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A level of precision to 'die' for

01 March 2009

The dimensional precision of dies that shape a beverage can has a direct impact on manufacturing quality and productivity. We find out how a CNC controller and drive platform has helped machine builder, Pride Engineering produce a die grinder that sets a new standard for accuracy and productivity in this field

Minneapolis based Pride Engineering supplies equipment such as air bearing grinders to the two-piece can industry, serving manufacturers in North and South America, Asia and Europe. Its Model 30 air bearing grinder, for example, satisfies the industry’s demand for faster die production, reduced setup time, and more productive tool room operation.

Can manufacturing is a high-throughput industry with conditions that can be quite punishing on production machines and tools. Aluminium or steel ‘cups’ are drawn out of sheet metal, then ‘ironed’ into their final shape by pressing under extreme pressure through a set of tungsten carbide dies, called a tool pack. Typical production is 400 to 600 cans per minute.

Minute imperfections in the die’s roundness, inside diameter, or other areas can cause excess friction and uneven wear, resulting in cans whose shape or diameter is off-specification, or problems such as ‘tear off’ where the can literally rips or tears as it passes through the tool pack. Dies normally have to be reground every four to five days to avoid these problems. As Pride’s vice president, Bruce Allyn explains.

“There’s a constant industry demand for tighter tolerances and higher quality die finishes. The Model 30 produces dies ground to extremely precise tolerances, for the closest thing to perfect roundness that’s physically possible.”

Previous versions of the grinder used manual setup and changeover procedures. Operators had to centre the die and find ‘zero’ before grinding could begin. During the Model 30’s development, engineers at Pride concluded that they needed a new CNC controller to achieve their targeted levels of accuracy and ease of use.

Working with local Rexroth sales partner, Motion Tech Automation, Pride decided on the IndraMotion MTX - a high-productivity machine tool CNC package for grinding and forming operations, comprising drives, controller and operator software, all optimised for machine tool automation. This open-architecture platform supports up to 64 axes of motion and 12 independent, short-cycle CNC channels. It provides fast PLC functionality and standard interfaces such as Profibus, SERCOS and Ethernet.

Model 30 includes Rexroth’s scalable IndraDrive family, offering drive-based position loop closure, various compensation functions, and optional features such as IEC 61131-3 compliant integrated motion logic, together with a range of high-performance servo (linear and rotary) and spindle motors.

Improved accuracy
The Model 30 air bearing workhead and wheelhead spindles are mounted together rigidly on a granite surface plate that floats on air, isolating the work from high and low frequency vibrations. During grinding, the die is held in the workhead, which can be pivoted in three axes while rotating it at precisely defined speeds against a separate grinding workhead. Pivoting the workhead, it grinds the different angles needed to achieve each die’s precise specifications. Dies can be ground to sub-micron accuracy with this system.

As the die’s diameter or other specifications change, the MTX automatically re-positions the grinding pivot point with great accuracy, automatic die advance or feed being achieved in one-micron increments. With Pride’s previous controller, the smallest initial feed movement it could handle was three microns, and that also required multiple commands.

A 12in IndraControl VPP16 HMI enables the correct die to be selected and allows the operator to enter specifications such as the tooling ball, land width and ring thickness. Pride developed the custom HMI screens using Rexroth’s WinStudio visualisation software. Once the die is loaded, the operator selects ‘cycle start’ to begin operation. The controller automatically moves the X and Y axes to the correct position to maintain the precise pivot point throughout the grinding process.

A key feature of the Model 30, ‘Pride Touch’, was developed in close cooperation with Bosch Rexroth, and takes advantage of IndraDrive’s distributed architecture. On earlier machines, it took typically two to three minutes to set the die on the chuck and find the Z-axis zero. Pride Touch enables the grinder to find zero automatically and start the grinding cycle, all in approximately 15 seconds. The Z axis moves the grinding wheel at a faster feed rate until it finds the die, minimising non-grinding time. Once the grinding wheel touches the die, a high-speed input is sent to the MTX controller to start the grinding cycle.

Controlled by a custom drive-based PLC technology function, the process takes advantage of the fast update times, for quicker, more precise measurement of the grinding wheel torque. The grind cycle is automatic and does not require operator supervision; when it is complete, a flashing light signals the operator to return to the machine.

The Model 30 motion platform includes Rexroth IndraDyn-S MSK motors for the X, Y and Z linear motion axes, driven via Rexroth ball screw units. In addition, a Rexroth IndraDyn-T frame-less torque motor drives the tool’s all-important rotary axis (which determines the die radius) directly. Eliminating mechanical elements from this drive path, such as gears, couplings, or joints, ensures smooth rotation and control - and hence high accuracy.

More cans - fewer dies
Rexam produces more than 54 billion beverage cans a year for customers in Europe, the U.S. and South America. The company recently installed a Pride Model 30 at its Northfield plant in Buckinghamshire, which is capable of producing in excess of five million aluminium cans a day.

The Model 30 improved die grinding productivity, as well as the quality and working life of the dies, according to Keith Berryman, one of the plant’s die grinding operators. The Northfield plant is currently producing record numbers of cans, yet its tool usage has actually decreased, since the Model 30 was installed, as Mr Berryman explains.

“Previously, we had to grind 30 or more dies a day; now, we’re under 20 dies a day. Since the machine grinds every die exactly the same, and every die is much closer to the specification, they last longer. In the past, I might have had to grind two or three dies to get one good one. With this machine, I know that once I’ve set the grinding parameters, it will hold the size I want, and I can produce dies much faster. I’m very impressed with how accurate it is, and how much time it gives me to carry out other tasks, because die after die, it grinds everything true.”

Mr Berryman also favours the Windows-based user interface, supported by Rexroth’s IndraMotion MTX. This was easy to learn and even easier to use, he says, and it has made him much more productive. Since die grinding is now fully automated, he can step away from the machine and see to other tasks without have to interrupt and slow down the process.

- Misplaced loyalty or common sense?

It wasn’t that long ago when we all thought tying ourselves to a single supplier was not the way forward, writes Les Hunt. The push for ‘open standard’ as opposed to proprietary’ fieldbuses, for example, was supposed to free us all up to choose disparate products from multiple vendors, confident that they would all work harmoniously once connected together.

In many ways, while certain issues have yet to be addressed, that philosophy for the machine automation market remains intact. But earlier this year, Bosch Rexroth’s Trevor Osborne and Stuart Williams made a very strong case to machine builders for returning to ‘brand loyalty’. Not to lock the machine builder into a single-supplier agreement, so much as to help them simplify machine design and build and thus speed time to market, reduce overall cost of ownership and minimise risk.

Rexroth has marked the start of 2009 by launching a strategy that takes full advantage of its wide technology offering. The company claims that a reduced supplier base translates to lower procurement and administration costs, and limits the complexity of warranty issues when things go wrong and there is no agreement as to who is culpable. With a single supplier, there’s just one point of contact and resolution is that much quicker. Moreover, a reduced supplier base means less time attending meetings and company pitches, and you can be sure that key components will ‘interoperate’ from the word go.

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