Peter Ulintz Peter Ulintz
PMA Technical Consultant

Transfer-System Considerations

January 26, 2022
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Consider transfer dies as basically line dies, with some minor but important differences. Die stations are timed together and spaced evenly apart, usually in a single press (Fig. 1). Rails with fingers or grippers transport and position stampings. The rails can mount inside of the die, to the outside of the die or to the outside of the press. Transfer dies also can mount in individual presses. During a press cycle, each rail travels inward, picks up the stampings with special fingers or grippers, and transports them to the next die station or press.

Fig. 1—Transfer dies evenly spaced and mounted on a master die set.Stampers can install transfer systems in a variety of ways: mounted through the press window, mounted to the front and rear of the press, mounted to a transfer plate, or mounted externally between presses.

With a hybrid transfer, a coil-fed progressive die that performs some cutting, notching, forming or blanking operations combines with a transfer die that performs the remaining work. Here, a stamping separates from the progressive-die strip and the transfer unit transports it to the additional die station(s). If necessary, specialized transfer fingers can rotate or flip the workpiece over as it moves through the transfer stations. In some cases, these systems use material inefficiently—depending on the blank shape—but they eliminate the need for offline blanking and a blank destacker. 

A hybrid process also allows a pitch distance that suits the part requirements without sacrificing material use, as the transfer portion is not restricted by the die progression. Increasing the pitch allows additional space in the die to accommodate side cams or to provide inserts in form stations to ease adjustability and maintenance. Reducing the transfer pitch (after large draw reductions, for example) conserves space within the die, which can allow the designer to specify more working stations under the ram than with a constant-pitch progressive die. 

2-custom-built-transfer-die-tooling-by-design-UlintzSpecialized systems integrated into the progressive die, hybrid systems usually are designed and produced by die shops rather than by transfer manufacturers (Fig. 2). One exception: the hybrid Micro Transfer produced by Jacar Systems.

Transferring the Workpiece

The two basic types of transfer systems, two-axis and three-axis (or tri-axis), can perform numerous motions. Two-axis systems, usually simpler and more economical than their three-axis counterparts, are limited in their application. Examples of two-axis systems:

  • Shuttles, though typically inexpensive, present some die-design challenges, especially when part lifting is required. Here, die lifters must accurately position all parts to the exact same height. No position gauges exist to contain a part in the die-open position—the stampings rest on flat pads or rails with nothing locating them in position before the transfer tools engage the parts and move them to the next station.
  • Crossbars use suction cups or magnets mounted to a tubular frame to lift stampings and move them between die stations. Crossbars typically find use in press-to-press transfer lines.
  • Walking-beam systems offer stampers the flexibility to run a transfer die in a press without the need for an externally mounted transfer system. Designed and built as part of the die, transfer components lift parts and transfer them between stations.

Two-axis systems operate in a single plane—the X-Y plane in a station-to-station transfer, or the X-Z plane in crossbar and walking-beam applications.

Lifting and Transferring the Workpiece

Lifting the workpiece in a die requires motion in the X-Y-Z planes. This three-axis movement allows the lifting of stampings sufficiently high off of the die, and placed within the perimeter-gauge boundaries in the next station.

Three-axis systems are commonly plate- or press-mounted, with tooling components designed to allow sufficient clearance for the fingers’ return path to the original start position while avoiding interference with lower-die steels, cams and die-guiding components. Designers also must consider the timing of die closure relative to the incoming fingers.

Some die-mounted transfers run at a faster stroke rate than externally mounted transfer systems, due to shorter travel distances. In-die transfer systems present the same challenges as traditional three-axis systems, along with having to deal with the added design complexity of in-die transfer components.

Positioning the Workpiece

When the transfer fingers release the workpiece, orientation must be maintained in all directions (including rotation).

A two-axis system uses spring pins to maintain workpiece positioning during finger retraction until the die closes. In a three-axis system, a combination of part geometry, nesting blocks and locator pins help to maintain location during finger retraction.

Pitch length also impacts part positioning. Due to the limited time in which a transfer system must complete all of its motions, a longer pitch will produce high accelerations, often resulting in unwanted vibration and unstable movement of the stamping. 

Less stiff than thicker materials, thin materials are particularly vulnerable to vibration. Meanwhile, larger, heavier workpieces require stronger transfer arms and fingers that gain their strength from increased mass. The increase in part weight, plus bar and finger mass, subjects the transfer system to higher inertia loads, often requiring slower operating speeds. In most cases, consider using transfer-motion simulation to determine a motion profile that produces acceptable levels of inertia, vibration and instability for optimized speed, accurate workpiece positioning and long transfer-system life. MF

Industry-Related Terms: Blank, Blanking, Die, Draw, Form, Forming, Grippers, Lines, Nesting, Notching, Plate, Ram, Run, Stroke, Transfer Die, Transfer
View Glossary of Metalforming Terms

Technologies: Tooling

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