Digital servo drive improves cutting efficiency

By equipping a high speed cut-to-length machine with an advanced digital servo drive, Baldor has doubled its accuracy, and produced an efficiency improvement

By equipping a high speed cut-to-length machine with an advanced digital servo drive, Baldor has doubled its accuracy.

Since most manufacturers of downstream cutting machines - and their customers - regard cutting accuracy as the single most important parameter, the improvement is highly significant and likely to lead to increased machine sales.

Manufacturers of cut-to-length products nowadays work to very precise dimensions, to avoid raw material wastage and maximise profit.

As a consequence, they demand increasingly higher levels of accuracy from cutting machinery.

Even small errors, over time, can account for significant wastage.

A typical example is the manufacture of windshield wiper blades, where the rubber extrusion now needs to be cut to an accuracy of +/-0.02in (+/-0.5mm) - anything outside this range is likely to be deemed unacceptable.

Even machines handling relatively bulky materials such as plastic drain pipes are expected to achieve a cut-to-length accuracy in the order of +/-0.04in (+/-1mm) or less.

The machine that Baldor upgraded is designed to cut plastic tube extrusion into 2 inch (50.8mm) lengths, at a rate of more than 300 feet (91m) per minute.

It is produced by GF Goodman and Son, a leading manufacturer of downstream cutting machines based in Ivyland, Pennsylvania.

Founded in 1922, this company has grown to become a major OEM of high speed cut-to-length machines for extrusion lines; its products include a wide range of rubber and plastic extrusion cutting systems, as well as rotary shears and slitters, and hydraulic guillotine bale cutters.

GF Goodman asked Baldor to investigate whether the accuracy of its plastic tube cutting machines could be improved without major redesign.

These machines are installed downstream of extrusion systems, forming one of the final stages of fully-automated production lines.

Goodman's current range of cutting machines employ an earlier-generation brushless servo drive and motor to control a rotary knife head.

The accuracy of the cut depends primarily on the response of the drive.

To maximise accuracy, the drive needs to move the knife at a consistent rate, and stop as repeatably as possible with no overshoot.

Goodman was typically achieving a figure of approximately 200microseconds with its existing servo systems.

However, even obtaining this level of performance was time-consuming, because the servo drive used analogue potentiometers for tuning purposes, which only provide limited resolution and offer no confirmation of performance.

Compared to the earlier servo used on Goodman's existing cutting machines, the Baldor FlexDrive model chosen as its replacement offers much faster I/O and servo loop update response times, and considerably more control over operating parameters such as stiffness, dither and overshoot.

The software also features auto-tuning wizards and a Windows-based graphical user interface to help minimise set-up time, together with an oscilloscope facility which provides engineers with instant visual feedback of the effect of any changes to tuning parameters.

This particular feature proved key to the cut-to-length application, enabling the performance of the entire servo drive and motor system to be optimised very quickly and easily, halving the response time from 200us to just 100us.

According to Duke Davis, Managing Director of GF Goodman, "This particular project has proved advantageous for everyone.

We have doubled the cutting accuracy, to the obvious benefit of our customers.

And because the upgrade is cost-effective and relatively simple to apply, we are now incorporating FlexDrive servo drives across our entire range of high speed cut-to-length machines." Over the years, GF Goodman's downstream cutting equipment has steadily increased in performance and sophistication.

While early-generation machines used simple mechanical or vacuum-operated clutch brakes, for example, today's high speed machines nearly all use servos.

This situation is compounded by the fact that most extrusion lines are continuous-flow processes - once the various resins have been fed into hoppers and the melt heaters and extrusion dies are up to temperature, production continues until the end of a batch.

Any downstream equipment failure has an immediate effect, making it crucial to minimise system downtime.

Duke Davis points out that this is one of the reasons that Goodman selected Baldor servos in the first place: "Baldor's main manufacturing facility in Fort Smith, Arkansas, operates a highly responsive exchange system, helping us to accelerate our customer support services.".

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