I believe this is a low-cost, effective tool for fitting pipe insulation, either fiberglass or Armaflex-type insulation. You need a plain mitre box, the kind for cutting wood. Then take a 9-in. or 12-in. drywall taping knife and sharpen the edge on a grinder.
Insert the insulation in the mitre box and swipe the taping knife through the mitre slot, at the angle you need. You will get a perfect cut and your elbows will line up perfectly.