"An Accurate CNC 4th Axis"...?

I read with interest, and a little concern, the article in MEW #175/176 on the construction of a so-called "Accurate CNC 4th axis".

 

The main cause for concern was the choice of the athor to use a simple toothed belt reduction of 3.6:1, between the stepper and the spindle, and his calculation that with 8 microsteps, this would result in 5760 steps per rev of the spindle. All very well on paper but...

 

Firstly, stepper motors are not precision instruments in themselves. The positioning of whole steps on a stepper motor is typically quoted at 5-10%, but if you start going down the microstepping route, all bets are off as the microstep accuracy is highly dependent on the ability of the microstepping drive to accurately position the microsteps (which it achieves by applying varying currents to the two motor coils), and this ability is generally pretty poor. So if you are looking for the best accuracy that you can get out of a stepper based system, then you are better off going for full stepping and uping the drive ratio accordingly, because at least then the accuracy should be as good as the motor is inherently capable of.

 

Secondly, the accuracy quoted for a stepper motor is under no-load conditions. Apply any rotational force to the motor shaft and it will move to a point where the restoring torque generated by the coils on the magnets of the rotor is exactly equal and opposite to the force applied. So, assuming the force isn't great enough to cause the rotor to turn to the next step position, then an applied force could cause a positional error of up to 1/2 a step of the motor (i.e., a step position error of 50%). Obviously, there are two ways that this can be ameliorated; you can use a socking great big stepper that generates a gozillion oz-inches or newton-meters of torque, or you can increase the drive ratio to suit the real-life working conditions. In a CNC mill, those conditions can be fairly "interesting"; slap even a modest stepper motor on the end of a 1mm pitch screw and the resultant force moving the axis can be quite large - for example, with my Taig CNC mill (see MEW 120, 121) I figured out that the 120 oz-in motors I used were capable of generating around 100 kilos of force at the tool tip, with a mechanical advantage via the leadscrew of around 125:1. Now, stick a 4th axis with a mechanical advantage of only 3.6:1 on the table of my Taig, and once you start taking a decent sized cut, something is going to give; that something is the posinal accuracy of the 4th axis. So what is the solution? Obviously, use a drive ratio for the 4th axis that is considerably greater than 3.6:1; perhaps slightly less obviously, use a worm drive rather than simple gearing. Why? Because if you use a worm drive with a reasonable ratio (say, 30:1 or greater) it is impossible to drive it backwards against even a small force holding the worm in position. This is one of the major reasons why worm drives, often as high as 90:1, as mentioned by the author, are used for 4th axis drives; they provide, in a single compact package, a large reduction ratio and the important characteristic that they can't be back-driven by cutting forces.

 

So, while Mr Gordon's device is nicely designed and well explained, I fear that building one in the hope of it being useful for real CNC work would be a waste of time.

 

A secondary cause for concern was the heavy weather the author made of his problems with the quality of toothed pulleys, and the astonishing solution that he mentioned in part 2 of re-machining one of the pulleys after layering on car body filler. Frankly if I had gone down that kind of route I would have kept very quiet about it when I wrote up the project! Treality is that toothed pulleys and belts of a suitable quality are readily available - RS Components for example - however, as mentioned above, this was in any case an example of the old Irish joke - "I wouldn't start from here!".

 

Turning a worm/wheel driven dividing head or a rotary table into a CNC 4th axis is a far more rewarding, and far easier, solution; it will give far greater precision than the one described in the articles, and will not suffer from its very obvious, and in my view fatal, flaws.

 

Regards,

Tony

Tony,

You echo my exact same sentiments with one major difference.

 

I went down this same design path some years ago but built a much larger unit. Powered by a large type 42 motor with 4:1 reduction on 1" wide L series timing belt to a slave D1-3 spindle to take the chucks off the lathe, in short a real hefty bit of kit.

I have recently completed a DivisionMaster conversion of one of Arc's 6" rotary tables as per the gospel (and pictures of John S).

