Gears. Modularity Counts!

by John

Making the big spur gear which pushes the gun carriage up and down the inclined chassis has been a bit of a saga.

For a start, I decided that fabricating it with lathe and mill was going to be very difficult, and it was an obvious candidate for casting. In bronze. After making a model with 3D printing in PLA.

So, I drew up a 3D model, saved it as an STL file, and printed it. But did not take into account shrinkage of the PLA part. Or shrinkage of the cast bronze part. So instead of 58mm diameter, the blank gear was only 57.4mm diameter. By reducing the number of module 1 teeth to 57, I could get a reasonable gear, and the teeth were duly cut.

But, module 1 teeth looked skinny and pointy and not correct. Plus, 3 of the cast gears were total casting failures and were discarded (remelted).

So, I machined off the module 1 teeth, made some bronze disks, and silver soldered them onto the cast central hubs and spokes, and machined the blanks to 60mm diameter. By this stage I had decided that the big gear teeth should be module 1.25. Chunkier. Looked the part.

But I did not have 1.25 module gear cutters. And no-one in our club had them for loan. So I ordered a set from China. Delivery any time up to the end of November!! Then I found 2 of the set from an Australian dealer, but they were priced almost as much as the full set of 8 cutters from China. But, thinking that they would arrive more quickly I bought them. They were Chinese. It is a seller’s market.

Then today, at our GSMEE meeting Swen P said that he had a set! And I could borrow them! So, gratefully, I did. And I cut the teeth this afternoon.

The module 1.25 gears at bottom. The module 1 gear top. Please tell me that you can see a difference.

While the teeth were being cut, I tidied up another bronze T rex.

Two of them now face off on my Trevithick engine. They should amuse the kids.

Next to cut the M1.25 rack. Should be straightforward.