This is one example of why.
I was milling 0.830" diameter 1.100" deep holes in MDF with a 1/4" two flute carbide router bit. The cutter was a little dull at the start, and not really the one I should have been using.
I was helical milling the holes in two passes.
The first pass was 0.450" with a 0.100" ramp down per revolution. I ran it at 39 IPM and it seemed to go okay.
The second pass was much more aggressive (because GWizard and the CNC Cookbook blog said it should work - with an end mill, not a router bit...) I helical milled to 0.830" diameter with a 0.300" inch ramp down per revolution at 32 IPM.
You can tell by the first hole that I was in trouble. It's oval shaped.
I was in the shop but not really watching the machine too closely. I heard it when it got into real trouble and ran over and hit the reset button (no E-Stop wired up yet - though it was almost on fire.)
Let's break it down a little:
- Wrong cutter
- Dull cutter
- Straight flutes on the cutter did not expel the dust well enough
- Overly-aggressive cut
- First run of a new tool path
If I had not been in the shop it probably would have caught on fire after a bit as it tried to drive the end mill sideways at 1.100" deep through solid MDF. You can see where it started burning.
As a side note: MDF is terrible stuff to mill. It dulls the cutters almost right away. I've been doing some reading and 2 flute end mills with TiAN coating are suggested. I have one coming to try out.