CNC Conversion Question

[Hillbillycnc]

Hello All
I converted my Bridgeport clone to cnc about a year ago. My Z axis was by powering the quil. I decided to remove the stepper motor from the quil and power the knee. Here's where my problem comes in. My Z motor was 640 in/oz and geared 3 to 1 on a 5 turn per inch ballscrew. I built a new motor mount for the knee, and for grins and giggles I put the 640 in/oz motor on the knee. To my surprise it drove the knee pretty well but could only get 30 units a minute out of it. I decided ti order a new motor and some different pulleys to see if I could get more units per minute out of it. I got a 1800 in/oz motor and enough pulleys to gear it anywhere from 1to 1 up to 3 to 1. I mounted the new motor and tested it, but can only get 30 units a minute out of it, no mater what combo of pulleys I use. I forgot to add I am using Mach3 to control it. Am I running out of pulses or anyone have any idea what is limiting me?
Thanks Barry

[NC Cams]

Try this:

crank the knee and then move the quill.

The amount of power needed to move the dead weight of the bed/knee combined with that to overcome friction in the ways is MASSIVE as compared to that involved with moving the quill.

If the amount of weight and/or friction you're trying to move overcomes the output torque of the motor (irrespective of gearing), you'll lose steps or could be possibley reaching current limit programmed into the drivers.

Perhaps you're at a pulse limit someplace which is why you're only limited to 30 units/min.

[Hillbillycnc]

Thanks for the response NC Cams

Don't you think the 3 times larger motor should turn faster with the same load as the smaller one? Less like you said I am reaching a current limit. The smaller motor is a 4 Amp motor and the lrger one is 6 Amps. Is anyone else powering the knee of there mill with steppers and having the same issue?
75VDC Power Supply
Qecko 201
Still have acme screw in knee and bevelled gears

Thanks Barry

[NC Cams]

You're confusing size with speed and torque. And it is an easy mistake to make until/unless you learn how to read/interpret motor power curve graphs....

Motor SPEED is a function of the voltage and TORQUE ultimately is a function of the current draw that the motor is capable of handling.

For a given wind configuration, say one that will allow 3000 rpm at 100 vdc terminal volts, a 4 amp motor will turn EXACTLY the same SPEED as a 6 amp motor.

However, the 6 amp motor should have a much higher stall torque (and more power) than the 4 amp motor.

Roughtly speaking, the stalled rotor power of the 4 amp example would be 400 watts versus 600 watts of the 6 amp - 50% more power at the same speed.

The 6 amp'er would also probably have more at any comparable RPM point.

Unless you up the motor voltage or go to a motor with a higher speed at the given voltage you have to work with, you ain't going to move anything any faster, Be careful however, as higher speed motors often give up torque to gain speed - YOU make up the power loss via gearing....

Pretty basic stuff once you understand/do the math....

[Hillbillycnc]

Thanks NC Cams

I think I get it now. 30 in/min X 30 revs per in = 900 motor rpm
I must be right at the peak torque curve for my motor at 30 in/min.

Thanks Barry