Page 28 - MetalForming August 2013

Servo Presses
approaching zero strokes/min. 3) The press can stop in mid- stroke and hold an accurate posi- tion while exerting force on the
tool.
4) It’s now unnecessary to com-
plete a 360-deg. rotation of the driveshaft, always in one direc- tion, during production. The press can make partial strokes and even restrikes without completing a 360-deg. rotation.
Slow Down to Run Faster
It seems illogical that the new capa- bilities provided by servo presses revolve around the ability to slow down during the stroke, yet increase overall stroke rate.
Press technology—conventional mechanical presses and servo press- es—is analogous to dragster and stock- car racing. While the dragster driver accelerates in a straight line as quickly as possible, a stock-car driver negoti- ates a set of curves in the track—he must slow down and accelerate rapid- ly and repeatedly. Conventional mechanical presses operate similarly to the dragster—they run great full out with no corners (or complex tooling operations) in sight. But, they lack an ability to accelerate or decelerate once
Conventional
mechanical presses
and servo presses,
and their applica-
tions, are analogous
to dragster and
stock-car racing.
Conventional
mechanical presses
operate like the
straight-line-running
dragster—one speed,
with no ability to accelerate or slow down once in motion. However, tooling operations in the pressroom often force presses to negotiate curves. Here’s where the flexibility of a servo press pays off, due to its ability to slow down as it enters each curve (or die operation) and then quickly accelerate down the straightaways.
26 MetalForming/August 2013
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in motion. Maximum press speed is dictated by the most challenging die operation.
Yet, most stamping jobs are loaded with curves. A series of operations (or curves) must occur during each press stroke. Among these curves (die operations):
1) Eject the part(s) completed in the last stroke.
2) Feed the material into the next tool or station.
3) Close the tool to bring it into con- tact with the part.
4) Form the part by continuing to close the tool toward the bottom of the stroke. During this travel, the part may be pierced, cut, bent, coined or drawn. Often, several of these operations occur at various die stations and at different points in the stroke.
5) Close fully to 180 deg. of stroke, setting forms and finishing coining operations.
6) Shed scrap from cutting or pierc- ing operations.
Each of these operations can become a bottleneck, and the ability of the servo press to slow down and nav-
igate those curves, and then accelerate to a higher average speed in the straightaways ultimately results in an overall increase in stroke rate.
Negotiating the Curves
Ejecting the last parts formed— Particularly during transfer-press oper- ations, part ejection can be impacted by press-opening speed. If the die opens too quickly, the part can jump out of the die unpredictably, driven by spring or cushion forces. This can prevent proper ejection and stop the operation.
Feeding the material—There always is a maximum speed at which materi- al can be advanced for the next press stroke. During transfer applications, rapid opening of the die can cause loss of control of the part. As the die opens, it is helpful to move rapidly to allow clearance for part movement. Then we may want to slow the process down to allow the transfer tooling to index the part. With transfer complete, we can advance quickly to the point where the die again contacts the workpiece. With progressive dies running in a servo
“Conventional mechanical presses operate similarly to the dragster—they run great full out with no corners (or complex tooling operations) in sight. But, they lack an ability to accelerate or decelerate once in motion. Maximum press speed is dictated by the most challenging die operation.”