Another change gear ??

[GK1918]

Although not a problem just a need to know. My stack of change gears, and these are big gears, on the
old GK doesnt seem to make sense of what the tooth count is and what I can barely make out on its
chart. Right now its set for Acme #8. verified with a pitch gage and its gearing doesnt match the chart??
No big deal we only use it for tapers, turning facing and drilling. I think a 100 yrs ago mfg.s came up
with their own numbers. I measure, compare to my south bends still doesnt make sense. So is there
any kind of rule of thumb for change gearing before there were standards. Nothing on the net on
this one. Just wondering. sam

 

[British Steel]

Not so much a rule of thumb, there's a formula to work it out...
Tpi = leadscrew tpi x driven gears / driving gears...

so, if you have a 4tpi leadscrew and want to cut an 8tpi thread, the driven gear needs twice the tooth-count of the driving gear - an idler gear is both a driven and driver, so no effect on the calculation. Example: spindle gear is 36 teeth, leadscrew gear is 72 and leadscrew 4tpi would give 8tpi.
;
For metric threads the thread pitch is given by leadscrew pitch in mm x driver / driven, as the metric world is t'other.way around....

Cutting either metric.on an inch.lathe (or vice versa) will need a conversion factor, usually a 100/127 gear pair if it'll fit in the gear cover...
When it gets confusing is module and diametral pitch threads (for worms, hobs for gear cutting etc.), where there's a factor of Pi to factor in as well....

Dave H. (the other one)

 

[GK1918]

Thanks Dave, all whats there now is a 64 T stud gear and a 64 T screw gear which equals 8 TPI or I
should say thats what it comes out to be.
sam