Question: When routing mortises like in “A Guide to Guide Bushings” (FWW #207), I use a bit that’s the same size as the mortise, but my mortises always get packed with wood chips, which really slows down the process. How can I avoid this?
Problem: Clogged mortise. A packed mortise slows down your work, and leaves a less-than-perfect cut.
-- Ross McPherson
Answer: Excavating the waste when you’re routing a mortise improves the quality of cut and keeps wood chips from getting caught in the mortise. Here’s how I keep my mortises clean.
Start with a router that can be connected to a shop vacuum. This way you can remove the chips from the mortise as they are created. You should also use an upcutting spiral bit, which pulls the chips up and out of the mortise.
It is also essential to choose a guide bushing that allows plenty of clearance for wood chips to flow past easily. In my experience, 1/8 in. of clearance is plenty. That means, when cutting a 1/4-in. mortise, use a 5/8-in.-dia. guide bushing.
Technique matters, too. Cut the mortise using multiple shallow passes, increasing the depth a little with each cut. As a general rule, the depth of each cut should not exceed the diameter of the bit that you’re using. This will reduce the volume of chips created at one time, so they don’t get packed in the mortise.
Use a vacuum. Attach a vacuum to remove debris from the mortise while you’re working. Use a spiral bit and a big bushing. An upcutting spiral bit pulls
chips out of the mortise. To make a path for those chips to exit, use a
bushing that leaves at least 1⁄8 in. of clearance around the bit. Take multiple passes. Cutting in stepped increments will prevent the mortise from getting jammed with chips. Clean, trouble-free cuts. By collecting dust as you rout, the pattern and mortise remain clean and ready for the next pass.Photos: Staff; Drawings: Kelly J. Dunton
From Fine Woodworking #239
, pp. 84
February 5, 2014