Maintaining precision at high speeds
July 29, 2008
The catch to operating at higher production speeds is that metalformers often see jumps in scrap rates and accelerated wear-and-tear on the press and tooling. New, and some tried-and-true, mechanical press technologies help metalformers strike a balance between high speed and high precision.
When metal forming companies confront "my labor is cheaper than your labor" price attacks from competitors in developing regions of the world, their production engineers are challenged with finding innovative processes to win business contracts without sacrificing profit margins.
One way to reduce cost per part is to establish processes that will increase parts-per-minute production rates. However, the catch to operating at higher production speeds is that scrap rates often jump, and wear and tear on the press and tooling accelerates.
Fortunately, new—and some tried-and-true—mechanical press technologies help metal formers strike a balance between high speed and high precision that allows them to grow their business and profits. Here's what some innovative metal formers do so they don't have to compromise employee wages or quality to win bids.
Bronze bushings (left) typically are used in slow-speed, center-loaded applications. But at higher speeds and more stringent precision specifications, antifriction roller bearings are suitable, because they run cooler, last longer, and sustain tighter tolerances.
Presses are equipped with bearings to reduce friction between sliding and moving parts (see Figure 1). Using the wrong bearing design or using the proper bearing but installing it incorrectly can result in excessive vibrations, poor part quality, and, in extreme cases, catastrophic failures.
Most mechanical presses have bronze bushings in their construction. Friction bearings or bushings are less expensive, but they run hotter and wear out faster, which eventually increases press clearances and reduces press accuracy.
High-speed, high-precision presses are equipped with antifriction roller bearings to support the drive shaft and slide guides. These bearings reduce friction by eliminating sliding between surfaces and replacing part contact with rolling components. Antifriction roller bearings also run cooler and maintain tighter tolerances than friction bearings or bushings.
But not all antifriction bearings perform as well as others at high speeds. To maintain high precision, the roller bearings must operate at near-zero clearance.
In addition, presses that use engineered lubrication systems allow bearings to run with less internal clearance because such systems reduce thermal expansion effects.
Stampers should examine the manufacturing processes of the precision roller bearing assemblies in the press to ensure that critical dimensions are precisely achieved and efforts have been made to maximize the accuracy and life of these bearings.
Production engineers are advised to consider their production rate and part tolerance requirements to ensure installation of the best bearing design for the application.
Large, complex progressive dies, or dies that extend beyond the press bed, can cause off-center loading. Presses built with a multipoint, antifriction roller bearing gib guidance system with zero clearance counter off-center loading.
Presses cannot produce consistently precise parts at any speed unless the ram moves straight and square. This becomes increasingly challenging in off-center loading applications (see Figure 2).
Conventional press designs cannot tolerate high off-center loads while maintaining acceptable ram motion. One way to counter off-center loading is to use a higher-tonnage-capacity press. That may mean starting up and running a 250-ton press to run a job that could be done on a 100-ton press. Not only does the bigger press use more energy, it also typically runs slower and requires more floor space.
What determines whether a press can handle both off-center and uniform loading? The slide guidance system does. Many mechanical presses come with a bronze gib guidance system, or a post-style slide guidance system. While these systems provide adequate guidance for some applications, high-tolerance jobs require more precision (see"Burr Oak Tool Keeps Cool at Sizzling Speeds"sidebar).
High-speed precision presses with high off-center loading capacity incorporate multipoint, antifriction roller bearing gib guidance systems installed with zero clearance. These designs allow shops to match the press capacity to the job requirements. In addition, this saves floor space and energy.
Hydraulic withdrawal systems with 8 inches or more of lift allow operators to easily and quickly make repairs and adjustments for routine die maintenance, minimizing downtime.
Robust construction and die area access also factor into production rates (see Figure 3). Even if presses run at higher production rates, forming lines can quickly lose time gains if operators constantly have to make press adjustments. This is a concern especially when operators have to install a lot of different dies.
If the press isn't rigid enough, excessive deflection can directly affect part quality. Worse yet, hours of nonproductive downtime can be required to shim die components to compensate for these deflections. It is critical that the press has an extremely rigid crown, bed, and slide. Press manufacturers should be able to provide deflection analysis based on the load applied by the tooling
Another factor that hampers production rates and increases cost per part is downtime related to tooling maintenance. Most high-speed applications run in presses with less than 3-inch strokes. The short stroke is good for high-speed production, but bad for tooling maintenance. Many times the die must be completely removed from the press to perform even the simplest maintenance. This halts production for an extended period of time.
Stampers should look for a press with a hydraulic withdrawal feature. Not only does this feature allow operators to easily and quickly raise the slide assembly up to 8 in. without affecting shut height, it also provides press overload protection and makes it impossible to stick the press on bottom.
The turtle may have won the fabled race against the hare, but in today's economic climate, slow and precise doesn't win bids over foreign competition's lower wages.
Fortunately, press technology such as antifriction roller bearings installed with zero clearance, multipoint guidance, and hydraulic withdrawal help make high precision at high production rates achievable.
By using a press specifically engineered for high precision at high speeds, shops can increase production rates and win more contracts without sacrificing quality, employee benefits, or profitability.
Burr Oak Tool Inc., Sturgis, Mich., manufacturers equipment used to produce heat exchanger coils for the heating and cooling industry. The company used conventional presses for many years to produce the fins in a heat exchanger coil.
However, as demand for air conditioners grew, fin designs became more complex and required tighter tolerances. Soon the tooling and press precision required to produce these fins exceeded the capabilities of its current presses. The company needed presses that would help it keep its cool when precision and production rates started sizzling. That's when Burr Oak started a new company—Oak Products Inc. (now known as Oak Press Solutions)—to build its own presses in 1965.
Over the years these Oak presses have addressed one of the company's main problems: the off-center loads inherent in the dies used to produce fins for HVAC units. In most cases, two-thirds of the progressive-die load was applied over the first half of the die. It also was common for 30 percent of the die to extend beyond the press bed. Burr Oak Tool needed to handle the extreme off-center load without compromising the fin production rate and precision needed.
"The Oak Press Solutions presses are equipped with a unique ram guidance system that provides the stability required to handle the extreme off-center loading," said Ned A. Haylett, Burr Oak vice president. "With material thicknesses as thin as 0.1 mm, tooling and press precision are critical."
Burr Oak Tool can successfully fend off low-wage competitors when its presses protect their complex progressive dies from premature, costly wear and perform millions of cycles with minimum scrap, Haylett added.