A few years ago I bought some macro extension tubes so that I could experiment with macro photography. Unfortunately, the tubes didn't work very well; the lens wouldn't lock into the tubes, so using them was very risky. I put them aside and moved on; they were only $6 anyway.
Fast forward a year and we have built a 3D printer: time to fix these extension tubes!
Looking inside the tubes I could see that the lever that holds the lens in place was too thin. It could easily slip past the slot in the lens, preventing it from locking.
The obvious solution here is to modify or replace the locking lever. I decided to 3D print a new lever.
This new lever would be thicker and fit into the slot of the lens better. Designing it was a bit tricky, since the lever was a very random shape. I got out my calipers and did my best to recreate the lever. Then I printed it... And it didn't fit. Time to try again! I continued tweaking the design and reprinting until it fit and worked properly.
With the new lever installed the macro extension tubes work exactly as intended. This is one of the smallest parts I've ever printed and I'm very impressed with how well it turned out; 3D printing is awesome!
Instead of buying a new set of macro extension tubes and hoping they were better quality, I was able to fix the ones I have for a total cost of about 1¢. Add that to the list of everyday uses for 3D printing!