Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: JohnHaine on February 13, 2011, 06:35:01 AM
-
I'm slowly getting to grips with my Myford Super 7 CNC conversion. Initial results were promising but I have got caught up in a "problem loop" with the accuracy of the X-axis (i.e. cross slide) feed.
I have fitted a new leadscrew which is 20 tpi though I actually work in metric. For complicated reasons this is driven through a timing belt with ratio 4/7 from a standard stepper with 8 times microstepping on the driver. "Steps per mm" then works out to 2204.724. The problem I have found is inconsistent diameter turning using the turn wizard. The diameter ends up up to 0.01 mm or so different from what it should, sometimes larger and sometimes smaller than nominal.
I have set about calibrating the leadscrew pitch using a 123 block measured by micrometer to ~25.41 mm (actually measured to a micron) and using the procedure recommended in the setup instructions. I am setting the slide to a zero with a DTI against the block, the latter mounted against an angle plate on the cross-slide, then removing the block and moving the slide under manual control to zero again and looking at the movement on the DRO. The DTI is a very nice surplus indicator that has a travel of +/- 25 microns and indicates to 0.5 micron.
Apart from a small indicated error of about 0.08 mm over the travel, I get relatively inconsistent repeatability if I move the slide back 25 mm and then bring it back to zero, of up to about 0.01 mm. I can reduce this by firmly pushing the slide back against the screw thread before reversing the feed to come back to zero - then I can get to a few microns. So I was wondering if anyone could help with the following questions please?
Are my expectations of accuracy too ambitious? If turning down to a specified diameter using the wizard, what is a reasonable accuracy to expect?
Moving the slide repeatedly backwards and forwards to the same nominal position, what repeatability can be expected?
The leadscrew is lubricated with a molybdenum grease, but it seems to me that this may be forming quite a viscous film between the screw threads and the nut, the thickness of which will depend on the force on the screw - would I be better using a light oil?
I don't think I am losing steps, but I was wondering about just how repeatable microstepping is - has anyone any experience of problems from this source?
Any answers to these or other suggestions would be most welcome! Thanks in advance,
John.
-
I think you have answered the question yourself when you say
I can reduce this by firmly pushing the slide back against the screw thread before reversing the feed to come back to zero - then I can get to a few microns.
I think you are seeing a fairly good repeatability from the setup you have, microstepping will have some effect, cant give you specific numbers but I would say it safer to think of a uS could be anywhere between one or the next. Your gearing may also introduce inacuracies. Light machine with flexing, stiction etc will introduce innacuracies but the biggest issue is likely to be the screw.
Having said that have a look at the repeatability of some industrial machines and you may be surprised, their positioning will likely be something like +- 0.005mm, repeatability slightly better, probably 0.005mm.
Hood
-
Just had a look at HASS, theire positioning and repeatabilty seem to be standard accross their range, it is positioning of +- 0.005mm and repeatability of 0.003mm.
Their machines are by no means the best, in fact they could probably be classed as fairly light in indutrial terms but compared to your myford they are huge rigid mosters, so I would say you are doing well ;)
Hood
-
I don't think I am losing steps, but I was wondering about just how repeatable microstepping is - has anyone any experience of problems from this source?
Hi John. Microstepping should NOT be included in the "theoretical" calculation of accuracy. Purely from the motor accuracy perspective, full steps are usually stated as being within +/-5% accurate. Microsteps are less than that and are unpredictable. (Note the difference between accuaracy and resolution). Whilst full step accuarcy is purely down to motor manufacture, microstep accuracy is down to a combination of the motor/driver/PS/mechanics etc. As just one example, sometimes, a motor will not even move until it's had 3 or 4 microsteps, then it will "jump". It will do this sometimes and not at others - maybe.
Microstepping's purpose is purely to get smooth(ER) motion than full stepping.
Ian
-
This is fron the Gecko site regarding the linearity of microstepping.
"Another factor affecting accuracy is motor linearity. Motor linearity refers to how the motor
behaves between its ordinal step locations. Ideally a 1.8 degree per step motor should move exactly
0.18 degrees for every step pulse sent to a 10 microstep drive. In reality all step motors exhibit some
non-linearity, meaning the microsteps bunch together rather than being spread evenly over the span of
a full step. This has two effects: Statically the motor position is not optimum and dynamically low speed
resonances occur because of the cyclic acceleration where the microsteps are spread apart and
deceleration where they bunch up. Figure 17 shows a motor with excellent linearity and one with
horrible linearity."
-
How do you measure .5 micron? And you said you are using a leadscrew..
Ray.
-
All, thanks for the good advice, I have now at least re-calibrated the "steps per" for the X-slide and things are much more understandable now. I can see that microstepping is not working exactly as one might naively expect and I'm seeing an effective "average" single step size of around a micron which is double what it "ought" to be. I have also reduced the axis acceleration - I wonder if the slide was slightly over-running when the motor slowed.
