Need help evaluating these servos

[pstockley]

I am trying to decide if these servos would be ok for my Hardinge TFB lathe (11") conversion:

http://www.k2cnc.com/motorDC_more.asp

My goal for rapids is around 75 in/min. I will be using a ballscrew of about 0.2" pitch so this will require 375rpm to achieve my desired rapid speeds. The tricky part is deciding how much torque I require. I've either got to measure the cutting torque when the ballscrews are mounted or go with an educated guess. I have a feeling that I will need about 15 lbs/in (240 oz/in) of continous torque at the ballscrew for typical cutting. Peak would need to be a lot higher for acceleration. Also the worst case would be drilling small holes (1/8") using the saddle (z axis) which could be considerably more than 15/lbs /in.

I was planning on a 4:1 reduction so my rapid speed would result in a top motor speed of 1500rpm

Assuming that I go with a 90v power supply, if I read the graphs correct:

1) Running at 1500 rpm would produce 14 kg/cm (208 oz/in) drawing 6A. With a 4:1 mechanical advantage this would be 832 oz/in.

2) Usable peak torque appears to be around 24 kg/cm (330 oz/in) drawing about 10.5 amps at around 250rpm (50 in/min). With a 4:1 mechanical advantage this would be 1320 oz/in (82.5 lbs/in)

Unless my calculations are all wrong, my gut tells me these would work OK. What do you think?

[andy55]

the formula for the force is

F = (M*2*pi /s )*eta
M=torque of motor (Nm)
s=pitch of screw (m) (=how much travel per revolution)
eta=efficiency of ballscrew (typically 80%)

M = 14kg*cm = 14*9.8 * 0.01 Nm = 1.4 Nm
s = 0.2" = 0.2*25.4*0.001 = 0,00508 m

so, with 1:1 gearing (not recommended I guess...) you would get
F= 1380 N and rapids of 7600mm/min
with 1:2 gearing double that (and half the rapid speed) etc.

for comparison eg. the pro-grade Haas office lathe is quoted as having 5000N of thrust in X and Z axes with rapids 19000mm/min...

[pstockley]

Thanks that helps a lot. With a 4:1 reduction I would be around 5000N but at a 10th of the speed of the haas!

I think i will look for some slightly bigger servos to give me some more head room. I have a line on some others from a local source in Canada.