Compression bits

I know what compression bits are, but I’m struggling to understand what use case they have in a hobbyist’s tool palate. Can anyone explain this to me?

In most cases when carving wood, they will afford a cleaner top surface of your carve. Instead of pulling the material up, which causes “tear-out” and rough corners that require more hand finishing, they push the material down when cutting, leaving a much more desirable and cleaner finish appearance.

@quesohusker Here’s a photo of some calibration cuts I made in some really crappy plywood. These shapes were cut with a 1/4" compression bit.

I needed a very crisp edge since I was running trials to determine laser offsets needed to integrate my laser with V carves and the like. I used the raw cuts with no sanding or cleanup. You can see where they were outlined with a laser.

Very clean look with the compression bit.

The whole answer is they’ll give you a clean cut for both the top & bottom of a piece of wood.

If all you care about is the top surface, a standard downcut but works fine - it pushes down into the fibers as it cuts. But the bottom face will suffer some amount of tearout because of that downward cutting pressure.

Conversely, if you want a clean bottom surface with little or no tearout but don’t really care about the top, you can use an upcut bit. The upward cutting design of the flutes pulls the fibers upward so on the bottom of the material you won’t get tearout. (You may get it on the top depending on the material though.) This has the added advantage of clearing chips easier as they’re carried up & out of the cut.

A compression bit has upcut flutes on the tip of the bit (up to 1/4" from the end of the bit) and then downcut flutes the rest of the length. The trick with these is your plunge has to be deeper than the upcut length otherwise you’re just hitting the top of the material with an upcut bit and it will produce tearout. Typically that means your pass depth has to be 1/4" or more so you get to the downcut portion of the bit on your initial cut into the wood.

The Jenny bit 1F sells has only a 0.100" upcut portion so you can have a more conservative plunge of 0.125" (for the qtr inch bit) and still engage the downcut portion in your first pass.

All of this applies mostly to plywood because the veneers are more prone to tearout than sold hardwoods. So you might find that you don’t get much value out of a compression bit vs a standard upcut on hardwoods.

BTW, upcut bits can be run more aggressively in hardwoods than downcut bits because the chips are evacuated (lifted up & out) from the cut vs a downcut that is pushing those chips down into the cut. That’s why good dust collection & dust shoe are really important for downcutting - you’ll need to have the vacuum sucking the chips out since the bit isn’t flinging them out.

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