I bought two identical looking used quick release bench vises from a local woodworking supply store. One works pretty good and one does not. The problem is that when trying to tighten the vise, the screw thread disengages from the part that is supposed to hold it tight (the part I'm holding in my hand in the pic). This part looks like it should work fine, the mating threads look okay to me. I've taken them both apart to inspect them, and I can't see any significant differences. When tightening the vise, this part sort of lifts up out of the mating threads and the jaws of the vise slip.
Does anyone know how to solve this problem? Are generic vises often a hassle? Any suggestions or thoughts would be appreciated. A vise that doesn't tighten is not very useful to me.