Die Science: The basics of servo-drive presses

As materials change and the competition gets tougher, metal stampers must invest in technology that helps them to remain competitive. In stamping, efficiency is everything. Stampers must produce higher-quality parts in shorter cycle times using materials that often have poor formability. This is an exceedingly difficult task.

To complicate the problem, stampers often are forced to reduce piece-part cost by using very little or no lubricants. This usually drives up the tooling cost because the dies now need to be coated with expensive antifriction, antiwear coatings.

One way to increase efficiency is to use a servo-drive press.

Crank-drive Presses

To understand the advantages of using a servo-drive press, you first need to understand the basics of how a more traditional press operates. Both types of presses have advantages and disadvantages.

The most common press type used in sheet metal stamping is the traditional crank-drive press. It works in the reverse manner of a steam train. The train transfers the linear motion of a long, heavy-duty rod into rotary motion—as the rod (or pitman arm) moves back and forth the wheels of the train rotate. In contrast, a crank-drive press converts rotary motion into linear motion. It transfers energy from a heavy, spinning flywheel to an eccentric crank that drives the press ram up and down.

Not all crank-drive presses have this simple design. Some may contain several different types of links and mechanical drives. A draw link-drive press, for example, slows down when it gets to a certain point in the stroke. This allows for the metal to flow during drawing.

In general, crank-drive presses have a lower initial cost than servo-drive presses. They are available in high tonnages and come with high-speed capability. The presses are suitable for cutting and blanking.

They have a fixed stroke length, and a great deal of tonnage can be lost when the ram is not at the bottom of the stroke. The press has a fixed ram motion, so acceleration and deceleration are not possible without changing the press linkage. At slower speeds, the energy available is lower.

Servo-drive Presses

Servo-drive presses use servomotors to control the ram motion of the press. This servo motion allows the stroke length, motion, and ram speed to be changed so that the single press can be used for a variety of stamping operations.

Servo-drive presses can be expensive compared to comparable-tonnage crank-drive presses. They also are more complicated to maintain and troubleshoot electronically and mechanically.

Variable stroke length allows for a variety of parts to be made in a single servo-drive press. Ram motion and speed can be altered to aid in metal forming operations such as deep drawing. Generally, deep drawing is best-suited to slower ram speeds. Slowing down the forming speed helps to reduce friction, thus reducing the heat generated, which is important when forming high-strength materials such as dual-phase and high-strength, low-alloy (HSLA) steel. In addition, the punch-to-metal contact speed can be reduced, allowing metal to flow into the drawing cavity.

Progressive dies primarily need a press with fast speeds, and the servo-drive press can run short stroke lengths. For instance, you could run a progressive die for a simple bracket at 100 SPM using a 5-in. stroke length and later that day run a deep-drawing die to make a part 6 in. deep requiring a 16-in. stroke length at 20 SPM.

The servo-drive press can be tuned to maximize the output of automation systems such as transfers and progressive dies. If a standard crank-drive press with an 18-in. stroke is used to make a 1-in.-tall bracket that is 3 in. wide, about 10 to 12 in. of the ram movement is taking place while no work is being performed in the die, and all part transfers have been completed. With a servo-drive press, the ram stroke and speed can be maximized so that no time is wasted. This can double the number of parts made in a given period of time.

Because the ram of a servo-drive press can stop at the bottom of the stroke and remain idle for a programmed amount of time, it is suitable for hot stamping operations. And because the ram can dwell at bottom dead center, the amount of springback in stamped parts is reduced.