for hours. Once the particles are in the air, you have to scrub the air or change it. For complete protection you must constantly wear a full-face, highest quality respirator while in the shop, and/or have a high quality dust-handling system that deals with these smallest of particles at the source while constantly purifying the air of floaters that escaped initial collection.
No one wants to wear a respirator all day long, and such dust-handling systems are difficult to design, more difficult to install, and cost a LOT of money. We wear the full-face respirators when working with machines that generate considerable dust, and we keep the dust handlers running any time a machine is running, and often for some time thereafter.
Most consumer tools come with 2 or 2 1/2 inch dust ports. The next step up is 4 inch ports; larger ports are found only on the largest machines in production shops. The bad news is that, under most circumstances, even a 4 inch port, good ducting and a good 110V blower is insufficient to capture the dust at the source. For example, engineers recommend an airflow of almost 800 cubic feet per minute to capture the majority of the dust generated by a typical table saw (the figure covers saws up to 16" in diameter -- smaller saws do not require quite this much airflow). At 110V a really good blower can push out a little over 1000 cubic feet per minute if it has little or no line resistance. An ideal, simple system would have 6" ports and 6" or larger ducting all the way to the blower. To keep larger chips and particles moving through the lines, the air must have a linear speed around 4000 feet per minute. CFM = FPM x π * r^2, so solving for the duct radius, r: r = sqrt(CFM / (FPM * π)) = sqrt (800 / (4000 * 3.141)) = 0.25 feet. The needed duct diameter is therefore at least 6 inches to meet these minimums. A (much) more powerful blower might allow the selection of smaller ducting, but we are talking probably about a 5HP blower, very efficient designs, etc.
The above reality points out the raw futility of about 90% of consumer dust collection systems in use. They are usually underpowered at the blower, do not actually retain the small and damaging particles (but pump them right back into the air), and do not move enough air due to hose and ducting restrictions to collect the problem particles in the first place. They collect the chips that might be unsightly, but recirculate the ones that are actually dangerous. When designing a genuinely useful system, most designers end up with a cyclone-style system, a 2+HP motor, and 6" or larger ducting. But we went at this a little differently due to space and financial constraints.
First, there are two reasons for choosing the cyclone/impeller - style system: when properly designed it does divide out small particles quite well, allowing the bags and cartridges to do what they do best without getting clogged quickly; the impeller can handle the hits it gets from the larger particles and debris. In our system we attempt to get all of the larger particles to drop out before getting to the blower, and we simply blow the air containing the small particles into the outside atmosphere where God's nature takes over (and does an excellent job, I might add).
Most of our tools have 4" ports. The NX31 table saw has a silly 2 1/2" port under the table saw blade, and an even sillier 1" one above the blade. Not ideal, but better than nothing... maybe. We use a 4" hose to connect our tooling to a shop made particle separator -- basically a poor-man's cyclone assembly, sans the impeller, on top of a 30 gallon can. The highest linear air velocity will be in the 4" line between the tool and the can, and it is almost always enough to keep the hoses clear/scoured and have some air movement at the tool. However, a lot of dust still gets spun free of the collection hoses by the machinery. There just isn't enough air moving to capture the wind from a 12" blade at 3500 rpm.
Side note: We used to have the commonly-available trashcan separator lid. It is a piece of you-know-what. We then made our own separator lid modeled after a cyclone pump and experienced an immediate, significant increase in airflow. It used to be that we couldn't get our 4" flex hose to pick up dust off the floor without placing it directly over the dust. Now, it sucks up almost anything -- even heavier chips and chunks -- within 6" of the opening. It looks like we might even need to do some redesign on our trash can separator because the increased flow is keeping it a little too clean.
The first real velocity drop takes place in the can. The velocity drop, swirling motion and turbulence allow large particles to fall out of the air stream and into the bottom of the can. A 6" line proceeds straight up from the center of the can lid, up a couple feet where it transitions to 8" and connects with our main, 8" duct. I've opened up the 8" line for inspection and cleaning from time to time while the blower was running. It can really move air when the resistance is gone.
One time a couple years ago "the help" used the system to vacuum the shop and allowed the can to overfill. This was an earlier stage in our design and a lot of large particulate was pulled up into the 8" horizontal line where it sat until I opened the inspection/cleaning port one day. The blower was on, and with the decreased resistance, it cranked up and scoured the line clean. All of the garbage got sucked into the blower and it began shaking badly. It is a squirrel cage blower, but with some tedious cleaning and picking, all was fine.
Ignoring leaks, there is the same volume of air moving throughout the entire system, but the linear air speed drops by 75% between the 4" and the 8" duct. Anything of measurable mass is going to fall out and back into the can (hopefully). Only the smallest, lightest particles will remain suspended in the air flow, getting pushed through the 8" line, to the blower and outside. This way, no chips hit the blower.
This kind of system makes for some cold days in the shop in winter and hot days in the summer, since any air it moves gets replaced by air pulled into the shop from the house or outside, but it keeps it much cleaner. If you have a gas furnace or anything of the like, however, it is very important not to implement this kind of a system, as it could pull gas into your house and shop, blow up the whole building, and basically ruin your whole day.
