Servo Motors

Richard,

I must confess that I had not considered that 550W might be needed for a pen-turning lathe. ... I was thinking of something scaled-up a little from the Sherline version:

http://sherline.com/product/6500-stepper-motor-mounting-kit/

[quote]

NOTE: Stepper Motor RPM Range

We have had customers who want to use the stepper motor (on the headstock) as a dual purpose motor. They want to use it to index the headstock to do machine work (mill flats, or cut out inlay areas on a pool cue). They also want to use it as a drive motor to turn stock (to replace the DC motor).

So, they keep asking what the maximum RPM of the spindle is with the stepper motor and cog pulley mounted to it.
Max RPM = 225
Min RPM = 6 – 10

[/quote]

MichaelG.

Edited By Michael Gilligan on 07/06/2017 12:48:21

While not an absolute distinction there is generally a fundamental difference between the two drive options:

A servo system uses sensor feedback to move the driver to the required position, and is thus a closed loop system, by definition

A stepper motor system does not usually use feedback and is therefore an open loop system

For the relatively slow helical patterns shown I would think that a stepper motor spindle drive is more useful, and simpler. Basically a 4-axis mill albeit it with limited rectilinear movement options.

Andrew

OK.

I think I have gathered the following:

1. A stepper motor can't really spin the spindle fast enough to be usable as a lathe for my purposes (650rpm minimum). I'm using carbide inserts on plastics, and they work really well at pen diameters with a speed of about 900 rpm.
2. Unless a big gear reduction is used, I can't really get the holding torque on the spindle from either a servo motor or a stepper motor for engraving.
3. It is highly unlikely I will be able to use the same motor to rotate the spindle for the lathe feature and the engraving feature without at least some form of configuration change on the machine.

While this is not entirely welcome, it's not unexpected. So, thank you all for your help. I think the solution will be to have a spindle encoder permanently fitted. I shall use the standard Clarke 550W motor to power the spindle when it working as a lathe. Then take the drive belt off and engage a stepper motor through a big reduction to turn the spindle when engraving. Whether the reduction will be through a worm or belt drive, I shall have to investigate. Zero backlash worm drives seem to be quite unusual.

Having established this, the controller needs to be identified, but that is a problem for another time.

Your help has been greatly appreciated & has clarified the scope of the problem I face.

Regards,

Richard.

You shouldn't need a "big reduction" between the stepper and spindle - you could drive the spindle directly in many cases, depending how good your driver is. You are talking about machining a pen barrel here. However, you need to ensure that you have enough (micro)steps per revolution to give a smooth, almost stepless movement.

BTW, you can get "closed loop stepper" systems, where the stepper has an integrated encoder. When operated with a high microstep setting, this can provide very accurate positioning. And being closed loop, you generally don't need such a powerful motor. The Leadshine ones even have the driver built in to the motor so you only need to provide the step/dir signal and the DC power (24 or 36V).

Murray

The two motor option is common practice on industrial machine tools, an AC motor for rapid traverse and a DC motor for feed speeds. The changeover is usually automatic though.

Mike