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General Mach Discussion / lathe/threading question
« on: August 15, 2010, 05:19:08 AM »
hey, im totally new here, though i retrofitted my Bridgeport series II a couple years back and am quite happy using it with mach3 so far.
I recently picked up a Mori SL1 and am hoping that just sticking with mach3 is an option. What im wondering is this: how can the synchronization of the spindle with the other axes possibly be accurate using a VFD where the steps are converted to a 0-10v analog, it just seems that theres no real position control using this method. id like to use a smoothsttepper (+ any simple step to analog converter board) and a sensorless vector drive or something along these lines, but cant find anyone producing threads with this method and reporting meeting ANSI specs with the results- they just look good on a webcam, which doesn't really tell me if they are good threads. if anyone is making good threads with mach and not installing a servo for the spindle let me know how... please? the machine has an encoder on the spindle already, and wouldn't it just be a matter of a single setting in mach to use this instead of the single pulse per rev method ive read about? would this help, or is the control loop so slow anyway that it wouldn't make any difference?
one final, possibly silly question- a VFD 'steps' through a finite number of digital states when producing a wave anyway, so why cant an actual step/dir signal be used to 'step' it through these states, bypassing the idea of frequency (in the ac sense) altogether. it is a digital device, so why the analog input- should there not be an inexpensive VFD or vector drive which take step/dir commands that make a 3 phase motor behave essentially like a stepper- or even take pulses from an encoder and perform as a servo? i just don't see the intrinsic difference between a real servo and what id imagine a vfd and 3 phase motor could approximate given tiny changes which would negligibly effect the cost of the hardware itself and pretty much just be a firmware change- apart from things like low near zero rpm torque etc- not talking about a real c axis, just good threading. thanks to anyone who even read this long post, and my apologies for my ignorance of the basics of spindle synchronization and threading using mach3!
I recently picked up a Mori SL1 and am hoping that just sticking with mach3 is an option. What im wondering is this: how can the synchronization of the spindle with the other axes possibly be accurate using a VFD where the steps are converted to a 0-10v analog, it just seems that theres no real position control using this method. id like to use a smoothsttepper (+ any simple step to analog converter board) and a sensorless vector drive or something along these lines, but cant find anyone producing threads with this method and reporting meeting ANSI specs with the results- they just look good on a webcam, which doesn't really tell me if they are good threads. if anyone is making good threads with mach and not installing a servo for the spindle let me know how... please? the machine has an encoder on the spindle already, and wouldn't it just be a matter of a single setting in mach to use this instead of the single pulse per rev method ive read about? would this help, or is the control loop so slow anyway that it wouldn't make any difference?
one final, possibly silly question- a VFD 'steps' through a finite number of digital states when producing a wave anyway, so why cant an actual step/dir signal be used to 'step' it through these states, bypassing the idea of frequency (in the ac sense) altogether. it is a digital device, so why the analog input- should there not be an inexpensive VFD or vector drive which take step/dir commands that make a 3 phase motor behave essentially like a stepper- or even take pulses from an encoder and perform as a servo? i just don't see the intrinsic difference between a real servo and what id imagine a vfd and 3 phase motor could approximate given tiny changes which would negligibly effect the cost of the hardware itself and pretty much just be a firmware change- apart from things like low near zero rpm torque etc- not talking about a real c axis, just good threading. thanks to anyone who even read this long post, and my apologies for my ignorance of the basics of spindle synchronization and threading using mach3!