Tuesday 23 August 2011
John Bullar looks at a classic piece of joinery
Scarf joints are used to fix timbers together end to end. They are used sometimes in cabinet making, such as with rare wood that comes in short lengths, but more often in furniture repair or household joinery. The idea is to turn two short timbers into one long one without changing the external profile, or making the wood significantly weaker.
This article focuses on one type of scarf - the double-tenoned variety which is locked together in the middle with wedges, needing no glue. It would normally have been cut with a saw and chisel but here I will do most of the work with a router.
Normally, you would be joining two similar pieces of wood but for this demonstration joint, I have used two different hardwoods so it shows up more clearly in the photos. It is equally effective for softwood, provided there are no knots near the joint. Each piece is of the same section, which in this case happens to be 50mm square. Any rectangular section could be used, so long as the wood is thick enough to take the joint
The joint is cut as two identical sections, one of which is then turned over to interlock with the other. It consists of a tenon at the end, then a shallow socket followed by a deep socket with a mortise undercut in it. The cuts are all straight, which makes it is easier than a dovetail, for example, but it can still be confusing. To help me get my head around an unusual joint like this and check the dimensions, I start by drawing it out like this, full-scale on a piece of paper
A router rebate cutter designed for milling is ideal for this joint. The one I used was 31.7mm in diameter, with 5.7mm deep edges. It makes a straight side with a flat bottom. It is also capable of undercutting up to about 5mm or more - ideal for making the square grooves which form the mortises at each end of this joint.
Alternatively, if you prefer, it is quite practical to make this joint with hand tools, using a chisel to chop out the mortises
Mark out the depth by scoring two lines on the side of the wood with a marking gauge. The outer two sections must be the same depth. In my example they are 20mm each, while the centre section between the lines is 10mm. These dimensions will give plenty of thickness on each side of the finished joint, while 10mm is enough for the thickness of the tenons at each end and the wedges in the middle
The length of the joint is marked out in five positions and then a square is used to draw five lines across the side. These correspond to the positions of the shoulder lines that will be left when the joint is cut. Both halves of the joint are symmetrical so that when one end is turned over, it will fit the other
Areas between the lines will be cut away, and so I shaded them in with a pencil. This isn't strictly necessary but it certainly helps me anticipate how the joint will be formed, so I certainly recommend it first time
Using a router to cut this joint means that both pieces of wood can be shaped in one operation - not only saving time but ensuring that they are identical. Line up the ends and sides and then clamp the two pieces together in the jaws of a wide vice or a Workmate
With the router unplugged, mount the milling cutter in its collet and stand it sole up to measure the underside. You need an accurate measure of the offset distance between each of the cutting edges and the edge of the router sole
The guide square
This guide square is made from two straight edged pieces of MDF, screwed and glued together. It is a simple little jig but a most valuable one, and well worth the few minutes it takes to make
One arm of the guide is clamped against the side of the pieces of wood mounted in the vice, while the other arm provides a secure straightedge. Use the offset distance that you measured earlier to work out where to position the square guide for each cut
The depth stop on the router needs to be set. With the router still unplugged, plunge the router down until the cutter's bottom edge just reaches the lower line previously scored with the marking gauge on the side of the wood. Power up the router and cut waste away from the deep main socket in the normal way as a series of deeper and deeper passes
Routing the end mortise
With the main socket shaped as a straight-sided rebate, it is time to undercut the mortise at the end of the joint. With the square guide moved back to its new position, the router is locked at full depth to slot out the lower half of the mortise
I used one more pass of the router to make the upper half of the mortise. This time the plunge was locked at a height where the upper edge of the cutter met the upper scored gauge line. Depending on the dimensions of your cutter and joint, you may need more passes of the router at intermediate levels
I turned the wood over to make the final router cut. This made a narrow rebate along the edge of the wood, forming the shoulder of the end tenon. When making cuts like this, extra care is needed to keep one side of the router sole flat on the face of the wood. It could easily wobble, resulting in a wavy edge
I used a bandsaw to cut away the final shallower piece of waste because it is easier and more secure. It would be possible to rout it but because so much of the original face has now been cut away, you would need to set up extra supports to hold up the router sole
All the cuts are now complete and it should be possible to test fit the joint together. Don't force it but if the joint is too tight, shave off the excess with a shoulder plane or chisel
With both end tenons engaged in their mortise slots and pushed home, there will be a square or rectangular hole down the centre of the joint. Cut a piece of hardwood to a thickness matching the hole sideways but slightly oversize in both other dimensions
Cut the small piece of hardwood diagonally to form a pair of triangular wedges. Even if the joint is made from softwood, I still suggest you use hardwood for the wedges so they won't compress or jam
Drive the wedges into the hole in the middle of the joint, locking it securely together with one wedge from each side. You can use a mallet but I prefer to use a strong clamp to squeeze the wedges together, as it is more controllable. Finally, saw of the ends of the wedges flush with the sides and plane them.
Joints like this, made without glue, have a timeless appeal. If you are not convinced, there is no reason why you should not apply glue before assembly. You may want to practise this joint on some scrap to check the dimensions and methods before using it as part of a project. With careful measurement and cutting, the double tenon scarf joint is excellent for small, load bearing construction, and also an attractive joint to look at!