# Servo motor size

17 Apr 2012 10:19 #19306 by jeffie_3
Servo motor size was created by jeffie_3
Does anyone know how to calculate a motor size? Thinking about put a control on a lathe I have. Not sure how large or small motors to use. Looking to find someway to figure that out.

Thanks,

Jeff

17 Apr 2012 12:51 #19313 by andypugh
Replied by andypugh on topic Re:Servo motor size
jeffie_3 wrote:

Does anyone know how to calculate a motor size? Thinking about put a control on a lathe I have. Not sure how large or small motors to use. Looking to find someway to figure that out.

My experience is that the highest force required is that needed to push a drill. (you can mount drills on the tool post, and then use peck-drilling cycles, rather than motorise the tailstock).

So, you could try trapping bathroom scales between the chuck and the tailstock, and measuring the force at typical tailstock hand-feed force. (you could also measure typical cross-slide and longitudinal feed forces too, you may need to use a smaller X motor for mounting space reasons)

The force generated by a ballscrew is approximately 2 * pi * torque / lead (using consistent units, so 1Nm and 5mm pitch is 1256N force and 1 lb.ft and 10tpi is 753lbf.) Calculate what motor torque you need to achieve the measured forces, using a ballscrew pitch which gives you the speed you want at the rated motor speed. Don't forget to factor in any gear reduction you have.

Then add a suitable fiddle factor for luck. (quite a big factor if you are using leadscrews not ballscrews)

As a guide, my 3Nm steppers on a 5mm pitch ballscrew and 2;1 gearing are a bit weak for drilling, but OK for turning on m7 8x20 lathe. A similar torque servo with a greater gear reduction would probably be adequate (you can afford a greater reduction with a servo as they can spin a lot faster than steppers)