Zero Clearance Insert, So, Zero Dust

[Howard Acheson]

>>>> on the intake opening necessary at the front end of the base to vent through through the shroud inlet I would be grateful.

The formular for number of square inches in a circle is A=3.14 x r squared. For example. If you have 6" of exhaust opening, you need 3.14 x 9 = 28 1/4 square inches of intake opening.

[Bruce Seidner]

>>>> on the intake opening necessary at the front end of the base to vent through through the shroud inlet I would be grateful.

The formula for number of square inches in a circle is A=3.14 x r squared. For example. If you have 6" of exhaust opening, you need 3.14 x 9 = 28 1/4 square inches of intake opening.

I believe I get that dust collection is about both the volume and velocity of moving air. The Pentz example of controlling the movement of a balloon in a room blowing at it through a straw versus sucking at it some distance away is not lost on even one of little brain like myself. Since restricting the volume of an inlet would increase the velocity through that inlet it could be directed at the saw blade and arguably getting a bit of material out of the gullets. This would be at the expense of volume, and because opening enough intakes to avoid restricting the CFM of the native system is important it seems to negate the idea of directing inlet ports to the blade.

Am I missing something here, I want my cake but also realize I can't eat it too. Perhaps directing all 28 1/4 square inches of intake, with multiple inlets starting from the outside wall of the cabinet and ending in some sort of blower tube directed around the blade would make a difference? But here again I am likely missing some simple fact that with 28 square inches coming in the velocity through these inlet tubes would not increase despite my thought that they should.

The final collection and outlet would still be the same at the bottom return to the central system. Given the speed of the blade and the way it cuts and pulls material from the stock would such a directed intake be meaningful? I somehow doubt it but have no real objective way to do the mental experiment. Perhaps the only way to have the velocity needed to blow material off a 90mph blade with its teeth full of material is the use of a water pick like nozzles of compressed air introduced by an outside compressor. But I wonder if its worth going to the bother. I have an extra compressor and when I rebuild my cabinet will do a science project and report back.

[ian maybury]

It's probably a bit of a 'pays your money, takes your pick' sort of deal Bruce. As you say restriction is required to get high velocity, but this will reduce the cfm flowing - which will (a) reduce the size of the low pressure volume created around the intake to capture dust, and (b) if severe enough may reduce the velocity in the return duct enough to compromise transportation. I guess it's about balancing at least these considerations in a design that maximises the bang for the buck in all respects.

ian

[Alan Schaffter]

I missed this thread until Bruce sent me a PM, but here is my take on the whole issue, much of which repeats what others have said. First thanks to Bruce for the post retirement promotion! I retired as a mere Commander - Lieutenant Colonel equivalent for those not familiar with Naval Officer ranks.

ZCI or regular insert- no affect on TS dust collection- both are too small for enough flow to pull dust into the cabinet.

Extra holes in the insert- won't work either, they are blocked by the stock being cut.

As far as the cabinet by itself- it needs almost no SP of CFM to capture dust ejected there. The smallest DC will keep anything already in the cabinet from escaping.

However the transport of dust in the duct and efficiency of a cyclone or other separator will be affected by insufficient flow- dust may settle in the ducting and may not be separated in the cyclone and pass through to the filter(s) if flow is insufficient or restricted.

For all of the above to be right, as mentioned by other here- the area of the opening(s) in the cabinet for make-up air entering must equal or be larger than the exit.

So what do we do about the dust being thrown off the top of the blade at, as someone calculated, nearly 100 MPH and even worse, directly at the operator by the way!?!?! (depending on pulley arrangement blade tip speed will be over 100 mph. ( 3.14 * 10 * 60 * 3450 / 12 / 5280)

One solution is over-blade collection with adequate CFM and velocity. The problem with that is most blade shrouds potentially work fine when used with sheet goods where they seal well, but what happens when you cut thicker stock or use a jig where the shroud must lift which can allow dust to exit from the sides.

Lets go back and look at what happens when you have a tight fitting shroud and are cutting sheet goods- a tight fitting shroud will drastically reduce flow! One partial solution I have tried, is to make my over-blade dust pickup with a bristle skirt instead of hard plastic sides- the bristles allow air infiltration, conform better to irregular surfaces, and provides a kinetic barrier that stops or slows the dust being thrown off the blade so it can be collected. So far, so good.



But what do you do with any type of over-blade dust shroud when you have thick or tall stock, when you use a tenoning or other jig etc. when the shroud no longer seals well?

The only solution is to prevent the dust from being carried above the table in the blade gullets. This requires two things- (1) all DC make-up air needs to be directed past the blade, and (2) one or two manifolds with compressed air nozzles that blow on the part of the blade below the table at the gullet area. The manifolds would need to move to adjust for blade tilt and elevation. I'm not sure what pressure/velocity/CFM is needed and whether a standard shop compressor can meet this demand. A low pressure compressor driven by the saw's motor might work. Somebody want to check this out?

