question on the theory of zero clearance inserts

[James Baker SD]

The basic idea makes sense, support the workpiece near the cut to prevent tearout. But then the details get confusing. Seems to me there is very little cutting going on at the sides of the blade between the arbor and up to just behind the teeth, so does having zero clearance there help any? Now if the cutting is happening at the very end of blade (and maybe a little ways on either side), then it would seem you would need a different zero clearance insert for each height of cut as well as each blade tilt. If I cut a 3” thick piece of stock, the blade will be high, if I lower the blade to cut a piece of ¾ ply, then the insert will no longer be zero clearance where the cutting is actually occurring. Seems it would get expensive and confusing to have many different inserts for different heights. What I am missing here? Do you just leave your blade high for all through cuts?

James

[Steve Wurster]

I think the issue is with the material that is directly to the left and right of the blade where the cut is occurring. That material could chip out if there is nothing below it. That's the point of the zero-clearance insert: it backs up that material. The height of the blade is (theoretically) irrelevant; as long as there is something backing up the material adjacent to the cut, that material won't chip out.

I guess you could adjust your insert to only be zero-clearance from the arbor forward, since the cutting occurs in that area and nothing should occur on the back part of the blade. Might even support better dust collection that way.

This backing-up-the-cut idea is also the reason people use auxiliary fences on their miter gauges. If the material at the very end of the cut has no backup in the vertical plane (as opposed to the horizontal plane of the table), it might also chip out. But a fence on your miter gauge that was cut to be zero-clearance helps minimize that.

[Danny Thompson]

ZCIs provide at least three benefits:

A ZCI supports the wood throughout the cut, thereby maintianing the position of the board square (vertically) to the blade. This 1) ensures a square cut (vertically) and 2) reduces the chance of kickback.

Separately, the ZCI supports wood fiber on each side of the tip of the blade, regardless of any gap in front of the blade, thereby reducing blowout on the bottom side of board.

ZCIs do not prevent blowout on the trailing edge (nearest the user). It can be reduced by a backer board or cross-cut/miter fence.

[David Kumm]

To me it's more about not having the thin offcut fall into the arbor and shoot out like a spear. Dave

[Jeff Duncan]

I agree with David. I can't imagine the kind of close tolerances you would have to have for a zero clearance blade to really truly make a significant difference in tear-out while cutting. Not only would your zero clearance have to be perfectly fit to the blade and perfectly flush to the top of your machine, but your wood would have to be perfectly flat whilst traveling across that surface!

What does make a difference is using good quality blades and setting them to the right height. The best way to see real world differences in cut quality is cutting melamine. If your blade is too high you'll get chip-out on the bottom, too low and you'll get a rough cut on top, regardless of the insert. I'm still using an oversize insert on my Wadkin as I haven't gotten around to making one to fit the slot, and the biggest problem I have......offcuts falling into the saw;)

good luck,
JeffD

[John TenEyck]

Really? I see significantly less tearout when I use a ZCI vs. a wide clearance insert, especially on fragile things like melamine and plywood. Same thing on my RAS; when I put a sacrificial piece under the work the cut edge is much cleaner. Sure, a really good blade has less tear out than a poor quality one, with or w/o a ZCI, but the cut quality still improves with one. If you only use one blade with your ZCI the clearance will be defined by that blade as it cuts up through the insert. You can't get much tighter tolerance than that. If your workpiece is not flush with the ZCI then either your ZCI is not installed correctly or your workpiece is not flat.

John

[glenn bradley]

There are three camps on ZCI's more or less; 1. safety, 2. tearout protection and 3. both.


Seems to me there is very little cutting going on at the sides of the blade between the arbor and up to just behind the teeth, so does having zero clearance there help any?

If the sides of the teeth weren't important, folks like Freud wouldn't spend the extra time and money on their trick tooth geometry so, yes it helps there. This is probably where most of the fibrous tearout on veneers comes in.

254730

I actually tried expanding the slot at the rear for better above table dust collection (a helpful tip I saw somewhere) but, the jury is still out on that one.

254732


Now if the cutting is happening at the very end of blade (and maybe a little ways on either side), then it would seem you would need a different zero clearance insert for each height of cut as well as each blade tilt.

I don't worry about the non-ZCI at the front. I am concerned with tearout as the side of the tooth passes out of the material. As far as having many ZCI's? Yes, I make them by the dozen and discard them as they wear. They are a wear part and the fibers being pulled through the slot make it sloppy over time. The period of time varies with what the ZCI is made from.


Seems it would get expensive and confusing to have many different inserts for different heights.

