Zero-Clearance Insert: Universal Good?

So when I started out, my very first power tool was a Makita 10" sliding compound miter saw.  Much later, I found woodworking and devoured all I could learn.  Naturally, one of the first things I was told to do was make a zero-clearance insert for my miter saw.  "It will give you much cleaner cuts", I was told.  Who wouldn't want that?!

Now, I wonder why that's such a wide-spread belief.

Let's look at the forerunner to the "miter saw ZCI", the table saw ZCI.  The table saw ZCI is a solid throat insert that you cut by raising the blade.  The kerf opening is exactly the width of the blade (duh, the kerf!).

For future cuts, as the blade's teeth swing around and come down on the stock, all of the fibers are supported except those being cut.  The cut is clean.

Now look at the ZCI for the miter saw (yes, I left the one I made in there for the time being :)

Same idea, cut by the blade as you drop the blade.  Same idea, the fibers are supported as the blade swings around to make the cut.

But, wait.  The blade for a miter saw does turn the same way, but from above; that reverses the attack on the stock.  Essentially, as you slide the spinning blade into the stock, the teeth come up from below into the stock.  When the tooth hits the stock, the ZCI is already out of the picture.  The teeth will blowout the top of the stock and give you chipout there (more later).  This is why you'll also find (good) recommendations to cut on the miter saw with the good face down.  This is because you're gonna get chipout and it's gonna be on the top, regardless the ZCI.

On the other hand, a "zero-clearance" fence that you cut does give you the same support as a table saw ZCI for the back end of the cut.  Without it, you'll get chipout in the back of the stock.

Now, one way to reduce chipout on the top is to make a scoring pass much like a scoring blade on fancy-shmancy Laguna table saws (no insult; I think they are way cool).  To do this, drop the blade and push through the cut by barely skimming the surface.  Come back and do the real cut.  The skim pass will use the "bat ears" of the ATB blade to score the fibers and kick some out.  The real pass will then have that top cut layer as a fiber-support layer.  Simple and works like a charm.  Eyeball it; you don't need to set a depth stop unless you are doing dozens of the same-thickness boards.

More thoughts on the miter saw ZCI: used like a chop saw, a SCMS can get some benefit from the ZCI.  What?! Yes, if you just plunge down into the stock without using the slide, the part of the board between you and the arbor will have the teeth cut in a downward stroke thus gaining the benefit of the ZCI.  The part of the board between the arbor and the fence, however, still has the teeth cutting on the up-stroke.

So those of you with a lovely Festool Kapex miter saw, quit sweating that a ZCI kills your dust collection.  Leave the factory insert in place for the phenomenal dust collection and leave the ZCI idea in the land of unicorns.  (hey, lend me your Kapex and I'll do an experiment!)

I have some other disagreements with common "woodworking myths".  Stay tuned :)