Dresda, as I mentioned, the indicator has a full-scale range of +/- 25 micron and resolves to better than 0.5 micron. And it cost a few pounds from a surplus stall - a good buy (though this is the first time I've had a use for it).
Could I (re)ask a couple more questions please?
Is my surmise about leadscrew lubrication correct - is it better to use a light oil rather than thick grease to ensure that one doesn't get a variable thickness layer of grease which could affect the position? And what is people's experience of the accuracy achievable with Mach?
Thanks again,
John.
-
I would say a light oil is probably better.
Regards accuracy of Mach well that is really a non issue I would say, I very much doubt anyone has hardware capable of the accuracy that Mach is capable of ;)
Hood
-
I can see that microstepping is not working exactly as one might naively expect and I'm seeing an effective "average" single step size of around a micron which is double what it "ought" to be.
Lost me there John. Microstep errors are non-accumulative so in your case with a microstepping factor of 8, the average should be an 1/8th of a full step.
Ian
-
Hood - thanks, will go with the oil option. I should have chosen my words more carefully - Mach is capable of very high accuracy, I was really asking what people had achieved in practice with typical lathe conversions.
Stirling, yes, it should be but doesn't seem to be!
-
My lathe will repeat as close as I can reasonably measure, would say less than 0.005mm
My lathe however has ground high precision ballscrews with virtually no backlash, has a resolution of 0.000625mm fromr AC servos and weighs 3 tonne so its really not a good one to compare yours to.
Hood
-
Thanks, that's good information though as it tells me I probably can't get mine much better than it is now so I should stop worrying and start making chips!
As always, thanks for everyone's help and advice.
John.
-
If turning down to a specified diameter using the wizard, what is a reasonable accuracy to expect?
Actual turned down diameter will only be as accurate as your lathes "complete system" and additionaly accuracy can be related to surface finish when measurements
are taken on the diameter of the piece. Depends on spindle runout, surface finish, etc.
You have what you have.....!
Moving the slide repeatedly backwards and forwards to the same nominal position, what repeatability can be expected?
Depends on the motors / electonics / drives but also on the mechanics of the system ( backlash / ball screw accuracy).
On my converted lathe repeatable backlash is like .0004 on the Z and even better on the X. The x axis ball screw is a high end ground ball screw.
So in testing using a .0001" indicator or opticaly compared it's on as compared to Mach's DRO.
but I was wondering about just how repeatable microstepping is
Well, for my engraving machine i could not measure it ie; didn't have enough light at 400x to read the calibrated glass scale with a microscope.
I have found different steppers will microstep better than others....Russ gave an example from Geco for the reason.
I can however easily split .001 into 10 parts by jogging in .0001" steps. Now remember that my steps per unit is 253000, thus resolution is
.000004" but practicaly the screws ware only calibrated / certified to .000040" acuuracy over the total movement. As noted by others you will find that
there will be no movement and then the stepper will jump to the next postion and the number of steps can vary over one revolution of the motor. Change the motor and
you will see a difference if you graphed and compared the movements to steps. So not all motors are created equaly.
Now if you had a pulse counter you may also find out that even the tension of a timing belt can influence your readings as even though the motor turned from a pulse there was no
mechanical movement due to the belt tension. We did some testing and found that Mach / the PC provided the pulses but the drives, belting, motor, etc didn't mecessarily react to them all.
Things are just not perfect......!
And what is people's experience of the accuracy achievable with Mach?
Mach is more accurate than your ability to measure the movement..........!
If you have a quality system then you will have accuracy.
RICH
-
Thanks for the comments Rich...all goes I think to confirm that given that I'm using a standard screw rather than a ballscrew I'm probably getting as good as I can expect.
This was all a bit of a diversion back to the lathe from my main project at the moment which is converting an X1 mill to CNC, so I will stop worrying and get on with the job! Maybe one day I will return to the lathe and fit ballscrews to that.
Thanks again,
John.
-
John,
I fogot to mention that I have a Sherline lathe and it has a threaded screw. I was impressed when i checked the screw in that it was quite accurate in lead going both directions. The problem tough was backlash, even though i had the anti backlash attachment for it.
You could only get so much of the backlash out and then it had to be re-adjusted if used frequently. I will also remark that my tool makers microscope has threaded screws along with an anti-backlash setup and it is extremely accurate. Manny movement tables
use screws, but those devices don't see the wear that a running machine machine will produce. Thus a step up would be ball screws and provide for longevity. Since your interested in accuracy, then each component of the mechanical system needs corresponding attention to selection. You ge a lot for your money but pay more as the accuracy of component is upgraded....ball screws are a good example. Try to make your axis have no backalsh or absolutey minimise it along with a nice balll screw and i think for prcatical use you will be happy with your machine.
RICH