Cutting miters on a router table?

Matthew asked a real head-scratcher:

My first project was a fairly successful open top box made of maple. I made a nice little jig for making 3/8" finger joints on my router table. Recently, however, I picked up a copy of your book Taunton's Complete Illustrated Guide to Box Making. Great book! I love your jigs. To me, it seemed the next great improvement on my box would be to try your suggestion of a finger joint with a miter at either edge. I also thought it would be nice to try a different wood, so being a sailor, I chose Mahogany.

I have a Craftsman table saw, and it is the style that, instead of a standard miter track, it has a sled. As such, I don't feel that it can make cuts as accurate as one might need for such fine detail. Maybe I'm wrong...

Instead, I've been trying to do as much as possible on my router table, including making the 45 degree miter cuts. To increase accuracy at the router table, I've even gone and upgraded the miter track on table as well as purchased a new fancy Incra miter gauge.

The problem I seem to be having is that when I make the miter cut on the table, with the gauge set for one 45 the cut comes out beautifully, but when I rotate the miter 90 degrees to do the other cut, the bit seems to chew up the wood, sometimes splitting off pieces all together. Attached is a picture.


Do you have any suggestions for me?

Matthew,
The problem you are having has to do with the feed direction of the router bit. On the cut that comes out clean, the bit is cutting downhill with the grain. On the other cut, the router is cutting uphill, against the grain, so it gets fuzzy at a minimum and at worst will tear out chunks. This has to do not only with the feed direction of the stock, but also is related to the rotation direction of the router bit. You could test some of how this works with a hand plane. Try to go against the grain and you get crappy results.

The illustration below shows a rather complex solution to enable your table mounted router to do the job. It entails a regular cut, similar to what you do that is successful, and a cut on the other side of the router bit to cover the part that is not working well. This involves a climb cut, which is generally not recommended because it will pull the stock into the cut and often out of control. But, in this case the cut is very small, extremely unlikely to throw the wood and jig across the room and you should be able to hold the stock securely by clamping it to the fence. The position of the fence in this jig is critical as it being tight on the cut (zero clearance) is required to prevent tear-out, and you will notice that the fence on the left is behind the work piece and the one on the left ahead. This arrangement is to provide backing to the cut.

This is a pretty sophisticated technique, so please let me know that you get it thoroughly before you try it. And this is not particularly an easy jig to make. To be honest, I've never made one myself, but just invented it in response to this request. But I believe it would answer the problems you are having. Let me know how it turns out. Matthew, this is a great question to help readers understand the impact of router bit rotation direction. Thanks for the question.