More Precision, Faster Setup for Die Grinding Machine

In the world of can manufacturing, the quality of the dies that shape each can directly impacts manufacturing quality and productivity. A die ground to ultra-precise tolerances will last longer and produce fewer tear offs, where the can literally rips or tears during forming. Addressing the demand for more precise, longer-lasting dies, Pride Engineering of Minneapolis, MN, developed the Model 3...

10/20/2008


In the world of can manufacturing, the quality of the dies that shape each can directly impacts manufacturing quality and productivity. A die ground to ultra-precise tolerances will last longer and produce fewer tear offs, where the can literally rips or tears during forming.

Addressing the demand for more precise, longer-lasting dies, Pride Engineering of Minneapolis, MN, developed the Model 30 Air Bearing Grinder, a five-axis air bearing carbide die grinder that sets new standards for grinding accuracy, ease of use and fast, efficient grinder setup and changeover.

For can makers, it’s the type of tool that can dramatically enhance manufacturing and toolroom efficiency. To help meet these goals, Pride Engineering built the Model 30 with Bosch Rexroth IndraMotion MTX CNC controller and drive platform.

Precision to “Die” For

Pride Engineering is one of the leading manufacturers supplying the two-piece can industry with equipment such as air bearing grinders. The company serves can manufacturers in North and South America, Asia, and Europe.

The Model 30 is one of Pride Engineering’s most advanced offerings, built to satisfy a growing need by can manufacturers for faster die production, reduced setup time, and more productive toolroom operation.

Integrated motion and logic support the Model 30

Integrated motion and logic support the Model 30

Integrated motion and logic support the Model 30 "Pride Touch" control feature. Source: Bosch Rexroth

Can manufacturing is a high-throughput industry with conditions that can be quite 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 rates vary from 400 to 600 cans per minute.

Minute imperfections in the die’s roundness, inside diameter, or other specifications can cause excess friction and uneven wear, which may lead to cans whose shape or diameter is out of tolerance, or cause a problem called “tear offs.” Dies can be used for four to five days of round-the-clock production before re-grinding.

Previous versions of the grinder used manual setup and changeover procedures. Operators had to carefully center the die and find “zero” before grinding can begin. During the Model 30’s development, engineers at Pride concluded they needed a new CNC controller to achieve their targeted levels of accuracy, reliability and ease of use. Working with local Rexroth distributor Motion Tech Automation, Pride chose to use the Rexroth IndraMotion MTX motion system.

The system is a CNC machine tool 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 controller supports up to 64 axes of motion and 12 independent CNC channels. It improves machine tool performance with short CNC cycle times and minimum PLC program processing times, helping tool builders achieve reliable high-speed machining and reduce non-productive setup and changeover times. It also uses an open architecture with standard interfaces such as Profibus, SERCOS, and Ethernet.

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 IEC 61131-3 compliant integrated motion and logic.

Model 30 can grind dies to an accuracy of 15 micro-inches (.38 mm) total indicated runout. During grinding, the die is held in the workhead, which can be pivoted in three axes while rotating the die 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. The workhead must be positioned with micrometer accuracy over the pivot point to control the contour of the cut.

The operator fills in blank fields, and the MTX control locates the correct pivot point and grinds the die to specification. Source: Bosch Rexroth

The operator fills in blank fields, and the MTX control locates the correct pivot point and grinds the die to specification. Source: Bosch Rexroth

Faster, easier, more precise

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.

The interface cuts die turnaround time. From machine setup to die changeover, the platform provides a fast, automated, menu-driven process, so tool operators are able to handle a variety of tasks.

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

Next, the operator loads the die and 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.

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, a key feature that takes advantage of the distributed architecture, enables the grinder to automatically find zero and start the grinding cycle in approximately 15 sec.

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 signals the controller to start the grinding cycle. Controlled by a custom drive-based PLC technology function, the process takes advantage of the faster update times, for faster, more precise measurement of grinding-wheel torque.

The grind cycle runs automatically with no need for an operator to supervise the tool. When it is complete, a blinking light tells the operator to return to the machine.

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

Improved contour control

The motion platform on the Model 30 also includes Rexroth IndraDyn S MSK motors for the x , y and z linear motion axes, and ball screw units for smooth, reliable, precise motion. In addition, a high-performance torque motor is used for the tool’s rotary axis which controls the accuracy of the ground radius.

The frameless torque motor drives the Model 30

The frameless torque motor drives the Model 30's rotor directly, for a highly accurate radius. Source: Bosch Rexroth

The torque motor 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 said. “The frameless torque motor provides the most accurate radius possible, repeating to an accuracy of

Rexam PLC is the world’s leading manufacturer of beverage cans, producing more than 54 billion cans each year for Europe, the U.S., and South America. At the company’s Northfield plant in Buckinghamshire, UK, the die grinding system was upgraded with a Pride Model 30.

This plant has three aluminum can production lines that produce in excess of five 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 die-grinding operator Keith Berryman. Currently, the Northfield plant has been producing record numbers of cans, yet their tool usage has gone down.

“Previously, we had to grind 30 or more dies a day,” said Berryman. “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.”

“In the past,” he continues, “I might have to grind two or three dies to get one good one,” he said. “With this machine, I know that once I’ve set the grinding parameters, it will hold the size I want. I’m impressed with how accurate the machine is, and how much time it gives me to carry out other tasks, because it grinds everything true.”

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

“The Model 30 is an air bearing grinder, which is self-centering. There’s no clocking or truing of the die at the start,” he said, “and it grinds the die automatically, in about six minutes. It’s a great improvement over what we used to have.”

Since die grinding is now automated, Berryman can step away from the machine and handle other tasks without have to interrupt and slow down the die grinding process.


Author Information

Karl Rapp is machine tool branch manager with Bosch Rexroth Corporation—Electric Drives and Controls. Contact him at karl.rapp@boschrexroth-us.com .




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