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 only can 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 (see Fig. 1). 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 deg., while the curve for the 20-mm stroke (yellow) intersects this line at 148 deg. This means that the shorter stroke allocates 10 additional deg. of crank rotation to the forming or cutting process, which translates to 17 msec 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 (Fig. 2). 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 deg. is 25mm/sec., compared to 18mm/sec. with a 20-mm stroke at 148 deg.

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 deg. is only 10 mm. So, the comparison should be made with a 20-mm stroke of an eccentric press.