SLED RX: MORTISE & TENON JOINTS

Featured in the Sept/Oct 2007 Issue of Mushing Magazine: Here at the shop we cut thousands of mortises and tenons a year, and have purchased a machine that cuts them quickly and precisely. Still it is not unusual for us to end up manually marking and cutting a specialty tenon in the case of a repair or expediency. Sometimes it is quicker to cut by hand than the time necessary to change the machine around. While a bandsaw can be really helpful in the cutting of tenons, a good, small handsaw can do the trick. A quick note about tools that I reference and their ability to perform a task. Only if you use sharp bits and blades will these results occur. Dull blades and bits do not cut straight and often remove more chunks of wood than you wanted. Do not make the mistake of ruining wood, or whole projects, because you scrimped on a $5.00 bit. You will more than likely be cutting mortises early on in your project. The two types I will be talking about are blind mortises and through mortises. As it sounds, the difference is whether or not the mortise extends completely through the wood. Generally the through mortise is easier to cut, hence its popularity over the years. With a little extra work though, a blind mortise can add an extra layer of strength, especially in your stanchions. The blind mortise is always used on your runners wherever the stanchions are attached. You can fake a blind mortise here by laminating your stanchion platforms separately and then cutting through mortises. Glue the platforms on your runners and you have created nice blind mortises. If you cannot use that shortcut because of the type of runner you are using, you will have to cut a real blind mortise. It does not seem to matter if your mortises are rounded on the ends or squared off. When I started building sleds most of my joints were rectangular, but since we have used the multi-router they have become rounded. It will all come down to what tools you possess and how many joints you are talking about. If you have a lot of mortises to cut, and have access to a plunge router, you can build a jig to position your cut on your runners and stanchions. This requires an involved initial setup but will cut your mortises quickly and accurately. Using a drill press or a hand drill is an easy way to remove a lot of wood quickly and can give you two nicely rounded ends on your joint. If you are in a situation where you are only using a wood chisel to cut your mortise, it would be advisable to square off the ends for ease in cutting. Mark the mortises out on the top of your runner or stanchion side. I like to start my through mortises from the outside of the joint and work in. For blind joints we start in and work out. If you are using a drill bit, mark a center line on your joint to give a consistent guide for your drill bit. Start your drilling with a bit that is the same width as the mortise. I like to use a brad-point bit in this situation because of its ability to stay centered on the cut and not stray. If you want to cut a though mortise, just drill all the way through. But if you are cutting a blind mortise, mark the depth you want on the bit by using either tape or a maker, remembering to include the point. Drill a hole on each end of the joint, making sure to stay on the inside of your lines. If your joint is long enough you may drill a third hole to expedite wood removal. For through mortises you can use a jig-saw or a chisel and remove the bulk of the remaining wood, and then a flat bastard file to smooth it out. In the case of a blind mortise, carefully remove wood with a small wood chisel until the sides and bottom are even and smooth. Set aside for final fitting with a tenon. Tenons are on the whole easier to cut than mortises. Mark out the tenon on your stanchion or crosspiece as shown in photo 1. Using either a bandsaw or hand saw make your cuts to remove the excess wood. This leaves you with a rectangular tenon. Using a flat file, shape the tenon to fit the mortise. Make sure that the tenon slides in and out of the mortise freely, but not sloppily. When you are satisfied with the fit, take your file and soften the sharp edges at the end of the tenon and the lip of the mortise. This will help prevent future cracking or breakouts. When using through mortises you have a choice of cutting the tenon flush with the outside of your stanchion or leaving a bit of tenon extending out, usually ¼ to ¾ inch. This difference will determine how the sled is tied at the joints. A flush joint or blind mortise can be tied so that the twine just loops around the center of the exposed or blind tenon. If the tenon is extended you have to tie your joint with twine that wraps above and below it. A consideration when using extended tenons is again the type of trails you mush on. If you are running through tight, narrow trails these tenon ends can catch on trees and brush and be damaged quicker than flush or blind joints. Check out the next issue for more on tying your sled together. The type of sled you are building can help determine the size and type of joints you use. For smaller, sprinty type sleds, your joints can be looser because the sled will need to flex more. For these types of sleds you are often using wood that is smaller than normal because weight is a consideration. Therefore, do not make the mortises too large as this may cause the stanchion to crack or breakout. In all cases make sure your mortises are no more than ½ the width of the stanchion or runner. For reference, mortise and tenons are normally ½” by 1 ¼” for regular size stanchions and ¾” by 1” for sprint stanchions. As you finish the various parts of your sled, give them an initial coat of oil so that when assembly starts all joints and tie areas have already been finished. This is especially important in the case of your mortise and tenon joints where oil or varnish may not penetrate when tied. This will help prevent rot damage later on. So get all your pieces finished and ready to tie together because we will be writing about assembly and knots in the next issue. Until then, happy mushing thoughts.David Klumb has been making dog sleds in Fairbanks, Alaska since 1980. David and his wife Joanne ran the 2006 Serum Run. For more information on Laughing Husky dog sleds, visit www.laughinghusky.com.