Imperial Transposing set for 280 Synchro

[rpwilson]

Here's some photos of my latest 280 Synchro project, an imperial transposing set to allow a metric lathe to cut imperial threads. The standard set up supplied by Denford could do 21 pitches from 4 TPI to 28TPI, but I added 3 more gears so I can now do 27 pitches from 4 TPI to 56TPI, which pretty much covers all the British and American threads I'm likely to want to do. The only exceptions seem to be 44TPI which is a BS Cycle thread for spokes and nipples, 11 1/2TPI which is an American pipe thread, and 19TPI, BSP, but 1.3mm pitch on the standard metric set up gets close enough for me, especially as pipe threads tend to be short.

The gears in a transposing set are 18DP, rather than the 16DP in the normal end drive, so I had to cut the lot, a total of around 550 teeth! They should be 20 degree PA, but I couldn't find all the cutters I needed, so I used 14 1/2 degree PA instead, it doesn't matter because they don't mesh with any existing Denford gears. Actually, they are now identical with Boxford gears which are 18DP 14 1/2 degree PA, and have the same bore and width as the Denford ones.

I haven't tried it out on an actual job, but I have scratched a few threads, inc 4 TPI, and they seem to come out at the right pitch according to my thread gauge.