How much taper on sliding dovetails?

[Just4Fun]

Thanks Custard. I'll take your advice and make the joints as you suggest. That layout makes perfect sense when you explain it but I would have got it wrong.

 

[dzj]

"I'd have five tenons, each 24mm x24mm. The tenons would have to be square, or nearly square, as the wedges must run horizontal. "

Some authors suggest diagonal wedges for square tenons. Makes the joint tighter on all 4 sides, so they claim.

 

[custard]

I cut a tapered sliding dovetail this morning to try and answer to your question of "how much taper?". This is how I cut this joint for drawer dividers.

Step one is to lay out for the saw cuts. I pencil to a 1:7 dovetail slope but use a knife gauge to establish the "floor".

To get the taper I simply saw from the outside of the pencil line at one end, to the inside of the pencil line at the other. You can just make out the taper in this photo, the "fat" end of the dovetail is on the right, and the "thin" end is on the left.

So across the 89mm width of this workpiece the dovetail goes from 18.3mm at the thin end,

To 19.3mm at the fat end,

So that's a total difference of 1.0mm, or 0.5mm of taper on each side over 89mm of length. I'll leave someone else to figure out the geometry, but as degrees of angle the taper is barely anything. However, "barely anything" is really is all you need.

The tricky part of this process, besides being able to use a back saw with reasonable accuracy, is the transfer in order to layout the female part of the joint. You still have your knife gauge set for the depth of the cut, so that's easy. And you know the slope of the dovetail is 1:7, or whatever slope you personally prefer, so that's easy too. But you have to establish that very shallow taper of 1.0mm.

In order to do this I mark out two pencil lines to show the precise thickness of the "rail" piece, or the male dovetail piece, or whatever other description you choose to give it. I then carefully centre the thin end on those lines and using an ultra fine marking knife make two knicks deep in the corner of the dovetail and right at the very edge of the "stile" piece or female dovetail piece.

Then you repeat that process at the other side using the thick end,

Connect up those tiny knife nicks and you've got the taper. From there complete the layout with your 1:7 dovetail layout gauge and your knife gauge,

And then saw to the lines with a back saw, and clear out the waste with a chisel and a router plane. If you've done it right you'll find the joint closes up really easily to about 25mm from the end, but then almost immediately tightens up to the point where it's very hard to make any more progress using finger pressure. Instead of pounding away with a mallet I prefer to close up the joint in a more controlled manner using a cramp.

 

[custard]

Hmm? Seems I cocked up the final photo. Instead repeating one of the previous photos for a second time, this is what you should have seen!

With a couple of strokes from a hand plane to remove the pencil marks and clean up the face, you'll be left with a perfectly square and furniture ready joint that hardly needs any glue at all.

 

[Just4Fun]

I cut a tapered sliding dovetail this morning to try and answer to your question of "how much taper?".

That's really helpful of you. Definitely action above and beyond. Thank you.

I have been pondering on this for a few days and my gut feeling is that it should be possible to derive a rule of thumb for the taper angle based on the length of the joint, which I guess is the most important factor that determines ease of assembly. To do this would need data about a lot more joints than we have, so it is unlikely that this will ever be done. It is a shame that the test joints I cut went into the wood burner or they would have provided a little more data.

I would expect your joint, with 178:1 taper, to be harder to assemble than my joint with 133:1 taper, but the reverse was true. (What wood did you use?). I think the fact that my joint was 4.5 times wider (longer?) than yours explains this, as there is correspondingly more friction.

I cut another test joint, different to my previous efforts in 2 ways. First, I cut a taper on both sides of the joint rather than keeping one side flat. Second, I used a more pronounced taper, going from 20mm to 11mm in a joint 81mm wide. The result was really interesting. The joint falls together without pressure to within 1mm and is still sloppy at that point. From there it is easy to push it together with fingers only, so it is a perfect flush fit. At that, it is nice and tight and surprisingly difficult to disassemble. At 9:1 this taper is very different to the tapers you and I tried before.

My intention was to create a ridiculously loose fit as one end of a range with our previous joints at the other end of the range. In reality I would be very happy to use this taper in a real project. The downside is that this taper is much harder to achieve accurately. For my test I cut it wider than the housing and trimmed it to width after fitting it. I don't think I could reliably achieve this fit any other way.

 

[custard]

Interesting.

One of the issues with more pronounced taper angles is that it removes more wood from the male/tenon component. If it were a shelf for example then you'd want that component to be as thick as possible in order to retain strength.

I used Cherry for that test piece, I wouldn't expect a massive difference in results from most temperate zone hardwoods.