Stepper/Servo select

[sarel.wagner]

After reading numerous threads on the subject, both here and elsewhere, I am still not able to do a proper selection of motor (be it Stepper or Servo) based on fact and science.

I build IC engines as a hobby, and because I am not a PRO I use a simulation tool to build a virtual copy and simulate the results. This works for me because I do not make stupid mistakes in component selection. When I build the engine I know the best selections were made and the combination will work and is optimised. No wasted money.

In searching for something similar I read about Kollmorgen's Motioneering free application. Indeed what I found was heaven as it will simulate the load (X,Y Z Axiz) and give a scientifically based answer on what motor is required. Simple eh?

My problem being I am a noob when it comes to CNC, I do have a Lathe but no Mill. Never having done CNC conversion or having worked with a Mill before I am at a loss with some of the inputs required.

I am planning on getting a RF-45 clone and based my input to do the calculations on this, please see below:

Here are my inputs into the Motioneering app, only Z-Axis given here, some are guesses and I do not even know if they are close:

Z-Axis
Load Coupling: Inertia direct entry: 1400 gm-cm2 (Stepper Rotor Inertia, I ignored the actual coupling inertia, but should not I guess)

SCREW: Diameter 20mm, Length 350mm, Lead 5mm/rev, Nut preload 10N-m, Efficiency 90% (Not sure about the NUT pre-load value)

FORCE: Thrust and Continuous Force (Unsure what to enter here, please explain what these parameters do and how to best use them)

PART/TOOLING: 10KG for a part (This represents the weight of the tool and stuff clamped to Z-Axiz)

Slide: Weight 150kg (330lbs), Elevation 90degrees(Z-Axiz), Gib force 20kg(This is a Guesstimate), Speed at Thrust Force, 0mm/sec(do not understand this parameter), Speed maximum traverse 65mm/sec (150ipm)

Please comment and give me some direction.

Rgrds

[sarel.wagner]

Screenshots of the Kollmorgen app Motioneering and my inputs.

Rgrds

[H500]

Just ignore the trivially small stuff, like rotor inertia. Applying the force, acceleration and power equations on the slides should give you everything you need. With steppers, you need a very large safety margin, so nothing is gained from rigorous analysis.

[sarel.wagner]

Just ignore the trivially small stuff, like rotor inertia. Applying the force, acceleration and power equations on the slides should give you everything you need. With steppers, you need a very large safety margin, so nothing is gained from rigorous analysis.

Thank you for the insight. What would typical values be for the Force(Thrust and Continuous) acceleration and power?

I am also learning all the CNC and motion related stuff as I have no prior experience, besides its educational do rigorous analysis.

Rgrds

[H500]

I have a sx3 mill. I didn't bother with any calcs for the x and y, since I was certain that a 280 oz-in with a 60v driver was more than enough. If you prefer to, then use the slide friction and theoretical cutting forces.

For the z, I calculated the watts it would take to lift the 100 lb head upwards at 100 IPM. I doubled it and then checked the motor's torque vs speed curve to determine the desired gear ratio.

[Al_The_Man]

For servo's here is some motion profiles.
Al

[Larken]

Here are my inputs into the Motioneering app, only Z-Axis given here, some are guesses and I do not even know if they are close:

Z-Axis
SCREW: Diameter 20mm, Length 350mm, Lead 5mm/rev, Nut preload 10N-m, Efficiency 90% (Not sure about the NUT pre-load value)
FORCE: Thrust and Continuous Force (Unsure what to enter here, please explain what these parameters do and how to best use them)
PART/TOOLING: 10KG for a part (This represents the weight of the tool and stuff clamped to Z-Axiz)
Slide: Weight 150kg (330lbs), Elevation 90degrees(Z-Axiz), Gib force 20kg(This is a Guesstimate), Speed at Thrust Force, 0mm/sec(do not understand this parameter), Speed maximum traverse 65mm/sec (150ipm)
Rgrds

Its a fair amount of mass with the 20mm screw and 300lb verticle lift, but the 5mm pitch helps. Definitely need a large size 34 or even a size 42. You will probably need a counter-balance so it doesn't drop when power is turned off.

