Since the stretchers on this coffee table are curved, it’s easier to use loose tenons for all of the joints. I start by cutting mortises in the ends of each of the stretchers, using a jig that holds two stretchers in the same orientation as when they will be glued up.
A plunge router with a 1/4″ spiral up-cut bit cuts the mortises. To ensure the gap between the two mortises is consistent on all of the legs and stretchers, I use a jig with a 27/32″ spacer to offset the router so the gap between the two mortises in the stretchers is the same as the gap between the leg mortises.
To cut the two mortises on each of the legs, I use a shop-made horizontal mortising machine with a jig to hold the leg in the correct orientation. A plunge router with a different jig could also be used. The same 27/32″ spacer is used to ensure that the gap between the two mortises will mate with the stretcher mortises. In order to prevent blow out when cutting the mortises, I temporarily fasten a piece of veneer to the outside of the leg using double stick tape.
This table has six legs and six stretchers, so I require 12 floating tenons. I usually make some extra just in case. The size of the tenons must match the size of the mortises in the stretchers and legs. This coffee table uses tenons that are 1/4″ thick, 1-1/2″ wide and about 4″ long. I mill up three or four pieces of stock 1-1/2″ wide by 24″ long and just over 1/4″ thick, then plane the pieces to exactly 1/4″ thick.
Next, I round over the edges of the strips on a router table using a 1/8″ radius round-over bit and cut the strips into 4″ lengths. I always lean towards quarter-sawn material, while rift-sawn stock is my second choice. During assembly, I keep the grain orientation of the tenons symmetrical, mainly for visual reasons.
To pillow the front edge of the tenons, I dry assemble each joint, making sure the tenon is fully seated in the mortise, and mark the flush location of each tenon against the face of the leg. I then cut the tenons approximately 1/8″ beyond the line and pillow the ends using files and sandpaper, being careful not to shape past the pencil lines. Next, I cut two kerfs in each tenon. The legs in this table are about 1-1/2″ thick, so I make the kerfs about that length.
I use a 1/4″ diameter chainsaw file to enlarge the mortises in the legs and make them slightly wedge shaped (wider at the front of the leg than at the back). This allows the wedges to expand the tenon at the front and mechanically lock the tenon into the leg.
I make wedges using a simple 4″ x 16″ wedge-cutting jig made of plywood that runs against my bandsaw’s fence. In the long edge of the jig I cut a 1-1/2″ long wedge shaped notch. Mill a block of wedge material 1/4″ thick by 4″ wide by 1-1/2″ long. Push the end of the block into the side of the wedge jig and run the jig through the bandsaw blade, cutting a wedge. Flip the block over before cutting another wedge to ensure the grain continues to run lengthwise.
Pre-finish all parts prior to gluing up the table. This includes pre-finishing the ends of the tenons where they are exposed. Use blue masking tape to cover all glue surfaces.
To keep the parts aligned correctly I glue two stretchers and one leg at a time, with the help of a jig. I dry fit all of the parts and wipe some wax on the pre-finished surfaces of the legs and the ends of the tenons so glue doesn’t stick to any exposed surfaces. I use Claphams beeswax polish from Lee Valley. Be careful not to get any wax on the tenons or in the saw kerf for the wedges.
After disassembling the joint, I apply glue to the mortises in the stretchers and to the ends of the tenons, and insert the tenons into the stretcher mortises, tapping them into place. Next, I apply glue to the mortises in the leg and a little to the ends of the tenons, and use clamps to close the joint. I immediately remove the clamps, then glue and insert the wedges with alternating mallet taps to ensure the wedges finish at the same depth before re-clamping the joint. The clamp caul I use has been relieved where it will fit over the tenon.
Once the glue has set I trim the wedges to match the pillowed tenon and remove any glue squeeze-out. Extreme care is needed not to damage the finished surface. Re-apply some finish and wax to the ends of the tenons, as required.
Accuracy is Everything
In order to accurately rout the mortises in the stretchers, a well thought out jig is used in conjunction with a spacer. The spacer is easily moved from one side to the other in order to position the router perfectly.
Milling the Leg Mortises
A second jig positions the leg to cut mating mortises. Note the spacer on top of the workpiece. For the next cut, it will be placed below the leg.
A jig is used to accurately position two stretchers and a leg during each stage of assembly.