I can get about 60 of them out of a 5' x 5' piece of 1/2" BB ply. Even if I through in a set of allen screws for levelers for each and every one they are still under $2 each. Seems pretty reasonable, especially since I generally make them out of scrap. Whenever I have pieces that are about the right size they get stacked in the corner. When I get down to where I have only 3 or so blanks on hand I take an hour or so (I also have to drill for my MJ splitters) and run off a batch. After using a new blank, I mark it on the underside with a felt tip marker as to which blade or blade type it is for. Easy-peezee.

254733

This doesn't mean everyone should do this and it doesn't mean you're doing it wrong if you don't but, please at least make one that fits so-so for the safety issue of thin and small projectiles :).

Here's a worn one that was on the scrap heap:

254743254744

[Prashun Patel]

When you (I mean *I*) cross cut plywood, the tear out occurs to the left or right of the blade - the grain runs that way. On the top of the wood, the wood is it's own ZCI; that's why the tearout occurs only on the bottom of the blade.

[Alan Bienlein]

Been doing this for a very long time and the only consistent benefit to a zci is safety in not letting thin pieces fall in between the blade and the insert.

For a zci to prevent tearout it needs to remain in contact with the material you are cutting. If it raises up just a little then there is nothing there to support the bottom of the material. This is where using the proper blade for the operation and material you are cutting comes into play.

I'm still using the same zci since I got my current saw back in 2005 and don't see a reason to replace it. If I'm getting tearout on what ever I'm cutting then I need to A) change to the proper blade or B) remove the proper blade and replace with a sharp one.

[Larry Browning]

Tearout is usually only an issue on a crosscut. You only need to worry about tearout on the sides of the cut. If there happens to be tearout at the front of the kerf, it does not matter because as you continue with the cut the tearout is removed by the next tooth. I suppose there could be some tearout in this area on a stopped cut, but that is also pretty minimal because of the way the grain runs. It probably is true that the tearout is caused by the leading edge of the tooth, but the tearout happens one either side of the cut so the side is what needs support not the leading edge.

[Jim Becker]

ZCIs support the material right up to the cut line, reducing the chance of tearing from the downward (and upward) motion of the blade, even when the blade is as sharp as it can be.

[Ben Reese]

It is not only a matter of support when crosscutting but as a matter of support when ripping narrow pieces or using a molding head cutter or a dado blade as well. The support for the dado when cutting could prevent the work piece from "grabbing" The blade. When using a molding head cutter and in ripping it will keep work pieces from rocking down into a wide slot and grabbing the blade. Personally, I also have made a ZCI for my blade when I use it on a 45 degree angle.

[Mike Manders]

Tearout is usually only an issue on a crosscut. You only need to worry about tearout on the sides of the cut. If there happens to be tearout at the front of the kerf, it does not matter because as you continue with the cut the tearout is removed by the next tooth. I suppose there could be some tearout in this area on a stopped cut, but that is also pretty minimal because of the way the grain runs. It probably is true that the tearout is caused by the leading edge of the tooth, but the tearout happens one either side of the cut so the side is what needs support not the leading edge.

Exactly what I was about to say. The blade height doesn't matter because any unsupported fibers are about to be cut anyway as you keep pushing the piece through. Only the sides of the kerf need support, and that's what a ZCI gives you.

[Metod Alif]

A ZCI used just once it is actually no more - you know, like a...v;)n.
I make me a new one every few months, using the current one as a template. No level screws, just a bit of tape to level well. Not for controlling the tearout, only for keeping track of small cutoffs. The original insert plate has the slot too wide for my liking. A good blade minimizes the tearot. When that's not enough, cover the cut line with masking tape. Peel it off in the direction toward the cut line. Not always perfect but helps.
Metod

[Joe Angrisani]

.....No level screws, just a bit of tape to level well.....

Saw a neat trick on one of the TV shows.

Rub a little wax on the cast ears of your table saw. Put a dab of hot glue on each ear and let it cool for about 5 seconds. Lower the blade and place the new ZCI in the opening. Use two starightedges or square pieces of wood to push the ZCI down flush with the table, both ends at once. Let it cool another 20-30 seconds, and presto, flush ZC with no setscrews. The hot glue will have made perfect sized custom pads at all four ears.

[Lee Schierer]

I designed this ZCI for my Craftsman TS and a machinist friend made it for me. The inserts are pretty easy to make and I can whip out a bunch of them from scrap poplar, though I prefer maple. 254911

There is no need to have a different ZCI for each blade height, one slot will suit all heights of cut. I do have other inserts for different widths when I cut a dado.