Traditional mechanical presses achieve the highest production speeds when running relatively flat parts with simple, shallow forming requirements that typically can be processed from coil stock through a progressive or transfer die.
These presses typically feature a fixed-stroke length, although variable-stroke length presses are available from some manufacturers. They also offer variable slide velocity (strokes per minute [SPM]) where a single cycle of the press is fixed. With these machines, the working energy depends on flywheel mass and speed while full press tonnage capacity (rating point) is only near bottom dead center (BDC) of stroke. Users rely on these machines because of their simple setup and operation, high stroking speeds, high accuracy and repeatability, and special slide motions, such as link motion.
Mechanical presses equipped with a crankshaft or eccentric gear drives produce a symmetrical slide velocity between the top and bottom of the press stroke (see Figure 1). This system has part production limitations due to excessive slide velocity through the working portion of the press stroke.
To improve and expand part production capabilities, Stamtec Inc. offers multiple modified link motion drive options, including draw link, progressive die link, vertical link and knuckle-joint motion, which changes the motion of the slide. These modified drives are proving to be effective, significantly improving common stamping operations.
Link motion drives
Link motion drives are designed to stamp parts that require deeper or more complex forming and can increase productivity 20 to 40 percent. With a link motion drive, slide velocity decreases by up to 40 percent during the working portion of the press stroke (see Figure 2).
Material flow in the non-working portion of the stroke increases by an offsetting amount to maintain high production rates. The results are high-quality parts produced at higher production speeds. In addition, the slower speed of the working stroke reduces die impact, punch penetration, snap-through, noise and vibration, increasing machine and die life. To take advantage of these benefits, several types of link motion mechanical presses are available, including:
Draw link: Draw link motion is most commonly used when performing deep-drawing operations. Reduced slide velocity through the working portion of the press stroke allows metal to be stretched and formed without tearing. This reduction provides more constant slide velocity (see Figure 3) during the drawing operation, improving part quality and reduced side wall thinning.
While draw link motion is ideal for deep-drawing operations, the small amount of time the slide is near the bottom of the stroke limits the performance in other operations, including blanking, coining, forming and progressive die.
Progressive die: Also known as drag or slider, this motion is specifically designed to enhance progressive die operation performance. Progressive die link motion reduces slide velocity through the working portion of the press stroke by 20 to 40 percent.
Applying pressure increases metal plasticity and forming capabilities. This is enhanced the longer pressure is applied to the metal. The reasons that plasticity matters are far-ranging, including:
- Less springback during forming operations
- Achieving desired shapes without re-striking during coining operations
- Less material cracking during pierce and extrude operations
- Higher SPM during in-die tapping operations
- Very high SPM during shallow draws
Lost time created by slide velocity reduction in the lower portion of the press stroke is regained by increasing velocity through the upper portion of the stroke (see Figure 4). With progressive die link motion, SPM is not sacrificed, and operating speeds can be increased by 25 percent or more.
Vertical link: Vertical link motion can more than double how long the punch is engaged with the material (see Figure 5). Vertical link motion is common in coil-fed forming operations where pressure is applied by the press.
Like progressive die, vertical link motion enhances press performance by operating at a significantly reduced slide velocity. The increased slowdown increases the flow of material and improves the production of net shape parts where secondary machining operations have been eliminated.
Knuckle-joint: Knuckle-joint motion is a modified slide motion specifically designed for near net-shape forming where a very high degree of metal flow is required. The knuckle-joint motion (see Figure 6) not only slows down the slide through the working portion of the press stroke, it actually produces a significant dwell at the bottom of the stroke.
Knuckle-joint motion technology can benefit a customer thanks to its high rigidity with minimum deflection and long BDC dwelling time. It’s also geared toward thicker materials, wider stamping applications, net-shape blanking and coining, and prolonged tooling life. As a result, users see improved quality, precision and stability of produced parts as well as reduced noise and vibration.
Overall, link motion drives can improve die life by reducing shock and vibration. The reduced slide velocity in the working portion of the press stroke allows progressive die and vertical link motions to reduce shock and vibration by 30 to 70 percent during blanking operations. This reduces both press and auxiliary equipment noise levels and wear.
Shock and vibration are highest while the punch is engaged with the material and lower die section. The reduced shock and vibration also reduce maintenance costs for issues related to guards shaking loose, press wiring connection failures and press frame stress cracks. The lower working velocities also create savings in lubrication.
Feeding systems
Feed timing is traditionally based on the degrees of crankshaft rotation. However, with link motion presses, slide velocity is increased through the upper portion of the press stroke, leaving less time to complete the cycle.
Crank motion drives (see Figure 7) produce a symmetrical slide velocity between the top and bottom of the stroke. The same amount of time is required for the crank to rotate between the top half and bottom half.
With link motion drives (see Figure 8), slide velocity is reduced through the working portion of the press stroke, meaning more time is used. In order to maintain SPM, slide velocity is increased through the top portion of the stroke.
With link motion drives, there is significantly less time available to feed material through the die. Unlike using degrees of crankshaft rotation, link motion presses use milliseconds to complete the feeding cycle.
Mechanical presses with integrated feeding systems driven from a power take-off on the crankshaft will see higher acceleration with link motion. In this scenario, the press speed may need to be reduced. A servo-driven feeder with no mechanical tie to the crankshaft, however, will require no press speed reduction.
Overall accuracy
On the upstroke (press lifting) and downstroke (pushing toward the floor), inertia forces are created that can impact press stability and performance. To reduce these inertia forces on presses with link motion drives, dynamic balancing blocks can be applied (see Figure 9).
Dynamic balancing blocks can greatly reduce high-speed slide inertia forces. This helps the press maintain bottom-of-stroke repeatability and run at higher speeds with less vibration. The effect is greater part accuracy and reduced press and die wear.
To maintain cycle rates, slide velocity must be increased through the upper part of the press stroke to compensate for the reduction in the working portion. Immediately increasing velocity after the slide passes through the bottom of the stroke would also increase spring-loaded stripper velocity and stripper bolt wear.
Progressive die and vertical link motions do not increase slide velocity until the stripper has been lifted off the material (see Figures 4 and 5). This results in decreased stripper velocity and reduced load on the stripper bolts.
To recap, link motion drives can decrease slide velocity by up to 40 percent during the working portion of the press stroke while producing better quality parts at higher production rates. And, slower speeds can reduce die impact, punch penetration, snap-through, noise and vibration, increasing machine and die life and reducing operating and production costs.
