Press selection—sorting it out Part I

Are you in the market for a press or press system? Has your search generated a lot of questions? Does it seem that the more questions you ask, the more questions you need to ask?

The research is justified. Choosing the correct press can mean the difference between being in the black or in the red. In today's global marketplace, if your per-part cost is not low enough, you may not get the job—or worse, you won't make a profit on the job. You need a press that will be a good fit for the application.

Current demands for a higher degree of part accuracy, higher productivity than before; the ability to stamp a range of materials, including high-strength metal; and for the flexibility to run short and long runs, small and large parts, perform shallow and deep forming have complicated the selection process. Also, while a specific part may indicate a specific press, today's mercurial business climate means that you also must keep an eye on future demands, perhaps selecting a press that can grow with you as your needs change.

This article is in no way intended to be an end-all to press selection—entire books filled with the subject burst their bindings. It is simply intended to point out some press characteristics and differences, and highlight some considerations to help narrow down your choices to prepare you for the important consultation with a reliable and reputable press manufacturer who can fin-tune your selection before you make a purchase.

Matching Press Characteristics to Your Applications

The following questions–and their answers, provided by industry equipment manufacturers and experts–are intended as a general guide to help you simplify the daunting task of selecting a press or press system.

1. What Press Type is Best for My Speed, Volume Needs?

The demand to reduce part price has required higher production speeds, noted Dennis Cattell, senior project manager, The Minster Machine Co. (see Speed Comparisons sidebar).

Speed—Materials. "The material type and grade, lubrication type, and press stroke determine the maximum production speed that you can run," Cattell said, adding that press manufacturers provide charts showing the slide velocity throughout the press cycle.

[Rule of Thumb] "Most machines are equipped with a variable speed," Cattell said. "Although the press has the ability to run faster, you cannot run the slide velocity beyond the material's formability limits by without splitting or tearing the metal.

[Tip] Speed—Slide Velocity. "Often when stampers select a press, they request the longest press stroke possible to be ready for any unforeseen job they may quote in the future. Unfortunately, a longer press stroke comes with a higher slide velocity and, normally, a slower overall production speed—thus, a smaller part per million [PPM]—efficiency will be lost" Cattell said. "With today's new materials, accepting this penalty is a mistake. With high-strength materials, it is very important that slide velocity be kept to the minimum."

Speed—Volume. "The first step in this complicated, lengthy press selection process is the same," said Dennis Boerger, project manager, AIDA-America. "Without exception, the part characteristics—physical size, quantity, material type and thickness—dictate proper press selection.

"If a part's volume requirement dictates running it automatically at 20 SPM [strokes per minute] or a million parts per week—such as a small electronic component—the correct choice would be a mechanical press," Boerger explained.

"However, for an automotive outer body part with a production run of just eight per minute that requires that tonnage be applied throughout the stroke, then a hydraulic press may be the best choice," Boerger said.

Speed—Mechanical-drive Systems. "Each drive system has a unique motion characteristic related to its mechanical response to the drive motor," said Andreas Kinzyk, director, metalforming system sales, Schuler Inc.

"Mechanical press drives operate with a variable stroke rate; however, time-travel ratios are fixed, even with an optional adjustable stroke," Kinzyk said. "Therefore, the slowest part of the forming process determines the overall cycle speed and subsequent production rate. "In some cases, this may compromise the efficiency of the overall process.

"Thus, to optimize the production rate and forming process of a mechanical press, the goal is to develop a variable time-travel ratio for the slide – as can be done with a servo-mechanical drive design," Kinzyk said.

Enter Link-drive, Servo. "Mechanical presses equipped with link drive (also called alternative slide motion [ASM]) allow you to reduce the slide velocity going into the working portion of the press cycle; then, once through BDC [bottom dead center], the slide can return quickly to the top of the stroke, Cattell said. Alternatively, infinitely adjustable stroke presses [servo, hydraulic, and pneumatic drives] enable you to fine-tune the stroke quickly to the part length and die design to achieve the maximum SPM possible."

"With an eccentric drive, the slide motion is a sinusoidal curve; the slide speed decreases before BDC and also decreases at TDC, and increases during closing and opening," Kinzyk continued. "Due to the eccentric-drive system, the tonnage around BDC increases with constant motor torque. This suits the forming and transfer process in general," Kinzyk said.

"Eccentric-shaft, or crankshaft, presses generally have a small clearance and a shorter stroke, AIDA's Boerger said. "Some crankshaft presses have torsional twist deflection during forming, which requires the press to strike the material harder to achieve the required part characteristics. An advanced crankshaft press design minimizes this problem.

"Link drive is engineered to reduce contact velocity over crank motion by up to 50 percent, enabling mechanical presses to accommodate deep-draw applications," Boerger said.

Direct-, single-, or double-gear-drive selection is driven by the part application, size, the speed required to produce it, and torque. Direct drive is very high speed—thousands of (SPM); single-gear drive is hundreds of SPM, and double-gear drive operates at about 40 SPM, Boerger said.