Need Help!- motor sizing Q's (should i even bother with these motors)

[margni74]

Listed (way) below are the specs on some Litton servo motors I got from eBay well over a year ago. It looks like most people are using 100-200 Cont/600-1000 Peak torque motors. I don't want to waste my time building the motor mounts and power supply if my paltry motors are just going to be worthless.

My gantry-moving axis has a 5 turn/inch screw. I measured the amount of torque needed to move the gantry with a Snap-On lb-in wrench at it was less than 5 lb-in. Actually, it was probably < 2 lb-in because the smallest division on the wrench is 5 lb-in and it didn't even look like it was getting halfway to the first division. Anyhow, call it 3 lb-in. Meaning there is a 48 oz-in minimum requirement just to move the gantry. My motor provides a continuous 27 oz-in, so geared down 2:1 = 54 oz-in or 3:1 = 81 oz-in. not much margin for error there.

I'm not sure what kind of additional force is going to be imparted on the tool when trying to cut wood, mdf, foam, probably some AL.

The torque requirement for the on-gantry axis is an unknown at this point as it isn't 100% complete. The screw is a much larger pitch of 0.6 tho and could require even more torque so it might need to be geared down in the 4 plus :1 range.

I already have 3 gecko drives (2 320s, 1 340), so I'd like to stick to servo motors.

Peak Torque 250 oz-in
Continuous Stall Torque 27 oz-in
Terminal Voltage 36 VDC-
Rated Speed 3500 RPM
Rated Current 2.75 A
Torque Sensitivity 12 oz-in/amp
Back EMF 8.82 V/KRPM
Terminal Resistance 2.30
Terminal Inductance 3.10 mH
Rotor Inertia .004 oz-in-sec^2
Friction Torque 3 oz-in
Thermal Resistance 5.4 degrees C/ watt
Damping Factor .1 oz-in/KRPM
Weight 38 oz

[Al_The_Man]

If you can gear down to the maximum that will allow your desired IPM rate, which will increase the torque proportionally.
The break-away torque is one thing and also the cutting forces, but usually the biggest issue on sizing is the motor to load inertia value, and this is influenced greatly by the required accel/decel rate, which is often overlooked or ignored.
Al.

[margni74]

Originally Posted by Al_The_Man If you can gear down to the maximum that will allow your desired IPM rate, which will increase the torque proportionally. The break-away torque is one thing and also the cutting forces, but usually the biggest issue on sizing is the motor to load inertia value, and this is influenced greatly by the required accel/decel rate, which is often overlooked or ignored. Al.

What is break-away torque? Is it the same as stall tq?

Should the force required to cut add *that* much to the amount of torque needed? Isn't the cutting tool doing most of the work ... so the torque needed to turn the screw should not change?

Im not sure what 'load inertia value' means. It indeed seems that no one building these machines as a hobby takes this into account as I cant really find mention of it in the build logs.

As for desired IPM... I calculated 708 IPM maximum with the motor directly coupled to the ballscrew for a jog. This was taking into account my motors 250 cpr encoder, the screw pitch, and a projected maximum of 59,000 #pulses/sec that I could get from my machine running emc2 to ouput for a max motor rpm or 3500. So different with a few different gearing setups:
1:1 = 708 IPM
2:1 = 354 IPM
3:1 = 236 IPM
4:1 = 177 IPM

somewhere between 2:1 and 3:1 would probably do nicely.

[Al_The_Man]

The break away torque is what you measured with the torque wrench.
An example of the inertia would be if your torque wrench was operating a ball screw that moved a gantry weighing 100lbs.
At just a couple of rpm, it may require little effort, or torque, now accelerate the same BS from zero to several inches/minute, your torque will vary depending on how fast you want that load to reach the required IPM.
As for instance a gantry performing a right angle, one axis will rapidly decelerate, followed by rapid acceleration of the second axis.
If you download one of the sizing programs, it will show dramatically the different results when the accel/decel is changed.
http://www.electromate.com/technical...morgansoftware
Al.