Re: CNC lathe threading?

["Barry A. Boes" <bab-at-boesusa.com>]

Well, I don't exactly have a general purpose solution, but I can now
cut arbitrary and very nice (to me) threads on a shopmaster 3 in 1
using EMC.
   I'm holding in my hand a rod with 1/2" of m12x1 threads cut in 6
passes.  The store-bought nut feels great on it.

   I'm really looking forward to real servo to spindle motion
synchronization, but for now if there are any other hackers out there
who've just gotta, here's how I cut NC threads on the shopmaster with
EMC and linux using no tachometer or spindle position sensor of any
kind!

#1) put the shopmaster on it's slowest setting.  Most of this works
because the SM has belt reduction and an AC motor which means for
very, very constant RPM for light cuts.  I tested this by having the
program cut a 0.2" pitch thread in two passes.  The first pass was
made then the program waited for two minutes ( > 240 revolutions) and
cut the second pass.  The second pass was right on -- cutting with the
front half of the cutter just as I wanted.

#2 cut the thread as below assuming 122 RPM.  Watch the cutter.  If it
cuts on the back half of the thread, rerun the program with a slower RPM
in 0.1 RPM increments until your cutter cuts on the front half.  Now
you know your machine's exact RPM.  Of course you can skip this step
if you have a very accurate low speed tach.


How I cut the threads :

threader -d 0.017 -r 121.9 -t 25.4 -l 0.5 > 1mm_threads.nc
emcrun (bring up EMC)
setenv GCODEFILE 1mm_threads.nc
emc_codetimer

threader is a C program that generates NC code to cut the threads with
special control comments that runthread reads.

emc_codetimer is emcsh with the guts replaced with a program that simply
feeds the gcode file into EMC except for two special timing commands :

(#GT ) -- Gets the current time
(#WT ###.###) -- Waits until current time = an integer multiple of ###
   from the last GT command


Attached are the programs if anyone's interested :

threadgen.cc

emc_codetimer.cc

1mm_thread.nc

globaldefs.h

functionlib.h