Sizing Stepper motors for CNC

I'm just starting to plan the cnc conversion of my mill (Myford VMC)
I made some static measurements on the torque needed to move the X axis
feedscrew and this comes out to about 1.25NM (or 176 oz-in).
It seems that motors are specified by their holding current on the data sheets. So my question is how does that translate to the torque needed to turn the feedscrew? Also how much extra torque should I include to allow for reaction to cutting forces.
Thanks in advance
Bob
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Bob Minchin wrote:

That seems a little high.
Are you thinking about fitting ballscrews?

Badly.
Steppers only give about 1/2 to 1/3 the holding torque when moving at reasonable speeds. Generally speaking, aim for 1/3.
Roughly, it goes
stopped            holding torque moving slowly        1/2 moving slow-medium pace    1/3 moving medium-fast pace    1/10 moving fast        0.0000000001
Also how much extra torque should I include to allow for

For a cnc mill with leadscrews, not much. Two reasons, first when cutting the machine is moving slowly and the available torque from the stepper is therefore greater, and second for accurate work you don't want to take large cuts - and a cnc mill can easily do lots of small cuts rather than one big one.
I don't know the VMC, but I hear the ?similarly-sized? X3 works okay on 3Nm / 425 oz-in motors, and also on 2.2Nm motors when fitted with ballscrews, though some people find those figures a little underpowered.
For something slightly smaller but of personal experience, I have 1Nm motors on my BCA, geared 2:1, with 2mm pitch leadscrews, and it's just about okay. It's okay for slow speed machining, but fast transverse isn't fast - though if I upped the voltage (24V at present, limited by the driver) it'd probably be reasonable.
I'd use at least 3Nm, and more if I could afford it. Generous power on the steppers makes a lot of other problems go away. Less than 3 is probably too little, anything over 7 is wasted.
-- Peter Fairbrother

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wrote:

If you had fast traverse on a BCA, the table would fly across the workshop ;-)
Mark Rand RTFM
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Peter Fairbrother wrote:

Ah, I missed the static part of that, sorry.
That's probably quite reasonable for a static value, and a stepper will put out at least 1/2 it's holding torque, if not more, when moving very slowly.
(which it will/can do - cnc programs [*] allow you to set the acceleration, so the first few steps, to overcome static friction, can be done very slowly and at high torque)
[*] I'm using emc2 aka linuxcnc, but if you are feeling not-nerdy mach3 is easier for not-nerds to use. Both programs allow you to set acceleration, and a whole lot of other stuff.
-- Peter Fairbrother
11-14 microns, and falling
Sunny day, been in the garden and drinking beer. 90% chopped down a tree - I didn't actually chop it down, but I decided 99% that it had to be chopped, and the rest is - was I right when sober, sanity check - then actually doing it.
Done this way 'cos you can't unchop down a tree (also it's probably extra dangerous to chop down a tree after drinking beer, trees can take their revenge even on the sober).
Reminds me of (ordinary subtractive, cutting) machining and going a little too far - you then want a putting-on-machine.
Hmmm, the 3D printing/rapid prototyping/direct part manufacturing processes mentioned recently are all additive, putting-on-machines ...
hey, I just stepped my toes into cnc, now I want to do do 3D additive manufacturing too? - well, yes ...
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Peter Fairbrother wrote:

Thanks Peter,
I can fit in a 3Nm motor and almost certainly a 2:1 timing belt drive. Eventually I'll fit ballscrews but for now, the motor will drive the exisiting 4mm pitch leadscrew to give me X axis power feed to start with with CNC to follow. The VMC has a similar table size/travel to the X3 but is a knee mill and looks more solid. I'll not try moving the knee under CNC control but just use the quill for z axis travel.
Cheers
Bob
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Bob Minchin wrote:

I wouldn't initially try with a 2:1 timing belt drive, just 1:1 - but ymmv.
Otherwise, I'd thoroughly agree with that idea: single axis power feed, see what a 3Nm motor can do and whether you need/want more (or less). I plan to do that on my X3, as a test.

Generally speaking, the Z axis motor is spec'd to be twice as powerful as (or, more usually, it operates at twice the ratio of) the X and Y motors.
-- Peter Fairbrother

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Peter Fairbrother wrote:

Hi Peter,
I'm curious why you don't advise a 2;1 drive. I'd have assumed that other than minor efficiency losses that this would double my available torque? I've just measured the motor I had from the 'junk box' and it seems to offer 80 oz-in holding torque so that is going back in the junk box and I've ordered a 3Nm one from Zapp. They seem to get good reviews on ebay and are slightly cheaper buying direct - just under 40 delivered. http://www.slidesandballscrews.com . I'll let you know how I get on with it.
When it comes time to do the Z axis on the quill, I currently have a normal rack and pinion drive and when milling manually, I lock the quill. Is it normal to drive the pinion on CNC conversions (via some plain gearing or timing pulleys) and rely on the stepper holding torque or to fit a worm and wheel drive to give positive quill locking during cuts? Another option would be a ballscrew I guess.
cheers
Bob
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Bob Minchin wrote:

I wasn't not advising it, though I wasn't advising it - err ...
You need two practical axis speeds on a mill, fast traverse and cutting speed(s). For a small mill, especially one with leadscrews, the traverse is the one to get right first, then the cutting speeds will most likely be okay torque-wise without you having to do anything about it.
I'd simply suggest first trying without gearing down. 3 Nm should be enough for milling purposes - if it isn't then gear down, but to get a decent traverse speed running ungeared may be better.
Of course this depends on the driver, set current, supply voltage, leadscrew ratio, tightness of the slide and so on, and it's very variable - trying first with one motor is a very good idea, and I kinda wish I'd done it.
It's worth repeating: generous motor torque makes a lot of other problems disappear. Conversely, too little torque makes problems much more noticeable.
Also, for controllers, microstepping is good, though about 8 steps is enough - it makes the motors run much more smoothly, though it doesn't affect the possible speed or torque.
That's my 2p-worth, but I'm not an expert on CNC by any means!
-- Peter F

