Q: I work in a stamping facility in the Midwest. We have 40 high-speed stamping presses from 20 to 80 tons, with maximum speed of 800 strokes per minute (SPM). The actual speed at which the press should run is a constant battle between the toolroom supervisor and the production manager. The toolroom supervisor prefers the presses to run slower, claiming they "run better" at lower speeds. The production manager obviously wants to increase press speed and throughput. Any advice on how to balance speed versus maintenance?

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A: I understand exactly the toolroom's concerns. If a punch breaks while running a die at 200 SPM, it generally does less damage than if the same punch breaks while running at 800 SPM. The safety device that detects the punch break can stop the press more effectively at lower speeds than at higher speeds, preventing the press from making another hit. In fact, because of the inertia of the ram at very high speed, the press could make two more damaging hits even after the stop circuit is activated.

Another problem with running at high speed is that it makes setting the raw material feed more difficult. The feed has to be tweaked slightly as speeds increase because of the inertia of the raw stock slipping through the feed rollers. At 800 SPM, raw material is fed more than 13 times per second!

Raw material payoffs, the straightener, and other auxiliary equipment used to support the production line must be set up and operated precisely, and this is simply more difficult at 800 SPM than at 200 SPM.

Another reason the toolroom tends to run a stamping line more slowly is to compensate for nonrobust tool construction. You can maximize speed and accuracy only when using robust tooling. Design extra-thick die shoes top and bottom, keep the top weight to a minimum to reduce inertia when stopping, and install large roller guide pins, thick internal die plates, large surface area down stops, and a thick press bolster, with scrap and openings under the die minimized.

I have seen a 3-foot-long die with 2.0-in.-thick top and bottom die shoes, using four 1-in.-diameter guide pins on each corner, working 2.7-in.-wide by 0.045-in.-thick 1/2 hard stainless steel with oversized openings in the bolster so the working area of the die is not supported. A tool build and press setup like this cannot support the stamping without the tooling flexing and moving. All of these issues combine to make a recipe for production variation.

Running the press more slowly just makes it easier to run nonrobust tooling and presses that may have too much play in the guides and crankshaft.

But let's assume the equipment is in good shape. When designing a progressive die build, be sure to follow these guidelines:

• Minimize strip lift. If the tool requires lifters, make sure they are as short as possible. This directly affects press stroke.
• Minimize pitch or feed length. The raw material is stationary at press bottom when stamping. As the press opens, the rollers need to begin feeding. The raw material has inertia and resists acceleration. If your feed window is one second, feeding 1 in. of material per second requires feeding at a speed 10 times greater than feeding 1/10 in. per second. The slower the feed (in inches per second), the less possibility of slippage.
• Minimize the press stroke. Press ram speed is not linear. The ram moves fastest at the 9 and 3 o'clock positions and comes to a stop at the 12 and 6 o'clock positions. If you reduce the ram stroke from 1.0 in. to 0.5 in., you can double the press SPM without changing ram speed.
• Make part ejection robust. Get the parts out of the die as soon as possible. For the most robust part ejection, blank the parts through the die. Removing parts on a carrier strip is the second best option. Blowing the parts out of the die is the slowest and least robust method.

When all is said and done, you always want to maximize the speed at which you run your stamping dies. Maximizing speed primarily starts in the design phase.

Here's Why Stampers Should Use the Shortest Possible Stroke Length

The influence of stroke length on metal-stamping productivity generally is not well understood on the plant floor. Press operators often will say, "This job can only run in this press," or "We can't run this job as fast as it should," without being able to explain why. While many reasons exist, most often the blame lies in the integrity of the press and its stroke length.

Many stamping operations occur under less-than-ideal conditions, and the impact of stroke length often is overlooked. Four reasons why metal stampers should strive to use the shortest possible stroke:

• Reduce wasted time. Unnecessary ram movement wastes time and energy. A shorter stroke reduces thermal losses and guide wear.

• Minimize dynamic forces in the press structure. Moving the ram and the upper die down and up over a shorter distance in a given amount of time reduces the harmful forces within the press frame and drive system.

• Better forming. Forming time is inversely related to stroke length. Time allocated to shaping the sheet metal is most critical and allowing more time for forming yields better results and a more stable process. As seen in Fig. 1, the ram-position curve for a 40-mm stroke (orange) intersects the material thickness line at 158 degrees, while the curve for the 20-mm stroke (yellow) intersects this line at 148 degrees. This means that the shorter stroke allocates 10 additional degrees of crank rotation to the forming or cutting process, which translates to 17 milliseconds at 100 strokes/min—a 45-percent increase in forming time.

• Longer tool life. Shorter strokes reduce the velocity at the point where the die punches contact the workpiece material. As a result, cutting punches are less likely to chip or become dull. In keeping with the previous example, reducing the stroke length by half decreases the impact velocity by 28 percent: ram velocity of the 40-mm stroke at 158 degrees is 25mm/sec., compared to 18mm/sec. with a 20-mm stroke at 148 degrees.

Note: We included the 40-mm link motion to illustrate that its forming time is longer and the impact lower. This comes, however, at a cost of significantly higher forces within the press structure. If we couple a mechanical feed to a link-motion press, the working portion of the stroke is half of the stroke of an eccentric press. Therefore, in the example above, the working portion between 90 and 180 degrees is only 10 mm. So, the comparison should be made with a 20-mm stroke of an eccentric press.