Lee Valley Vise Rack Stop
This simple device is designed to help prevent racking when clamping work at one side of a vise. Simply stack up as many of the 0.10" thick shims as you need to match the approximate thickness of your work, up to a total of 4", and insert it into the vise opposite the work being clamped.
For thicker work, you can add a piece of scrap lumber to occupy the bulk of the space, and then use the rack stop to fill in the remainder. This keeps the jaws aligned as force is applied, eliminating potential for damaging the workpiece, the vise mechanism or both from racking.
Secured on a tension-adjustable steel shaft with hard maple end knobs, the 5-1/2" long shims are durable high-density polyethylene, resistant to stains and adhesives.