Basic CNC

This is a stepper speed controller that does not require a computer for control. It has limit switch inputs but will not move a specific number of pulses.
http://www.ebay.com/itm/9-24V-Input...59466c&pid=100338&rk=3&rkt=30&sd=281570110066

Once you add distance traveled then you are into CNC control. Most if not stepper drives have several settings for steps per revolution and power output.

The down side of putting handles on the steppers is that steppers cog when turned and cause the hand feed to be ''lumpy''. The other thing is that the steppers become a generator when turned by hand and if connected to the drive, require extra effort to turn. For hand feeding you would want to provide a switch to disconnect the motor wires from the drive if the motor is mechanically connected to the lead screw.

A belt non-cog system would work on the Y and Z, also on the X for power feed, but for accurate positioning it would require a timing belt..
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Thanks Jim,
Would using a Variable-reluctance stepper motor get rid of the "Lump" problem of hand operation ?
Just turn off the power and manually feed ?
 
Good question, and one I am not able to answer.

A brushed DC servo motor would not cog. There are DC servo drives that will take a step & direction input just like a stepper drive. Higher cost than stepper systems but would make a great basis when you are ready to convert to CNC. Something in the 600 oz/in (~30 in/lb) range world work in your application.
 
Since I was thinking of being able to drive a rotary table and/or indexer, figured I'd better stick with steppers.
I'd think that would be a better option for controlling rotation for operations like gear cutting.
Not sure if servo motors work well for this application or not and didn't think mixing and matching both stepper and servos would be a good idea.
I read about Variable-reluctance stepper motors here: http://www.freescale.com/files/microcontrollers/doc/app_note/AN2974.pdf
Says they don't exhibit magnetic resistance when rotating unpowered. Did a quick Google search for them and didn't find a whole lot so not sure how available they are.
 
A servo motor or a stepper will do exactly the same job and just as accurately if set up correctly. It's really a matter of the drives, the feedback (encoder), and the controller. Adding feedback to a stepper system technically turns the stepper into a servo. I have a mix of brushed DC servos(X&Y) and a stepper on my Z axis and it works well. I consistently hold 0.0001 on my Z. See that build here http://www.hobby-machinist.com/threads/z-axis-cnc-conversion.21060/

I am going to build a RT with a stepper on it for my 4th axis. Going with a stepper just based on cost, not function.
 
Lot of good info.
One question, you stated that a 1200 oz-in stepper would work for my application. You also posted that a 600 oz-in servo would work as well.
I thought I understood the differences between steppers and servos from reading earlier posts in this thread, but I either missed something or don't quite understand.
Why would a servo half the power rating work for the same application as a stepper with twice the power (meaning oz-in rating) or did I miunderstand your replies ?
 
Nope, you have it correct. BUT.... You normally size steppers twice the torque that you size DC and AC servo motors because the steppers can decouple under load and lose steps the other motors will just stall, but won't lose position. A stepper with feedback (servo) may decouple but won't lose position.
 
So, if I understand correctly and for example, I could get by with using a 600 oz-in stepper for my intended use since I would not (at least for now) be using a CNC program to run them realize that they would be undersized for a full CNC conversion or is there something I'm missing as far as the stepper controller is concerned ? Or does it have more to do with the way the different motors work ?
 
On a machine of your size, the 600 oz-in would probably work OK for CNC with ball screws. Acme thread leadscrews require quite a bit more torque for the same load.

A lot of it has to do with the way the motors are constructed and how they are controlled. By decouple in the case of a stepper I mean that the torque overcomes the magnetic force inside the motor, and the magnetic field continues to spin but it can't grab hold of the rotor.

A brushed DC motor has no way to decouple, it just keeps producing torque, even if stalled.

A BLDC (AC servo) is a 3 phase motor and normally won't decouple, but I think it can, but I have never seen it happen. I think the controller would shut it down before it reached that point. Somebody correct me if I'm wrong on this point.
 
The torque ratings issue is quite a bit more complicated that steppers being able to "de-couple" or lose steps, all though that's part of the reason you need a higher torque rating with a stepper.

The rated torque for a stepper motor is the "holding torque" which is the torque the motor can resist when the shaft is not moving. The minute you start stepping, the torque drops by 20% or so. And as you move faster, the available torque drops even further, essentially down to nothing at high speeds. Large steppers may have only 25-30% of their rated torque at 1000 RPM. Also any vibration in the motor drive train is effectively added to the torque seen by the motor, further reducing the available torque to drive the load.

Servo motors have 2 different torque ratings - the continuous torque (this is the rating most often cited) and the peak torque which may be 2x - 3x times the the continuous torque for a large motor. With a servo motor, though, the torque does not drop off with the speed - most servo motors will run at the rated torque at speeds up to 1000 - 3000 RPM. The other great thing about servo motors is that the controller can run them a more than the continuous rated torque (up to the peak torque) for short periods to overcome static friction.

Also keep in mind that the force generated by a motor directly coupled to the lead/ball screw will produce a force (in pounds) of: (rated torque in oz-in) x (number of threads/inch) x 2 x pi x (lead screw efficiency) / 16. Therefore, 600 oz-in of torque applied to a 5 TPI ball screw (~90% efficient) would produce a force of 1059 lb. This is a fairly dangerous amount of force that would likely damage something on your machine. 200 oz-in would be more appropriate, but if you are using a 600 oz-in stepper, you may only have 200 oz-in of torque available at higher speeds.
 
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