Timing belt pulleys -- manufacture

As an introduction let me say what we do. We build special purpose machines
like Ink Mixers and Pencil production machines to name the most popular.
We use poly vee belts now as being more suitable for our work but before
that we used a lot of timing belts. As a lot of our work is one off's
development plays a large part and before invertors we would spend hours
changing pulley ratios to suit the machine. To this end to save time and
costs we made our own.
At the time we couldn't find much info on these, no WWW and manufactures
kept information close to their chest. So I worked backwards from known
pulleys and got our own formulae sorted out.
Now I have shelves of books on the design of these and just looking through
them last night all the calculations that are so freely in print work out
exactly to the ones we arrived at in about 79 - 80
Truly gratifying to read the penciled in corrections in the margins of
these earlier books.

Initially we used to use a fly cutter as that was all we had. Later we made
a milling cutter similar to a gear cutter with 12 teeth. These were all for
the L series 0.375" pitch BTW.
We put the small radius on the bottom but as it was a lot harder to produce
we left the top radius off and actually formed this by polishing the
finished pulley on a course wire wheel. This deburred it and also took the
sharp edge off the top to give a decent radius.
We also found very little difference in tooth form between small and large
pulleys so decide to just use the one cutter, hopefully the belt taking up
any errors. in experience we found no problems. We ran up to about 3 Hp on
1" wide belts up to 1500 rpm, and on the smaller belts and pulleys we also
ran cutters in excess of 9,000 rpm. For these we usually used the XL 0.200"
pitch series.
As proof we have pencil sharpening machines in production that work at
22,000 pencils per hour. These have now been in service for up to 15 years
and are just coming back in for refurbishment. The pulleys are still OK and
won't be changed.

As I said earlier we now use poly vee belts which are not suitable for
CNC's as they aren't timed but if I was going through all this again I
would seriously look at the HTD range of curvilinear belts. Not having
studied them in great detail but it looks from a couple of simple
calculations that you can drill a series of holes and then turn the outside
off to get a pulley. A far easier job. I may even look at this in more
detail.

As most of the people on this list are hobby types and would probably be on
the XL series, possibly even the MP series I would have no qualms about
producing my own with a form tool as the speed and HP required are well
within anything these can handle.
--

Regards,
John Stevenson
Nottingham, England