Q: We are having a considerable amount of machining failures when machining the ends of rails when putting on a tenon. A few samples are enclosed. Can you help us with this problem?
A: The failure, which is characterized by chipped grain extending from the cut area into the useable portion of the rail, is due to mechanical failure of the wood. That is, the knife is breaking or splitting out chunks of the wood. This is more likely to occur in several instances:
- When the wood is very strong and brittle. Strength and brittleness are increased by over-drying the wood. This failure is likely even if over-dried wood is brought back up to a higher MC. For this reason, I suggest that a kiln never operate exceeding a 45 F depression (25 percent RH, 3.7 percent EMC) at 160 F dry-bulb. You might try drying one load with this change (if indeed it is a change) and see what happens. Drying time will not be extended more than a few hours - a small price to pay for better quality machining.
- When stock removal rates (amount of wood removed per tooth or per pass of the knife) are large. The knife has a choice of cutting the wood or breaking it out. It chooses to do whichever is easier. With high feed or stock removal rates, it is easier to break out a chunk of wood rather than try to cut all the fibers quickly. In other words, less energy is required to break the wood than to cut it; what we want is to cut the wood with a small cut with a sharp knife, which means less energy for cutting than for splitting.
- When tools are dull or have low rake angles. The tool needs to be sharp enough that cutting is easier than breaking (following the discussion of the item immediately above). Sharpness with carbide is sometimes harder to achieve than with HSS. In addition, we need a knife that is a little more slender that will cut easier than a fat knife that acts more like a plow.
- When machining against the grain. Understand that splitting failures will follow the grain; wood is naturally weak in splitting, sometimes called low cleavage strength. The split will go along the grain until the piece finally breaks out. If the grain dips slightly into the wood, the splitting failure will go into the wood and the chipped-out piece will leave a hole or void, as noted. Unfortunately, we cannot do anything in production about orienting the grain differently in most cases. In some cases with large pieces, we might mold the piece in one direction about halfway and then as we begin to mold against the grain, begin molding on the other side in the other direction so we are once again molding "with the grain."
- When there is no support for the fibers on the faces being cut. If each piece being cut had another piece held tightly right behind it to help support the fibers and prevent splitting and chipping, then we would minimize the failure. We might do this occasionally; one example is when drilling a hole in one piece in a home workshop - we put another piece of wood under the piece being drilled to prevent chipping around the exit side of the hole. (Although not practical, you can imagine that if you glued a small scrap piece of wood onto the workpiece, then this small piece would prevent the chips from occurring in the workpiece - the small scrap piece might chip out however.) Many tools provide this support by having some metal supports very close to the cutting area. For example, in a planer we have a chip breaker that is just several 1/1000ths behind the knife. On a saw, we may have a saw table insert that provides minimal clearance between the saw cut and the finished piece. It is difficult to provide such support in a moulder. In low production operations, sometimes a small cut line is put on the finished piece right at the spot where it is going to be machined. Then when the chip tries to form, the chip breaks right at the cut line, as this is the point of weakness. I've seen this done when crosscutting plywood to avoid roughness on the backside of the cut.
I hope this explanation gives you some ideas of what you can do.
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