Mortises by Router 3 Ways

Machined mortises are quick to cut and accurate.

By Gary Rogowski
Page 46-48

My old friend from college is a physicist who launches rockets into the sky for a living; let me just say that he is a very bright fellow. But he has also told me that the router is the quickest way for him to ruin a piece of wood. Well that can be true for anyone who doesn’t pay attention to some simple facts about the tool. Proceed with accuracy and clarity, and the router makes flawless cuts every time.

Here are three methods for router-cut mortises that guarantee success.

Cutting Geometry
Routers pull themselves into a board and cut the softest wood available, tearing a path without regard for the beauty and simplicity of a straight line. You need to restrain them and show them the way to cut to get good results.

First, you must understand cutting geometry. When making a top-side cut the bit spins clockwise. If you are using a fence or bearing-guided bit, this direction of rotation pulls the bit in tight to the work as you move the router from left to right along the piece.

All this gets reversed when you move to the router table because there, your router is mounted upside down. As you look at the router table, the bit spins counterclockwise, so you then move your work from right to left into the rotation of the bit.

Now that you understand the cutting dynamics of the router, let’s look at three ways to use it to make mortises.

Router Table Shim Method

I use the router table for one of my mortising methods. With this approach I prefer to take tiny nibbles – about 1⁄8″ deep in a hardwood such as oak or maple. But the problem with making a series of cuts is that I would often get a step on one side of my cut due to slop between the motor and the base as I adjusted the depth of cut upward. If, like me, your router doesn’t move precisely and without a hitch, there’s a trick to get good mortises using a router table – shims.

To achieve the smoothest mortise cuts, choose a spiral flute bit. Set the bit to its final depth of cut. There can be a scary amount of bit showing, so clamp as many 1⁄8″-hardboard shims as you need around the bit until only 1⁄8″ of it shows.

After each pass, simply remove one shim.

Work between stops on the router table fence and never move backward in the cut (left to right). The bit could grab your piece and pull it away from the fence – and even send it shooting across the shop.

Video: See how glue dries in a mortise that’s made with a window in a short online video, “Where Does the Glue Go.”
In Our Store: Learn how to accurately cut with handsaws on the DVD “Sawing Fundamentals” featuring Christopher Schwarz.
Web Site: Visit Gary Rogowski’s Northwest Woodworking Studio site for information on classes and to view a gallery of his work.

From the June 2013 issue #204