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Eliminating tapered tenons

Q:

Recently, I had to cut 12 "-thick, 212 "-long tenons in 34 " oak for a project. Each time, the results were a disaster with tenons measuring 916 " thick nearest the shoulders and 716 "  thick at the tip. I’m using a Delta tenoning jig and a thin-kerf blade that may deflect. Can you help?
—Bob Van Zandt, Lebanon, Ohio

A:

That thin-kerf blade could be part of your problem, Bob, but first let’s eliminate the other possible causes. Start by confirming that you’ve adjusted the blade 90° to the table top, as shown above left, not to the throat plate. Don’t just rely on the 90° bevel stop, which can become contaminated with sawdust and throw off the blade angle.

Now let’s focus on the jig. Adjust the tenoning jig fence 90° to the saw table, as shown above right, and be certain any workpiece clamped against the fence doesn’t accidentally touch the jig base. To keep your workpiece from tilting, adjust the backstop 90° to the table top. If you’ve followed these tips and the jig’s directions but you’re still having problems, try switching to a full-kerf blade. These thicker-bodied blades deflect less than thin-kerf ones.

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