July 2008 Edition

grinding

Precision to ‘die’ for

CNC and interface package help boost beverage can productivity

In the world of beverage can manufacturing, it’s all about the dies.

The higher the quality of the dies that shape each can, the better the manufacturing quality and productivity.

A Minneapolis company with the help of a Bosch Rexroth IndraMotion MTX CNC controller and drive platform sought a way to build a better die grinder — and along the way set new standards for grinding accuracy, ease of use, along with the benefits of fast setup and changeover.

Pride Engineering developed the Model 30 Air Bearing Grinder, a five-axis air bearing carbide die grinder to produce more precise, longer-lasting dies. The grinder is capable of grinding dies to ultra-precise tolerances to last longer, with fewer tear-offs.

Punishing conditions

Beverage can manufacturing is a high-throughput industry with conditions that can often be punishing on production machines and tools.

Aluminum or steel "cups" are drawn out of sheet metal, then "ironed" into their final shape by being pressed under extreme pressure through a set of tungsten carbide dies, called a toolpack. 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. As a result, can shape or diameter could be off-specification or the can could cause a problem called tear-offs where it literally rips or tears as it passes through the toolpack.

Dies can be used for about four to five days of 24/7 production before normal wear requires that they be pulled and re-ground.

"There’s a constant industry demand for tighter tolerances and higher quality die finishes," says Bruce Allyn, vice president at Pride Engineering. "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 carefully center the die and find "zero" before grinding began. During the Model 30’s development, engineers at Pride concluded that they needed a new CNC controller to achieve targets of accuracy, reliability, and ease of use.

Working with Rexroth distributor Motion Tech Automation, Pride found the automated CNC functionality it needed in the Rexroth IndraMotion MTX.

The IndraMotion MTX system is a machine tool CNC for grinding and forming operations. It contains all the components — drives, controller, operator software, and a powerful engineering framework — optimized for machine tool automation.

The system supports up to 64 axes of motion and 12 independent CNC channels. It is designed to improve machine tool performance with the shortest CNC cycle times and minimum programmable logic controller (PLC) program processing times. It also uses an open architecture with standard interfaces such as Profibus, SERCOS, Ethernet, and others.

Pride’s Model 30 also utilizes Rexroth’s IndraDrive family of intelligent drives, offering drive-based precision loop closure and compensation functions, and optional features such as integrated motion and logic.

Improved accuracy

In the Model 30, the air bearing workhead and wheelhead spindles are mounted together on a granite surface plate that floats on air, isolating the work from high and low frequency vibrations. Dies can be ground to an accuracy of 15 millionths of an inch (.38mm) total indicated contour (TIR).

During grinding, the die is held in the workhead. This can be pivoted in three axes while rotating the die at precise speeds against a separate grinding workhead. Pivoting the workhead offers different grinding angles to achieve each die’s precise specifications. The workhead must be positioned with micrometer accuracy over the pivot point to control the contour of the cut.

As the die’s diameter or other specifications change, the MTX automatically re-positions the grinding pivot point with cutting-edge accuracy. Automated advance or feed of the die can be made in one-micron increments.

With Pride’s previous controller, the smallest initial feed movement it could handle was three microns, which also required multiple commands — and was insufficient to the precise motion needs of today.

Faster grinding

The IndraMotion MTX interface reduces die turnaround time, always a crucial need in the industry.

Using the 12-inch Rexroth IndraControl human machine interface (HMI), the operator selects the die to run. The operator also enters specifications such as the tooling ball, land width, and ring thickness. Pride developed the custom HMI screens using Rexroth’s WinStudio visualization software.

Next, the operator loads the die and then selects the "cycle start" button to begin operation. The controller automatically moves the X- and Y-axes to the correct position to maintain the precise pivot point throughout the die grinding.

A feature of the Model 30 called Pride Touchâ was developed with cooperation from Rexroth’s applications engineers, and takes advantage of the IndraDrive’s distributed architecture.

On earlier machines, it typically took two to three minutes to set the die on the chuck and find the Z-axis zero. Pride Touch enables the grinder to automatically find zero and start the grinding cycle, all in about 15 seconds.

The Z-axis moves the grinding wheel at a faster feed rate until it finds the die, minimizing 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 Programmable Logic Controller (PLC) technology function, the process takes advantage of the faster update times, for faster, more precise measurement of grinding wheel torque.

The grind cycle is automatic. When it is complete, a blinking light tells the operator to return to the machine, since there’s no need for an operator to supervise the tool.

"A grinder is only profitable when it is grinding, not when it’s positioning," Allyn says. "The less time spent setting up, the more time spent producing."

The motion platform on the Pride Engineering Model 30 also includes Rexroth IndraDyn S MSK motors for the X-, Y- and Z-linear motion axes, and Rexroth Ball Screw units for smooth, precise motion. In addition, a high-performance Rexroth IndraDyn T torque motor is used for the tool’s rotary axis that controls the ground radius accuracy.

The IndraDyn T powers the rotary axis directly, with no gears, couplings, or joints, thereby eliminating mechanical elements to help ensure extremely smooth rotation and control.

"Controlling the radius of a redraw die is crucial," Allyn says. "The frameless torque motor provides the most accurate radius possible in the world, repeating to an accuracy of ±0.077 arc seconds."

More cans, fewer dies

Rexam PLC produces more than 54 billion beverage cans each year for Europe, the United States, and South America. The company’s Northfield plant in Buckinghamshire, UK, recently upgraded its die grinding system with the Pride Model 30.

This plant has three aluminum can production lines that produce over 5 million cans a day. At those production levels, the plant’s machine tool shop needs to grind 20 new dies a day, on average.

The Model 30 improved die grinding productivity, as well as the quality and working life of the dies, according to one of the plant’s die grinding operators, Keith Berryman. Currently, the Northfield plant has been producing record numbers of cans, yet their tool usage has gone down, attributed to the Model 30.

"Previously, we had to grind 30 or more dies a day," Berryman says. "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, the dies are lasting longer."

Die grinding is much more accurate and reliable with the Model 30, Berryman says.

"In the past, I might have to grind two or three dies to get one good one," he says. "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."

Before Rexam began using the Model 30, die grinding was a manual operation at Northfield, Berryman says. Any time he needed to step away and take care of other duties, grinding stopped until he returned. Now he’s much more productive.

"With the Model 30, it’s an air bearing grinder, which is self-centering — there’s no clocking or truing of the die at the start," he says. "And the Rexroth CNC system grinds the die automatically, in about six minutes. It’s a great improvement on what we used to have."

Berryman also says the Windows-based user interface, supported by the IndraMotion MTX, was easy to use, and has made him much more productive.

Bosch Rexroth

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