Joint Strength Contest And Repeat ability

Hey guys, the local woodworking guild here is having a joint strength contest. I decided to see if anyone here has done anything like this. If so post some designs, and I’ll mill them out, and we can all see how many pound of force the joint is capable of.

Using Red Oak 7" x 3" x .76", so base your design for that size.

CNC Heart Joint: If only because

I positioned and used the triquetra square to find 0, 0, 0 to home the machine on both pieces of wood. The piece of aluminum is bolted down and square to the spindle. I then used a spacer to line the wood to make sure it was square.

At first glance it looks good

Unfortunately, it doesn’t mate up very well. I’m super sad, I would have thought using homing with known offsets it wouldn’t have been such an issue. Has anyone else had this problem? I’ve never been able to get the repeat ability that I should.

If you watch the tool simulation in 360 it shows you it is going to be off by 1/2 the thickness of your tool!
Why… well I’m not a 100% sure lol.

Have you calibrated for belt stretch?

Cheeked the machine for squareness w/ a diamond circle square test?

Why not redo the CAM and cut the pieces sequentially at 90 degrees to each other?

Also double check the cutting diameter of your tool vs the shaft dia. I had some 3.175mm dia cutters that measured 3.16. doesn’t seem like a lot but when you are trying to get a press fit it will cause you some headaches. not to mention that for a while i was using a 3/16" cutter that for some reason i thought was 1/4".


That looks like it’s off by half the tool thickness which would happen if you zero to the edge of the table using the tool and then don’t apply an offset for half the tool width. I’m not sure how to apply the offset (I’m new to Carbide Motion) but I remember having to do that with the CNC routers I learned on.



if this is true then that is your problem, easy mistake to make that I’m sure everyone will make at some stage when they have been working too late at night.

Hello all,

I thought I would share the results of our little guild meeting. All the photos can be found here.

Not super scientific as there should be more samples, but I was really happy with how well the heart joint held.


That the lap joint and heart joint fare the same is quite the endorsement of modern adhesives.

I was surprised. The 80mm double tenon, the wood didn’t fail. There wasn’t enough glue used.

Yeah, I was surprised that that failed so soon — haven’t yet found time to look at the images yet, but that would make sense.