How I Fit a Through Mortise

ftt_1434One of the curious things about woodworking is that you don’t really know how thorough to be in one step until you are knee-deep in the next. With experience you develop a routine and can move along efficiently. Without experience you won’t understand what’s important and what isn’t and you’re likely to either waste a lot of time fiddling with things that don’t matter, or you’ll be reworking because you didn’t take into account something that really does matter. In a previous post , I discussed cutting a through tenon. The obvious next step is to fit that tenon in a mortise.

Mortise and tenon joints require “a good fit”. You should be able to press the two pieces together by hand, and when the pieces are together you should be able to pick up the tenoned piece and have the mortised piece come along with it. This means working precisely whether the tenon goes completely through or not, but a through tenon is a bit more demanding – the outside of the mortise needs to be as perfect as possible, and the tenon needs to fit all four edges of the mortise. With a non-through tenon you can leave some wiggle room on the ends and still have a sound joint.

ftt_9473There are several ways to make the mortise; a hollow chisel mortiser is my first choice, followed by a router. These leave the edges of the mortise in pretty good shape without much hand work. The next option for me is to remove most of the waste with a Forstner bit at the drill press then pare the sides with a chisel. I’m not a fan of chopping completely by hand with a mortise chisel, but that is also an option.

No matter what method you use, the layout is critical and the mortise is cut from both sides, with the cuts meeting in the middle. From the outside I use floats to clean up the edges that will show in the finished joint. The mortise can taper slightly (very slightly) toward the back to make fitting easier.

ftt_9484I get the mortises in shape before I layout the tenons. In theory they should all be identical but in practice there can be some variation in size. The more you practice the more consistent your mortises will be. I use a knife and marking gauge to layout the joints and take care to not work past the lines. If you’re cutting the tenon cheeks by machine make sure your machine set up is based on your widest mortise, if there is much variation. In a perfect world the tenons will slide in directly from the cut. If you can’t reach perfect aim for a tenon that is just a bit thicker than the width of the mortise.

You can fine tune your set up with scraps the same thickness as the tenoned pieces. If you can jam a corner of the just cut tenon into the mortise you have the thickness right. To layout the end cuts of the tenon, place the piece directly on the mortise on the show side and mark the ends of the tenon from the ends of the mortise.

ftt_9486From the back side use a chisel to put a slight bevel around the perimeter of the mortise. This will make it easier to get the tenons started and it will prevent the edges from breaking out when you take the joint back apart. You also want to bevel the ends of the tenons (leave the tenons slightly longer that the thickness of the mortised piece) If you leave the tenons proud you will eventually bevel back to just past the face. For now the bevel exists to make it easier to start the fitting and to prevent the grain from blowing out when the tenon eventually comes out the show side. Don’t fuss over the bevels yet.

Don’t expect the tenon to go completely through on your first attempt. The idea is to put the tenon (or tenons) in as far as you can without resorting to brute force. Then you take the joint back apart and do some detective work to find the area that is keeping the joint from closing. Remove the excess and try again.

ftt_1525I make my first test fit from the show side, even though the actual fitting is done from the back. That lets me know how close I am. The goal is to get the flat faces of the tenon (just past the bevels) into the outside of the mortise. If something has gone horribly wrong in the layout or cutting you’ll know without spending a lot of time, and you’ll establish the part of the joint that will show when you’re done.

It helps to mark on the tenon where the outer face of the mortised piece will be. Once you have the visible part of the tenon at the size you want it to be, the real finicky work is over. Flip the mortised piece over and put the tenon in as far as it will go. I usually tap the tenon into place with a dead blow hammer. Don’t wail on the thing, as you tap it into place you will hear a change in the sound when it has gone as far as possible. Take the pieces back apart and look at the tenon and the inside of the mortise.

ftt_1528If your lighting is good (and you’re under fifty) the high spots will be shiny. I usually take a soft pencil and scribble on the faces of the tenon before making a test fit. The graphite from the pencil will smear where the tenon is too tight in the mortise. Remove material until the pencil marks disappear and repeat the process. In theory you should only need to remove material from the tenon, but it’s worth taking a look inside the mortise. If you have to work on the mortise, be careful not to remove anything from the finished edges on the show side.

You can make the tenons skinnier with a shoulder plane, a chisel or a float. I prefer the float if there isn’t much material to be taken off. I think it is the easiest tool to keep flat and that’s the key to removing small amounts of wood in a controlled way. With a plane or a chisel it’s easy to go too far and taper the tenon. What tool to use depends on how hard the wood is, and how much material needs to be removed.

ftt_1576As I get closer, I take a look from the show side to see how it looks. Eventually the tenons come all the way through, but that doesn’t mean the job is done. You can’t really assess how well the shoulders fit until the tenons come completely through, and there may be additional work, as in the photo at right where another pair of mortises get cut to accommodate the tusks that hold the joint together.

If the tenons are left proud, now is the time to make sure how far beyond the face they go. I mark around the tenon with a pencil on the face, take the joint back apart and work the bevels back to the pencil line. That eliminates the risk of doing some damage to the finished face of the mortised piece while beveling. Fit the tenons on the least visible part of the project first. If you’re less than perfect on your first attempt it won’t be obvious, and by the time you get to more visible areas your practice will serve you well.

— Bob Lang


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