View Full Version : Chinese Lathe Change Gears

02-02-2010, 02:33 AM
I am trying to obtain the pressure angle for a change gear I need to make for Chinese Hafco AL340 Lathe. I have all original change gears and they are 1.25module but am only guessing PA. Is there any way to measure PA accurately?


Paul Alciatore
02-02-2010, 02:46 AM
Boston Gear has B&W profiles on their web site. Just pick the closest pitch and compare visually to both 14.5 and 20 degree pictures. With side by side view, you can usually easily determine if it is 14.5 or 20 degrees.

02-02-2010, 02:48 AM
I hope this helps:






02-02-2010, 03:05 AM
Thankyou Paul A. for your help. PA is 20deg.

02-02-2010, 03:09 AM
Thankyou oldtiffie. Your help is appreciated.


02-02-2010, 03:22 AM

If the gears are 1.25 module 20, then the base pitch should be .1453" (I'm rounding, it's actually .1452819") ;)

Base pitch = pi/DP * cos(PA)

If you do a span measurement over 2 different sets of teeth (e.g. over say... 4 teeth and 5 teeth, or whatever "pair" allows you to keep the jaws of your vernier clearly on the flanks/profile of the tooth form), the difference between those two measurements should be .14528".

e.g. Here is a 30 tooth, 1.25 module 20 gear, with measurements over 4 teeth and 5 teeth. The difference between those two measurements is .14528". (note that the number of teeth in the gear has no effect on the base pitch -- as long as the measurement is done over sets of teeth that are one tooth apart and the measurement is clearly on the flanks of the teeth, the base pitch will be the same).



02-02-2010, 03:23 AM
I had an amazing revelation regarding my Grizzly 3 in 1 G0516 lathe/mill. The mill's box column has a rack on it for moving the head. A longer rack is available for when you move the mill to it's own table which I did so I bought a new rack. One day I was fiddling with the old rack and parked a change gear on it - perfect fit!

Point is, for every gear there's a rack that is the pressure angle you seek. The following images were made on my scanner - not a photograph. Here's a link to a really big version. The rack can be drawn onto the image with reasonable accuracy given that there's just a few popular pressure angles out there for these lathes.

Large: http://MetalWorkingAtHome.com/images/rack-and-gear.jpg



02-02-2010, 03:52 AM
Thankyou Precisionmetal that worked out perfectly. I measured 44tooth gear.
Measurement .1458. That formula is very handy.