What size stepper motor do I-need.

[Boyan Silyavski]

Thank you very much for a very informative explanation on motor selection.
I'm upgrading an older machine that had two motors driving X axis gantry. How do I calculate the motor size with two motors on the same axis? Is it a simple as dividing by 2?
Gecko recommends a 40% increase in motor size from base calculations, do you agree?
Thanks again
Mike

Not so simple. First you need to know the moving mass. Then the desired speed and acceleration. Then you decide the motors and the ratios. Then you check if your BOB can cope with that. Then you check what your resolution will be. Some back and forth between all that and a final decision. At least that's my logic.

[Clive S]

Then you check if your BOB can cope with that.

Do you mean the drivers?.
Mike But as Silyavaki has said there is more to it. How about posting some pics of the machine for a better answer. ..Clive

[CNC Visions]

I did the calculation, 670 oz-in/4.6 NM is required to move the axis with one motor. I would normally add +30% for efficiency loss and use a motor in the 850/6 range. With all that, I have a pair of NEMA 34 frame motors that are 420/3. Will I be ok with these or should I go with something bigger? I'm sure that a frame size increase to 34 will slow down my Rapids.
My drives are Gecko 203V, I have several power supplies ranging from 36 to 70 volts available. I upgraded to a Ethernet Smooth Stepper with a high end BOB so I should be able to handle most anything.
Thanks again for your help.
Mike

[Clive S]

Well are you using ball screws, pitch, size, direct drive, etc etc. Without full info nobody will be able to answer you. I take it you don't want to post any pics . Generally nema 23 are more than enough running at about 70V. ..Clive

[CNC Visions]

Ball Screws, direct drive, .5"/rev, 48"X48" table, 2.2 KW water cooled spindle with 10" of travel. The machine is totally in pieces but I will post pictures as it goes back together. I'm replacing the rails and bearings so most of the machine is in tote boxes or stacked in a corner.

[Clive S]

Ball Screws, direct drive, .5"/rev, 48"X48" table, 2.2 KW water cooled spindle with 10" of travel. The machine is totally in pieces but I will post pictures as it goes back together. I'm replacing the rails and bearings so most of the machine is in tote boxes or stacked in a corner.

Ok then I take it you have read this earlier in this post from Jazzcnc

I'll save you a lot of headache and just say use 3Nm motors run at 65-70Vdc on 75-80vdc drives and you won't have any problems cutting anything.

Cutting speeds will depend on many factors like spindle power and tool material but 4500-5000mm/min wouldn't be out the way with 2.2Kw spindle using carbide tooling.

It might be better to keep away from direct drive and use belts and pulleys with a 2:1 reduction (that's what we tend to do this side of the pond) ..Clive

[CNC Visions]

I apologize for not reading everything, Jazzcnc's post would have answered my question.
I'm looking into your advice about getting away from direct drive. It would be much easier to optimize stepper motor performance with the ability to change the drive ratio. I'm not sure why this side of the pond prefers direct drive and drives on the wrong side of the road.
One of the reasons I joined this group was to look into different ways to do things.
Thanks to all for your help
Mike