Replacing DC motor with stepper motor

[sacem1]

We have a Heavy Horizontal Boring Mill Column Type which has basically two movements one for the column along the bed the second for the head up and down the column. They are both powered by DC motors which I would like to replace with stepper motors in order to be able to retrofit a PC based CNC control to the HBM.

The only motor availiable is the one on the horizontal displacement of the column wich has the following specs:

Brand: Flexitorq (The Louis Allis Co - Milwaukee, WIS)
Model: 3844531001 - Type: GPNB - Frame: A218 AD
HP: 3 - RPM: 1150 - Volts 240 - Winding: Shunt
FL Amps: 12.6 - Service Factor: 1.0
Field Volts: 240 - Field Amps: 0.64

I assume the vertical motor could have been the same as it seems by the frame conection and that both use the same identical speed reducers (both are still in the machine)

Now my question is how do I determine the size of stepper motor that will be able to replace this two motors?

If you go to stepper motors catalogues they never talk about HP or motor load just motor torque and rpm range, resolution, etc.

Thanks for the help.

Regards

SACEM1

 

[MikeHalloran]

1. They don't come big enough.

2.:
The very first generation of CNC milling machines used stepping motors, to allow relatively simple controls without the complications associated with position feedback.

When they hit a hard spot in a piece of aluminum, or the ways ran dry a little, or the cutter got dull, they would lose a step, destroying a workpiece. (My company used them for cutting big rings out of aluminum plate. A precision ring with a .001" step in it is scrap.)

The solution, the milling machine guys said, was bigger stepping motors. They ran slower. They also lost steps.

Then came hydraulic amplifiers on the stepping motors. They lost steps and gained steps and had other odd problems that were never adequately solved.

A long history says that stepping motors and milling machines don't get along well.

For hobbyist work, the several variations on "stepping motors driven from a printer port" work well enough.

For the sort of expensive workpieces that find their way onto HBMs, you really need something better.

Which does not eliminate the PC as a controller, but it does pretty much rule out the stepping motor.

You can choose to find that all out the hard way, of course, but I suggest that it will be cheaper to just hire a system integrator to retrofit real CNC servo drives to your machine.








Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

I agree, and if you can't find an integrator to do it, there are some very inexpensive self contained servo motors on the market now that may be more along the lines of what you need.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[cswilson]

There are a lot of what I call "stepper-replacement" servo drives out there that accept pulse-and-direction commands like a stepper drive would, but use them to drive a servo motor with real feedback. I know of decent commercially available CNC systems that use these so they can command them directly from a PC.

I agree with the above comments about the inappropriateness of stepper motors for metal removal operations, both because of the size issue and the step-loss issue.

Curt Wilson
Delta Tau Data Systems

 

[sacem1]

I had been wandering why you could not find bigger stepper motors, now thanks to you I know the reason, I thought that the problem was not knowing how to make an equivalent size of motor, but now I see this is not the case.

Ok so no integrators are around here, I am in Lima, Peru and the CNC machines all come ready to use and when anything goes wrong you have to bring a technician from other place which comes for a few days and at a very steep price.

So given my financial reasons I will have to learn how to deal with this things and make them work, so now I need to find the equivalent size in servo drives, will any one give me a hand whith some guidelines.

Thanks

SACEM1

 

[sacem1]

Another idea: would it be useful to stick with the DC motor and use a digital scale as positioning imput to the CNC control, the problem is controlling the machine movement accurately right?

Regards

SACEM1

 

[MikeHalloran]

Given that you've already got a CNC control on the machine, it must have a position feedback of some kind and a really strong servoamplifier on each axis already.

Exactly why are you hacking this machine?

What part is actually broken?



Mike Halloran
Pembroke Pines, FL, USA

 

[sacem1]

Hello Mike:

I bought the machine without any control or CNC or even an ON/OFF switch. The HBM is a very nice strong and incredibly well kept Giddings & Lewis but it seems that early in its life time his controls where lost, broken or stolen, whatever and has been in storage for at least a decade.

So now I have a HBM with main AC motor, X axis (column displacement) DC motor with no control, missing DC motor for Y axis (vertical headstock movement up/down the column) and two auxiliary motors botha AC one for a hidraulic clamping pump and the other for the clamp/unclamp motion of the tooling on the boring bar also withot any controls.

I have contacted the people at G&L but they do not have any electrical layout, drawing or even an operational manual, less a maintenance/repair one, They have furnished us with about 100 drawings covering only the mechanical layout of the components and the operation of a lot of hidraulic positioner that are used to change speeds, or it seems to us, the drawings do not carry notes just outlines of the pieces and numerical codes so descifring them is very time consuming,

So what I want is to rescue this nice piece of machinery from being sent to a junk yard and retrofitting it with a PC based CNC, I already have a supplier for the control, the interface cards and all the electronics involved at a reasonable price from the states but they do not carry motors big enough for this type of machine, and that's where you guys come into the picture, so here I am working the foundations to install the HBM and studying the mechanical part drawings (even if all seems to be in a very nice well kept condition)

Please comment

SACEM1
Marcelo Cabello
Lima, PERU

 

[MikeHalloran]

Ah.. pre- hacked.

So, now you need linear sensors, which have gotten much cheaper since that machine was built, and serious servo amplifiers.

The only problem I have with the integrated servomotors is that backlash compensation may prove difficult. Or there may be a way; I can't say I'm intimately familiar with them.

I suggest you keep shopping, and not actually order the controls until you have a better idea of how the control output will move the machine, and how the control will know how far the machine moved. Maybe the prospective control supplier will work with you; this can't be the first time someone has tried to put their controls on a serious machine.




Mike Halloran
Pembroke Pines, FL, USA

 

[sacem1]

Well I mfollowed your sugestion and have found a CNC cupplier which has experience in the same type and brand of HBM so now we are in the process of finding with them the best servos and amplifiers for the machine.

And you are right the backlash compensation with 2 axis working on rack and pinion movements is somewhat tricky, the pieces involved are not worn out because this HBM in particular was property of the USAF (just found out) and has seen really very little work done on it.

Still the question is how big should the motors be,

In the in-out movement of the spindle this machine used a plain 1/2 HP 1710 rpm motor hooked to a worm drive and the positioning was done by a set of clutches that conected the movement one way or the other, I think that has to be ripped off and replaced by a servo mottor with feed back to the CNC computer, but for heavy boring the original setup disconected the 1/2 HP drive and conected another clutch to the spindle main motor (20 HP at 1725 rpm) so maybe if we only conect the small motor we will end with a mamouth machine that is only capable of very light cuts.

To use one or the other thats the question.

Cheers

SACEM1

 

[BobM3]

I'm guessing the worm drives were self-locking (can't backdrive them). If so, their efficiency will be real low (probably <60%).

So see if they can be backdriven and if they can't make a conservative estimate that the gearboxes are no greater than maybe 70% efficient.