[David_Nolan]

Hi guys,

So I'm building a cnc machine, I'm trying to make it accurate within a thou, primarily for steel. First machine build.

I'm a little lost when it comes to half-stepping and micro stepping, I'm sure like everything it depends on a multitude of variables but maybe you guys can help me figure out what the best for me...

I'm using a chinese nema 23 stepper motor with a chinese stepper motor driver, driven by my arduino. So I want my machine to be as accurate as possible but I don't want to just set my driver stepping to the highest microstepping value and presume that's going to work out peachy.

Here's some stuff that might help you folks give me a good answer:

I'm pretty sure the driver is of the cheap and cheerful variety,

I want my machine accurate within a thou. (So if I need to get some more expensive drivers better to figure that out now then later.) ,

I will say the stepper seems to be obeying my instructions without issue so far on half step mode.

The range is 200 pulse/revolution (Full Step mode) to 25,000 pulse/per revolution with a lot of options inbetween.

So what microstepping value should I use?


Any help would be greatly appreciated as always, thanks guys.

[joeavaerage]

Hi,
the main reason for microstepping is 'smoothness of motion'. Its very tempting to believe that increasing microstepping
increases resolution but due to degrading differential torque that does not happen in practice.

You will see an improvement of resolution at half-stepping, and possibly at quarter-stepping, but beyond that not so.
Smoothness of motion increases as microstepping increases, however the signaling rate demanded of your controller increases also,
so if you choose a high microstepping regime then your controller may not be able to signal fast enough to get your machine to full speed.

Its a case of diminishing returns, while increasing the microstepping increases smoothness the difference between 8 microsteps/fullstep and 16 microstep/fullstep
is limited, while the signaling rate doubles. Most people chose 8, sometimes 10, sometimes 16 microsteps/fullstep as a good balance between smoothness of motion
WITHOUT really high signal rates.

With regular two phase steppers of 1.8 degree per step, or 200 fullsteps per rev:
8 microsteps; 8 x 200 = 1600 steps/.rev
10 microsteps: 10 x 200 = 2000 steps/rev
16 microsteps: 16 x 200 =3200 steps/rev

As I said before the resolution you can reliably attain is about quarter-stepping or 800 steps/rev irrespective of your microstepping regime.
To achieve 1 thou in steel will require an EXTREMELY rigid machine, and I would guess like most hobbyists machines the lack of rigidity will
set the effective accuracy NOT the resolution of the steppers.

Ask me how I know??? My mini-mill uses 5 phase steppers, ie 500 fullsteps/rev, through 10:1 low lash planetaries. Thus I can achieve a reliable resolution of
5000 steps/rev. When direct coupled to 5mm pitch ballscrews that results in a linear resolution of 1um. Sound impressive right? In practice the lost motion due to
lash and torsional rigidity result in a practical resolution of 4.7um. This works really well in plastics and soft metals like aluminum and brass but in steel the machine starts to flex
and about the best accuracy I can get is 0.02mm or 20um. My little mini-mill has cast iron beds and a 75mm x 75mm solid steel column with 15mm linear rails and I thought
would have been any amount rigid enough for steel, but its marginal.

I have used it extensively for over six years, so I am by no means unhappy with it, and I've done some great work with it but struggle in steel and stainless.
My new build mill is much MUCH bigger and many MANY times more rigid. The cast iron axis beds are 115kg each alone.....I hope to be able to do steel
and stainless but even then will have to be mindful of the rigidity limitations.

My advice is concentrate on making the most rigid machine you can....and then worry about steppers and resolution. You can always fit better steppers or servos later
if you want better resolution, but what you can't do, easily, is improve the stiffness of a too flexible machine.

Craig

[dazp1976]

If we say you've got a 1605 ballscrew (you didn't specify). Nor specified mm or inch.
For metric mm, talking thou's you need to consider kernel speed capabilities.

For 1,000th of a mm you'd need:
1000 per unit (x5) = 5000 per rev.
Pulse rates are done in seconds.
So if your kernel is 25khz that's (25000/1000 = 25, x60secs= 1500.
So your maximum achievable rapid velocity would be 1500mm/min.

At these stepper settings 50khz would be needed to be able to reach 3000mm/min max.
Target of 4500mm/min would need 75khz.. and so on.
I don't know what the Arduino is capable of but this kind of kernel speed is on the cusp for parallel type pc connections.

For inches it's not such a worry.
1 thou of an inch is 0.0254mm.
So setting the driver at 1/10th (2000) is the equivelent of 0.0025mm.
Miles within (potential) of what you're asking.
A kernel speed as low as 25khz would see an achievable velocity of 3000mm/min (118ipm).
50khz would see 6000mm/min.. and so on.

[peteeng]

Hello David - First build being a mill for steel you need to look at commercial machines as examples and start there to understand the size of machine you will need. As Dazp says (and many others will) your limiting factor is machine rigidity not the motion system. Commercial mills are not overdone. To mill you need very very stiff machines and you need to learn all about that first. A stepper has electronic accuracy and mechanical accuracy. Microstepping is a "soft" step that has a tolerance. The mechanical steps reset within a tolerance each full step. If you look up the tolerances and maths (+/-5% mechanical usually for a stepper) there is no value going past 8 or 10 usteps as then the electronic tolerance far exceeds the mechanical tolerance so the usteps becomes pointless as the mechanical tolerance is the limiting factor. Keep asking Q's there's quite a few to come.. Peter

[peteeng]

Hello David - When you say "accuracy" you have to be careful as that has a specific meaning to a machine builder and user. Accurate means that the number you program to is the number you get, maybe repeatability is a better term for you at the moment. This is quite difficult to achieve due to the tolerance stack that comes from the electronics and the mechanical parts. Then there's backlash and machine part compliance to build in. So a quick calculation of mech vs elec tolerance with a 5mm ballscrew shows that neglecting all the issues and just look at the pitch and steps its no value going past 3200 usteps. But to achieve this in realty you have to use a C0 spec screw, even then its per 300mm accuracy is 0.0035mm so its not there. Plus C0 screws cost a small fortune... Hope this helps... Peter

[David_Nolan]

This has helped thanks guys!

[David_Nolan]

I actually haven't decided on which ballscew I will be getting yet but it will be a ballscrew that is for sure! And I'm a metric guy...Most of the time anyway.