Thread: Zero clearance insert - Thin steel insert plate

Even when fully exposed the air speed through any 6 mm diam holes in the throat plate will be very small (as will be the flow ie <1 CFM per hole) but more importantly is will not be able to compete with the air speeds generated by the blade.
And when those holes are covered by wood no air or dust will be able to go through those holes.

Worse still, holes drilled in the back half of the throat plate will actually lead to fine dust being pulled out of the cabinet into the space above the table..
The photo below shows how the back half of the throat plate is under negative pressure and the front half is under positive pressure.
This is caused by the blade dragging air from the back to the front of the table - this is also why the back of any guard should be completely open
In addition the blade pulls fine dust up from the cabinet even through a zero clearance throat plate
Note" DC not running in this photo
IMG_2252.jpg

The majority of the air generated by the blade is forward along the table top, so the best thing to do is defect that air upwards by using an angled guard, into the dust port - that's why having the dust port as forward as possible on the guard is desirable.

Now here is the air movement with the DC on
IMG_2253.jpg

This setup has so much suck I have to be careful when docking matchbox size or smaller pieces otherwise they will get sucked away. If I don't need these then it doesn't matter but if I do, I control that by lifting the guard up a couple of mm so some air gets sucked in from the front reducing the amount of air that flows in from the back - this leaves small pieces in place on the table.

The reason the hole near the end of the slot works for chips is because that is the point where most of the chips are made and dragged towards - it's THE point source of the dust. The fine dust is usually still sprayed out towards the front of the table.

Originally Posted by ian

from my observations, this dust is entrained with the blade and is "released" when the teeth start cutting.
I can't see how holes in the insert can help all that much because when the saw is cutting the holes are covered by what is being cut.

What am I missing?

Dunno. Just does. Less dust with in the shop, more in the bin, when the holes are there.

I'm pretty sure the hole at the front of the zero clearance insert idea came from WoodPixel. I use engineered flooring, the dark brown MDF looking stuff, for zero clearance inserts. Works a treat, very stable, machines easily and has a nice hard wearing and flat surface on the base.

Originally Posted by ian

from my observations, this dust is entrained with the blade and is "released" when the teeth start cutting.
I can't see how holes in the insert can help all that much because when the saw is cutting the holes are covered by what is being cut.

As usual what you see with dust is not necessarily what happens. There may well be more sawdust in the bin, less around the workshop and less dust let behind on the throat plate, but that is the stuff you can see. The fine stuff which is what matters is invisible and goes where ever the air goes and without effective OH collection most of that is usually sprayed straight out the front of the blade.

The only hole that I've found that helps with chips is the one at the end of the throat slot.
The same thing happens on the BS although with the BS the location of the slot is counter intuitive

Lots of mini-BMH holes in the throat plate and the slot facing the operator
UTDC.jpg

Reversing the slot so its on the other side of the operator
Reversetheslot.jpg

Opening up the slot to double the original width.
Reverseand-open-slot.jpg

Originally Posted by BobL

As usual what you see with dust is not necessarily what happens. There may well be more sawdust in the bin, less around the workshop and less dust let behind on the throat plate, but that is the stuff you can see. The fine stuff which is what matters is invisible and goes where ever the air goes and without effective OH collection most of that is usually sprayed straight out the front of the blade.

the attached shots are of the underside of a deWalt 745 bench top saw after about 10 m of cutting softwood. The saw was brand new before cutting commenced and was connected to a Rigid wet / dry vac when in use. (the blade guard was removed for these shots.)




What I interpret is happening is that the blade gullets are pressurised while they are in the cut. On existing the wood the air in the gullets decompresses spraying dust everywhere. Most of the larger chips are sucked away by the vac, but the really fine stuff tends to stick onto the first surface it hits or stays air borne. You can see some of the fine stuff on the inside of the blade guard where the suck from the vac was not sufficient to keep it moving.