With the mortises plunged in the legs, the braces need to be fitted to the leg and seat. This involves some tight tolerances and compound angles, but is quite acheivable when approached systematically. Trace your laminated brace onto some scrap plywood. Do most of your fitting with these plywood models. It would be a shame to waste a good brace!
Square the Mortise
The bottom side of the routed mortise must be squared to receive the brace. Ash is quite hard, so I honed my 1/2” mortising chisel before making the cuts.
Plane the Brace
The laminated brace had some waves in it from the clamping pressure. The brace must be flat where it contacts the bottom of the seat. These are planed flat with a sharp smoothing plane.
I ripped the braces about 1/32” oversize to allow for precise fitting to the mortise. The holdfast stabilizes the brace as I plane the brace to final thickness.
Determine Angle of Brace in Mortise
The legs holes are drilled at a compound angle through the seat. That makes determining the contact angle of the brace to the leg mortise a challenge.
Put the brace into the mortise and sight the twist angle of the leg that results in the leg brace landing flat on the bottom of the seat. Keep the leg in this position and use a protractor to find the angle that tracks down the center of the leg. Transfer this angle to the contact surface of the brace.
Here the brace-leg mortise contact surface is trimmed to that angle. You need to do a good job of estimating how long the brace needs to be. The brace needs to be 1/2” tall at that end. If you make the angled cut line too deep into the curve, you will not have the required 1/2” thickness at the end of the brace and it will land below the surface of the leg at the bottom of the mortise.
Extend Mortise to Provide Clearance for Brace
The brace form I used is one I recycled from previous projects. The resulting brace contacts the leg on this rocker high on the leg. I believe that my rear legs are angled too far back and will likely bring them about 5-10 degrees more upright on subsequent rocking chairs.
This photo shows the brace not making contact with the existing mortise. I determined that I needed to extend the mortise higher on the leg to provide clearance for the brace. I remounted the leg to the lathe and extended the mortise at a shallow depth with a plunge router.
I then chopped the mortise extension at a shallow angle to remove the minimum amount of wood from the leg.
Another view of the mortise.
The brace has adequate clearance in the leg mortise.
Cut Brace to Length
Use your plywood template to determine the final cut on the brace that will rest at the bottom of the leg mortise. I sneak up to this cut in 1/16” increments and test fit between cuts. If you cut the final brace too short, you’ll have to glue in a spacer at the bottom of the tenon, or laminate a new brace…ouch! I’ve done this in the past.
Here’s the final fit on one of the rocking chairs. Two screws and glue hold the brace at each contact point on the seat and the leg. I’ll drill holes to receive round washer head screws and plug the holes with cherry buttons during assembly.
My next step is to cut wedges and saw kerfs in the upper and lower leg tenons. I’m getting close to assembling the bottom half of the rocking chairs!
-- Mark, Minnesota