 

There have been references to light and heavy cuts. If rounding off, say a piece of MS 3/16" thick with an 8mm end mill what sort of depth of cut should the DM powered RT be able to cope with? It seems that this setup would not pull the skin off a rice pudding, but not sure if I am expecting too much.

 

TIA

 

Pete

Posted by Spurry on 28/04/2011 16:01:38:

 

Pete -

 

There's no substitute for trial and error.

 

Regards,

Tony

Hi Pete,

I am not clear whether your statement " would not pull the skin off a rice pudding" referrers to the belt driven version or the rotary table version. In your question you do not sate the diameter of the disk you are asking about. The larger diameter the disk is the larger the torque required so cutting say a 15" diameter disk you would have to take much smaller cuts than for a 3" disk.

I had only skimmed the first part of the article on the 4th axis CNC and assumed that there must be some kind of brake mechanism to be shown in later parts. On reading it properly I could see no mention of this so I totally agree with Tony's and John's comments.

Les.

My own experience of measuring repeatability and accuracy of my CNC converted super-7 cross-slide bears this out. i am using 8 "microsteps / step" and certainly don't obtain the predicted accuracy or resolution, and the repeatability depends on the applied load. Seeking advice on the Mach forum it was pointed out that stepper motor accuracy is poorly specified for microstepping, since it is achieved by applying linearly interpolated current values to the windings between full steps and the torque/current curve is probably non-linear so there is, even in the best case, a non-linear relationship between demanded and actual position. This will be made worse by the effect of cutting torque and variable friction since I suspect that the full holding torque of the motor probably isn't available. I've seen adverts for specially made steppers with a linear microstep characteristic in the US "digital machinist" magazine to improve performance, but no idea how much these are or whether available over here.

 

The main reason for microstepping seems to be to allow better driving of the motor at higher speeds without exciting resonances. For accurate positioning looks like it's better to have a large mechanical reduction (worm/wheel) and rely on whole steps.

The other problem is that every time you increase the microsteps you reduce torque so at some point under load you don't have enough torque to make the move.

Stiction also plays a big part in that nothing happens until suddenly it moves so may steps in one go.

 

Ideally full or half steps are the best options if you can get the mechanical advantage.

 

John S.

Les

 

My query relates to the use of Arc's 6" geared rotary table. One of the jobs was to round the corners of some MS 1/2" thick plate to a radius of 8mm. From memory the plates were 2" x 2 1/2". Cutting full depth, I found the maximum infeed per pass to be about 0.1 mm or less.

 

Next up was some 3/16" x 3/4" MS bar with a radius of 8.50mm on each corner. I was just curious as to what experienced users of the Divisionmaster/Arc rotary table had found to be a suitable depth of in-feed.

 

So in both cases cutting was taking place very near to the centre of the RT, as the radii indicate.

 

Pete

Hi Pete,

I do not have a stepper / divisionmaster on my rotary table (6" Vertex) but I have considered putting a stepper motor on it and designing my own controller as it sounds like an interesting project. (No criticism of Tony's Divisionmaster.) I have used my rotary table for cleaning up disks of 2 mm steel and 20 mm chipboard both about 16" diameter. I did not find that any great effort was required to turn the handle but I was taking very light cuts.

 

Les.

One thing to bear in mind is that the Divisionmaster controller can only drive up to 2A/phase; if the motor that the OP is using is rated at significantly more than 2A then the controller won't be driving the motor as hard as it can go. Also, be sure to set the current level on the controller appropriately...I'm pretty sure that the default setting is less than the full 2A/phase capability.

 

Regards,

Tony

Thanks for the replies gents. Much appreciated.

 

Pete

Posted by Les Jones 1 on 29/04/2011 15:32:35:I do not have a stepper / divisionmaster on my rotary table (6" Vertex) but I have considered putting a stepper motor on it and designing my own controller as it sounds like an interesting project.