I would guess a 750-1.5kw servo for the Z axis. This is much too big for a stepper. X and Y probably could be smaller, since they are horizontal.

Larry K

[H500]

I would guess a 750-1.5kw servo for the Z axis. This is much too big for a stepper. X and Y probably could be smaller, since they are horizontal.

That sounds like a behemoth. I doubt that my motor put out more than 60W. It can handle the 100lb head quite easily.

[Larken]

A 10Hp colombo weighs about 60 Lbs, and we use a 890 oz/in size 34 stepper. (Not a ball nut but a Kerk nut.)

You use a 270 oz.in size 23 to lift 100 lbs ?? wow (Maybe there is less gravity in toronto)

What kind of a machine do you have that has a 100 lb head ? Is that a 5tpi screw ?

[Larken]

Personally I suggest you choose the servo, because it is more accurate and smooth.

What size of servo would you recommend for the specs above ?

[sarel.wagner]

Thank you gents. Have been reading up what I could find, not a real lot. As I am not an Engineer some of the calcs is way beyond me, but I am a fast learner.

Reading your responses in the last few posts is exactly why I want to understand motion control as best I could. Guessing and following advice blindly, that's just not me.

I managed to come to the conclusion that a RF-45 and Clone cannot be run properly by stepper, or cannot be run well if ya like. Each axiz demands between 230 and 400 watts to drive em, squarely in Servo territory.

Attached the Z-axiz PDF from Motioneering as well as the complete export. If one of ya will be so kind as to check my inputs and answers, will really appreciate that! (Delete the .txt extension from ZX45-sarel.exp.txt) and import in Motioneering.

I do not have my Mill yet and had to change my plans from the ZAY7045 to the ZX-45 as the ZAY is only available with MT3 spindle and that sucks, no tooling available but the cheap China imports. We use a lot of R8 in SA so Industrial supply abound with anything R8

Some of my input values are guesses or values obtained here in various build threads. Please advise if anything is off realistic values.

Thanking ya all in advance.

Rgrds

[H500]

A 10Hp colombo weighs about 60 Lbs, and we use a 890 oz/in size 34 stepper. (Not a ball nut but a Kerk nut.)

You use a 280 oz.in size 23 to lift 100 lbs ?? wow (Maybe there is less gravity in toronto)

What kind of a machine do you have that has a 100 lb head ? Is that a 5tpi screw ?

If you don't use ball screws, then your motor needs to be 2 to 3 times bigger than mine. But 890 oz in still seems like an overkill for a tiny 60 lb head.

I have an SX3 mill with 5tpi ball screws. Just for fun, lets do some rough calculations.

Head weight+slide friction = 120 lb = 55 kg = 540Newtons
Gear ratio = 1.6
Ball screw ratio = 5

Motor torque drops to about 70% of static value as soon as motor moves:
Motor torque = 280 x .7 = 196 oz in / 16 = 12.25 lb in

Convert motor torque to ball nut force, assuming 80% mechanical efficiency.
12.25 x 3.14 x 1.6 x 5 x .8 = 245 lb

This is about double the weight of the head. It's a reasonable amount of over design.

Ignoring accel and assuming I want about 120 IPM, the required minimum motor power is:
Speed = 120 /60 = 2 ips x .0254 = .0508 meter per sec
Power = 540N x .0508 = 27W

If I didn't make a mistake somewhere, the calculations show that my motor is easily up to the task.

The sx3 uses thrust bearings which rubs on the sides when turning. Acceleration is also not negligible. I am also using only 34v for now, so the motor torque drops as the speed increases. In practice, I am getting 60 to 80 ipm reliably. I fully expect to get 120ipm or more once I replace the bearings and use a 50v power supply.