Is .001" error realistic on a stepper retrofit?
16 Dec 2015 18:44 #67049
by delco
Is .001" error realistic on a stepper retrofit? was created by delco
So my boss asked me to retro fit an old Lucas 4 horizontal mill. This thing is MASSIVE probably 12 feel tall. It obviously uses lead screws which he doesn't want to change to ball screws. Convincing him to re manufacture them is a possibility but I'm not sure if he'll go for it.
He said he wants to achieve .001" error but I feel like that just won't be possible with lead screws and stepper motors. I feel like even with ball screws and servos that'd be pretty challenging. Speed isn't critical so maybe we could gear it down like crazy but that still doesn't change the backlash on the ballscrews. I know LCNC can account for backlash error but again I don't know if .001" error is realistic even with the backlash error accounted for.
Can anyone give any insight to the feasibility of this project?
He said he wants to achieve .001" error but I feel like that just won't be possible with lead screws and stepper motors. I feel like even with ball screws and servos that'd be pretty challenging. Speed isn't critical so maybe we could gear it down like crazy but that still doesn't change the backlash on the ballscrews. I know LCNC can account for backlash error but again I don't know if .001" error is realistic even with the backlash error accounted for.
Can anyone give any insight to the feasibility of this project?
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16 Dec 2015 18:53 #67050
by Rick G
Replied by Rick G on topic Is .001" error realistic on a stepper retrofit?
I would start by finding out just how much mechanical back lash there is now and how tight you can get it with what you have to work with.
Most likely the weak link will be mechanical not the steppers.
Backlash compensation can help but depending on the cut and the forces applied, but it is best to get rid of as much mechanical backlash as possible first.
Rick G
Most likely the weak link will be mechanical not the steppers.
Backlash compensation can help but depending on the cut and the forces applied, but it is best to get rid of as much mechanical backlash as possible first.
Rick G
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16 Dec 2015 18:59 #67051
by delco
Replied by delco on topic Is .001" error realistic on a stepper retrofit?
Just so I'm clear, you think it could be possible if we eliminate enough backlash and possibly find the biggest planetary reducer????
As soon as he demanded .001 error WHILE sticking with the lead screws I figured the project was dead in the water.
Just taking a wild guess, I'm assuming something on the order of a nema34. these lead screws are like 15 feet long and about 2" thick.
As soon as he demanded .001 error WHILE sticking with the lead screws I figured the project was dead in the water.
Just taking a wild guess, I'm assuming something on the order of a nema34. these lead screws are like 15 feet long and about 2" thick.
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- Todd Zuercher
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16 Dec 2015 19:07 #67052
by Todd Zuercher
Replied by Todd Zuercher on topic Is .001" error realistic on a stepper retrofit?
That may be possible. But what was the machines capabilities before? If the machine wasn't able to hold those tolerances before, Linuxcnc isn't going to magically fix those mechanical deficiencies.
However considering the size of the thing, stepper motors almost certainly will be woefully inadequate. Not so much from a lack of accuracy but from lack of power. How were the screws of that thing driven before?
However considering the size of the thing, stepper motors almost certainly will be woefully inadequate. Not so much from a lack of accuracy but from lack of power. How were the screws of that thing driven before?
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16 Dec 2015 19:13 #67053
by cncbasher
Replied by cncbasher on topic Is .001" error realistic on a stepper retrofit?
firstly accuracy is built in the machine , if the machine has backlash and is worn ,leadscrews worn etc then your going to probably spend more than it's worth to mechanicly fix it .
the electronics are the least of your worrys . electronics wont fix the problems .
if the screws are 2" dia , you'll be looking at motors with at least 6 to 8 nM and probably more
you need to take into consideration the Mass that needs to be moved as well ,
look more into the mechanics and how worn it actually is
the electronics are the least of your worrys . electronics wont fix the problems .
if the screws are 2" dia , you'll be looking at motors with at least 6 to 8 nM and probably more
you need to take into consideration the Mass that needs to be moved as well ,
look more into the mechanics and how worn it actually is
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16 Dec 2015 21:48 #67054
by Rick G
You might want to look at Nema 43 or even Nema51 up to around 50nM are available. But again start by finding out the condition of the mechanics first. If the machine can hold .001 the electronics certainly can.
Rick G
Replied by Rick G on topic Is .001" error realistic on a stepper retrofit?
Just taking a wild guess, I'm assuming something on the order of a nema34. these lead screws are like 15 feet long and about 2" thick.
You might want to look at Nema 43 or even Nema51 up to around 50nM are available. But again start by finding out the condition of the mechanics first. If the machine can hold .001 the electronics certainly can.
Rick G
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17 Dec 2015 10:53 #67074
by andypugh
With screw mapping it ought to be possible to do much better than that. But only in conditions when the load-face of the screws does not change. ie, if you write the G-code to stay on one side of the backlash, it should be possible.
As an example, this is a screw-mapped 5-axis machine with the original leadscrews riginal leadscrews. You can see a "flick" on reversals, but otherwise it does pretty well. This uses a screw-map created using a laser measurement system.
If you write G-code that makes cuts in the way that a manual machinist would make the cuts, then it is probably possible.
If the 2" screws are 4TPI and the motors are belt-coupled at a 2:1 ratio then each full motor step is .0006". Microstepping will give you a bit better than that, as would a 4:1 reduction.
But, for this machine I think linear scales directly on the axes and servo motors are likely to be a better idea. Steppers big enough for this machine are likely to be quite expensive themselves.
The new closed-loop steppers might be worth looking at as a half-way house:
www.ebay.co.uk/itm/121516261126 for example.
As a first step, find out what torque it takes to turn the screws on a typical job, and work from there.
I think that the design brief can just about be achieved. I suspect that the machine is likely to get converted twice. But a low-cost conversion followed by a better one taking into account all lessons learned isn't a the worst way to do things.
Replied by andypugh on topic Is .001" error realistic on a stepper retrofit?
He said he wants to achieve .001" error but I feel like that just won't be possible with lead screws and stepper motors.
With screw mapping it ought to be possible to do much better than that. But only in conditions when the load-face of the screws does not change. ie, if you write the G-code to stay on one side of the backlash, it should be possible.
As an example, this is a screw-mapped 5-axis machine with the original leadscrews riginal leadscrews. You can see a "flick" on reversals, but otherwise it does pretty well. This uses a screw-map created using a laser measurement system.
If you write G-code that makes cuts in the way that a manual machinist would make the cuts, then it is probably possible.
If the 2" screws are 4TPI and the motors are belt-coupled at a 2:1 ratio then each full motor step is .0006". Microstepping will give you a bit better than that, as would a 4:1 reduction.
But, for this machine I think linear scales directly on the axes and servo motors are likely to be a better idea. Steppers big enough for this machine are likely to be quite expensive themselves.
The new closed-loop steppers might be worth looking at as a half-way house:
www.ebay.co.uk/itm/121516261126 for example.
As a first step, find out what torque it takes to turn the screws on a typical job, and work from there.
I think that the design brief can just about be achieved. I suspect that the machine is likely to get converted twice. But a low-cost conversion followed by a better one taking into account all lessons learned isn't a the worst way to do things.
Please Log in or Create an account to join the conversation.
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