This is a radius gauge, for figuring out the radius of an object. Works for inside and outside curves. Guages for radii of 20 to 69 mm, in 1 mm increments. There are several plates, grouped by radius. Plate30.stl contains guages for 30 to 39mm, etc. Print out the plate(s) that you want. After that, stack the gauges from that plate and measure the thickness. Then choose the bracket that matches that thickness and print it. So use bracket19_5.stl if the thickness of those 10 gauges is 19.5 mm. Then attach it all using a M3 x 25mm flathead screw, similar to this one http://amzn.com/B000FN3NS8. The reason for the multiple bracket versions is because the actual thickness of each gauge depends on how you leveled your build platform-- and thus how much the first layer or two got smashed down. The variation won't be huge, but multiply that by the 10 gauges per bracket and it can add up. I highly recommend using a dark, opaque filament. Black, blue, etc would be good. Natural or anything translucent would be bad. The reason for this is you want to see the gap between the gauge and the object you are measuring and it's hard to see the gap unless it blocks the light fairly well. I printed these at 0.2 mm layer height, 20% infill, and 2 extra shells (3 total).