Sunday, March 29, 2009

tearout on 1/8 in. box joints?

A reader asks:
I have been practicing making boxes with using a 1/8" box joint at the corners as per your video. I have been having a problem with tear out at the cuts and the blades I have tried don't give a flat top to the cut. I was wondering if you had a recommendation for a blade that is good for this type of cut.
Forrest Manufacturing makes a Woodworker II blade that has a special grind (Special #1 OD Grind) for square cut at the top, and they also make a special blade for larger box joints that stacks one way for 1/4" and reverse order for 3/8". I wrote reviews for both of these blades for Fine Woodworking. The special #1 OD grind is also excellent for cutting miter key slots. You can see the special grind on my well tested blade in the photo above, and if you compare it with a standard grind, you would notice that the teeth are not quite as pointed. This blade is great whenever I want to make a square top cut, for example fitting small panels for lids or bottoms. So it is useful well beyond box joints and key slots. I have also found that some combination blades have a square cut and work well.

The tear-out at the back of the cut most often comes from the stock being unsupported, lacking sufficient backing material. This happens when you make a deep cut and then use the same jig for shallower cuts. That height difference is the space that allows tear-out to happen And it doesn't need much to make a mess of your work. The easiest solution is to add a fresh accessory backing piece. Use some 1/4 inch Baltic birch ply, a piece sized approximately the height of the backing piece on your jig. Push the guide pin out of the way, with the blade lowered just enough so that it will cut the height of the pin. Make a through cut with the piece standing on edge, and then place that cut over the pin. You can use double stick tape or clamps to hold it in place. If the tear-out you've experienced is the result of diminished backing in the jig, this will restore the quality of your cut.

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