This is a tool that is used to guide a hack saw blade for making a square cut on mountain bike (or any 7/8" diameter) handlebars.

It is a two piece tool that bolts together (threads are printed in the bottom half) to clamp the bar. 1/4" rounds (I used dowel pins) are pushed in the tool to guide the saw blade and provide wear resistance. I spaced the pins wide enough so that you can use a grit blade and cut carbon handlebars as well.

I used this to cut carbon bars that were still mounted to my bike. The clamp held everything solid and I had no issues making a clean, square cut. Then I used my other thing (https://www.thingiverse.com/thing:4280437) to make an angle cut so that I could install WTB PadLoc grips (the best grips I have ever used, fwiw).