Table saw dust collection working too well?

Question

I've done some dust collection upgrades to my contractors saw. Essentially this involved putting a large dust hood under the blade and motor assembly. I then used duct tape to seal off all the holes.

The theory I was going for is that if all the air flows through the hole in the clearance plate, it'll mostly get sucked into the cabinet and then through the large dust hood, into the my dust collector.

Turns out, this theory worked very well. To test, I crosscut a bunch of small pieces of pine into probably 1x1 inch squares using a miter gauge. What I noticed was that the squares weren't moving along past the blade but rather getting "trapped" next to the blade due to the suction.

I'm not 100% sure if this is actually a problem, any kickback would be directed to the right of the blade, where I'm standing on the left hand side of the blade.

To be sure, I would like to get an understanding of potential dangers.

Answer 1

I would like to get an understanding of potential dangers.

Any time you push wood into that spinning blade, the part between the blade and the fence is at risk of binding and being thrown across the room in an event called "kickback".

This can happen with any tool with a spinning blade, but the table saw is the most dangerous because the wood becomes a missile when this happens. With a miter saw, for example, the kickback shoves the wood into the corner between the fence and the base, causing the motor/blade/handle assembly to rise back up. With the miter saw, the blade is automatically protected by the closing blade guard, and the whole thing is mechanically limited in its travel distance.

It can also happen when cutting with a hand-held circular saw. Usually, though, the saw will stop (especially if it's battery powered) because the blade will bind in the wood and it won't launch too much wood/equipment too far.

To give you an idea of the forces involved in table saw kickback, there was a 2x4 shaped chunk taken out of the chalk board^* in my Junior High School shop class (back in the dark ages when there was such a thing as "shop class"). This was caused by someone attempting to rip cut a 2x4 on the table saw. The 2x4 got caught and kicked back, throwing the board across the room and hitting the black board. Note that this board flew at least^** 30 feet (~10m) and traveled above all the student desks and the teacher's desk before hitting the chalk board. Fortunately, everyone was in the shop area and not in the desk area, so nobody (but the saw operator) was in the line of fire.

When cutting small pieces such as your 1x1" squares, you should always use a push stick on the piece between the blade and the fence. The stick protects your fingers from the blade; it also ensures even pressure on the wood as it travels, reducing the risk of twist which increases the risk of kick back; and it ensures that you can push the off-cut out from between the blade and fence as leaving it there also increases the risk of that piece being thrown back.

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^*As a follow up, that chalk board wasn't replaced during the 3 years I attended that school and probably longer. Partially because replacing it wasn't cheap, but also because it was used as an object lesson in the dangers of kickback when using the table saw. It was quite sobering to a group of early-teenage kids walking into class and seeing that dent that wasn't there yesterday. Turns out, it had happened during the class right before mine.

^**Junior High School was ahem a few years ago. It was probably more like 50 feet, accounting for the rows of desks, but my memory isn't precise enough to recall exactly. I'd rather underestimate than be accused of fearmongering and over exaggeration.

Answer 2

To work correctly, the dust extraction on a table saw should not be creating that much vacuum. Dust extraction works by moving the sawdust with air, and for that to work on a table saw, it has to move a significant air stream past the saw blade and into the extraction system. Sealing up the saw so that the only way air can get to the blade well is through your (hopefully zero clearance) insert is going to result in too little air flow to move the dust and chips, of too high a velocity air flow through the insert. It's also going to cause the problem you document. The pinning is probably not that big of problem though - if you have a zero clearance plate, the force that can be exerted on an offcut is not going to very great compared to the mass of most offcuts.

Given the effort you've put into sealing the bottom of your saw, I would look into the blade well, and ask where air is able to enter the well that will cause it to flow past the blade in a forceful stream.