Heisz wooden bar clamp

[Sussertown]

I created John's bar clamp. It looks great but when I try to clamp down on something the moveable jaw slides back.
The photo is the Heisz project. Mine looks nearly identical. Who knows the answer to this mystery. I'm very disappointed right now.

[olderdan]

It looks to me as if the clamping block is too square on the rail to jamb in position, it needs to be able to tilt more. You could screw a thin plate on the rear to engage in the threaded rod, if all else fails I would drill a series of holes along the length of the rail and just use a pin through as in a sash cramp. Seems a shame to lose all that nice work.

[Sussertown]

It's a piece of threaded rod that get glued inside the moveable jaw. In the print I made the photo looked like residue from a rod. Now I realize it's an actual piece of rod.

[Christophe Mineau]

Yes, thé threaded part, it's just too thick, too well guided along the bar. Imagine if you get it even thicker, you would obtain a slip fit, which is the contrary you want in a clamp.
This piece must rock a little bit in order to get locked on two points on the bar. So I would make it thinner or open a bit the square hole