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Mortising by Machine


Q & A: Mortising By Machine


I’m having trouble with my mortising machine. I cut mortises by slightly overlapping each hole, but I’ve broken a few bits and my chisels have overheated and turned blue. Is there a problem with my method?


Yes, there’s a better method than overlapping holes. When you overlap, you’re essentially cutting a hole with three sides. It’s like trying to drill overlapping holes with a twist bit. The bit wants to follow the path of least resistance. It walks into the neighboring hole, bending as it goes. When a mortising bit cuts an overlapping hole, it bends, too, and rubs against the side of the chisel. Both get hot. The chisel turns blue as the temper is drawn out, and the bit eventually snaps.

The solution is to drill four-sided and two-sided holes. Drill the two ends of the mortise first. Because the bit meets equal resistance on all four sides, it goes straight down.

Now, instead of overlapping, leave a space about half the width of the bit, and drill another hole. Keep skipping half a hole like this until you get to the other end of the mortise.

Next, drill two-sided holes in the half spaces. There’s equal resistance on opposing sides of the bit, so the bit travels straight down.

This story originally appeared in American Woodworker June 1999, issue #73.

June 1999, issue #73

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