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CAD/CAM optimizes punch-press tool performance.

CAD/CAM optimizes punch-press tool performance

In many areas of metalworking, the machine-tool builder who develops a new design also must make special tools to meet requirements of the new machine. In special situations, the builder will seek outside toolmakers to assist in the task. This results in asking competitors to become allies, but the builder is happy when his tooling goals are met and his customers get higher productivity--making the machines look good!

When the makers of CNC turret punch presses bid for a quantum leap in tool design, it was Mate Punch & Die Co, Anoka, MN who answered the call. The firm's engineers developed the Marathon line of tools that can work in existing turret systems. Tool-design goals included:

* Long life for lower workpiece costs.

* Precision performance to satisfy today's close-tolerance specifications.

* High-speed capability.

* Quick-change capability for reduced downtime.

* High productivity for efficient shop operation.

Some of these lofty goals appear to be in conflict, but, as with other products in the '70s and '80s, engineers worked with computers to solve the trade-off puzzle and exceeded performance objectives. Engineers created a modular closed cutting system, while computers organized details such as shape and size of components. The man-computer team thus developed the Marathon line of turret-punch-press tools.

Computers didn't stop with development of the new product, however. Today, they continue to serve through computer-aided design (CAD) of customers' particular tooling requirements.

Unlocking the past

Original toling for NC turret punch presses was designed around 1957, when Wiedemann introduced numerical control. The same style tools serve many of today's much-faster machines, but Mate designers have found that such tooling is obsolete for many of today's applications.

The older tooling was designed for manual machines and early NC units that had a top speed of 60 hits/min (hpm). Incremental gains from new materials, higher tolerances, better holders, and improved toolholder systems have helped, but as speeds increased to 100, 150, 200, and 250 hpm, with nibbling at 450 hpm, the improvements have not kept pace with punch-press design.

For one thing, changeover is too cumbersome. To change punches and dies with conventional tooling, an operator must unbolt the tool, pull it in to square it on it's side (0 deg and 90 deg positioning), drop it into the turret, and, finally, manually key it in fox X and Y positioning. Furthermore, he must deal with different holders: one to turn on an angle, and one for length adjustment.

Marathon combines these functions in one tool, so the operator can set both angle and length right on the tool before it goes in the turret. Line-up is better because he can choose various tool sizes as needed.

"Most turret-punch-press tooling is carried in a soft holder that lacks needed precision," says Dean Sundquist, managing director of Mate. "That's why we build precision into the tool system itself. We use a keying system that keys through the stripper plate--as far out from the point as possible. This helps compensate for errors. It can key in eight positions, so at any time we can choose another pin position to get any angle with the same tool. The angle is not slaved to the tool, but rather to the stripper plate itself."

Closed cutting system

With conventional tools, punches can "skid" sideways during the punching cycle, which causes additional tool stress that greatly reduces tool life and limits application. Also, rubbing on the OD of the holder in the ID of the turret can result in wear and loss of accuracy.

Marathon tools, on the other hand, are each configured in a complete self-contained assembly, inside a hardened-and-ground holder. Guiding of the punch in the die is controlled at three points by the tool itself, regardless of any instability of the punching machine. Furthermore, the design eliminates the high-pressure rubbing on the tureet by the punch holder.

In this tool system, the ram effectively strokes inside, and the OD is static. Thus, all punch sliding is on the inside, guided over the full length of travel for each stroke. The tool absorbs all of the sideways radial force internally, so therehs virtually no wear on the turret--no rubbing--just up and down motion.

Because the closed cutting system is more stable than the conventional open system, there is less inpact each time the ram hits. Thus, the design can take advantage of harder, longer-life high-speed steel such as M-2. The controlled conditions of the newer machines (turret punch presses built in the last 10 years) also facilitate application of precision tooling.

