March 1, 2004
Demands on stampers are similar to those on every component manufacturer these days: Make it right and progressively cheaper year after year. Older press technology may not be able to provide the speed, throughput, die life, and uptime required to reduce production costs. As a result of research and development, press technology is rapidly advancing to handle today's complex parts.
Five realities that many stampers face on a daily basis to meet cost objectives are as follows:
Reality No. 1: Produce multiple parts (two, three, or four or more) with each stroke to minimize labor cost per part. Low-tonnage presses and small-bed presses can't accomplish this.
Reality No. 2: Produce a complex part in a single press. Progressive die and in-press transfer processes are a trend, but the press must have the capacity in terms of bed length, tonnage, and work energy to accommodate 12 to 14 die stations and operations, not just four or five (see Figure 1). To cut costs, secondary operations must be eliminated.
Reality No. 3: Produce more parts per hour on one press. To be profitable, stampers must achieve maximum output speed that the part material will allow without cracking. Link presses speed up during every portion of the stroke, except when the dies are actually forming the parts. This is the portion of the stroke that produces the slowdown feature.
Older crankshaft or eccentric-gear presses that don't have a slowdown feature must run at slow speed all the time to produce a quality part. Also, blank feeding, transfer, and press load/unload equipment must be designed for a faster pace to operate in the press angle window that a link press requires.
Reality No. 4: Routine and unexpected downtime jeopardizes production and increases costs. Modern press technologies can help increase die life by offering advanced sensing and monitoring systems that can prevent expensive damage to the dies or press.
Reality No. 5: Part tolerances are tight and non-negotiable. Drifting tolerances are unacceptable. Accurate gibbing and a rigid press frame are essential to produce good parts (see Figure 2).
An in-press transfer process allows a stamper to produce a complex part in a single press.
Bed Size and Press Windows. Modern stamping presses should have large column openings for installing transfer automation. These larger windows allow transfer rails and connections to extend beyond the sides of the press. Bed length is a significant factor; even small parts (less than 10 inches long) can require a minimum 204 in. of bed length if 12 to 14 operations are needed.
Press Deflection and Compression. The use of finite element analysis (FEA) during the engineering stage has improved press structure design. A lower deflection rating achieved with this analysis helps to lower tonnage requirements because the bolster and ram face deform less while forming a part between the two surfaces.
On modern presses, the press structure is designed for maximized stiffness via FEA calculations. A press that is rigid in design reduces vibrations during blanking and will increase die life while producing an accurate part.
Gibbing System. In addition to a rugged press frame, an accurate gibbing system for the slide is key to maintaining and prolonging die life. A gibbing system should be designed so it will neither grow nor shrink because of variations in shop temperature. Press guides should be positioned in the area of the tie rod box where the press frame is the strongest.
Gears. To achieve the high speeds required for today's production rates, a press's gears must be designed for longevity with the proper material and lubrication. The connection from gear to shaft has evolved from a key to a wedge-locking device to eliminate clearance between the gear and shaft. This eliminates the rocking action associated with worn keys, which causes clearance over a few million strokes, resulting in rework.
Clutch/Brake. New clutch/brake combinations produce more revolutions per minute (RPM) than ever before. Improved brake inertia keeps stopping time to about 300 milliseconds, even on high-speed presses. Hydraulic clutch/brake combinations are suitable for high-speed single-stroke applications because the heat created can be removed through liquid cooling.
To reduce maintenance, advanced monitoring systems track clutch pressure and heat to ensure that press components are not exposed to damage or wear.
Press rigidity and a low deflection rating can lower the tonnage requirement for a job because the bolster and ram face are less deformed while forming a part between two surfaces.
Flywheel Energy and Hydraulic Overload Protection. Quill-mounted flywheels can be made larger to produce more energy because the flywheels' weight is fully supported by the quill, which is mounted solidly to the press crown.
In the past extra flywheel energy had the potential to damage the press, but this has changed since the invention of hydraulic overload protection, which is installed under ram connections. A direct-acting hydraulic overload sensing and dumping unit removes oil in less than 10 milliseconds, which protects the dies and the press.
Increased flywheel energy creates tonnage higher in the stroke, while still providing plenty of tonnage at the bottom of the stroke for coining and heavy blanking. This technology can eliminate a secondary operation in another press or allow a higher-tonnage job to be run in the press with the tonnage staggered in the die. It is important that tonnage created high above the bottom of the stroke is not higher than the recommended torque curve. Electronic tonnage curve monitoring can prevent this from happening.
Tonnage Monitoring. Advanced electronic devices on mechanical presses can monitor the amount of tonnage used during any part of the stroke. This ensures that the clutch is not being abused. The use of PCs with faster processors means that frequent readings are possible during the stroke. Older tonnage monitors without curve monitoring cannot read tonnage at positions such as 2 in. up, which means overload conditions may exist without anyone knowing it.
Link press options should be investigated if aluminum or extra-hard material is used or parts are cracking in a standard crankshaft or eccentric-gear press.
Figure 3A link drive reduces the ram's impact velocity while exceeding the production rate of a conventional crankshaft-driven press. A slower forming speed can produce a good part, reduce tooling maintenance costs, and be combined with optimized approach and return strokes to provide more SPM.
A link drive reduces the ram's impact velocity while exceeding the production rate of a conventional crankshaft-driven press (see Figure 3). This slower forming speed produces a good part, reduces tooling maintenance costs, and can be combined with optimized approach and return strokes to provide more strokes per minute (SPM).
Link presses can be grouped into three categories: blanking link, combination link, and draw link.
Blanking Link Press. This type of press has a stroke up to 12 in. and slows down 50 to 60 percent, with a typical slowdown of 3 in., before bottom of stroke. It can be used for blanking, heavy progressive dies, forging, and coining.
Combination Link Press. Because of its versatility, this press often is used by contract stampers. Its stroke ranges from 14 to 28 in. and it has a slowdown from 4 to 7 in. before the bottom of stroke. Production might increase up to 100 percent in some applications with a 50 percent slowdown.
Or a stamper can opt for a reduced production rate to increase die life.
A 3/4-in. slowdown on the upstroke is available for in-die tapping units because they require a slow upstroke portion to reverse the inertia of the tapping unit.
Draw Link Press. Strokes on this type of press are from 28 to 48 in. and slowdowns are from 7 to 14 in. As the name implies, it is ideal for draw work, forming, coining, and limited blanking. The press performs the job with less heat and can provide a production rate increase up to 100 percent over standard eccentric-gear presses. Reducing slide velocity at the impact point by 50 percent reduces shock and vibration, increasing die life for some applications.
All of these link press types can help a stamper meet productivity and cost-reduction goals. It's important to match the press, link style or otherwise, with a coil feed line or in-press transfer system to maximize throughput gains.
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