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Peter Fairbrother wrote:

I've since done some reading and see why gearing is not necessarily a solution. Stepper torque falls almost linearly with speed so gearing up such that the motor turns faster provides no more torque. With some loss in the gearing, it can mean you are worse off by gearing. Essentially a stepper is a constant power device over most of it's speed range (once it is moving and not going too fast that is) It seems that 1) power is everything and 2)for a given motor, running from a pulsed controller with as high a voltage as possible will get best results.
More power Igor!!
Bob
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On Mon, 05 Apr 2010 22:35:28 +0100, Bob Minchin

Bob -
Stepper torque is almost constant from zero RPM up to the "knee" on the torque/RPM curve; from there up it pretty much halves with every doubling of RPM (so not linear at all - hence the speed axis on motor torque curves is usually a log scale).
So, while what you say about gearing is true above the "knee" RPM, below that speed you are indeed getting a torque advantage (while also reducing the "knee" speed at the same time of course). And as it is potentially at the low speed end that you need extra torque - both for overcoming stiction in the ways and for cutting forces, gearing down can indeed be useful. Granted it will reduce the rapids you can achieve, but unless you are going into production, it isn't clear that that is a big issue.
Beware of the "power is everything" approach; it isn't. Yes, you can increase the torque generated by a stepper by upping the voltage (broadly speaking, double the voltage gives double the torque for a given RPM or double the RPM for a given torque), but there is an upper limit that will be governed by what your drivers can handle, both in supply voltage and current. Another important consideration when choosing a stepper therefore becomes its winding inductance; if the inductance is high, then it will need less current to drive it, but the speed achievable for a given dynamic torque will be less (a stepper generates back EMF when it spins; this effectively subtracts from the supply voltage. More inductance means higher back EMF for a given rotational speed.)
So, if you were to choose two motors, both with the same current rating, but one with double the holding torque (and consequently, a higher inductance), then you would get the interesting result that the lower powered motor would be capable of delivering higher speeds, given the same supply voltage and current setting. I demonstrated this conclusively (at least to my own satisfaction) when I re-converted my Taig CNC mill (see http://www.jeffree.co.uk/pages/taigcncpt2.htm ). The original setul used high inductance 200 oz-in motors and a lousy half-step drive system. I tried two different replacement motors; one about 140 oz-in, the other about 200 oz-in but with a higher inductance. The low end performance of both was ample for the job, but the smaller motor gave me faster rapids. Hence, I ended up using the smaller motors.
Regards, Tony
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wrote:

...And one other thing before I forget; there is a popular misconception that microstepping is always preferable to half or full stepping. They do indeed have many advantages (smoother operation etc.); however, if getting the most torque is your goal, then full stepping is the answer.
A full step is achieved by energising both windings using 100% of the rated motor current. A half step is achieved by energising one winding with 100% of the rated current, and leaving the other winding at 0%. Given that the actual torque achieved is the vector sum of thetorque contributed by each winding, the full step gives you root-2 times the torque of the half step.
Hence, in microstepping drives, where your goal is to have the same torque generated at each microstep, at the full step position the winding current will be 1/root-2 times the max current, and the torque achieved at each microstep will be the same as at the half-step position.
Regards, Tony
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Tony Jeffree wrote:

Tony,
Thanks for your detailed replies and the link to your website article.
I have ordered a 3Nm motor 4.2amp, 0.65 ohm, 3.2mH. I have a driver module from Lester C which is 55v 3 amps - not quite capable of the full current but exceeds the 20x voltage rule. Currently my mechanics allow for a 1:1 timing belt drive which is readily changed to 2:1 if needed. So I am hoping for reasonable speed performance. The current plan is simply use this with the existing acme leadscrew for power feed only. Hopefully I'll get reasonable low speed power for cutting, a reasonable fast feed traverse (no cutting forces) and something to play with in terms of supply voltages and speeds.
The next steps will be to fit ball screws and steppers to a free standing XY table to get 2 axis CNC by mounting the xy table onto the Myford Mill table.
Finally I'll add a programmable Z axis to the quill of the mill for 3 axis operation. Not sure how I will do this mechanically yet to achieve good holding torque/reaction to cutting forces.
regards
Bob
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<....>

Hello Bob, It's a while since I did any of this and I can't put my hands on the technical details at present, but IIRC one other issue is the inertia of the system at start which can lead to the motor stalling. Specifically, the inertial varies with the square of the ratio and you can easily find that the biggest inertia for an instantaneous start is the pulley on the leadscrew. Inertia is a very significant parameter which you do need to consider. As commented earlier it makes life much easier if you ramp the step speed and this is particularly significant for fast traverse. I will try to dig out the BergerLahr design manual when I get a chance.
Richard
--- ---
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Richard Shute wrote:

Thanks Richard, I'm hoping the ramp up & down in mach3 will help with the inertia issue.
Bob
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On Sun, 04 Apr 2010 13:54:12 +0100, Bob Minchin

Just for info
I fitted an ELS to my Denford Viceroy Lathe. I used a 3Nm Nema 23 motor (25 from Motion control products) running at 2:1 to a 3mm pitch leadscrew. 50v drive. Not stalled it yet and the "Rapid" whilst not really fast is adequate for me.
Richard
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Richard Edwards wrote:

I think I have a very similar motor on order from Zapp.
Bob
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