preventing tearout while cutting miter key slots

Andrew Craig wrote the following question:

I have been making boxes with inspiration from your books for awhile now and have recently run into an issue. When I cut the keys on the mitered corners of a recent box made out of clear vertical grain douglas fir, I had a problem with the grain tearing out on the back side of the kerf, making the finished product unacceptable. Do you have any tips for eliminating this in softer woods? I use the table saw jig that you show how to make in your latest Taunton book.

Tear-out is almost always a problem related to the backing for the cut. If you have made deeper cuts previously with the jig than what you are planning to do now, place a piece of thin 1/8" ply over the cut so that the blade will be cutting into fresh backing rather than empty space. As an alternative, if that doesn't work, you can do something that people tell you never to do. Make a climb feed cut. This means that you start your cut with the jig on the back side of the blade and pull it toward you with the box in place. This would be extremely dangerous if you were making a large cut, but in cutting a key slot, the amount of material is so very small that it is safe as the blade has very little contact with material. When I have experimented with this technique, I actually stood on the back side of the saw. This is a technique which I have to urge people to be very clear what they are doing... Something you would never do on a large cut, or long stock as the action of the blade would pull the stock into the cut and hurl it at you or pull your fingers into the blade. So I would check the integrity of the backing first before trying what would generally be considered the more extreme technique. I should note that the climb-feed cut I have described should NEVER be made with a new jig. The blade cutting into the new jig would be very forceful due to the amount of material engaged with the blade. The blade would throw the jig across the room and damage anything in its path.

I got complementary copies of Taunton's new jigs publication "Essential Shop-made Jigs" in the mail today and the cover image is of the jig that Andrew is using to make his miter key slots. The image above is from the magazine, but I won't spoil the whole thing. It is full of useful jigs you can make yourself. It should be available on the newsstands on March 31.