Now that the dissection is complete it is time to cut the mortises, or is it morti? I’ve been a big fan of the Mortise Pal jig and continue to be. The more I use it the faster I can set it up and adapt it to various situations. Let’s review a couple of notes. Be sure to use the guide bushing that comes with the jig. You may want to use a 5/8”, but the jig’s bushing is slightly larger. It will slide easily in the guide, but unlike the 5/8”, it will produce a mortise that precisely matches the diameter of the bit.
Second, the guides are labeled according to the length of travel using the correct bushing. This means the centerpoint of the bit will travel that distance. With this in mind I work backwards to calculate the template I need for a certain mortise. Say for example I’m going to cut a mortise 3/8” x 1 5×8”. Using a 3/8” bit I subtract 3/8” from 1 5/8” to get 1 2/8” (or 1 1/4”). One way to visualize this is the bit will cut 3/16” (1/2 of 3/8”) on each end of the mortise greater than (or further than) the center point of the bit. To me it is counterintuitive as I would normally use the router to cut a longer mortise instead of using a larger bit.
Since the jig would not fit between the back slates I ripped a 3/4” MDF scrap at the same width as the vertical support and cut through it to produce mortises in the back leg support. I had previously marked a centerline for the mortises and extended it across the top of the scrap guide piece to align the jig. After the initial cut I removed the jig and guide piece and used a 3/8” straight bit with a roller guide to make the holes deeper.
The arms are a couple of 2×4” glued together and ripped to width. The back was cut at 8 degrees on the chop saw the match the slope of the back. Since the vertical (front) and horizontal (back) mortise/tennons are oriented about 90 degrees to each other it would be very difficult if not impossible to install the arm rest with snug mortises. I cut the two mortises on the vertical arm support a little longer so I could slip the tennons in the back and engage the front before tapping the arm in to place.
Not a bad fit. Next we’ll rough cut the arms and assemble the bench and do some final shaping with the Rotex.
-- tim hill www.newcalshop.com