This is a re-work of "Parametric Magnetic Driver Bit Handle" to be used with Needle File Sets. I Narrowed the handle a bit, adapted the shaft hole, and added a slot for a 6-32 square nut to help hold the needle files in place with a thumb screw. Works really well + a good amount of grip for large hands. (no, its not parametric anymore, just expand the hole a tiny bit with a hot drill bit, if the files wont go in.)I did this mod for a set of "HF" needle files. these files are not exactly a good example of precision engineering, there shafts differ in diameter greatly even along the length of the same file! but what i did is size it for the smallest, then i worked the smallest into the shaft and wiggled it around a bit, then moved up to the next largest,. and so on,. until the largest could just barely fit, then the thumb screw helps lock in the smaller ones. its intentionally a very tight fit. Square Nut slot is Sized for 6-32