 

I appreciate that you have a background in electronics so may prefer to do your own thing. But it may interest you and others to know that a DIY dividing head/rotary table controller has been developed by Steve Ward (aka 'Kwackers) and is described/discussed on the CNC Zone here:

 

Rotary Table Indexer

The main block of documentation is at Post No.45 and the most recent PIC firmware /Manual versions can be downloaded at Post No.420. This project has been around since 2006 and is now pretty mature, relatively cheap to build and has good documentation (not to mention the exemplary support Steve has given via that thread). A video of one constructed by M. Parker-Lisberg can be seen here:

 

 

— and another build by C Raynerd is available here:

 

C Raynerd

 

Joe

 

 

Posted by MICHAEL WILLIAMS on 02/05/2011 11:33:12:

Actually, you are wrong.

 

The situation is greatly improved if you use a worm and wheel; the positional error of a stepper motor isn't accumulative, so the accuracy of positioning improves by a factor equal to the worm/wheel drive ratio.

 

Direct indexing using full steps of a stepper motor gives you a positional accuracy of ~ +- 5% depending on the quality of the motor (and also gives you a very limited number of divisions, as there are only 200 discrete positions that the motor can index to). Stick that motor on the end of a 90:1 worm drive and the +- 5% reduces to +- 0.055%, modulo the accuracy of the worm and wheel.

 

This fact leads to a very usable means of improving the accuracy of dividing plates on a dividing head or rotary table; you cut one set of dividing plates as accurately as you can, then use that set with the reduction drive on your dividing head to generate a second, more accurate, set.

 

Accurate, manually operated division systems that work on that principle have been around for far longer than rotary encoders. The only fundamental difference between a manual dividing system using a high reduction ratio and the stepper-driven equivalent is that you push a button rather than cranking a handle.

 

Regards,

Tony

 

Regards,

Tony

Posted by MICHAEL WILLIAMS on 02/05/2011 12:09:58:

Hi Michael -

 

That is of course true (and I did point that out in what I posted, although maybe not as explicitly as you state here).

 

However, I take issue with your statement that you cannot contrive greater accuracy in a system that is not accurate in the first place. If that was universally true, then it would be impossible to construct measurement systems that are more accurate than the ones we had before, and we would not have the instruments we have today.

 

Regards,

Tony

 

Are we in danger of chasing ghosts here ?

 

If you go to the Divisionmaster legacy site still on the web at

 

Divisionmaster Legacy Site and under examples of use, near the bottom where it shows a big Hoffman dividing head it reads:-

 

Top right shows gear cutting in progress, cutting the seventh tooth - he was cutting four 27-tooth gears simultaneously. Bottom right shows the display of DivisionMaster at this point - the actual position is 93.330 degrees. The theoretical position should be 93.333 degrees; John said that the positional error on any one of the 27 moves was never more than 0.008 degrees.

 

can anyone measure an error of this magnitude in the home workshop - or even, pray, an armchair ?

 

John S.

29 seconds of arc? Easy!

 

Actually, no it isn't. I'd have to raid the optics lab at work to do any measurements that accurately. And I think I'd have to do it there, as well - don't have a stable optical bench at home. Also, a surprising number of things can disturb a measurement that small...

 

But I agree with Tony completely - in most disciplines, the vast majority of advances in scientific measurement in the past have relied entirely upon contriving at ways of analysing, and subsequently reducing the errors in previous measurement methodologies.

 

I wonder how easy it would be to contrive an experiment to determine how positionally accurate any given stepper motor actually is, within reason? I wouldn't have thought that it was beyond the bounds of possibility...

Posted by Steve Garnett on 02/05/2011 16:59:41:

Edited By Steve Garnett on 02/05/2011 17:02:28

A lot easier just to look at the spec sheet

Regards,

Tony

Posted by Steve Garnett on 02/05/2011 17:34:34:
Oh come on, Tony. You've done enough electronics not to trust one of those, haven't you?

I'm afraid that is true....

 

Regards,

Tony

 

 

Any comment on the practice of building the teeth up with body filler then re-machining ?

 

John S.