Transfer Automation—Options Abound

When a metal-stamping process engineer looks at a part design, many questions come to mind:

• What is the base-material type and thickness?

• What are the flat-blank dimensions?

• What are the daily, monthly or yearly volumes, and how long will the part be in production?

When quoting a new part, the engineer may propose a process that fits existing equipment, rather than developing a more efficient process for the part at hand. One of the most important decisions the process engineer may make: Should the part be processed in a progressive or transfer die?

Transfer dies are basically line dies, with some minor but important differences. Die stations are timed together and spaced evenly apart in a single press. Parts in a transfer die often are transported and positioned by rails and grippers, mounted either inside the die, on each side of the die or to a transfer system mounted to the outside of the press. During a press cycle, each rail travels inward, grips the parts with special fingers or grippers, and then transports the parts to the next die station.

Common Transfer Processes

Transfer systems can perform numerous motions; the two basic types are two-axis (2D) and three-axis (3D or tri-axis). Two-axis systems work in a single plane—the x-y plane in a station-to-station transfer, or the x-z plane in crossbar applications.

Examples of two-axis systems include:

Shuttle transfer—These setups, while typically inexpensive, can present some die-design challenges, especially when they require part lifting. Here, die lifters must accurately position all parts to the exact same height; there are no position gauges to contain the part in the die-open position. The parts rest on flat pads or rails with nothing locating them in position before the transfer grippers engage.

Crossbar transfer—Here, suction cups or magnets mounted to the crossbar system pick up the parts and move them between die stations. Crossbars typically find use in press-to-press transfer lines.

Progressive/walking-beam transfer —This hybrid process offers stampers the flexibility to run a tool in a traditional straightside press without having to deal with externally mounted transfer systems. Transfer components are designed and built as part of the die.

Three-Axis Transfers

…typically work in an x-y-z plane. They move inward to grip the parts, lift the parts vertically, index them to the next station and then lower the parts onto the die. This three-axis movement allows parts to be lifted sufficiently high off of the die and placed within the perimeter gauge boundaries in the next station.

Examples of three-axis systems include:

Externally mounted tri-axis transfer—These systems may be plate- or press-mounted. Tooling components must be designed to allow sufficient clearance for the fingers’ return path to the original start position, to avoid interfering with lower tools, cams and die-guidance mechanisms. Designers also must account for upper die components, considering the timing of the tools relative to the incoming fingers.

In-die three-axis transfer—Some in-die transfer tools may run at a faster stroke rate than externally mounted transfer systems. In-die transfer systems present the same challenges as regular tri-axis transfers, along with having to deal with the added design complexity of in-die transfer components.

All transfer processes allow the tool to be designed at a pitch (the distance between die stations) to suit unique part requirements, without sacrificing material use, since transfer processes are not restricted by the part progression. The ability to increase transfer pitch allows the addition of inserts to form stations, to provide ease of adjustability and make it easier to deal with sheet-thickness variation and springback. And, the ability to reduce transfer pitch (after a large draw reduction, 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 setup.

Finally, transfer processes can eliminate expensive offline operations, allowing the stamper to produce completed parts from blanks or coil, lessening the likelihood of shipping incomplete parts.