Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Quote from: Jim on July 25, 2026, 06:48:41 pm

You're overthinking a simple procedure.

No, I'm not!

You show plenty of pulleys and some appear to have U-grooves while others look to have V-grooves. As I said before, I'm sure both work and likely work well for hobbing, but the simple question is whether you find one style to work better than the other for your purposes???

If you really cannot tell any significant difference, then that is valuable information as well.

BTW, that may well be a record number of photo attachments ..... well done!!!

I'm still of the view that you're overthinking and complicating something that, in practice, is actually a quite rudimentary straightforward process.

Whether they're steam engines or hit-and-miss engines, these machines are typically driving small accessories using most commonly 2 mm drive belts.

You asked a focused question but then went onto say, "I'm sure both work and likely work well for hobbing," which makes the question about whether the groove should be 90°, 120°, or some other angle academic.

I've personally hobbed hundreds of pulleys, both for myself and for many other hobbyists who have asked me to make them, and they've all worked perfectly well for their intended purpose.

I suggest that if you really want to investigate the academic and practical finer details of whether 90°, 120°, or another angle offers a measurable advantage on such small drive belts, then the best way for you to get that answer that you desire, would be through your own practical experimentation. Head out to the workshop, dust of your lathe and switch it on, turn up a selection of brass or aluminium pulleys, make a jig to suit the bore, grind a range of HSS cutters with different included angles, hob the pulleys, fit them to engines driving accessories, and compare the results. It doesn't take long to turn up a pulley, make a jig, put a tap in the three-jaw chuck and hob a pulley.

Once you've completed the practical testing, I'd genuinely be interested in reading a detailed write-up of your findings. A systematic comparison of the different groove angles, along with the research methodology, observations, and conclusions, would be far more valuable than theorising about it beforehand, I personally don't have the time nor the inclination because and I quote you once again "I'm sure both work and likely work well for hobbing".