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Author Topic: Lathe Spindle Synchronisation  (Read 640 times)

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Lathe Spindle Synchronisation
« on: June 23, 2018, 01:25:10 PM »
I have had to replace the servo motor on my small CNC lathe with a 1.5hp 3 phase induction motor driven from single phase through a VFD.
My question is, would it be possible to synchronise the spindle for screwcutting. -  If so can I use the simple "hall effect switch" and magnet on the spindle method.
Your advice appreciated.
Re: Lathe Spindle Synchronisation
« Reply #1 on: June 23, 2018, 03:26:49 PM »
yes, that is how the majority of Mach users synchronise their spindle for threading.

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Re: Lathe Spindle Synchronisation
« Reply #2 on: June 23, 2018, 08:34:08 PM »
Hi leadhead,
I have not experimented with spindle synchronization so my opinions are just that and probably not worth a great deal.....

I have heard that in order for Mach3 to reliably read a spindle index pulse it needs to be of several milliseconds duration. Most lathe threading seems to occur at 500 rpm or less
or 8.33 ms per rev. If you needed a 1 ms pulse for Mach3 to recognize it reliably that would require that your Hall sensor have a magnetic sector of some 30-40 degrees in order
that the pulse be wide enough.

I have read a number of post we users have used an optical sensor rather than a Hall sensor using a CD at the spinning disc with a notch cut in its periphery. Just the perfect
use for the wife's bloody Barry Manillow CD I would have thought!

My wife left with my best friend...
     and I miss him!
Re: Lathe Spindle Synchronisation
« Reply #3 on: June 24, 2018, 01:05:25 AM »
I suppose my question is more. Can an induction motorised spindle be synchronised?
Re: Lathe Spindle Synchronisation
« Reply #4 on: June 24, 2018, 02:03:23 AM »
when threading Mach requires an accurate speed, that is not to say it has to be exactly 400 rpm say, but if it slows slightly as the cutting forces act on it to 390 rpm it needs
to know that it is 390 rpm, not 392 or 387 but 390 rpm. The reason is that Mach needs to synchronise the Z axis movement to the rotational speed, any error will result in a thread
with the wrong pitch.

An asynchronous motor will slow under load even if the VFD provides fixed frequency input voltage. If the load is but a small percentage of the spindle ultimate torque
then the speed change will be small, a few percent. Provided your index pulse remains active a small change in speed is not disasterous to your thread, Mach will accomdate small
changes by tweaking the Z axis advance rate.

If there is a large change in speed or it changes speed quite rapidly then Mach struggles and wont cut good threads.

You may have noticed that on the Ports an Pins/Spindle Setup page the opportunity to operate your spindle closed loop. Mach is not a good candidate for closed loop operation
because its so slow to respond but its still better than nothing. The speed is measured by the index pulses, per normal, but if the speed slips below target Mach will increase the PWM output
voltage which will in turn cause your VFD to up the ante. The feedback loop is slow to respond so this works well only with spindles of significant inertia and plenty of torque relative to the
cutting torque. If you expect this arrangement to be able to maintain spindle speed when the cutting forces are as much as 50% of your motors max torque you'll be disappointed.

I recently bought a second hand Allen Bradley AC servo of 1.8kW as a spindle motor for my mill. Its speed, and even more impressive, position accuracy is a real eye opener. Its not that its a great
deal bigger than your induction motor but rather the servo drive/servo combination have a feedback refresh rate of 20kHz. This is broadly true of any feedback control system, assuming the motor has
the torque to drive the maximum expected load it will do so accurately only if the feedback refresh rate is fast enough. For this reason you may have seen large production CNC mills that have
20 plus horse power spindles do a good job of rigid tapping, one) because they have plenty of torque and two) they have very high refresh rate feedback systems to take advantage of that torque.

If it is within your means replacing your spindle motor with an AC servo would be a very strong performer for you when threading. You might be surprised how economical second hand servos and drives
can be and some of the Chinese offerings are very good value for money.

My wife left with my best friend...
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Re: Lathe Spindle Synchronisation
« Reply #5 on: June 24, 2018, 02:53:33 AM »
It is more accurate to say that Mach3 synchronises the feed to the spindle.  Given that you replaced the h/s servo motor with a 1.5 hp induction motor I don't think you will see much variation of speed with lightish cuts.  My Super 7 has half that power in a single phase induction motor and I don't get any threading problems due to the spindle slowing down.

As Joe indicates, Mach3 uses the spindle index pulse both to measure the spindle RPM, which it uses to calculate the Z feed rate for the pitch you want; and also to synchronize the Z feed, starting it at the same relative position for each pass.  To a limited extent I think it can adapt to small changes in speed caused by cutting force, but with only one sample per rev it can't do a wonderful job.

I think one can obsess about Mach3 threading - given a reasonable amount of motor power and if you don't take aggressive cuts, it WORKS.
Re: Lathe Spindle Synchronisation
« Reply #6 on: June 24, 2018, 05:14:12 AM »
Thank for the information gentlemen. Pondering my options as to whether it is worth doing. -  I have a manual lathe standing next to the CNC lathe

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Re: Lathe Spindle Synchronisation
« Reply #7 on: June 24, 2018, 09:20:30 AM »
Not all Hall affect sensors are the same in their ability to trigger depending on rpm.
So read the spec's on it. A reed hall sensor would be to slow. Optical sensors work but the hall's
make for an easier instalation.

The motor rpm should be +-1 rpm , and if you measure it make sure your rpm measuring device has
a  % resolution that allows to that +- value.

The rpm should be manualy set for the motor. DO NOT have the motor constantly adjusting itself
when threading. Mach adjusts in the NEXT pass / pathing move  for variations in motor slow down.
You don;t want the two fighting each other!
Even though Mach threading has  the ability to address a rather large slowdown, it is not the way to
do threading. Adjust cut type and / or depth of cut to address motor torque available.

Mach synchronises the threading  / feedrate based on the it's true rpm dro and watches the rpm
for changes during threading. Spindle speed averaging just uses start and few other readings of the rpm
to set the feedrate for the next thread pass.

The more motor HP you have the better you will find that spindle slow down won't be a problem.
All depends on the material, depth of cut, cut type etc and this is covered in the threadnig on the lathe wirte up.
Accurate axis screws with no backlash is most desired.