How would you cut butterfly shape floating tennons?

[Ruhi Arslan]

I would like to get suggestions on how to cut long butterfly shaped floating tenons. I am planning to use them to attach end boards to the work bench top. They need to be roughly 26" long, 1 1/2" thick and 3/4" wide at the widest ends. I made a trial run. Mortises are relatively easy. First, I cut a straight dado and got the final shape with a 14 degree 3/4" dovetail bit on the router table. Of course, I need to be able to do the same with a handheld router on the top but it seems like I cannot get the tenon right. If proves to be too difficult and easy to mess it up, I may go with the straight mortise and tenon with pegs to pull the ends together. Thanks for the advice in advance.

[Wes Grass]

I think I'd try the same router bit, and flip the piece 4 times. And I'd make it a bit taller to start with, with a bit of one end cut down to finished height so I could test fit it. That would leave a small flat at each corner to give a solid reference against the fence.

[Larry Edgerton]

I have a router bit that is made just for that. Cuts both sides at once. I can't remember where I got it, but I think it was Ballew Saw and Tool. Check there. I had a hard time making them in multiple passes as well.

Larry

[Jamie Buxton]

I'd make that piece on the tablesaw.

But I wouldn't design the table that way. You're making a sliding dovetail with zero taper. If you cut it tight enough that it will really act as a dovetail, you won't be able to slide it the 26" width of the table. It'll bind up before you get it home. And then you'll be in a pickle: the tenon half-way sticking out of the table, locked in place, and the glue setting up as you look at it.

[Tony Bilello]

I'd make that piece on the tablesaw.

But I wouldn't design the table that way. You're making a sliding dovetail with zero taper. If you cut it tight enough that it will really act as a dovetail, you won't be able to slide it the 26" width of the table. It'll bind up before you get it home. And then you'll be in a pickle: the tenon half-way sticking out of the table, locked in place, and the glue setting up as you look at it.

+1

That was my very thought as soon as I read the OP

[Mark Rakestraw]

Ruhi, what Tony and Larry say about glue is true enough, it would freeze halfway through. But since the table side of the dovetail key is crossgrain you wouldn't glue the key in anyway. The whole point is to allow movement. The other methods are fine, but if you want to use the key, which would look very cool, here's a way to do it. Lubricate all but the front three inches of the channel in both the bench end and the endcap with a little parrifin. Drive the key in from the front most of the way through. When you only have two or three inches of the key sticking out the front apply glue to that portion of the key only and quickly drive it home.
Mark

[Tony Bilello]

190433190432Breadboard Edges are usually made in one of the 2 ways I sketched on the attached jpgs. If you just want a butterfly key for looks, just make it on the end pieces. Keep in mind that the end pieces wont change their dimension from the front of the workbench to the rear, but the workbench surface that it attaches to might move a total of 1/8" to 1/4" throughout the yearly high and low humidity cycles.

[Larry Edgerton]

Ruhi, what Tony and Larry say about glue is true enough, it would freeze halfway through. But since the table side of the dovetail key is crossgrain you wouldn't glue the key in anyway. The whole point is to allow movement. The other methods are fine, but if you want to use the key, which would look very cool, here's a way to do it. Lubricate all but the front three inches of the channel in both the bench end and the endcap with a little parrifin. Drive the key in from the front most of the way through. When you only have two or three inches of the key sticking out the front apply glue to that portion of the key only and quickly drive it home.
Mark

Agreed. Further, there is really no gain in making it in one piece. I do mine in three, a center section with no glue, about two thirds of the length, and the ends driven in from both sides glued at just the ends as Mark has indicated so that the dovetailed key does not move at the end that shows. Your strength is from the breadboard end, not the dovetailed key, so seperating it does no harm and makes it easy to get together.

[Carl Babel]

Here's another idea: make it a real sliding dovetail with a non-zero taper. Make the key extra long, slide it in as far as it will go, and saw off the ends. You could even leave the key ends a little proud and sand them smooth - kind of like a Greene and Greene affect. Okay, maybe I am getting a little carried away...

The key ends wouldn't be the same size, but no one can look at both of them at the same time. Just look out for those persnickety types that approach your workbench with a micrometer.

[Ruhi Arslan]

Thanks for the ideas. After trying to make an accurate double dovetail on a trial model several times and being satisfied how well it fits or rather how it doesn't, I decided to go with the "traditional method" as in Tony's sketches. I couldn't make the keys pull the end board towards the core. It proved to be too tedious to get it right. I am going to leave the bottom side uncut as is the first figure to have more meat left for the dowels to work against. Time to cut the ends... I am going to use the Festool 75 to cut most of it from the top and flush trim the remainder with the router bit with a guide bearing.
Thanks again.