I designed this to be able to use my Dremel tool & extension shaft as a small cutoff saw. It can cut about 1/4" (6mm) diameter stock in a single cut, or 1/2" (13mm) by turning the stock.
Requires an extension shaft with a 1/2" diameter handpiece; unfortunately, a lot of the extension shafts currently available have contoured handpieces and won't work.
ZIP file contains source files from Solidworks 2011.
You will also need a 4" (100mm) length of 5/16"(8mm) rod, a 6-32 or M4 screw to hold the handpiece, a 3/8" OD x 1.5" long spring, and a 2"x4"(50mmx100mm) piece of clear plastic for a guard.
Ream the 5/16" hole in the base for a tight fit on the rod, and ream the hole in the arm for a slip fit. Assemble the base and arm with the rod, and put the spring into the spring pockets. Put a 90 degree bend into the guard, and attach the guard to the arm with a small screw. I used 1 screw, so the guard can be rotated off to the side to tighten the cutoff disc to the handpiece. I used 3mm acrylic because I have a bender & lots of scrap; lighter plastic would be ok too.
I would recommend attaching the saw to a sturdy flat base to keep everything from flexing and tipping.
Use 1.25" reinforced cutting discs.