This is a lens holder for a lens that is 50 mm in diameter. A semicircular slot 3 mm wide holds the edge of the lens. Adjust the SCAD file for the lens and mounting hole dimensions you need . Lens holders are useful for demos and optical experiments. The specific lens shown in the photos is "50mm Optical Glass Lens Collimator Plano-Convex for Led Projector" which is Ebay item 161126916191. On my printer, the slot ended up at 2.7 mm wide, about 0.3 mm too narrow for the 3.0 mm lens edge, so I had to scrape it out a bit with a knife point. Unless your printer is more accurate than mine, you might want to enlarge the slot a bit in the SCAD. For a different 50mm lens holder that fits on a yardstick, see http://www.thingiverse.com/thing:49942 The blue item is what I printed from the enclosed file. The black one is a similar shape I made by hand from Kato polymer clay, which turns from clay-like to a hard plastic (PVC) after baking 10 minutes at 150 C. That was my very first try at a clay model- crude, but functional.I tried printing this in ABS two different times on a Makerbot dual with 110C heated bed. The first time it worked fine, and that's the one you see in the photo. However I printed without a raft and it cracked off the bed just at the moment that the printer finished, so I got lucky. The second time it popped free at about 1/5 done, so maybe I need a raft or some different settings. The hand-carved polyclay version is a lot less clean looking, and more work to make, but it also does the job and didn't require a printer at all! Although having the actual 3D item in hand was very helpful in carving out the rough copy.