Setting out 'impossible' dovetails

[Eric The Viking]

I mean the sort used for front bench legs (Roubo-style):

I've a mind to try out my new Japanese saws (Happy Christmas to me!) and was wondering about making some of these to form the joints at the corners of some wooden shelf brackets. The brackets will be braced, but this joint, especially if I pin through the tail, ought to hold nicely against the tension forces it's under. I'm not really relying on it for strength, but it would be nice if it actually was strong too!

I know the recommended angles for the ordinary sort of dovetail, but what rules, if any apply to these, e.g. what width of tail (as proportion of the overall stock), and what slope to pick for the hidden face, and for the visible dovetails? I've only ever seen these used for bench and stool legs, with the grain running crosswise. Will I make it significantly weaker if both pieces have the grain running 'the same way'?

The pic is from Sketchup and that dovetail is based roughly on thirds of the width. I *think* there's no splitting force on the socket, as long as it's glued (there would be if the inner faces can slide over each other).

It's partly for the exercise, so anyone who says "just use a metal bracket" is both right and wrong at the same time ;-)

Other thoughts appreciated...

E.

 

[MickCheese]

Sorry but I am struggling with how this will even go together so no help I'm afraid.

Mick

 

[AndyT]

It's not quite what you want, but the old 1921 Woodworker Handbook (available as a download from Project Gutenberg) covers something similar and may help:

Read online from here https://archive.org/stream/woodworkjointsho00fair#page/194/mode/2up

or download as a pdf etc here: https://openlibrary.org/works/OL167..._used_with_four_hundred_and_thirty_illustrati

Roy Underhill shows off one at 22:05 in this video http://video.unctv.org/video/2365001342/ and goes on to show a mallet joined with a double version.

 

[Jacob]

I can see how it goes together (I think :shock: ) a sort of diagonal slide?
Rules for DTs are just for guidance - you can do anything from zero (box joint with right-angle tenons and housings) to 45º ish (depending on the timber) as it's more about good fit, glue area, and holding without having to use clamps. Ditto your christmas challenge joint I guess - as long as it fits and there is no part too thin for strength.

 

[Richard T]

Your sketch up drawing looks like a meatier joint than what I remember of R. Underhill's on his french bench or the one that Andy has provided above but maybe in marrying end grain that is a good thing ???

 

[Eric The Viking]

Thanks. I get a strong feeling they guesstimated the angles too!

Am I being daft to think this will work with the grain in the same directions (sort-of)? I notice the puzzle-joint version has the grain crossing.

@Mick: It slides together at 45deg to the pieces. The "tail" starts at 50% of the thickness of the stock and slopes out to full thickness at the shoulders. It's basically a squared-off conical peg, but on a skew so that where the sides slice through the 'cone' they look like dovetails.

There is still a "dovetail effect" though: The way I hope to do it (left hand version in my image), the joint can't pull apart straight upwards or to the front of the bracket (faces backwards to show the joint in the pic.). That means the two pieces can't separate unless the joint fails completely, so the shelf can't tip over.

(just seen Jacob's post): That's exactly it. I had to play around with the sizes and angles to get it to work, but the two Sketchup pieces do slide and are only snug when the joint is fully together.

 

[Billy Flitch]

Eric just give it a go its good practice for chisel work try it out on off cuts first .I can remember making these as an apprentice marking out is the secret to success could give you a few tips but only if your interested? There is quite a few of the dove tail puzzles their not all the same, the box with the double dove tail on the corners stumps people.Enjoy it. Billy