Page 30 - MetalForming August 2013

Servo Presses—Why Pay More?
“Any given material will have a maximum limit on slide velocity for impact, bending, coining and drawing operations. By setting the ideal speed for each of the forming operations, but only being restricted to the that specific speed during that forming operation’s travel, we can increase formability without restraining production rate.”
press, this feed time also presents an opportunity to improve productivity. On very long progressions, the stamper can slow the ram somewhat during the stroke to allow time for the feed, but avoid bringing the ram to a complete stop. Conversely, on short progressions we can reverse direction without wait- ing for full 360-deg. rotation—pendu- lum mode. By shortening the distance the ram must travel between strokes, the press cycles more frequently with- out increasing ram speed.
Closing the tool—The ability of a servo-driven press to adjust impact velocity at the instant when the tooling contacts the material can greatly improve die life, by reducing reverse load. Also, excessive impact velocity can cause spring strippers to bounce.
Forming the part—Several types of forming operations have a maximum speed at which they can function while maintaining a good balance of part quality and tool life.
Drawing: To avoid tearing the mate- rial during drawing, stampers use spe- cial tool coatings, lubricants and even premium-priced draw-quality steels. Servo-press technology allows a stam- per to reduce slide velocity during the drawing portion of the stroke, without
losing press energy and stalling the ram. This pays off in several ways, including reduced lubricant con- sumption, and the ability to make good parts with lower-quality material and less-costly tooling, while maintaining tool life. Part quality actually can improve, since a slower draw speed minimizes heat generation to prevent distortion.
Cutting/blanking/piercing: During these operations, excessive punch velocity at contact with the workpiece material can lead to significant reverse load. The ability of a servo-drive press to slow punch speed to nearly zero and yet still produce enough torque, ton- nage and energy to cut can reduce reverse load by as much as 90 percent.
Bending: These operations often
cause springback. With some materials, stampers will program a servo press to briefly remain closed, or even restrike the part, to help set bend angles and minimize or eliminate springback. Also, bending can cause workpiece cracking—a servo press can provide extremely slow movement through the bending moment, avoiding cracking to allow room-temperature stamping of materials typically hot formed.
In-die tapping: This operation’s lim- itation occurs during the first turns of the tap. Stampers must honor the max- imum rotation speed that will yield a satisfactory tapped hole while opti- mizing tool life. Although the slide may only travel 0.200 in. or less to perform the restrictive portion of the tapping operation, with a conventional mechanical press the maximum tap- rotation speed—controlled by the tap- ping unit’s gearing and the slide speed —will dictate the press stroke rate. With a servo press, impact velocity and ram speed can be optimized over this brief travel distance to match the tapping need. Then the press can accelerate during the remainder of ram travel, greatly increasing output.
Coining: When coining and per- forming similar operations where the material is compressed into a plastic state and flows into cavities, the oper- ation generally benefits if the tool remains closed longer than when the job runs on a traditional crank-motion press. Slowing down when the ram is at 180 deg. will assist that material flow.
Shedding scrap: To ensure that scrap sheds properly, in some cases the ram must not move too quickly when opening. A punch pulled too
More on Servo-Press Applications
MetalForming has presented several case studies on how metal stampers are lever- aging servo-press technology, including one in this issue on Milwaukee’s Acro Metal Stamping (pp. 32-33). Here’s one more, worthy of a trip down memory lane, from our September 2012 issue.
Taking on very complex and increasingly large, high-tonnage automotive stamping jobs—that’s how Batesville Tool & Die (BTD) CEO and president Jody Fledderman char- acterizes his company’s focus over the last several years, as well as its approach to research and development.
Describing the first production job run on its new servo-drive press, Jay Fledderman describes the productivity and quality gain realized from moving a progressive die from a conventional 1200-ton press to the servo-drive press.
“The die stamps seat side brackets,” Fledderman says, “to the tune of 36,000 parts/week, about 20 percent of press capacity.” Brackets are of 0.156-in.-thick 80-KSI high-strength low-alloy steel. They’re stamped two-out from 15-in.-wide strip over 10 die stations.
“We had run the job for years on one of our two 1200-ton presses,” Fledderman continues, “learning to live with and compensate for as much as 145 tons of reverse tonnage. While the addition of hydraulic dampers on the press brought snapthrough force down to 115 tons, our press operators and diesetters found the dampers burden- some and cumbersome.”
Moving the part to the servo-drive press and redesigning the process to reduce ram speed at certain portions of the stroke, during piercing and coining, brought snapthrough down to a manageable 55 reverse tons.”
28 MetalForming/August 2013
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