Combine that with a Sawstop and you have a neat arrangement.

[Kent A Bathurst]

So what do we do about the dust being thrown off the top of the blade at, as someone calculated, nearly 100 MPH and even worse, directly at the operator by the way!?!?! (depending on pulley arrangement blade tip speed will be over 100 mph. ( 3.14 * 10 * 60 * 3450 / 12 / 5280)

Oooops!! I just got spanked!!. >>100 mph it is.

Apologies to Howie, and thanks to the Commander!!

My joke about CERN is still funny, though..............

[Alan Schaffter]

That's OK, blade RPM varies from saw to saw anyway. Some Deltas spec at 4200 RPM which would equal 124 mph. Bottom line is, it is no easy matter to stop it.

If you think about it or dig through Bill P's site you'll see that once even slow moving dust gets more than 2" - 3" away from a typical DC suction source it is difficult to capture. Part of the problem is that instead of drawing from the cross sectional area of the duct (pi r sqrd) you are drawing from almost all directions- the volume of a sphere (4/3 pi r cubed).

[Kent A Bathurst]

OK - I'll give you that. Thanks. Plus, you were kind enough to give me an "easy out"

Now I'm wondering what your expertise/postings in the Navy were. I couldn't toss off the calc for volume of a sphere..........

However....yes, you are drawing from all directions. But, the cabinet is enclosed [disregarding the above-the-table problems], so the dust can't go everywhere and anywhere, it can only wander around the inside of the cabinet. Eventually, it has to come within the range of the suction. Plus - back to where I started - by sealing everything, and then adding air inlets in judicious locations and in the proper sizes, one can influence the flow of inlet air and, by extension, the dust, toward the pickup point.

BTW...in a previous life, the company I worked for had a hundred+ large-frame saws, that would cut the multiple angles on 2x material for roof trusses. Up to 20'+ lumber. 5 blades. Impossible to capture the dust. The best way to manage it was with water misters overhead, which would add weight to the dust so it would fall down for collection in the conveyor belt that caught the end-trim pieces. I'm not yet ready to consider a mister system throughout my shop, tho........

[Bruce Seidner]

Well, the conversation about dust, as fetching a topic as I know of, almost always comes back to basics. I shared my excitement about directed inlets and even compressed air nozzles with Bill Pentz. This fellow has the patience of a saint. I can't imagine the volume and diversity of email he gets. (And yes, Kent, I have made donations to his site.) He reiterated as many have here that the real problem is keeping sufficient flow back to the system to capture the fine dust with sufficient CFM and hoods close to the source to prevent spread. Regardless of my 7th grade science club flash of insight to use compressed air like a water dental pic on the gullets, there is going to be big time dispersion of fine dust generated by a TS and there needs to be a system below the table and above the table to capture it. So while nozzles are cute on dogs they don't add much to the bottom line work of keeping our workshop air safe. Alan's innovation of keeping the volume sufficient up top is important because while the TS cabinet is relatively easy to contain, once it is sprayed out the top one had better have a way to capture it because as the MDF users will attest you can be cutting in the basement and later find dust in your bedroom on the second floor.

[Ben Hatcher]

I've tried several strategies to collect dust on my contractor saw. It came with a blade shroud which I found worked best when removed and placed on a shelf. As Kent suggested, I, too, sealed the cabinet/table openings with foam, carpet foam in my case but left the curved tilt slot to direct airflow along the blade towards the lower back where my collection port is located.

At one point I built a cabinet that enclosed the motor and attached my dc to a port in the bottom of the cabinet. The collection was pretty good and the saw was a lot more quiet. But, my motor got packed full of dust pretty quickly. Some kind of fresh air intake may prevent that, but the right kind of motor, tefc, is probably the best bet if you're going down that road. I've since switched back to a slotted plate on the back and a tapered chute on the bottom. I plan to upgrade the port to 6" once I finish some other shop projects I'm working on.

[Kent A Bathurst]

heh-heh-heh

[raul segura]

I would seal as much of the box frame as possible, then place a strategic adjustable hole/flap etc. in a place that will carry away anything that may want to stay. You can then adjust this to suit your needs over head/under table. (it would be great if you could have the fresh air coming in passing the motor to). Measuring suctioning pipe SQ. IN. to fresh air inlet sounds right to me.

From my experience,Not to much, I find I have more dust coming from having a half exposed blade such as when you shave an eighth in. of the side of a board. Dust goes up and can be picked up by your overhead but in this case the dust flies to the left and in your face (I never keep my face in line with the blade). Ive never seen an over head collector address this issue. I would think you would need the side of these overheads to be adjustable automatically dropping lower etc.
1.PNG3.PNG4.PNG
1. adjustable wall. 2. leading edge of blade 3.adjustable slot.

This is a sketch of what I m talking about in adjustable over head wall.
If I had a contractor saw I would probably fill in as much space in the box as I could get away with.