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Author Topic: Mach3 spindle RPM OFF!  (Read 2385 times)

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Re: Mach3 spindle RPM OFF!
« Reply #10 on: January 26, 2018, 01:21:54 AM »
Hey Thanks for the education on that!!  Ambitious I am.  I built a 12 Tool ATC carousel for my G0704 just to see if I could do it.  Everything is done just need to mount it up to the table.  The ATC cannot be mounted to the G0704 as it is too big (heavy) for the Z column.  I have everything drawn up in CAD for the ATC mount, just have to cut the steel and weld it up.  The whole enclosure was designed for the mill and the ATC.  I think I will forgo the servo spindle motor on this machine.  I want to learn everything I can on this machine then jump to a used Haas, that will be a while though.

Again Thanks for the straight talk I appreciate that.

Re: Mach3 spindle RPM OFF!
« Reply #11 on: January 26, 2018, 01:31:31 AM »
another distinguishing feature of rigid tapping is that a thread tap goes direct into the spindle and the spindle has to supply the torque necessary to cut the thread.
Not to much torque required with small, say less than 1/4'' but try driving a 1/2" tap in stainless steel and you could be up around 20-40 ftlb. Can your spindle do that,
mine sure as hell cant.

I bought an Allen Bradley 1.8kW servo to make a spindle. It is direct coupled so has a max rpm of 3500 rpm and a continuos stall torque of 6.2Nm and a temp overload
torque of 24Nm. I haven't tried rigid tapping with it yet but I expect it to go to 1/4 -5/16 with 70% engagement and maybe up to 3/8 with 60% in mild steels. Nothing is surer
that it won't do much more than that...no matter how grunty I think it is!

My wife left with my best friend...
     and I miss him!
Re: Mach3 spindle RPM OFF!
« Reply #12 on: January 26, 2018, 01:58:08 AM »
you will no doubt have seen proper pro CNC mills with spindles of 10-15-20-30 and more horse power. One reason that they appear so grunty is that they require
very large and generous torque at slow speeds for rigid tapping but the same spindle needs to be able to do 15000 rpm. The combination of lots of torque and high speed
is high power and with high power comes high cost.

One technique to try to overcome that combination is called 'field weakening'. An ordinary AC servo has a permanently magnetized rotor, usually with powerful neyodium magnets.
The strong magnetic field means that the motor can generate high torque but it also means that it generates a high back EMF at high speed. The high back EMF defeats
high speed operation. With some clever control techniques its possible to apply current in the three phase windings which have a component that opposes the magnets of
the rotor thereby weakening them temporarily. So you can have high strength magnetic field at low speed for good torque but can reduce the magnetic field at high speed to
reduce the back EMF for even higher speeds.

While its possible to do that with any field controlled motor its not that common. My Allen Bradley servo does not use it. I notice that some of the high end manufacturers use
it on their axis servos to get amazing high speed traverse speeds, but they all use something similar in their spindles. You want one....join the queue....and bring your cheque book.
I'm building one, I have a big kick-arse  3kW servo and I'm building my own Field Oriented Controller and it will have field weakening to extend it from 3000 rpm to 4000rpm
and still maintain 12Nm continuous, 48Nm peak.
My wife left with my best friend...
     and I miss him!
Re: Mach3 spindle RPM OFF!
« Reply #13 on: January 26, 2018, 01:55:28 PM »
On commercial CNCs with ridgid tapping 3 phase motors and inverters are the norm for spindle drives.  On the Fadal I ran you had to use the high speed range for ridgid tapping too. So I believe the answer here is that the spindle position encoder is used to sych the Z axis to a spindle that is changing speed and direction, instead of trying to closely control the spindle speed. The problem here is that Mach can’t close the loop this way without help from a motion controller.
Re: Mach3 spindle RPM OFF!
« Reply #14 on: January 26, 2018, 04:02:12 PM »
Hi Gary,
yes you are right Mach can't close the loop, but it doesn't have to if your servos/spindle is up to it.

If you Gcode a coordinated helical move, Mach will issue to the controller the position/velocity/time data and the machine
will follow that. If the spindle has the necessary torque it will perform the helical move very accurately. If it has insufficent
torque it will lag behid. IF WE HAD a closed loop controller the controller would try to compenste by speeding up the spindle
to catch up to where it should be. Just because our hypothetical closed loop controller says 'speed up' the spindle can't,
I mean if it didn't have enough torque to keep up before a closed loop controller doesn't magically increase its torque.

Its very similar to the argument about open loop steppers and closed loop steppers. People imagine that closed loop steppers are
going to work better, they aren't, they are no more powerful than open loop steppers. Either the stepper has the grunt to keep
up or it doesn't. Sure a closed loop stepper might try to correct itself but usually fails because it doesn't have the power and
so fault out 'following error'.

I have as I stated in this thread bought a second hand Allen Bradley AC servo, 1.8kW. I have now become an absolute convert,
why bother to have Mach close the loop when the manufacturers servo drive closes it SO SO SO much better!

My wife left with my best friend...
     and I miss him!