Stripping close

It's important to keep the stripping mechanism close to the cutting edge of the tool. The closer the fit, the cleaner the punched hole. Many conventional tools employ urethane stripping material. It keeps stock flat, but it's perishable and wears out relatively quickly. You can get it close to he cutting edge, but you may not always get exact keying or matching to the cutting edge. You may not get clean cuts every time.

Mate uses built-in springs in the stripper system to provide long life--several hundred thousand hits. And, the stripper has precision fit--a fit not practical till wire EDM came along, according to Mate. This provides perfect guiding right at the cutting edge.

Flatter sheets during punching improve hole accuracy, but the workpiece itself puts its own limitations on tolerance. Although the punching machine and tools provide [+ or -] 0.002" repeatability, the sheet stock can grow so much that you can't hold that precision in some products.

The stripper plates are self-locking for quick change without wrenches, and the overall self-contained construction of the tools means there are no loose parts subject to loss. Jun most setups, the tool is self-oiling. It captures oil from the surroundings and pulls it down into the internal components.

Slug ejection

Slug pulling is a nagging problem for all NC punch-press operators. A pulled slug will stick between the top of the die and the workpiece and interfere with the next hit. Before punching can be stopped, the loose slug can ruin the punch, die, and workpiece. To alleviate this problem, Mate developed the Slug Free[TM] die.

Slug Free dies are double-tapered to eliminate upward movement of the slug. The punch squeezes the slug through a constriction that acts as a one-way door. The constriction or pressure point is the intersection of the two tapers. The entry taper (constricting) reduces in diameter to the pressure point. The exit taper (relief) begins at the pressure point and increases in diameter to the bottom of the die. This allows the slug, once it passes the pressure point, to fall a way without jamming.

The constricting taper is very slight, allowing the die to have a 0.125" sharpening life without significantly changing die clearance. Because the Slug Free system uses the punch stroke to clear slugs, stroke length is a critical operating variable. A die penetration of at least 0.100" is required to push the slug past the pressure point.

Much of the Mate product line is custom made. Here's where the CAD/CAM system really pays off, working with wire EDM. In fact, the wire electrode is said to be the perfect cutting tool. It requires no wear compensation, and it's performance is set by electrical parameters under strict computer control.

Mate has found that four wire EDM machines can replace 12 small NC mills. EDM is not fast, but it offers precision, repeatability, and overall predictability--essential factors in building good punches and dies. At present, the firm uses 10 Charmilles Robofil 200 machines.

Computers, which ultimately feed the EDM equipment, are tied together in a network system. CAD/CAM programs work through a mini main-frame that services various workstations throughout the facility. CAD programs on the workstations feed to the main frame for storage, then to the wire EDM and other machine tools.

A single program designs a Marathon punch, die, and stripper. Most of the parameters are already built in, and there are many subroutines available from canned programs to help speed the rest of the tool-design process. For instance, there is a Louver Tool program that does most of the thinking and calculation for louver tooling. The designer simply enters overall size and louver angles. The software does the rest.

Also, the designer can call up other subroutines to generate standard shapes. Some 40 percent of incoming orders are for standard punches and dies, and 60 percent are for custom-made sizes, shapes, and assemblies. In all cases, the use of the CAD/CAM system helps save design time and sets the stage for statistical process control of tool-manufacturing operations.

What of the future? Machine designs are gettting even better, and you might ask when we'll need the next round of improvement in punch tools. "Probably not for a while," according to Jack Schneider, chief operating officer of Mate. "There seems to be a practical speed limit. Ram speed is no longer the limiting factor; table speed and acceleration are the true limits. Significant productivity gains will come in the form of control: control of machine and control of the punching process. We expect that our new tooling systems will operate very well in this new environment. While we continue to improve our product, we don't expect to make any major design changes in the next 10 years."

For more information, contact Mate Punch & Die Co, PO Box 728, Anoka, MN 55303.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Miller, Paul C.
Publication:Tooling & Production
Date:Sep 1, 1989
Words:1572
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