Chapter 6, Lesson 2 of 3
GOAL: To gain an understanding of how to use a router and a table saw to make a mortise-and-tenon joint.
Using machines to cut a mortise-and-tenon joint opens the door to a wide variety of inventive and personalized methods. Inevitably the methods you choose will revolve around the machines you have available and the jigs you design and build. Because most small shops are equipped with a table saw and a plunge router, those are the tools used in this lesson, which also will include two helpful jigs.
Two helpful jigs for making mortise-and-tenon joints
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Mortising Jig
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Router Guide Collar
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The router guide collar rides inside the aperture of the mortising jig, which has been sized for the mortise, plus the offset value for each side. The offset is the distance between the edge of the bit and the outer edge of the guide collar.
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Tenoning Jig
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Set the blade to height and begin by cutting the inside tenon face on the spare rail.
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Saw all shoulders using the miter fence. Note that the rip fence is well behind the workpiece so the falling piece doesn’t fly back.
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Successful machine-made joints require that similar parts be milled and prepared to exactly the same size and shape. The mortise and tenon must align and have a snug fit if the joint is to serve its purpose and the finished piece is to come out as planned. It’s also good practice to mill and prepare an extra of every type of part involved in the joint to help with machine setup and fine tuning.
Assume for the purposes of this lesson that you are making a square table using four top rails tenoned into four legs. That means you would prepare a total of five rails and five legs. It’s important to mark the pieces so you can keep track of inside and outside faces and top and bottom edges; these marks will help you orient the pieces properly when making cuts. To accurately, efficiently and safely machine mortises and tenons, you must carefully control the movement of either the machine or the workpiece.
To enable this, as well as to make accurate cuts repeatable, you’ll want to build the two jigs pictured in the illustration at right.
The mortising jig is made from several pieces of ¾" plywood and a piece of ½" MDF, and you’ll need a guide collar, or bushing, for your router for the jig to work (see illustration).
A rectangular opening, or aperture, is cut in the MDF baseplate corresponding to the size of the mortise, plus the amount of offset between the router bit and the router guide collar. The base blocks are attached to the baseplate; their edges are parallel to the aperture, which has been positioned carefully to locate the mortise. The distance between the edge blocks is equal to the dimension of the workpiece face in which the mortise is being made.
The tenoning jig is made from pieces of ¾" plywood and a piece of poplar or plywood (see illustration). It will allow you to safely and accurately run rail pieces on end past the table saw blade to cut the tenon.
Routing the Mortise
Drill out the center of the mortise with a drill bit smaller than the size of the final mortise. That way you can plunge to the full depth of the mortise with your router and make the opening in one operation. If you don’t drill out the material beforehand, make the router plunge cuts in several passes at increasing depths.
As for the router bit, a straight bit or spiral bit will work, but it must be long enough to reach the bottom of the mortise. It needn’t be the full width of the mortise; the size of the aperture in the jig controls the size of the mortise. When you have routed the mortise, square up the ends with a sharp bench chisel.
Cutting the Tenon
Clamp the workpiece into the jig, making sure the rail is firmly seated in the jig and that the clamp head is firmly seated against the workpiece. Set the blade height to cut the tenon length. Adjust the table saw fence to cut the inside face of the tenon. Make the cut, then repeat it on the remaining four rails.
The second cut is critical because it determines the thickness of the tenon. Position the jig, make a test cut on the spare rail, and check the thickness. Make the full cut, then remove a part of the waste with a handsaw to expose enough tenon to test fit in the mortise. Repeat the full cut on the remaining rails. Using the same blade height, test procedures, and sequence of cuts, saw the tenons to width.
The simplest way to saw the shoulders is by using the miter fence to guide the piece across the saw blade. Set the blade to cut almost to the tenon, and use the saw fence to position the spare workpiece. Be careful to use a stop block so the waste piece doesn’t get trapped between the blade and the fence and kick back.
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