Q: I’m having trouble routing rabbets with a straight router bit. I have the router mounted to a table, and the cutting depth starts out OK, but by the end of the cut the rabbet tends to get shallower by as much as 1/8". I’ve checked carefully to see that both the wood and router table are flat, but still my rabbet cuts are getting shallower towards the end. Why is this happening?
A: Since you’re sure that the router, bit and fence are all secure in your setup and the wood is properly flat, I can think of a couple of possible reasons for this problem. One option is that waste sawdust from the cut is building up under your workpiece. As you cut, more sawdust is being created and captured under the stock so the change in depth is gradual. You need to make accommodation for that sawdust to be extracted or cleared in such a way that you eliminate this problem.
Another possibility is that you are gradually tipping the back end of the board up off the router table near the end of the cut. It’s easy for this to happen, especially on long boards, if you don’t hold the workpiece down with a featherboard and or an outboard support device of some kind. Either way, the good news is that since the cut is too shallow, a second (or third) pass will likely cure all.