Wednesday 11 July 2018
The slip joint is great for batch jobs such as cabinet frames
Lay the sawn wood flat on the bench against an endstop. Plane end to end until long, wide shavings curl out of the mouth â€“ these prove that the surface is flat. Turn the wood over and plane the other side until the thickness is suitable and even. I planed the oak down to 18mm thick
Stand the wood on edge against a stop. Pinch the front end of the plane between your fingers and thumb so the fingernails guide its sole straight along the narrow edge of the wood. Turn the wood over and repeat until the edges are at right angles to the faces and parallel to each other
A well adjusted bandsaw is a pleasure to use, and essential for accurate work. Use a fresh, sharp blade and check that the tension is firm but not too tight. With the machine unplugged, pinch the blade and wobble it. There should be a few millimetres of bending movement in the blade with little or no rattling from the guides. The blade should move freely up and down without sticking
Check that the cutting edge of the bandsaw blade is vertical to the table. Larger machines have adjustments which can be used to correct misalignment in the tracking if necessary. Smaller ones should be accurately preset. The guides may need readjusting as well
The angle of the blade from side to side is fixed but the angle of the table can usually be adjusted by releasing a clamp beneath it. The table must be precisely at right-angles to the blade, even if this means the table is not perfectly horizontal
Bandsaw blades have teeth that are set alternately to left and right. This makes their slot, known as a kerf, slightly wider than the blade and prevents it from jamming. You need to measure the thickness across the teeth to see how wide the kerf will be. Alternatively, cut a short slot in a piece of scrap wood and use this as a thickness gauge
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
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 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 woodâ€™s thickness. I used 6mm MDF to match the 18mm oak
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
The bandsaw 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
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
Place two thick shims plus two thin shims against the fence to position the wood for sawing the inside of the peg
Place one thick shim plus one thin shim against the fence to position the wood for sawing the outside of the socket
Place two thick shims plus one thin shim against the fence to position the wood for sawing the inside of the socket
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
There are several ways to remove waste from the socket. I carefully slanted the angle of the wood to allow the bandsaw to cut away the waste, then nibbled down to the line with fine movements. If you do not have much experience in using the bandsaw, it may be better to remove the waste with a drill before bandsawing then chisel down to the line
The waste from either side of the peg is removed with a single saw cut. This can be done using a sliding fence on the bandsaw table, being particularly careful not to cut into the thin peg itself. Alternatively, use a tenon saw
The two halves of the joint are 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 off on the outside
Clamp the slip joint with softwood pads to spread out the pressure while the glue sets
The slip joint is a wide, flat, machine-cut joint, easily repeated for small batch jobs such as the corners of cabinet frames. On the face of it, the slip joint has the same basic geometry as a traditional bridle joint, but it is made in a very different way. Its wide, thin proportions give it a massive gluing area.
Slip joints were much used by the late James Krenov – one of the great cabinetmakers of recent times. With the aid of a bandsaw and after a bit of preparation time, you will be able to make a series of identical well-fitted slip joints, each one taking just a few minutes to cut. Once one joint fits well, the rest follow automatically.
The slip joint's main use is for the corners of low profile frames. Frames need good stable wood that will not twist or bow when moisture level changes. In this example, I used straight-grained oak. You might prepare your wood with hand tools or machines but if you use an electric planing machine, it's best to finish with a hand plane afterwards. Work with the grain to remove any surface ripples.
Be prepared to experiment at first – the secret of making well-fitted slip joints is in the thickness of the shims. Once the shims are correct, keep them safe and all the slip joints you make after that will fit equally well.