Volume: 140
Issue: August - September 2012

June Meeting: Mortise and Tenons

Story by Ron Corradin
Photos by John Griffin-Weisner

The June, 2012 Guild meeting was hosted by Greg Flanagan at The Mill in Minneapolis.  Several members took Guild President Charlie Kocourek’s advice and brought in their first woodworking project.  They were well done, especially for first projects, and they included:

Table
Pencil holder
Stool
Jewelry box (2)
Horse profile bookends
Abstract sculpture
Spool rack
Rocking horse
CD rack
Clipboard
Flute Hanging lantern
Bow saw
Bookends
Bench made from 100+ year old wood from a corn crib

        Instant Gallery: Members' first projects

The M&T

The topic of the meeting was mortise and tenon joints. Five different approaches to making them were demonstrated, some producing mortises and tenons with rounded corners and some with square corners. Mortises 3/8” wide would be cut in table legs to receive 3/8” wide tenons cut on the side pieces to make small end tables..

Guild members were told the mortise and tenon joint is the strongest, most fundamental woodworking joint. It has the mechanical strength to resist torsional forces and plenty of glue area that does not use end grain. Mortise and tenon joints have been found in thousand year old furniture in Shanghai, China. Traditionally a tenon is one-third the thickness of the board, and two to three times as long as the board’s thickness. So a tenon on a ¾” thick board would be ¼” thick and 1-1/2 to 2-1/4“ long.

Traditional Hand-Cut

In the most traditional approach to making a mortise and tenon joint, Jeff Hand used a 3/8”mortising chisel, a backsaw, and layout gauges. First he laid out the size and position of the mortise on the table leg with a marking gage. Then he re-set the marking gage and laid out the tenon dimensions on the table’s side piece.

Jeff clamped the table leg to his workbench, mortise area up, and began cutting the mortise with shallow cuts of the 3/8” chisel. The chisel’s width set the mortise’s width, and the how hard the chisel was hit set the depth of cut. After each set of cuts Jeff would use the chisel to lever out the waste until he got to the desired depth.

  Hand cutting mortises with chisel         Cutting tenon with backsaw

To cut the tenons, Jeff clamped the side piece vertically in his workbench vise and made the cheek cuts with a backsaw. This takes practice to do well. Clamping the board to the workbench he made the shoulder cuts, again with his back saw. Any cleanup in either piece was handled with a shoulder plane or a chisel.

Mortising Machine/Drill Press & Table Saw

Cutting mortises on either a mortising machine or a drill press with a fence was demonstrated by Jeff Anderson, using the Mill’s Jet mortising machine and their Jet drill press.

 

A mortising machine or mortiser has an auger bit turning in a hollow, square chisel to cut square holes for mortises. It works quickly – drill a line of square holes in a row and you have a mortise. The Jet mortiser had a vise on a carriage with hand wheels to control left-to-right and in-and-out movement. The long handle on the mortiser and a stop on it controlled the depth of cut. This design helped to keep things straight and square.

 

The drill press used a simpler version of the same idea. There, the leg was held against a wooden fence that was clamped to the drill press table.  A 3/8” forstner bit cut a line of overlapping holes for the mortise as Jeff moved the table leg along the fence, fully withdrawing the forstner bit after each cut. The material between the holes was cut out with a chisel to straighten the mortise sides. Tom Caspar of Woodwork magazine recommends clamping another block above the work piece to guide the back of the chisel when doing this.

Ed Neu used the Mill’s SawStop table saw to cut tenons. First he used the saw’s fence and a miter gauge in the left miter gage slot to make the shoulder cuts. He set the distance between the blade and the fence to get the correct tenon length, then gradually raised the blade height on successive cuts to get the right tenon thickness and width.

Ed then used a Delta tenoning jig to make the cheek cuts. The jig rode in the right miter gauge slot. The work piece was clamped to the jig, perpendicular to the table. With the saw blade raised to the height of the cheek cut, Ed gradually moved the work piece towards the blade with an adjusting screw on the jig. Each pass narrowed the tenon by twice the cut width. Once the tenon thickness was right, the other tenons could be cut without further adjustments.

The last cuts were on the narrow faces of the tenons. For this, Ed went back to using the miter gage and the fence. He used the distance between the fence and the blade and the blade height from the previous shoulder cuts. To make the new cuts he either made multiple passes past the blade along the tenon’s length, or just pushed the work piece along the miter gage into the blade at its highest point. The tenoning jig would also have worked for this.

A note on safety: While there were no loose pieces to get pinched between the fence and the miter gage and get shot back at the operator, some of these cuts required removal of the saw guard. So be careful.

Router with Leigh M&T Jig

Steve McLoon took a more high tech approach with a Leigh model FMT mortise and tenon jig. A router was mounted to the jig’s base plate. The base plate had two pins, one for cutting the mortise and the other for cutting the tenon, used in conjunction with the vertical guide. To cut a tenon, Steve clamped the work piece to the vertical guide. Once set up, he could do any number of identical tenons. (Different guides are available for different size mortises and tenons.) Using an up cut spiral router bit, Steve first did a climb cut in the clockwise direction (seen from above) all around the tenon. Then he did a regular cut in the counter-clockwise direction. With the guide pin in the tenon track the router bit made the cheek and shoulder cuts simultaneously. Steve said to be careful at the corners, which were the biggest source of mistakes.

To cut the mortise Steve clamped the table leg to the vertical guide and made a series of plunge cuts with the router. This time the base plate’s pin followed the inside of the guide, instead of the outside of the guide as it had for the tenon. He warned against using back-and-forth router movement when the bit was turning.

Steve claimed the jig was less complicated than the Leigh dovetail jig, and compared using it to programming a VCR to record a TV show.

M&T with a Carved Pattern

Finally, Mark Laub demonstrated making an insert mortise and tenon joint. In this technique, a pattern carved on the end of the tenon protrudes through a matching hole cut in the far side of the mortise.

The actual mortise and tenon are cut by hand or machine. The far side of the mortise is left 1/8” thick, and the tenon is ¼” longer than the depth of the mortise. Mark uses a Dremel tool mounted on an aftermarket luthier’s base with a small down cut spiral cutter (and a hose from an aquarium pump to blow away the dust) to route a pattern through the far side of the mortise. Often he cuts an Asian kanji character. Then he traces this pattern through the mortise on to the end of the tenon and mills away the end of the tenon so the pattern stands ¼” proud of the tenon’s end, and 1/8” proud of the mortise’s far side. This is neither a half-blind mortise and tenon, nor a thru mortise and tenon, and the results are impressive. The technique is described on the back cover of issue 191 (June 2007) of Fine Woodworking.

Summary

Guild members got a visual lesson in how tight a mortise and tenon joint should be. If it won’t go together, then it’s too tight. If the tenoned piece falls out of the mortise when it hangs from the mortised piece, then it’s too loose.

Getting a bigger hammer will not fix a too-tight joint. If you have the opposite problem, gluing veneer to the tenon’s faces (or even heavy brown paper if veneer is too thick) will fix a too-loose joint. Allow the glue to dry before reassembling the joint. Further cutting and sanding may be necessary to get a good fit.