Question about the parallel guide mortise in a leg vise.

[Tony Shea]

I am getting ready to install my leg vise on my bench but am having trouble wrapping my head around the mortise for the parallel guide. I plan on using rollers front and back to make the operation of my new wooden screw nice and smooth. Therefore the parallel guide really needs to be flush with the bottom of the vise chop. How can I create a strong enough mortise to house this parallel guide with such little room on the end of the chop? I noticed on Derek's bench that it looks as though he created almost a half lap joint. That connection just doesn't seem that strong to me. Any suggestions would be greatly appreciated.

[Paul Saffold]

The tenon's shoulders, and pegs, if you peg or drawbore them, are taking all the are force exerted on the joint when clamping. When moving the chop out (opening) the force on the parallel guide is negligible.

[Jim Ritter]

Yes Tony the joint is all under compression when it is in use, so it is the shoulders that do the work. Pins help keep it aligned. For the rollers to be effective the beam must be square to the chop also.
Jim

[Michael Peet]

Yes, drawbored M&T is how I did mine too.

237478

237479

Mike

[Derek Cohen]

Hi Tony

My joint is also pinned with a steel rod.

Regards from a wintery Cape Town

Derek

[Tony Shea]

So I suppose if I drawbore this joint I may be ok. It just seems like that little space of end grain on the chop just below the mortise would be a very weak point.

If I understand how a traditional leg vise works then you are creating a bit of a levering action due to the pin in the parallel guide. This action is wanted as it enables the clamping force to happen at the top of the vise chop before any other point on the chop. This is what makes this vise so strong. Due to this levering action I would think there would be lots of pressure created on that small piece of end grain left over in the mortise for the parallel guide due to the guide not being able to lever at the same angle as the chop. But I suppose the drawbore pegs should be enough to overcome the force put on the parallel guide?

[Michael Peet]

The force at the parallel guide / chop joint is directly inline with the parallel guide, same as the force at the screw is parallel with the screw and the force at the business end is directly against the side of the benchtop.

Imagine it as a bridge supported at the ends with a point load in the middle (the screw).

Mike