This is a replacement part for the thumb roller hook that broke on one of my dial calipers. I made the part by injection molding polypropylene. I included the model files used to make the aluminum mold.
I don't know how often these things break but I couldn't find a direct replacement for mine. I did see some company selling a similar part that might have worked for ~$8. I think I bought the caliper for about $15.
Additional information about my injection molding process can be found at http://www.scorchworks.com/Injection_molding/injection_molding.html
I used a CNC mill to make aluminum molds and injection molded polypropylene to make this part. The model is designed so that the mold can be milled with a 1/16 end mill. I used a Dremel 1/16 carbide grout removal bit for the bottom half mold because my standard length 1/16 end mill was not long enough. I used a 1/16 ball end mill on the top side to make it look nicer. The models for the molds do not include the gates for injecting the plastic. I milled the gates in manually after the main section of the mold was made.