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Old 09-12-2011, 12:20 AM
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Default A new joint?

I'm working on a table that will have stretchers joining the legs diagonally across the bottom of the table. The stretchers will be quite wide (4-5") and will cross at right angles under the table.

I've been musing on a way of having these boards meet under the table. I don't want to do a butt joint, as they're pretty weak. I was considering a joint that I think is a full lap joint - a slot cut half way up/down each board so that they can slot into each other. But that is not the strongest joint, and there's potential for gaps.

So I thought of a joint with mitered shoulders. In the absence of other names I'm calling it a "mitered full lap joint". I'm sure there's a real name for it, I just can't find it (happy to be educated).

I tried cutting the joint in three different ways. First I used some scrap pine, since it's nice and soft and would be easily squished into a light-tight joint:


I had to hammer the pieces together. I can't get them apart. I cut the shoulders using a BCTW Japanese dovetail saw, and then pared out the bottom of the cut with a chisel. I wasn't very happy with the joinery - the slope of the shoulders was uneven, and there were lots of gaps.

So then I tried one where I cut the sloping shoulders with my TS55 on the guide rail, and pared out the waste (and flattened the bottom of the dado) with a chisel:


This one turned out a little better, and I could get it apart (internal views later). I got a better sense of where the joint would screw up - mostly on getting the bottom of the dado flat (the joint is really just a dado with 45 degree sloping sides). So I tried one other method.

The third time I used a V-groove bit in my router, and snuck up on the line using the microadjuster. After cutting the 45 degree shoulders with the V-groove I flattened the bottom with a 1/4" spiral upcut bit. I cut the channel out with the BCTW dovetail saw, and popped it out with a chisel:


I was pretty happy with how it turned out. It's incredibly strong and rigid. It holds together without glue, and has no deflection from 90 degrees. Here's what it looks like inside:




And here's what it looks like half assembled. The two pieces just slide together.


I think this would be a perfect joint to do with hand tools. For instance you could bang it out pretty fast on a JMP with a chisel or small shoulder plane to clean out the waste. Or you could use the V-groove blade on the HP6 with a fence to follow the line, and then use the dado blade to clean out the waste. Or you could cut the shoulders with a shoulder plane resting on a 45 degree fence to get the angle right, and then chisel out the waste.

Whaddaya think?
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Old 09-12-2011, 12:28 AM
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Default Re: A new joint?

Looks good Peter.
You may also consider a sliding dovetail.

Scott w.
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Old 09-12-2011, 12:56 AM
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Default Re: A new joint?

This solves the problem I always had with that kind of lap joint, that the parts could twist at the joint, but it also significantly weakens the overall load carrying ability of the members. Which may or may not matter in your application.

It does look pretty cool. Here are a couple of router bits that could make it easier and neater.



It would also be a good joint to start on the JointMaster.
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Old 09-12-2011, 01:02 AM
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Default Re: A new joint?

You might call it a Jack Mitered Cross lap Joint or JMCL joint for short.
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Old 09-12-2011, 01:16 AM
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Default Re: A new joint?

Peter,

The HP-6 "V" grove sole and iron won't cut that large of a 45. The rabbet sole with blade and the 45 degree fence probably would with the center cleaned out first. The JMP would work great for the 45s as long as they're shorter than about 6 inches and you could probably set up a stop and use the Kerfmaker to get things exact.

Looks like a great project; I'm looking forward to seeing it progress.
-Rutager
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Old 09-12-2011, 02:05 AM
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Default Re: A new joint?

Joint Maker Pro, a red saw, and a chisel.

It should be one of the demonstration projects at WIA.
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Old 09-12-2011, 02:49 AM
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Default Re: A new joint?

Quote:
Originally Posted by MichaelKellough View Post
Joint Maker Pro, a red saw, and a chisel.

It should be one of the demonstration projects at WIA.
I was thinking the same thing, Michael. We might have to steal a chisel from L-N, but they're always game.
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Old 09-12-2011, 02:51 AM
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Default Re: A new joint?

Quote:
Originally Posted by scottw View Post
Looks good Peter.
You may also consider a sliding dovetail.

Scott w.
Thanks, Scott. That's an interesting idea - I hadn't thought of a sliding dovetail. The downside would be that I'd have to cut one of the pieces in two, and I think you'd end up with a much weaker joint.

The strength and rigidity of this joint comes from the mitered shoulders - they support each other in that plane. It's kind of cool-looking, too.
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Old 09-12-2011, 02:55 AM
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Default Re: A new joint?

Quote:
Originally Posted by MichaelKellough View Post
This solves the problem I always had with that kind of lap joint, that the parts could twist at the joint, but it also significantly weakens the overall load carrying ability of the members. Which may or may not matter in your application.

It does look pretty cool. Here are a couple of router bits that could make it easier and neater.



It would also be a good joint to start on the JointMaster.
Those bits would work, Michael, though you'd have to be careful of the depth of the plunge. You'd have to make it so that the shoulders of the cut were exactly the width of the crossing piece of wood. I'd rather do it in three stages, to allow flexibility: left shoulder on each face, right shoulder on each face, central dado on each face.

I'm betting that there are quick and dirty jigs that you could invent (using scraps from the stock you're using) that would give you perfect accuracy every time, without measuring anything.
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Old 09-12-2011, 02:57 AM
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Default Re: A new joint?

Quote:
Originally Posted by rutager View Post
Peter,

The HP-6 "V" grove sole and iron won't cut that large of a 45. The rabbet sole with blade and the 45 degree fence probably would with the center cleaned out first. The JMP would work great for the 45s as long as they're shorter than about 6 inches and you could probably set up a stop and use the Kerfmaker to get things exact.

Looks like a great project; I'm looking forward to seeing it progress.
-Rutager
The 45 degree cut only needs to be a few millimeters deep, Rutager. Then you can cut out the middle with a chisel, shoulder plane, or dado plane. I used Gillian's tiny shoulder plane from Lee-Valley that Fred gave her for Christmas:
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