With a reamer and a matching tenon cutter, making perfect fitting tapered mortise and tenons is a breeze. Recently, I've been trying to come up with a simple process to make a first rate tenon cutter to match the reamers that I sell. This is the simplest that I've come up with. If you want detailed instructions read the text, but you can probably follow the process from the pictures alone.
Start by drilling a hole (the size of the narrow end of your desired tenon) through a block of wood. The block of wood should be close to square in cross section and it should match the width of the blade you plan to use for the cutter. For this rounder I used a two inch wide blade, so I started with a two inch square piece of wood that was about ten inches long. Drill the hole closer to one edge, but far enough away from it that there will still be material between the edge of the blank and the edge of the reamed hole.
|Sighting to make sure the reaming is staying on course.
Ream the full depth of the hole so that the reamer is just barely cutting at the narrow. I hold both the reamer and the tenon cutter blank in my hands and turn them both. In hard wood, this will require some torque and it may be easier on your hands to clamp the blank to the benchtop and use two hands on the reamer. I like to hold both in my hands so that I can flip them over and check the narrow end to make sure the small end of the reamer stays roughly centered in the hole as I ream.
|The marks in the hole show that the reamer has cut the full depth.
When you can see that the reamer has cut the full depth of the blank. Stop reaming.
This will leave just a sliver of wood above the hole on the wide side and a more substantial amount on the narrow end of the taper. Plane the thicker side so that there is an equal amount of wood above the hole on both faces of the blank.
|Note that in this picture the waste has been sawn off, but you are viewing the blank from the narrow side which is why there is so much material still above the top edge of the hole.
|Edge of the hole peeking through the face.
Plane until the edge of the hole starts to come through the face that you are planing. Try to keep the width of the opening equal across the face of the blank as you plane down. When the width of the opening is even across the blank I clean up the edges of the opening with a knife and start to test the cutting action of the blank.
|Width of the opening is now even across the width of the blank.
|Lines layed out across the blank. Excess is trimmed away with a knife.
|Blade clamped in place to check the cut.
This part in the process is where this tenon cutter shines. With the blade in place, I use a hammer to tap the blade into place and check the cutting action. In the photo above, you can see that the blade is just barely cutting. If I adjust the blade forward or back, it stops cutting. So to get the tool to work correctly I need to plane a little bit more off the face of the blank, which will make the blade cut more aggressively. I took a couple of passes with a handplane and then re-tested with the blade.
|There's that pencil sharpener shaving I was looking for.
When your tenon cutter is working properly it should cut along the full width and the tenon should fit tightly in the hole at both sides with no space visible between the tenon and the cutter. When using thin blades, like the old Stanley stock blades, the clamp has to be positioned relatively close to the cutting edge. It matters less with thicker blades that are less prone to deflection.
|I clamp the tenon cutter to my workbench and turn the spindle through the cutter with my hands.
I'm dreaming up more elegant blade clamping systems for a commercial version that I will probably offer for sale at some point, but the simple cutter illustrated above functions perfectly and can easliy be made in under an hour and yields perfectly sized tenons every time. The process is the same for making tenon cutters for straight tenons.
These cutters are easy to tune and so fast to make that you can have a small arsenal of them on hand for creating a wide range of tenon sizes, tapered and straight. And they finally give you a use for all those vintage Stanley blades you have tucked away in drawers and boxes.