I was cutting a small item on my Primo and broke the bit, but don’t know why. I am cutting a 0.625" piece of red oak with a 1/8" compression bit (Spetool W02004, 3/4" cutting depth). This is cut in 5 passes, 0.1252" each pass. On my first try, the bit jammed as it started the 5th pass, and I was able to stop the machine immediately. I repositioned the router and tried to cut again, but this time the bit broke before I could stop it. Again, this happened at the start of the 5th pass. I am running this at 18000 rpm, with feed rate of 20 inches/minute. Why did it do 4 passes with no problem and then break on the 5th?
First thing that comes to mind is tramming. If your router is out of tram each step down is increasing the side load on your bit and the cutting forces in general.
Armchair analysis considers that the 5th pass is finally starting to generate enough downward movement of chips in the compression geometry that they can’t clear fast enough, and it’s clogging the flute(s), causing excessive load on the bit, or some other interaction with the downward geometry… That would be my explanation for why it’s failing on the 5th pass. The downward geometry is really kicking in.
Edit: manufacturers site includes this:
How deep should I cut for them to work properly?
Your first pass must fully engage both up‑cut and down‑cut zones—typically half the diameter deep at least to activate both.
Which implies that at 0.125”+ DOC, you’re already more than fully engaged with the downward geometry from your first pass. Since the first four passes succeed, it’s able to clear at least some chips when the inflection point is well below the surface (relatively speaking). So there just may not be enough room at 0.5" of down geometry to push chips up/back/out.
And what Reilly says. Compression bits are really best for laminated or engineered stock (plywood or MDF come to mind), and most natural stock is probably better served by upcut or straight flutes with finish passes.
Both responses are valid responses! I will also add you have chosen oak to work with on a compression bit! Try o flute upcut with blue tape on the top to stop tear out, also use a finishing pass.
Thanks for the comments. I’ll check the tramming of the router and try a different bit.