Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: ballistic42 on January 25, 2018, 05:25:52 PM
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I put M3S1000 in the MDI and the spindle turns on and runs CW (which I want) but the RPM goes to 1250rpm. I put a magnetic sensor on the pulley so I know exactly what the spindle is really spinning in RPM. I check it with my hand held tach as well and its right on. Back ground on my setup. In the C32 instructions it says to run the spindle at max RPM (4000 for spindle) and adjust the trim pot for 10VDC, I did that. When I tell Mach3 to go too 4000RPM it goes to 4100RPM. Very confused on how to make this RPM match the requested RPM and help or ideas would be awesome.
Tread Mill motor
2.5hp
21 tooth timing belt pulley
DC board KBMM 225D set 90vdc 1.5HP resistor
spindle set up
18 tooth timing pulley
G0704 mill spindle
System setup
C32 board (CNC4PC)
ESS (smooth stepper)
encoder to C52 board 3rd port on the ESS(Smooth Stepper)
Frequency set to 300
PWM % 5
PWM checked
Smooth Stepper config set to PWM
Freq 300
PWM checked
Spindle Pulley settings
Pulley 1 max speed set 4000rpm
ratio set to 1
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Hi,
the DC voltage to frequency converter is unlikey to be that linear. As it stands you have 100 rpm error at 4000 rpm
ie 2.5% error, is that not close enough?
Craig
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I don’t think you will get better than that with a motor to controller mismatch like that. Are you running the motor in its normal direction? The brushes are offset for rotation in one direction only on many tread mill motors. Running backwards my treadmill motor went up in flames!
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I don’t think you will get better than that with a motor to controller mismatch like that. Are you running the motor in its normal direction? The brushes are offset for rotation in one direction only on many tread mill motors. Running backwards my treadmill motor went up in flames!
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Oh am I bad I meant CCW not CW. Thats what I was thinking that the mismatch would be to great. I think the machine will be a whole lot better when I get the leeson 2.5HP motor mounted and run it with the Hitachi WJ200 VFD Through Mach3. I will then be able to get what I want. I tried everything that I can think of and nothing gets them to match, oh well.
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Hi,
an open loop controller like Mach3 is not likely to result in a really accurate speed with either a DC motor or a VFD driven
induction motor.
You could achieve very tight speed control with a servo but a 2.5hp servo and drive will set you back a pretty penny!
Do you actually need such tight speed control? Forget bragging/swaggering rights, this is about what you need not want
you want.
Craig
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Well I know people who are getting the speed they want using mach3 through a VFD. Now how they are doing that I am not sure, but I do know that they are using a 3 phase motor. Any insight would be good.
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Hi,
a three phase induction motor and a VFD will usually get within a few percent open loop. Beyond that level of accuracy
you have to go closed loop.
Most VFDs will accommodate an encoder and will get you to very close, an AC servo on the other hand will be accurate to
within a few thousands of a percent and will maintain ANGULAR accuracy of better than a quarter of a degree.
Still I have to ask 'Do you really need that level of accuracy', I like lots of others spent a lot of time and money trying to get
the speed spot on only to find that except in a few unusual situations it doesn't matter that much.
Craig
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Well your right it does not but for later when I want to put a lathe together, from my understanding it needs to be spot on. I will have to do some more research on this very subject. Correct me if I am wrong with the 3 phase motor and the VFD combo I will be able to do rigid tapping? From what I have read that is correct but I could of read that wrong.
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Hi,
have you done any turning on an ordinary lathe? If you have then you'll know that speed needs to be right but its not supercritical.
For threading on a lathe accurate speed is not that critical but a CONSTANT speed is important. Any and every time you take a cut on a lathe the spindle slows a little
bit. Trying to minimize the speed variation is the key to threading with Mach3/4.
Rigid tapping is a term more commonly applied to mills than lathes but the idea is that a spindle turns a tap, usually fairly low speed, say a few hundred rpm, and the feed
axis advances the pitch of the thread with each revolution of the tap. When the bottom of the thread is reached the spindle decelerates, the reverses and the feed axis
follows suit and backs out. Whats critical is not the speed but the angular accuracy of the synchronization of the spindle/tap and the feed axis. Ordinary DC and 3 phase
induction motors with a VFD are not really suitable, for rigid tapping you really need a servo spindle, that is to say a spindle which can maintain angular position.
Rigid tapping is pretty ambitious for a hobby machine. Single point lathe threading is achievable.
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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.
Jeff
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Hi,
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!
Craig
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Hi,
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.
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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.
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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!
Craig