Slippage

Whilst goofing around the other day on t'interweb, I came across an interesting little snippet on the GMC site, which entailed cutting 'slip joints.' I've never heard of these before, but they are large, skinny bridle joints, suitable for the back panel of a cabinet, for example. This types of joint was much beloved by the late JK and he used them in all or most of the stuff he made.

I've cut these types of joints before by hand and although not difficult, they do take a measure of concentration to get just right, and often need to be adjusted for a decent fit with a shoulder plane or the judicious use of a wide chisel.

I copy/pasted the information from the site for a 10 point instruction doc:

1. You will need to make a pair of thin shims, the same thickness as the kerf, cut by the bandsaw blade. In my case the thickness was 1.2mm and I cut out a piece of mounting card for the shims. Do not worry too much about measuring this accurately, when you make a trial slip joint you may find you need a slightly thinner or thicker shim to make the finished result tighter or looser.

2. The peg in a slip joint does the same job as the tenon in a mortise and tenon joint and a pair of thick shims (6mm) are used to set the thickness and position of the peg. I recommend making the peg one third of the thickness of the wood. For this, each of the thick shims must be one third of the wood's thickness. I used 6mm MDF to match the 18mm oak.

3. Unlike hand cut joints which rely on accurate marking, slip joints are largely self-aligning once everything is set up. The only mark required is the depth of the joint which you gauge by laying the end of one piece of wood on the edge of the other, then run a pencil line against It.

4. The band saw fence is clamped and positioned with one thin shim plus the wood thickness away from the blade. I use a home made fence for this kind of work because It is more rigid.

5. Place one thick shim against the fence to position the wood for sawing the outside of the peg. Feed the wood slowly into the blade as far as the pencil marked depth

6. Place two thick shims plus two thin shims against the fence to position the wood for sawing the inside of the peg.

7. Place one thick shim plus one thin shim against the fence to position the wood for sawing outside of the socket.

8. Place two thick shims plus one thin shim against the fence to position the wood for sawing the inside of the socket.

9. You now have four slots or kerfs, carefully spaced so the inside of one pair matches the position of the outside of the other pair.

10. The two halves of the join tare now ready to slip together. Ideally, this will be a firm fit with even contact all over the peg and socket sides. If the fit is too tight, the joint will be forced apart. The solution is to Increase the thickness of the thin shims. If It is too loose, these shims need to be made thinner. Brush glue inside the socket, that way any surplus will be pushed Into the joint as you close It. rather than getting scraped on the outside.

I've been playing around with this technique today and found that with a little experimentation, a perfect bridle joint could be cut on the bandsaw in about 2 minutes, taking about a minute or so to get the correct thin shims made up. In my particular case, it happened to be a couple of pieces of picture framing card which were just about perfect.