I sometimes see woodworkers talking about a zero-clearance insert for their table- or band-saws.

What is it? And why/when should I use one?

  • To add to the dust collection argument. I own an Atlas table saw made way back in 1938. They cared so little about stopping dust back then that the stock insert threw more than half the dust in my face, on the floor and onto everything in my garage. A quick vacuum volume test showed around 30% made it into the catch bin under the saw, rest above the saw. The stock insert was so horrendously over sized for a standard 10" saw blade that even cutting a 1/4" vernier slat took many attempts to get right. After producing my first zero-clearance insert. So little dust got out through the top that I c – default_ex Sep 12 at 6:21
  1. To prevent small pieces from falling into the gap.
  2. To serve as a backing to minimize chipout on the back side of the cut.

I can't think of a time when you wouldn't be better off using one. However, sometimes it's sort of a pain to make one for every blade, or in the case of a Dado set every blade combination... So it tends to not be used for non critical cuts sometimes. But again, I can't imagine it not helping for any cut.

  • 1
    You touched on it but it would also be good to point out that a ZCI is always calibrated to a specific saw and blade, and will often no longer be "zero-clearance" even if used with the same blade on a different saw of the same model. The ZCI is also called a throat plate and is typically metal or plastic on older saws. On newer saws they're usually made of UHMW plastic, though many people make their own from plywood/MDF. Some modern aftermarket throat plates have easily-swappable plastic inserts to deal with the problem you mentioned, having to have a different one for every blade. – rob Apr 18 '15 at 1:10
  • A ZCI doesn't have a lot of value while ripping or when crosscutting with a sled. I have a ZCI for my crosscutting table saw (I dedicated one table saw to that operation), but I only use it when I'm using the miter gauge. It's otherwise totally redundant. – TDHofstetter Sep 7 '15 at 3:29
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    I disagree, on two points! 1) A ZCI is needed when ripping narrow and short pieces that could fall into the gap around the blade with a normal insert. 2) the crosscut sled itself should function as a ZCI to prevent chipout. – aaron Sep 8 '15 at 10:48

A zero clearance insert is an insert which exactly matches the width of the blade. You can make one by slowly raising the blade through an un-cut insert.

The advantage is as aaron says, they prevent things from falling through the hole, and more importantly, they reduce chip-out by ensuring that the piece has support and thus chips won't tend to get pulled away from the board. It's not perfect though, so if you care, still put in your sacrificial piece. In general, the less open space you have under your piece, the more control you have over it when cutting.

You should always use an insert that leaves as little gap around the blade as possible.

Let me add a couple things:

  1. Here's two references for making one: Mattias Wandel's method, and Woodmagazine's method. And here's what it looks like:

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They're typically made out of plywood or MDF to prevent wood movement.

  1. Others have mentioned that they're used for two reasons (1) to reduce chipout and (2) to prevent pieces from falling through the hold. Let me add a third: to improve dust collection. When you have a zero-clearance insert, it leaves less room for sawdust to spill onto the table (and directs it down the dust chute). Or at least that's what I've heard.

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