# The Real Numbers Behind Effective Dust Collection.



## JAAune (Jan 22, 2012)

I've known for a few years that throwing HP at dust collection is inefficient and doesn't always achieve optimal results. We use 100cfm shop vacs on our CNC routers and get almost all of the dust thanks to AXYZ-inspired dust boots.

Only this week did I find an article that explained the concept using actual numbers backed by objective, real-world testing.

ShopHacks.com: Table Saw Dust Collection

The gist of of it is that collecting dust is not about the cfm, it's about negative pressure. If the source of sawdust is enclosed and just enough openings are left to produce airflow across the dust source and into the duct, the result is near 100% collection. The amount of CFM needed is calculated by finding out how much air leaks into the enclosure and how much needs to be pulled out to obtain 1.4-2.8 WC of pressure.

I don't have the instruments to test the numbers provided by the author of the linked article, but based upon my personal experience designing jigs and machinery, his method is sound. He even mentions discussing the topic with Bill Pentz who agreed the CFM figures on Pentz's website assume people are using poorly-designed machinery.


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## bigblockyeti (Sep 9, 2013)

This is how I've approached collection on my Unisaw, most openings are sheilded in one way or another to allow minimal (not zero) air leakage into the cabinet at various locations to keep the dust flung from the blade in suspension to the port located at the bottom of the cabinet. The biggest caveat is the air being blown above the table, and work being cut as it acts as a centrifugal fan. The surface speed of each tooth acting as an impeller blade spinning at 4300rpm is far greater than a 1" router bit spinning at 23000rpm, coupled with the fact the dust boot on a CNC router would be around the entire circumference of the bit and the difficulty of surrounding 360° of table saw blade means more will escape, but I'm still constantly trying to make it a little better.


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## JAAune (Jan 22, 2012)

The spinning tablesaw blade issue is why overhead collection may be a necessity to get close to 100% collection. The dust source is not totally enclosed as long as any portion of the blade is exposed.

Something the article writer fails to mention is that the shop vac actually has an advantage over a dust collector whenever the openings are restricted enough to reduce CFM requirements below 100. Our MiniMax tablesaw does have a halfway-decent blade shroud so we haven't modified that machine yet. But I do want to improve it and test a shop vac and see if it's more effective than the 1.5HP collector currently dedicated to the saw.


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## JAAune (Jan 22, 2012)

I still don't have instruments to test vacuum pressure but here's some info on how I've modified one bandsaw to catch more dust.

First is the Grizzly 513 Anniversary edition. I'm assuming the standard 513 saw is exactly the same minus the black paint job.










The manufacturer did put some thought into dust collection. The cabinet seals tightly and there is a metal baffle directing sawdust into a 4" port under the table. There's even a nylon brush to knock dust off the blade and towards the duct. Unfortunately they didn't take the final step to enclose the gap between the table and the cabinet so a lot of dust escapes before entering the collection zone.










This is a short piece 3.5" ID flex hose from McMaster-Carr It's pretty springy and holds shape well. I think it's the neoprene-coated item# 5619K41.










Massive improvement. The piece of hose does have to come out if the table is tilted much but that's not often.


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## kelvancra (May 4, 2010)

Yes, that gap. I used PVC on my oscillating sander and that cranked up the collection on it.

So to my little Powermatic a bit of 6" PVC pipe and the collection in that area is much better. HOWEVER, doing logs, the gap between the bottom of the log and the table is a major dust source, so the only solution is another hose in that area.

The 531 just before the PM really builds up a lot of dust around the lower guides, and that is a major area of dust pollution all around the saw. As such, the DIY solution for collecting around the guides is a must. To that end, I was thinking of pulling from my Dust Deputy to see if that made a big difference. Next would be another Y to pull off the upper and lower collection port hoses.


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## rwe2156 (May 7, 2014)

I agree its the dust on on top of a table saw that's important. Collecting the cabinet is simply akin cleaning a dust bin. Its difficult to create enough negative pressure due to openings for the wheel slots and space between top and cabinet.

I open the blast gate and clean out my table saw cabinet once every few months.


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## tvrgeek (Nov 19, 2013)

I read this bit a while ago. It does not address that there is not sufficient velocity across the gullets to clear the dust before it exits back up. I want the air I breath to be clean, I don't care about inside a cabinet.

