Q. We have a machining problem. As we machine a piece, it is good until the last half inch. At this end, we all too often get a piece that chips out all the way to the end. Where should we look?
Answer: When a knife comes around and begins to take its cut, it always asks this question “Is it easier to cut the wood or to push it (or split it) out of the way?” In your case, it is easier to push it out of the way sometimes. [Incidentally, in parts of Canada, it asks this question in French.]
My answer begins with another question “What makes the wood so strong at the end that pushing or splitting is easier or, conversely, what is it that makes the end piece easier to split off?” From a practical standpoint, we have several answers that need to be checked out:
1. The knife could have too large of a rake angle, so it is acting like a splitting chisel or wedge and not like a cutting knife. In this case, on a microscopic level, there is a small crack that runs ahead of the knife, with the crack following the grain. As we get near the end, this crack weakens the wood enough that splitting the piece off is easier. If this indeed the cause, you would also notice some chipped grain here and there.
2. The knife could have a rake angle that is too small. In other words, the knife is plowing the wood off, and that continues to the end. In this case, we would also see more fuzzing in the rest of the piece…more than normal.
3. The energy required to cut a large amount of wood off in one cut is higher than a small cut. Plus higher feed speeds make it worse. Fast, deep cuts are tough so splitting is easier.
4. We know that drying lumber over 160F and drying to moistures under 5.5 percent MC make the wood more brittle. Brittleness means splitting is easier.
5. Dense wood is harder to cut, but splitting is still easy. Fast grown hardwoods with large pores like oak and slower grown softwoods are more dense usually.
Check these out. If these are not the cause, we will need more information.
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