|This is an “anti-rocking” aid. One end of a file or a sanding backer can be clamped firmly in it. The device helps keeps the sanding backer and sandpaper or file level and prevents it from rocking back and forth. There is a vertical rod with a universal ball-end that can be adjusted up and down for different angles. Through the ball-end slides a 3/8″ rod that has a clamping device threaded onto the front end. The tip of a file or sanding backer is clamped into this clamping device.
This tool is very useful when truing up plunge cuts, fixing grinding mishaps, or doing precise sanding or filing. In many ways, it is simply a poorman’s milling machine. I can’t claim all the credit for this; I saw something very similar in a shop photo of another maker’s shop.
The bottom photo shows a second clamp set-up I designed and built for holding round files more securely. It makes use of a small drill chuck and a bolt threaded to take the chuck, itself which is drilled and tapped to take the threaded end of the guide rod.