Ballscrews and CNC conversions

Jim Stewart · 2004-02-10T18:02:47+00:00

It seems to be a given that a mill needs ballscrewsfor a CNC conversion. I'm wondering why. Is itto eliminate backlash or to make the machine crankeasier? I have a knee mill and I was thinking ofthe following modifications in this order: fitDRO scales and readout, Fit DC motors to X,Y, andquill, close the loop with a microcontroller.Why would I need to use ballscrews in this application?The system would be closed-loop and the backlashwould be compensated.

Feb 10, 2004 23 Replies

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Ned Simmons 22 years ago

It's a very big problem if you're trying to optimize the response and stiffness of the system. Whenever you reverse the direction of the servo thru the backlash it will be momentarily uncoupled from the load. One case where you

*can* often ignore backlash is with a suspended or continuously overhauling load.

The backlash may not be a problem if you're willing to sacrifice performance, or if the reflected inertia of the load is small compared to motor's inertia. Sometimes these tradeoffs are justified, sometimes they can't be tolerated. A system where very short fast moves, high accelerations, and short settling times are necessary would be an example where minimizing backlash and maximizing stiffness are required.

I know, I've designed, installed and tuned at least 15 axes of servos on 3 different pieces of equipment over the last

2 years or so, and more than I can remember over the past

Ned Simmons

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Richard J Kinch 22 years ago

Yes. This is part of the tradeoff between keeping the stock acme screws for a CNC conversion, or converting to ballscrews. You can stabilize the controls on the backlashed acmes, but at a significant performance penalty, on top of the performance penalty for the acme screw mechanical inefficiency. It is not the case that backlash necessarily involve instability.

So if your goal is to convert a manual machine to "basic CNC capability at lowest possible up-front cost", then acme screws can be made to do the job, but if your goal is "CNC at best performance per dollar", then the more expensive ballscrews can be ultimately cheaper.

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Ned Simmons 22 years ago

Glad we're on the same page now.

Absolutely. As often as not there's no reason to spend lots of money on low inertia motors, low backlash reducers, expensive bellows couplings and preloaded ball screws. Sometimes a cheap DC motor, an Oldham coupling, and a timing belt is all that's required. The trick is knowing when you need to cough up the big bucks and when you don't.

Ned Simmons

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