By keeping the system running, we are constantly changing the air in the shop, too, so any dust that escapes collection at the source eventually gets picked up. We custom built the blower box to hold the largest 120V blower we could find. It takes about 3000 FPM to keep wood particles moving horizontally in a pipe. Given our tests, we know we are getting at least 1000 CFM at low resistance (with the inspection/cleaning ports open on the 8" line). We're sure that this drops once we add the can and 4" lines, but it is working thus far. Even if we are getting only half of that much air movement, our shop contains 8000 cubic feet of air, which means that our system "changes" the shop air every 20 minutes or so. I quote "changes" because it is really a matter of constantly diluting the dust-contaminated shop air with outside air, not really changing or replacing it. It isn't as if we get completely new, outside air every 20 minutes, but probably more like the inside air is diluted by 50% every 20 minutes or so.
It turns out that too much dust is actually getting pulled outside by our present system, not trapped in the can. Big chips are settling in our horizontal eight inch line, the heaviest chips staying in the collection can, and everything else is getting blown into our back yard. I upgraded all of our tools and collection lines to six inch diameter, and inserted a ClearVue cyclone separator. All of the changes took a couple days of work and about $1500 in parts from various sources.
As seen in the picture, the cyclone is supported by a wood frame and a rope/pulley system. It is merely resting on the can and a foam seal. When the can fills, a pull on the rope lifts the cyclone assembly off of the can so it may be dumped. It all takes about a minute. However, this meant that the exhaust line had to be flexible, so one of the duct joints is not sealed with tape, but covered with flexible eight inch hose so that it can move when the cyclone is lifted without creating an air leak.
The maker of the cyclone separator said that our blower wouldn't have enough power to work, but it turns out that it works fine as long as we don't get too much resistance in the line. When resistance increases, airflow decreases, and the forces that separate dust in the cyclone diminish, resulting in more fine dust getting pulled out the exhaust line. By upgrading all of our tools to six inch dust collection ports, the resistance is about as low as we're going to get it and still maintain the air velocity necessary to carry the chips. It is much better than our prior setup. I can hand-dump two cups of saw dust directly into the line and see almost nothing coming out of the exhaust.
We also have much greater airflow at the tools now. The Laguna/Robland NX31 was clearly not designed with efficient dust collection in mind, but now runs almost dustless. This required pulling all of the plates and plugs off of the blade shroud, covering and plugging almost every hole in the tool, and creating a tight skirt around where it meets the floor. Most of this was done with magnetic sheet material. Now there is significant airflow downward around the blade and chips and dust from cuts drop straight to the floor, where they are picked up by one of two four inch hoses that come together outside the machine into our six inch general collection hose.
With these changes, we now have much better dust collection at all of the tools, and far more dust is being trapped in our collection can and not being blown outside (where it makes a mess or might annoy neighbors).
Downdraft Sanding Table
I've been looking for a downdraft sanding table for use in our shop for a couple years without finding the exact one we wanted. These were the requirements:
- 33 7/8" tall to match our NX-31 combination machine and Sjoberg workbench
- About 2 feet by 4 feet footprint to move easily through the shop
- Heavy duty locking casters
- Can connect directly to our dust collection system -- doesn't need an internal blower and filters
- Can store sand paper and sanders
- Wired for power
- Can hold ~1000 pounds
Not finding anything meeting these desires, and seeing that commercial ones run about $2000, we made our own. It cost all of $139 for the MDF, casters and some 4" ABS fitting from Lowe's. It took about one full day to build.
I'm not sure about its weight capacity yet -- I may need to do some reinforcement on the lower deck. But otherwise it works great.
In 2007 when upgrading our general dust collection, we added sliding walls to the downdraft table to improve the direction of the airflow. Total materials for the table are now about $200.
We offer community classes on beginning and intermediate woodworking one evening per week. Give us a call if you are interested.
Inexpensive Router Table
No need to spend hundreds on a router table and lift mechanism. If you have a common Porter Cable router with a 6931 plunge base, all you need is about $3 in hardware and scrap plywood to make your own lift-table router. Assuming you already have the router and basic tools, this is all you need:
- 12" x 5/16" threaded rod
- 2 5/16" nuts -- preferably well nuts
- 5/16 fender washer
- Thick acrylic sheet (sometimes available free as a scrap from glass-shops)
- Set the plunge clamp so that the router is at about mid stroke and the router is resting upside down.
- Remove the plastic base.
- Remove the nylon lock nut at the top of the 5/16 threaded rod and the two thumbscrews.
- While applying hand pressure to the router to keep it from springing apart, release the holding clamp and slowly lift the base from the unit.
- Remove the spring and the plastic insert from the shaft that did not contain the threaded rod.
- Replace the spring and re-assemble the router.
- Turn the router right side up and file the paint from around the hole that previously contained the plastic insert.
- Affix a 5/16 nut over the hole using JB Weld or similar adhesive. Be sure not to mess up the threads or block the hole.
- Once cured, insert a ~12" x 5/16" threaded rod (or, better-yet, hex head washer and bolt of equivalent length) through the related hole in the base of the router, and through the nut added in the previous step. Place a washer on the end of the rod and another 5/16 nut. Affix the nut using JB Weld, super glue, thread lock or similar adhesive so that it is firmly attached to the rod and flush with the end.
- Using the thickest acrylic sheet available (up to 3/8"), affix your router to a melamine, MDF or similar board. Make sure there is an additional hole in the acrylic so you can access the 5/16 nut with a socket.
The rest is fairly obvious. We just could see spending $200 - $500 for something we could make for $20 using parts we already had or could get from the local hardware store. The great thing about this is that you can put 4" or larger dust ports on it -- commercial ones have only 2 1/2" or smaller -- and actually have a chance at collecting some of the damaging dust.
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