In article s.com,
Jon Elson wrote:
Todd Rearick wrote:
OK, so when one adds CNC to a mill or lathe, it's a good idea to upgrade to
[ ... ]
I suppose the CNC controller could perform the threading operation, but it
would need to know the spindle speed and phase...how is this normally done?
[ ... ]
The only way to do it without CNC software that supports threading is to
have the ballnut be bolted to the carriage in an easily-disconnected
way. Unbolt to thread, put bolt back in for CNC.
Very awkward, as I don't see how to make split nuts with a ball
race in them -- hmm ... maybe I *do*, but they would be quite expensive.
And normally, the CNC machine's leadscrew is not connected to
the spindle with a set of gears, so that would not work anyway.
CNC control that DOS do threading has an encoder on the spindle, and
"electronic gearing" that links the carriage motion to the spindle
rotation for a certain distance, then breaks back out of that mode
for the return pass. Note that almost all hobby-level programs do
NOT do electronic gearing. Every one I've seen does spindle
synchronization, then just does constant-rate motion for the thread.
Hmm ... while it is rather primitive in many ways, the
Emco-Maier Compact-5/CNC (which is now in the hobby price range as it is
now obsolete) *does* have "electronic gearing". You can hear the
stepper motor on the leadscrew change step rate as you vary the spindle
speed (from 200 RPM on down, as at faster spindle speeds the CPU (a poor
tired 6502) is just not up to the task. :-)
The one thing missing which I would consider to be ideal for a
really good electronic gearing would be a way to recognize when you are
running the spindle backwards by hand, and to step backwards to keep the
tool in sync. If you rotate the spindle by hand in the actual machine,
it counts the pulses as though it was going forwards, which confuses the
position rather thoroughly. :-)
This is just not as accurate as real gears or true electronic gearing,
where the spindle rotation rate is monitored all through the threading
pass. You can almost certainly tell that any program that uses a
one-pulse per rev spindle detector is doing sync, not electronic gearing.
The Above-mentioned Compact-5/CNC has two rings of holes on the
spindle encoder disk -- one with only one hole for indexing, and the
other with a large number of holes (I've never actually counted the
number, but at a guess somewhere in the 30s -- perhaps 32? Perhaps 36?
Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---