Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: raadu on May 22, 2014, 09:18:38 AM
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Hi all,
i'm researching building a 4' x 8' cnc router for a woodshop.
I was wondering if Mach3 has the capability to drive 2 stepper motors per side of X axis ( 2 pinions per rack for reduced backlash, electrically preloaded ).
That would be 4 motors for the X axis.
Motor A would drive in one direction while motor B would have less torque (dragging) to keep the pinion engaged.
When switching direction, the motors would switch also to B driving and A preloading.
(http://www.americanmachinist.com/Content/Site304/Articles/10_01_2004/driving0400jpg_00000002033.jpg)
Thanks,
Radu
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No, Mach3 can't do that.
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First question :
Could Mach4 be configured to do that?
Second question:
Could it work with Mach3 if there's a programmed micro-controller between the breakout board and the stepper drivers?
It would read the signal from Mach 3 and drive the steppers accordingly.
Is that worth looking into or would it conflict with the way Mach 3 operates?
Thanks,
Radu
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I guess the answers are no and no ? :)
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Don't be too hasty, I am sure others will be thinking about this. ;)
I am certain that your suggestion of a microprocessor is something that should be investigated as I think that could work.
Tweakie.
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Even if you could get this to work (and I have my doubts) why would you?
Surely, you'd end up with exactly the same effect that is achieved far more simply and cheaply by the old trick of using two pinions fixed slightly out of phase on the same shaft.
Ian
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Oh boy!, This is not supported by any stretch of the imagination...................................... but ;D I do not see why you could not have 2 drives and two motors for the backlash comp by sharing the step and direction signals to 2 drives. Maybe even drive 2 motors from the same drive. I think I would go with a single motor, some belts and pulleys and 2 gears myself since the backlash comp would not be electrical preload. The separate pinions would still have to be separated or opposing mechanically (spring, shock, spacer etc). However, the best solution would be to use precision parts with precision placement, set up correctly to eliminate backlash. I do think any of the three options above would work (each with benefits and drawbacks) but they are elaborate workarounds for inferior components IMO.
Brett
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Precise and Rack & Pinion don't really go together, certainly not in your typical home built machine.
Regardless, the OPs idea would be best acheived with a microcontroller, however it would also need stepper drivers that are capable of having the motor current varied on the go, which I think would be the biggest stumbling block.
Any delay in changing current levels could result in a jolt as backlash changes direction (motors would go from pulling apart to pushing together, or vice versa), which could lead to missing steps/jumping issues.
Just spring loading the pinion onto the rack, or using a anti-backlash pinion are far simpler solutions.
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Precise and Rack & Pinion don't really go together, certainly not in your typical home built machine.
Regardless, the OPs idea would be best acheived with a microcontroller, however it would also need stepper drivers that are capable of having the motor current varied on the go, which I think would be the biggest stumbling block.
Any delay in changing current levels could result in a jolt as backlash changes direction (motors would go from pulling apart to pushing together, or vice versa), which could lead to missing steps/jumping issues.
Just spring loading the pinion onto the rack, or using a anti-backlash pinion are far simpler solutions.
Exactamundo - in every respect. Plus - a stepper with less current, whilst having less torque, will still step, because the other motor will release the load - so - no drag anyway.
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The answer is NO and NO. Nor would you really want to ;)
NOW you COULD use a DC motor and use a low current to feed it creating a small counter force. But that does not really solve your problem.
(;-) TP
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Thanks for all the replies!
The main reason for this inquiry was trying to incorporate inexpensive, ready made, planetary gear assemblies (e.g. power tools) which would not be high precision.
Electrical preloading does work and it's being used in industrial machinery. However, one of the suppliers for the gears and motors for this application said the motor control is done by the CAM software...
Finding an affordable CAM capable of doing that would make this approach worth looking into.
Mechanical preloading ( gears + springs, etc) are only recommended for slow speeds.
Even if you could get this to work (and I have my doubts) why would you?
Surely, you'd end up with exactly the same effect that is achieved far more simply and cheaply by the old trick of using two pinions fixed slightly out of phase on the same shaft.
Ian
Yes, that should work but, from what I read so far, it only works for slow speeds.
Maybe use a wide Rack or 2 of them, side by side ? But that would increase the cost as well.