[will gilmore]

I'm building a small lathe. 5c headstock, 4" swing over slide, 8" swing over bed. About 12" Z and 10" X (gang tooling). Z ballscrew is 16mm 4mm/rev. Z ballscrew is 16mm 2.5mm/rev. I'm planning on direct driving the screws. Hoping to machine everything including tool steel. Also down the road I'm considering adding a stepper/servo to control the spindle and live tooling.

I have been planning on steppers because I'm scared of servos but it seems keling and others offer reasonably priced servos.

The big question is should I build for nema 23 or nema 34 motors? Nema 23 would be easier to package but its not a huge problem either way.

Will

[H.O]

Do you have any ballpark figures on how much force you need and how fast you want the machine to move?

If you, as an example, look at the speed/torque curve of Kelings NEMA23, 425oz-in motor (KL23H286-20-8B) the chart shows that you'll get roughly 1Nm at 2250 half-steps/second or ~340rpm.

Directdriving a 4mm pitch screw with a motor delivering 1Nm of torque will give you a linear force of 1Nm/(0.004m/2/PI)=1570N not counting frictionlosses etc but let's be conservative and say 1000N. That's 1000N (225lbs) of linear force at 1360mm/min (~50ipm)feedrate, at 600mm/min you'll get roughly twice that.

As for the rapids....at 5000 halfsteps/second (3000mm/min) you still get 0.5Nm of torque which gives you 785N of force. Lets say that the friction etc still "eats" 570N and you have 215N left.

The chart shows the drive being fed with 30V which to me sounds awfully low for a motor rated at 4.17V. Raising the voltage to 60V should give you a conciderable boost in high speed performance and allow you rougly double the rapid speeds calculated above.

/Henrik.

[southernexplore]

If you have a servo motor do you use the continuos torque or the max torque for the calculations and then would you multiply times gear ratio for the drive pulleys on the servo system?

Example motor http://www.kelinginc.net/K23-120-36.pdf

Tommy

[RICHARD ZASTROW]

IMHO continuous torque. I believe if you apply max torque the drives will "kick out". Some we've done will not even allow you to reach max and will shut down to avoid damage.

Gear ratio X continuous torque X efficiency
EX. 3:1 X 100 Nm X 90% = 270 Nm

Dick Z

[will gilmore]

Do you have any ballpark figures on how much force you need and how fast you want the machine to move?

If you, as an example, look at the speed/torque curve of Kelings NEMA23, 425oz-in motor (KL23H286-20-8B) the chart shows that you'll get roughly 1Nm at 2250 half-steps/second or ~340rpm.

Directdriving a 4mm pitch screw with a motor delivering 1Nm of torque will give you a linear force of 1Nm/(0.004m/2/PI)=1570N not counting frictionlosses etc but let's be conservative and say 1000N. That's 1000N (225lbs) of linear force at 1360mm/min (~50ipm)feedrate, at 600mm/min you'll get roughly twice that.

As for the rapids....at 5000 halfsteps/second (3000mm/min) you still get 0.5Nm of torque which gives you 785N of force. Lets say that the friction etc still "eats" 570N and you have 215N left.

The chart shows the drive being fed with 30V which to me sounds awfully low for a motor rated at 4.17V. Raising the voltage to 60V should give you a conciderable boost in high speed performance and allow you rougly double the rapid speeds calculated above.

/Henrik.

Thanks for the math. That chart is for the Nema 23 286 oz stepper. The Nema 23 steppers go up to 570 which seems like it should be plenty.