On Dec 27, 4:31*pm, "Michael Koblic" wrote:
...
I am still trying to get my head around the fact that a lathe cannot cut
both SAE and metric screws. Why? The screwcutting is a function of matching
the carriage travel to the number of spindle turns. As this is achieved by
gears, should one not be able to achieve a more or less infinite combination
to match both systems?
Michael Koblic,
The ratio between millimeters and inches is 25.4000000000, by
definition of the inch. The prime factors are 254 are 2 and 127, which
means that somewhere in the gear train a 127 tooth gear is needed. The
16 pitch one for my lathe is 8" in diameter, not easy to stuff into
the limited space available..
47/37 = 1.2702702..., not perfect but compact and close enough for
many uses.
To calculate gear ratios for threading you find a gear combination
that turns the lead screw at the same speed as the spindle. Usually
this is any 1:1 ratio. Those gears will give a thread the same pitch
as the leadscrew. Then for any other thread pitch you find gears in
the same ratio as the thread pitch to the leadscrew pitch. For example
my little lathe's leadscrew is 16 threads per inch. To cut 32 TPI I
need to halve its speed with any 2:1 pair that fits the available
space. One thread pitch equals half a turn of the leadscrew.
I made a spreadsheet with all the threads my larger quick-change lathe
will cut, and the corresponding metric modulus, none of which came out
even. Next I referenced a cell with 127/100 or 127/120 etc in the
formula, which shows the effect of those two compounded into the gear
train, one driven and the other driving the next gear. Some of the
thread pitch cells then displayed standard metric modulus values. 120
and 127 gave the largest number of fine lens threads so I bought them
from Boston Gear.
http://www.bostongear.com/products/open/change.html
IIRC 100 was better for standard metric screw threads.
Then that laser optics job stopped abruptly and I haven't cut a metric
thread since. My little gold and flat black precision mechanisms went
into the company museum.
Jim Wilkins