X-axis power feed w/o CNC

[dlenox]

I am looking to add a power feed to the X-axis of my mini-mill but do not want to have the hastle of CNC'ing it (and do not forsee the desire to CNC it).

Looking for recommendations for simple power feed with something like 0-24ipm. Do not want to go through the hastle of using computer as controller, but simple directional control (left/right) and a way to control speed.

Saw a unit sold by LittleMachineShop.com (mfg by Seig) and the motor is severly underpowered as well as limited to 12ipm, the item lists for about $150.

Also have seen where people used windshield motors as drive, but not sure if I want to go down this path.

I have seen some geared DC motors at http://www.surpluscenter.com/ along with motor speed controller. Total outlay would be about $55.

My question is : is it better to use a DC motor rather than stepper motor for a simple feed control?

[arvidb]

If you need constant speed (and I guess you do ) then the choices are the same as for a CNC machine.

If you use a stepper, you will need circuitry to output the correct commutation sequence for the stepper (i.e. drive it). If you go too fast or you ask too much torque from the stepper, it will stall and your part will have an error.

If you use a servo motor (whichever kind), you will need circuitry to read feedback from the motor and drive it at the set speed. Feedback device from the motor could be tachometer or encoder, for example. Again, if you ask too much of the motor, it will not be able to keep the correct speed.

So I guess both are ok - get what you can find!

Arvid

[dlenox]

arvidb,

the power feed is typically for moving the table fast, or when cutting just to save you the hasstle of cranking the wheel on the feed screw.

Exact feed speed is not required as long as it is relatively constant. I think that feedback might be overkill in this situation. That is why I was thinking about going the DC Motor route, and using either a motor speed controller. As long as the geared motor can turn the feed screw under the desired pressure (while milling/fast forward) at a roughly defined (and variable) speed, then that meets the desired goal.

[Al_The_Man]

If you just want rapid (rough) positioning What you can do is fit a DC servo motor, there is always plenty on ebay and also pick up a PWM drive like AMC or Copley, again cheap on ebay, and set up a pot for speed and direction and disable the drive when you don't need it, you do not need an encoder for this, just a motor with a tach.
If you use this method for cutting then you will have to visually monitor the feed as the tach will compensate to some degree, but heavy cuts will tend to slow the feed.
Al

[dlenox]

Al,

thanks for the info, is the tach really necessary? I would assume that this would help compensate for when the lead screw comes under a load, and I have not found lots of DC motors that have them.

[arvidb]

The question is, what does "relatively constant" speed mean?

If you are only using it to position the table between cuts, just use a DC motor without any feedback, together with some kind of drive to vary the voltage to the motor (PWM is what comes to mind). This will give you some control over speed as long as the load does not change. Be aware though that the load will propably vary just because of differences in the ways or ACME screw, if that is what you got.

Otherwise, go with Al's advice. Then you need a motor with some kind of feedback device (be it tach, encoder, resolver, as long as it's compatible with the drive), and you need a drive of course. Depending on the drive this will give you anything from ok to "perfect" speed control.

There is another way to go; some motor drives can sense the back-EMF voltage from the motor and thus use "sensorless" speed regulation. I don't know where you can buy these drives though. If you are good at electionics it should not be very difficult to build your own .

Arvid

[Al_The_Man]

Originally posted by dlenox Al, thanks for the info, is the tach really necessary?

The early DC servo systems all used tach feedback, this is/was known as the inner velocity loop, the outer loop was the encoder positioning feedback to the CNC.
The drive on on its own with the tach feedback can offer excellent speed control with the velocity feedback of a tach.
Most modern servo amplifiers offer different modes of operation, including velocity and the torque (current) mode which is now used in CNC systems without a tach neccessary. This method closes the loop by just the encoder back to the CNC controller.
Early tach servo CNC systems, required the setup first of the velocity loop, before connecting the CNC loop.
Al

[Hobbiest]

Go to the junkyard and get yourself a windshield wiper motor. Runs on 12v DC, usually pretty smooth. There are many different kinds of motors one could use, without even thinking about standard steppers or servos. Could use a motor from a dryer, but kind of on the big side, 115vAC, and spins at 1720RPM. Look around though, and I am sure that you can find plenty of choices. Why not use a variable speed drill motor?

[Hobbiest]

Funny...just saw the other thread with this same question, and couldn't figure out how everybody else copied my answer so quickly (kidding...kidding!)! I guess lunatic minds think alike!

[dlenox]

Hobbiest,

Sorry about the other thread, I felt that this was the more appropriate place for my question...

Good information from all it is greatly appreciated!

That is the great thing about this site - there are a lot of people that are very knowledgeable about a variety of subjects. There is always room to learn from those that have gone before us.