A year ago or so, a local avionics installation company asked if I could make an aluminum pad for mounting an instrument nearly vertically in the outside of a plane. I went and looked at it and made some measurements. I came home with a piece of aluminum, and proceeded to make the pad over the next few days. The bottom was curved, so I tried to use a ball endmill for a finishing pass. The ball endmill just skated over the existing surface. So I used a square endmill and a really small step over.
There are two potential causes. One is inadequate downward force to keep the bit cutting. The other is that the bit is not sharp enough to slice into the material. In the case of planing endgrain on a shooting board, a plane blade that is not really sharp will skate over, too, especially with white oak. One strategy to alleviate this plane blade sharpness issue is to skew the blade, which is the idea behind spiral fluted router bits. Another result of inadequate blade sharpness is tear-out.
Mechanism of the Cause of the Observed Endgrain Cross Cut Surface
Based on the above, there are two possible mechanisms that cause the observed surface:
- the bit is forced out of its path by the harder grain lines, and is allowed to do so by flex of the CNC router framework.
- the bit {prabably the flute edges) is inadequately sharp, and tears-out the softer material between the grain lines.
If the first mechanism is in play, then why are the valleys between the grain lines uniformly "excavated' when the grain lines not parallel to the bit axis would not allow the bit to sink into the material? It seems that the second mechanism is the more likely one in play. One observation supports it.
When milling MDF at proper feeds and speeds, the machine only produces chips. No fine dust. When milling white oak, there is a significant amount of fine dust along with the chips. Thus, the bit (a new one) tears-out the material between the grain lines.
Confirmation of this could be confirmed with an adequately sharp bit, and, more specifically, adequately sharp flute edges. Another possibility would be to try a non-fluted bit.