Ask the Stamping Expert: What do the various press styles have to offer?

Q: I am having a problem stamping a shallow drawn cup about 0.75 in. deep from 0.045-in. 304 half-hard stainless steel. I’m using a mechanical press. What do other press technologies have to offer?

A: Years ago I wrote a brief on the major differences between mechanical and hydraulic presses. Servo presses were in their infancy then, but they’ve now become very sophisticated and can compete with mechanical and hydraulic presses.

Here are the main considerations in comparing presses:

• Force capability - Hydraulic presses can generate forces up to 50,000 tons and are generally used for automotive and large drawing operations. Mechanical presses typically are built for forces up to 6,000 tons. Mechanical presses with very high force ratings tend to be much larger than their hydraulic counterparts. In servo presses, I have not seen anything larger than a 2,500-ton machine (which is the exception); the most commonly sold servos tend to deliver about 600 tons of force. Hydraulic and servo presses can provide the press rated force (PRF) at almost any point in the stroke. This is important, for example, if you’re running a 5-in.-tall drawn cylinder with an 8-in. stroke. A high force is required over a 5-in.-long portion of the press stroke. With a mechanical press, the PRF is at about dead bottom. If you are blanking 0.125-in.-thick steel with a press stroke of 1.5 in., no press force is required until almost the bottom of the stroke. In this case, a mechanical press might be the best choice.
• Overloads - Hydraulic and servo presses have built-in overload protection. You can control the ram force applied and set predetermined controlled limits to the force applied. If these limits are exceeded, the ram stops. In a hydraulic press, you can relieve the hydraulic pressure with relief valves to free up the ram. In a servo press, you can back up the ram. Mechanical presses also have overload protection via tonnage monitors that tell you if you exceed preset limits; however, they will not prevent you from exceeding the limits. If you force a mechanical press to apply more pressure than it is designed for, you can lock up the press at bottom dead center, damaging the press or tooling and causing downtime.
• Speed - On hydraulic presses and servo presses you control the ram speeds at different points in the press cycle. Mechanical presses have one set ram speed within the press cycle and generally have the highest stroking speed capabilities. But that does not mean the ram moves at a constant. In fact, it accelerates from zero at top to maximum velocity at halfway down the stroke and then back to zero at dead bottom. This limits the time available to perform stamping functions that require the press ram to be open. Let’s say you want to optimize the strokes per minute (SPM) in an application involving robotic pick-and-place of a nut in the tool for in-die staking. Hydraulic and servo presses can be programmed to advance rapidly down and up, stop for some seconds, and then cycle again, allowing time to load the tool. For the same operation in a mechanical press, the SPM would be dictated by the time and minimum gap required for the robot to do its job. This can have a significant impact on the number of parts per minute you can stamp. There are a few other considerations in your press choice. For instance, blanking and shallow forming close to the bottom of the stroke can yield different results with different press types. Ram speed is slightly different at different points in the press cycle in hydraulic and servo presses relative to mechanical presses.

Before you make a decision on press type, define the press bed size, tonnage, and stroke you need. Would a finite ram speed within the press cycle optimize output, or would a variable ram speed be more suitable? Understand where the forces will be required relative to the stroke and how much time is required for other stamping functions. Determine the overload protection you require.

In today’s lean environment, there is no room for not optimizing press SPM. When the tooling application is paired with the proper press, the results are optimized.