63-tooth change wheel for Portass PD5

Can anyone suggest the best way to get a 63-tooth change wheel for a Portass PD5 lathe? The dimensions seem to be slightly different from those for the Myford 7. Ideally I would like a steel gear, rather than aluminium or plastic.

Thanks - Rowan

Assuming it's an imperial gear, do you know the DP? Myford change-wheels are usually CI.

Edited By Brian Oldford on 04/07/2020 18:09:12

If I have measured/calculated this correctly DP is about 12 (a 65 tooth gear has an outer diameter of 106.8mm). What does CI mean for the Myford gear?.

Thanks - Rowan

Your gears are 16DP.

Pete beat me to it!

CI = cast iron

Andrew

The formula is: outside diameter = (number of teeth + 2)/DP

Just to be sure that is Outside diameter in inches = (Number of teeth + 2) / DP

For comparison Myford change gears are 20DP. According to **LINK** there's only a 75% confidence of it being 12 DP. whereas it's 94% for Mod 1.5.
The other important criteria id the pressure angle. Generally older gears tend to be 14.5° whereas more modern gears are mostly 20 °.

Edited By Brian Oldford on 04/07/2020 20:08:49

Would you care to reconsider your suggestion of a metric gear specification in the light of the following, courtesy of lathes.co.uk:

"Portass lathes date from the very early 1920s and were first badged as being made in the west of Sheffield "?

For clarity, I have greater than 94% confidence that it is Sheffield, UK.

If you are wanting to use the 63T gear to cut metric threads on your imperial lathe, you can do it without using a special 63 gear. Martin Cleeve's inexpensive book "Screwcutting in the lathe" has charts in it to cut all metric threads by using the original standard set of change gears in a compound set up. IE 20 to 65 in steps of 5. Accuracy is within 1 in 8,000, ie more precise than the orginal leadscrew will ever be. I've used his method on my ancient Drummond of similar vintage to your Portass and it works well. Cleeve's charts suit any lathe with an 8tpi lead screw, which is most old Brit iron including yours I believe.

Edited By Hopper on 05/07/2020 08:14:36

I know that a Portass lathe has imperial gears. I was simply reporting what that particular web site said.

Further to Hopper's comment, here is a table of metric pitches for a standard set of gears with an 8tpi leadscrew that I calculated:

Whether you can get a combination that will mesh on your particular lathe remains to be seen but might be worth a try if all else fails.

stay well,

Rod

Edited By Roderick Jenkins on 05/07/2020 09:49:24

I think this is an area where this forum could have a page that lists all known lathes changewheels specs and gradually build up the data from people with the actual lathe. It would alos help identify mystery wheels.
Lathe: DP/Mod, Pressure angle, thickness, hole dia, key type. Any other parameters?

As an aside the only 16DP gears I know of are the back gear on a Boxford.

I recommend against steel for changewheels. Too tough to cut . No strength is required in a small lathe so aluminium and good plastic like Delrin or Tufnol are adequate. Brittle plastic like Perspex obviously not suitable.

If you find it necessary to use an internet calculator to find/check the DP of a gear, do find a decent one.

By simple calculation, the DP works out to 15.98 - very close to 100% confidence that it is 16DP. From my simple calculation that should be clear there is no chance of it ever being 12DP!

Just using 67/4 (calculated by a very complex series of neural operations within my head, in less time than it takes to read it) gives me 16.75 and it is clearly slightly less than that.