[samualt]

I am having a hard time finding a ball screw that is 1" diameter and with 53" long threads. I know where one is but it is only 3/4" diameter. Don't want to spend over $150.

Question, if I'm using 1500 oz-in servo with a 3/4" diameter screw what bad things will happen?
I mean, what happens when you try to cut something real hard or real fast? Would the screw break? Would it bend permanently? Or would it just shake and be unusable? What exactly is the problem with too small of a screw?

Getting antsy to buy...........Sam

[HuFlungDung]

Sam,

1500/(16*12) = 7.8 ft lbs. This is not a huge amount of torque. By comparison, you can think of tightening up a 1/2" wheel stud to what, 50 ft lbs? Normal cutting action, even at high feedrate will not hurt it.

The high momentum of the spinning screw moving a heavy load (in rapid mode) can have completely different effects, though. The screw could get bent in the event of an accident.

Also, another option is to limit the current gain on your servo amplifiers so that they trip out before the motor reaches its peak draw. However, this will affect your accel/decel ramps in a negative way. But, this might be an option while you are in the startup stages, and are just test driving your machine.

Yes, small screws are more likely to vibrate and shake at high rpms.

[steveald]

Hi Sam,

According to PIC-Design, a ballscrew 3/4" in dia with a fixed support at one end (2 bearings 1.5 times the screw dia apart) and a simple support (one bearing) 53" from the fixed support will have a critical speed of 1200+ rpm. With a .2" lead, this would give you 240ipm before oscillation starts. With a .5" lead, 600ipm!! You could directly couple a servo and be right in the max torque range, but if you coupled the motor to screw using a timing belt, you would have something to give way before destroying your machine in a crash. At least I think it would work, but I don't want to test that theory, do we have a volunteer?

Please don't use my figures without double checking them, I'm not an engineer, I just ran your numbers through the PIC-Design critical shaft speed calculator at http://www.pic-design.com

Good luck finding what you need,

Steve