I have been frustrated with making radial roof ends for various types of cars. In the past, I have used a large metal divider with sharpened points to cut a curve in styrene. The problem is that I have not been able to cut a uniform radius on multiple pieces. I end up spending time sanding the parts to match.
My problem created this solution to make a smooth radius curves. I made a trammel bar with an Xacto blade on the end. The simple tool was made from .060″ styrene and a piece of .040″ brass wire. By locating the pivot point equal to the desired radius I was able to cut a 14′ 3″ radius for a caboose end and a different radius for the cupola roof. The whole deal is set up to fit on a piece of particle board.
The hole for the radius is drilled in the .060″ base. The brass pin clearly shows in the picture above. I position the part to be cut along a centerline on the particle board. It is held in place with tape.
The #11 Xacto blade is held in place with some .020″ styrene strips and a machine screw. The cutting process is done by moving the base from left to right. It is better to use several light passes rather than a few heavy cuts.
I am sure that there are other ways to do this but this is my attempt. The tool does the job for me.