Have you ever clamped a part in one side of a vise, only to find that the piece slips? If you had looked at the vise’s front jaw, you probably would have noticed that it was skewed, pinching just one corner of the workpiece. That condition is called “racking,” and virtually every vise suffers from it.
What you want is even pressure. The best way to create it is to clamp an object of equal thickness in the opposite side of the vise. I use a stack of worthless credit cards—the kind you get with junk mail—that are bolted together. By swinging part of the stack out of the way, I can easily match the thickness of any workpiece.
To make this anti-racking device, tape together your pile of cards and drill a hole through them. (This is the fun part!) Remove the tape and insert a bolt with a washer. On the bolt’s threaded end, slip on another washer and a wing nut. –Charles Mak