My tenon cutters are designed for the final fitting of the joint. The less wood taken off the tenon, the smoother the cut will be. It will also be less likely that the tenon will get cocked on the part.
I hold both the tenon cutter and the part in my hands, since I feel that putting the tenon cutter or part in a vise increases the likelihood the tenon will be askew. While using the tenon cutter, put pressure straight into the tenon cutter and pull very slightly up on the part. Pushing down can cause the tenon cutter to pencil-point the tenon. For tapered tenons, stop cutting when the end of the tenon is flush with the left side of the tenon cutter. For cylindrical tenons (i.e. spindle tenon cutters), you will get a 1″ long tenon if you stop with the tenon flush with the left side of the tenon cutter. For longer or shorter tenons, the point where you want the tenon to end (and the tenon’s shoulder to begin) should be centered in the tenon cutter when you stop cutting.
You may now have a shoulder on your tenon. For tapered joints this must be removed, for if it bears on the edge of the mortise it will keep the tenon from seating, causing a bad glue joint. Regardless, it tends to look bad. The part can be chucked in the lathe if the lathe’s center-mark remains, or using a drawknife or chisel will work fine. A spokeshave is more dangerous — it is difficult to tell when you are cutting the tenon itself.
ADJUSTING TENON CUTTERS
You will need to adjust the blade after sharpening, if the blade has been knocked, the tenon cutter’s body has moved from moisture changes, or in order to change the diameter of cylindrical tenons.
First turn or shave a testing stick. Dry oak works best. Turn your stick to <⅛” greater diameter than the entry hole of your tenon cutter. So, 15/16″ for leg cutters, ¾” for arm stump cutters, and 11/16″ for spindle cutters. Taper the end slightly to facilitate entry (or even turn a 6° taper to help pre-set your blade on leg and arm stump cutters).
Now loosely install the blade with the turned tenon inserted tightly in the tool if you did this in step 1. (If you don’t have a turned tenon, another tenon cutter can do this, or you can skip this part). Adjust the blade so that it seems it will just start to cut along the length of the tenon. In general, you want the blade located near the center of the blade-bed (again, bevel-down) and the cutting edge to just be proud of, and parallel to, the end of the blade-bed as seen by looking into the hole from the wide end. Tighten the screw lightly.
Spindle Cutters
Set blade parallel and just protruding into hole as noted in step 2. Rotate test stick with even pressure, pushing towards the small end of the hole. To increase the cut depth, put a screwdriver against the back of the blade and lightly tap the screwdriver with a hammer (putting the screwdriver directly behind the screw will drive the blade straight, putting it off center will rotate the blade). If you go too far, loosen the screw, pull the blade back and try again.
If the blade leaves a ragged burr on the stick, the stick is too big in diameter: turn or shave it smaller. If the blade doesn’t cut, look for burnish marks on the stick. If it is burnishing at the exit hole, either slide the blade slightly to the left (toward exit hole) to align blade better with the bed, or rotate the blade slightly to the left. If it is burnishing at the entry hole, rotate the blade slightly to the right. Rotating the blade may be enough to get the tool cutting, or you may also need to advance the blade.
When the cutter is cutting sufficiently, turn a tenon until it is flush with the exit hole. Remove and check the diameter of the tenon. This is best done with a test hole drilled in the same wood as your chair seat by the same bit you will use to drill your seat (make a series of holes, since they will wallow bigger as you test). You want a tight fit, but not so tight that you can’t twist the tenon at least half way into the mortise by hand.
If the tenon is way too big, it will stick in the exit hole of the cutter. To fix this, either rotate the blade to the left or tap the whole blade evenly forward slightly (most likely the latter). If the tenon is too small, try tapping the blade slightly to the right (towards entry hole). However, it may be necessary to back off the blade and start over. Also, check to make sure the cutter is forming a non-tapered tenon by measuring the diameter of the tenon with calipers in two nearby spots to see if it’s the same measurement. If not, adjust blade until it is. Give blade screw a final tightening with tenon in place to prevent any shifting. The screw can take a solid tightening, but excessive torque can pull the screw insert from the body. If you’re in a humid environment, a light coat of wax on the blade will help prevent rust.
Leg and arm-stump cutters (much easier than spindle cutters)
Rotate test stick in tool with even pressure towards exit hole until it cuts along the whole length of the blade. Check that the cutting action is smooth and even and not too aggressive. Make tiny adjustments to the blade position by clamping the tool in a vise and tapping a screwdriver gently into the back of the blade with the testing stick in place. Tapping either back corner of the blade will rotate the blade in that direction. Tapping in the middle should advance the blade evenly. The finer the cut, the less the tool will tear. Once it is cutting well, check the taper of the tenon with a 6° bevel template or the blade of your reamer. Rotate the blade blade as needed to produce the proper taper, then give the screw a final tightening with the tenon in place to prevent shifting. The screw can take a solid tightening, but excessive torque can pull the screw insert from the body. If you’re in a humid environment, a light coat of wax on the blade will help prevent rust.
Tenon cutters can be purchased here.