It also does not work as he claims. Tired it. Only after reducing the cabinet holes to about the same area as my port did I have much better success and then going from a 4 inch port to 6 inch.

What he does not do is differentiate the difference between the large dust ( nuisance) from the fines in the air (dangerous)

I do like that band saw mod. My BS has 2 4 inch ports, but does not get flow right as the blade exits the work, where you need it. That is the problem with TC collection. The airflow is in the wrong place.

Instrumentation? Who can't make a slack tube monometer? Just some vinyl tube and a ruler.

A top side collection is needed, not just for what is still carried by the blade, but for the additional small amount by the blade not being perfect in the slot and of course any time taking a shavings off the side of a board. Most top side ducts are way too small and again flow is not in the right place.

I will be looking carefully at the trunnion when I get my saw Monday. I think a focus jet like a leaf blower, across the lower shroud as close to the bottom on the insert to get the dust away from the blade is needed.

Notice the Pentz designed cyclones are designed to run at a higher vacuum than some. 5 HP designed for a 6 inch duct. Of course Pentz assumes the tools are poorly designed. They all are.

I wonder if the shape of the gullet could be modified to actively produce airflow across.


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## kelvancra (May 4, 2010)

It is unfortunate, for those of us trying to grab as much of the fines our tools kick off, companies don't make their collection ports like those on dust collectors so we could just unscrew the flanged splitter to mount a larger hose.

I made my own zero clearance plates for my little PM from some aluminum diamond plate I had. It was the perfect thickness and, installed upside down, nice and smooth. Before installing it, I drilled holes in it and that helped the pickup at the throat notably.

Another helpful thing on the bandsaw was, when I made my circle cutting jig, I didn't stop at the blade like most do. Because I wanted to cut large circles, I wanted all the counter weight I could get. This had the added benefit of solving the problem we ALWAYS get cutting on the edge of a piece of wood, be it with the bandsaw or a tablesaw.

With nothing to the other side of the blade, the dust being thrown off the blade is not forced down a kerf. It's free to exit before it can be pulled through the table top.

I have a carving machine (I'm the CNC-serveer motor part). Just hanging a four inch hose near the little Colt router makes a night and day difference. Not so with a shop vac and a 2-1/2, high velocity hose. At the end of a job, there is very little dust or chips to clean up.

My LITTLE planer hates 2-1/2 collection. Just like trying to vacuum pine cones in my yard after they've started to open, you'll be unplugging the hose frequently. The four inch [on the end of a 3hp-4 "bag" system keeps it all but spotless.

As with the planer, the router table keeps pretty clean. It also uses the big collector, but has a split at the unit, with the upper port a reduced 2-1/2.

Over on the lathe, I have to move a LOT of air to collect the fines and most of what's tossed off spindles. Other things require more directed collection. The "unused parts of" spindles can be kept off the floor using some 8" PVC with the ends capped, the front cut out and port on the back. It needs a very movable, articulating stand.

My zero clearance plates for the table saw could, probably, use a few more holes, if only to allow the collector to breath better. As it is, even if I'm not using the over-arm collector, I leave it on, via a Y, so I don't throttle the collector input to just the adjustment wheel slots, the splitter mount [out the back] and the zero clearance plate.

The miter was the other of the hardest of all the tools to deal with and I resorted to the Rousue (sp?) cover. An old big screen, projector case worked, but ate too much room.

Future projects include: (1) using a CPAP machine placed inside a box with at least two pleated filters in the MERV 11 TO 13 range for my version of a $400.00 air fed hood using a cheap HF shield; (2) a box with no less than two 5" thick filters, plus pre-filters, to scrub air; (3) a couple squirrel cages, described in my post above, for the ability to fire up the ELECTRIC leaf blower, to exhaust all the settled fines over to the neighbor's pool.


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## Finn (May 26, 2010)

I tried using shop vacs for dust collecting but killed three of them. They cannot endure long running times needed. I now have a 6" Dust collector system and have given up on shop vacs.


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## JAAune (Jan 22, 2012)

> Of course Pentz assumes the tools are poorly designed. They all are.
> 
> - tvrgeek


Most are badly designed but not all. Examining different tools and seeing what works and what doesn't is helpful.

