Trying to understand benefit of a compression bit. Lets say your using pine. Bit has 4mm of upcut and rest is downcut. Total depth of your project is 6mm. And each pass is 2mm. The frayed edges comes from the initial cut in the first 2mm pass. So how does the downcut portion of the bit helpful? Or is it only beneficial if you are doing the full 6mm in one pass? Dont think the longmill can do a 6mm in one pass. Or even 5mm (.197 inch).
@Clt49er You’ve hit the nail on the head. A compression bit only does its job if the first plunge is deeper than the upcut portion of the flutes. Ideally, you do the cut in one pass. However, you can do multiple passes as long as the first one is deeper than that upcut portion.
Being cheap, I have not bought compression bits. When I do jigsaw puzzles, for example, I do two toolpaths. The first cuts about half way through the material with a downcut bit. The second starts at that half-way point and cuts through the material with an upcut bit. No sanding on either side.
I appreciate, though, that I’m not in a production environment so the added seconds to change the bit does not bother me. 
5 Likes
Bits Bits sells a short flute compression with 0.12", 3.048mm upcut. It does well but I’m not particularly fond of doing a depth of cut that deep very often. I mostly do as Grant mentioned above, using two tool paths.
1 Like
Thank you both. Wanted to make sure I wasnt missing something. Got a few of these bits with no use.
Take my advise with a grain of salt because I don’t have any compression bits but I think they only really have value with through cuts. If your not going all the way through a downcut with ramps would be fine.
For through cuts I’ve heard of make the cuts with two bits, the first cut is over or under sized and then the compression bit cuts full depth but only with a small step over. That said if you have to use two bits it doesn’t really offer and advantage over Grants two bit method, at lease that I can see. I guess that’s why I haven’t purchased any.