Fishtail Joint #1: Fishtail Joint Setup, part 1

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Blog entry by danriffle posted 09-15-2008 09:06 PM 8185 reads 14 times favorited 8 comments Add to Favorites Watch
no previous part Part 1 of Fishtail Joint series Part 2: Cleaning Up Pins, Drilling the Tails & Final Fitting »

Box with Fishtail Joint
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.

Fishtail Drawing
[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:
Pin Cutting Setup

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:
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).
Fisrt Boring

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.
Spacing 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.)
Cutting the third pin

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

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.)
Offset pin boring

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

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

8 comments so far

View Ampeater's profile


440 posts in 3769 days

#1 posted 09-15-2008 11:53 PM

Great post.

I love to see joints made using homemade jigs instead of $400 purchased jigs. And besides, yours makes a very unique joint.

-- "A goal without a plan is a wish."

View motthunter's profile


2141 posts in 3821 days

#2 posted 09-16-2008 12:58 AM

those horizontal borers sure do come in handy

-- making sawdust....

View jeanmarc's profile


1899 posts in 3738 days

#3 posted 09-16-2008 10:11 PM

very nice

-- jeanmarc manosque france

View SST's profile


790 posts in 4217 days

#4 posted 09-17-2008 04:56 AM

I saw this link on the 10er users group post…I guess I missed it here. As a fellow 10er guy, always great to see good work being done with the old “Smiths”, and this isn’t just good, it’s outstanding. Way to go!

-- Accuracy is not in your power tool, it's in you

View danriffle's profile


73 posts in 3596 days

#5 posted 09-18-2008 03:24 PM

Thanks everyone. Glad you enjoyed it and looking forward to seeing what kind of variations you guys make to this joint!


View a1Jim's profile


117114 posts in 3599 days

#6 posted 11-02-2009 05:53 AM

I saw the finished piece but just found this blog thanks a lot.

-- wood crafting & woodworking classes

View Dovetail's profile


27 posts in 3135 days

#7 posted 01-03-2010 07:54 AM

Can you write this and publish it in a magazine? You might/should get compensated for it. Thanks for sharing though!

-- Dovetail

View WistysWoodWorkingWonders's profile


12758 posts in 3179 days

#8 posted 02-10-2010 07:00 PM

cool blog, thanks for sharing this technique…

-- New Project = New Tool... it's just the way it is, don't fight it... :)

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