Festool gets decent collection using 1.25" hoses since they design collection efficiency into their tools. It's not always sufficient but it's far better that the collection efficiency on most stationary tools (before modding).

Many industrial-scale machines house the entire cutting area in an enclosure then stick massive dust ports on it to control dust but that's not helpful to most of us. Few can afford the 6-figure, outdoor collectors they use to accomplish that.

The purpose of this thread is to direct more attention to efficient dust collection design for tools to maximize the benefit for every dollar spent. The majority of dust collection topics center on dust collectors and air filters yet the biggest gains are obtained from revisiting tool design.

I've modified our 14" Delta bandsaw to improve collection and will post pictures of that when I'm at the shop. It's a new mod using 2" hose and hasn't been tested thoroughly yet. My initial observation indicates it compares favorably to the Grizzly and its dual 4" ports. It's certainly an improvement over the badly located 4" opening that is built into the Delta.



> I tried using shop vacs for dust collecting but killed three of them. They cannot endure long running times needed. I now have a 6" Dust collector system and have given up on shop vacs.
> 
> - Jim Finn


I mentioned shop vacs as a generic statement. We're using Ametek vac motors with bypass cooling and build custom housings for them. They don't burn up. Brushes wear out but can be replaced. These motors can run for hours non-stop. The 220volt models are my favorite since they halve amp requirements.

Long term, I'm planning to scavenge the 3-stage fans off the Ameteks then fit them with pulleys to connect them to induction motors. Should be an interesting experiment.


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## JAAune (Jan 22, 2012)

> My LITTLE planer hates 2-1/2 collection. Just like trying to vacuum pine cones in my yard after they ve started to open, you ll be unplugging the hose frequently. The four inch [on the end of a 3hp-4 "bag" system keeps it all but spotless.
> 
> - Kelly


Shavings and small hoses don't mix. Our planers are connected to the dust collector. So are the stationary sanders. But we have to be efficient with dust collection capacity because if we tried to extract dust with pure cfm, we'd need a 15Hp, 5,000cfm collector to power a 3-man shop and that's not accounting for future growth. Putting that outside would require a makeup air system and a new heating system to reduce heat loss. Add in a 3phase panel, wiring and large diameter ducting and the cost for that system becomes prohibitive for at least the near future.


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## JAAune (Jan 22, 2012)

Here are the pictures of two more bandsaws.

First is a Craftsman 9" as it came from the factory. It's shelved at the moment due to a shaft issue on the lower wheel that I haven't fixed yet. It's also full of dust since it was last used inside a church steeple and didn't have dust collection hooked up.



















This machine also has an annoying gap between the table and blade but is otherwise has the best dust collection design I've seen on a bandsaw. Notice the front cover forms a complete seal for the entire machine. Internal baffles surround the lower wheel so air can only flow from the opening near the lower guides and into the vac port at the lower rear of the wheel housing. Without a vacuum hooked up, sawdust shoots out the open vac port. With a vacuum connected, collection is excellent.

Next up is a common Delta 14" saw but one of the newer ones that had upgrades like quick-release tension and a 4" dust port.










There are large gaps everywhere on this saw and sealing them up would not be easy. Most of the airflow into the 4" port is wasted drawing from everywhere except the dust source. Someday this machine will probably be replaced with one that has a better enclosure but for now, it's been modded.

Getting at the dust source on this tool was problematic. The lower guide assembly was filling up the gap between the table and cabinet and left no room for the common pipe-under-the-table method other people were using. Since the lower guides never get adjusted anyway, I opted in favor of removing the assembly for improved dust collection. There's now a plywood housing mounted under the table that surrounds the blade. It's hard to see without removing the table. The duct tape covers the slot that makes it possible to slide the plywood block into the blade. If the table needs to tilt, the duct tape can come off.










There are specks of dust around the saw. The dust collection is right at the source so it's very effective but it needs more time to clear the dust from the blade. The 1.25" long enclosure is too short.

I believe the Craftsman setup with a revised table that eliminates the gap between the table and cabinet would be the ideal to strive for. My long term goal would be to locate a bandsaw that is a viable candidate for a table replacement mod.


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