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 :)

Floor-standing Planers vs Lunchbox Planers

No, no, no, a lunchbox planer does not flatten your lunchbox or your big heaping pastrami sandwich.  It's a cute term of endearment for a benchtop planer.

They are very different beasts and it is well worth knowing the differences.  Here are my observations after going from a Ridgid lunchbox to a Powermatic 20" floor-standing planer:

Lunchbox planers have pressure rollers.  Basically picture a long roller pressed down with a spring.  Floor-standing models have power rollers.  These press down with much more force.

Let's break down this observation to see what it means:

Since the pressure rollers are just rollers, it will press down until it hits the tallest thingumajig you are running through it.  If a board has a rough surface, the roller will follow the highest point.  This isn't a big problem.  The problem comes when you start a narrow board through and decide to push a second board through beside it.  If those boards aren't exactly the same thickness, the roller will press on the thickest one and leave the other one free to kickback (fly backwards) due to the cutter's direction of cut.  This means you can't run two boards through simultaneously without risk.  If you insist on doing so, you could put one on each extreme side of the planer's width and likely get away with it since the springs on each side of the pressure roller would get some contact; side-by-side, however, is bad news.

On a floor-standing planer, the power rollers look like helical cutters with a long spiraling tooth. It is also set with springs, but decidedly stronger ones.  Further, there are anti-kickback fingers hanging down in front of the roller.  Each finger operates independently to prevent a board from kicking back.  If you run two boards of significantly different thicknesses through simultaneously, the anti-kickback fingers will stop the thinner one from kicking back, though it won't make any progress through the planer unless you push it.  I say 'significantly different sizes' because I've found that some difference is easily tolerated by the stronger power rollers.  I still keep them to the outside edges, though, and resist the temptation to put a third board between them.


Another difference these rollers make is in the woodworker's saying "never run a cupped board through a planer as it flattens it, thins it, and it comes back out cupped".  The saying is true, if you are using a planer with strong rollers and you are trying to take a lot off at once.

Assume a board is going through cup-down in a lunchbox planer set to take 3/32" off.  The roller will press down relatively lightly and do nothing to deflect the cup before taking off 3/32" off the top.  In a floor-standing planer, the rollers are set to press hard right up to the level of the cutters.  On this planer, it will press hard through the 3/32" difference between the rollers and cutters and may be able to flatten the cup with the stronger springs.  In this case, the board is flattened, some removed, then it springs back with a curved surface.  Hardly useful.  For floor-standing models, flatten the top of the cup by taking very light passes so the rollers can't deflect the board