Fishtail Joint #1: Fishtail Joint Setup, part 1

By popular demand, here’s the details on how to cut the fishtail joint. For this example, I decided to make a small sliding top box.

This is a somewhat complex joint to cut. There are a lot of steps to explaining it, though it goes a lot faster once you’ve done it. (There are a lot of pictures in this explanation as they will probably explain the process better than I will.)

The place to begin explaining the joint is with the tools used to make it. The essential tools are a plug/tenon cutter and forstner bit in the same size—in this case 1/2” (I use CMT plug cutters—they work beautifully). Also, you’ll need a drill press—I use a 60 year old Shopsmith 10ER which allows me to bore horizontally & vertically, but you should be able to accomplish all the cutting needed on a vertical drill press. A bench top drill press may be challenged with the end borings since the plug cutters are very long. This won’t leave of lot of room for your work piece, but I imagine some creativity could overcome that too.

Next you need to understand how the plug cutter behaves. It will bore a perfect 1/2 tenon up to 3” deep. The cutter also strips away 3/16” of waste around the tenon that it cuts. This is important for spacing the pins on the joint—3/16” is the minimum allowance between pins and that would be pushing the limits. A 1/4” space is better. So, a 1/2” pin with a 1/4” space between gives us a spacing of 3/4” for each pin & tail. (You could vary this with larger spacing as you see fit.)

To form the fishtail, the pins need to have a flat on them—not a fully cylindrical tenon. So, the plug cutter is setup to cut 1/16” off the edge of the work piece.

[The drawing shows the pin placement in both 3/4” and 1/2” stock on the right. Spacing for drilling the holes is on the right. The graph is shown in 1/4” squares.)

With all that in mind, here’s how I setup to cut the pins:

I’m cutting horizontally, using both the fence and the miter (clamped to the table) to give a stable square setup. You can see I have a test piece with a piece of scrap underneath. You should be able to accomplish the same on the drill press—a vice on the table would be a big help.
WARNING: use the clearest grain pieces for your work and your scrap blocks. If the plug cutter encounters something it doesn’t like, it will chew it up and fling it at you.

This is the first test boring:

From this I see that the pin is too high—it doesn’t drop off the bottom of the piece leaving a flat. Also with the test boring I can determine the spacing needed to cut the first pin. The first pin is cut on the right end of the piece, not the left. Having set the proper depth for the pins, I’ll next set the miter to cut the first pin on the actual work piece so that it will be 1/4” from the right end.

When I setup to bore the actual workpiece, I have a scrap block the same length as the workpiece. I set the table so there’s a 1/4” between the cutter and the work piece (using a piece of scrap 1/4” stock). Then I set the depth gauge on the quill to bore just over 3/4”—that will cut 1/2” into the work.

First boring looks like this (though I actually had a clamp on top of the work).

For the next two cuts (there are 3 pins on this piece) I use 3/4” thick spacer blocks between the miter and work piece. This gives me a perfect 3/4” offset between the pins.

And, the third pin (note the two spacer blocks between the miter and the work piece. Also the scrap block & clamp on top of the work.)

And, here’s what the completed end boring looks like:

As an added challenge, since I’m making a sliding top box, the two end pieces are different heights to allow top to slide out one end. So the other end piece only has 2 pins and they are 3/8” from the edge of the piece instead of 1/4” (but there is still a 3/4” on center spacing for the pins.) I’m able to use the same setup with a different spacer block. Because this piece is narrower I don’t begin the first pin with the work directly against the miter. Instead I use a 5/8” thick spacer to align my first pin (that big board I’m using as a spacer is a 5/8” piece I had on hand for another project.)

Then I can use the 3/4” spacer block to align and cut the 2nd pin. So, all the pins are cut…

NEXT: Cleaning up the pins, drilling the tails and putting it together