Sensors can verify that material has moved forward, product has been ejected from a press, or that cams are in the correct position. Proximity sensors from Turck also prevent damage from misalignment or mistiming.

The process of stamping is traditionally associated with the auto industry, and all of its requirements for specially shaped panels and parts. However, as technology advances, stamping and pressing are increasingly being used to produce medical devices and electronic products as well. The Precision Metalforming Association recently tracked a sharp business increase for the industry, even during this most recent economic downturn.

As for the actual processes, in stamping and pressing machines, sudden impact and high forces are par for the course — which makes adequate damping essential. Pressing and molding parts from steel also benefit from automation, particularly where raw rolls and sheets weigh many thousands of pounds.

After the working cycle, blanks must be extracted quickly but accurately, to maintain productivity but prevent tool damage; sensors and quick actuation prove useful here. In addition, material going through stampers must often clear tight spaces, which necessitates accuracy and smooth motion. Elsewhere, in progressive die applications, blank sheet metal is carried through a series of dies to produce a final product; one newer design element for such operations is gantries that travel with the die, which quicken the processes by making die changes easier.

Let's consider the specific motion technologies involved.

Die protection

A major challenge in stamping is that forces must be high, but cycle times must be short, to allow machine shops to output an economically effective volume of parts. For example, some stamping applications, such as those for large automotive frames or panels, run at 30 or so strokes per minute, while others, such as those for small electronic components, run at 1,400 to 1,500 strokes per minute. Therefore, die crashes are the main cause of downtime (and nonconforming parts) in metal stamping; what's more, a die repaired after a crash often produces imperfect parts.

One way to prevent crashes is to ensure that nothing is misaligned during a press cycle — with a system of sensors in the tooling, encoders at the crankshafts, and a controller to interpret their signals. Sensors from TURCK Inc., Minneapolis, detect speed, accuracy, target orientation, and position, even for part ejection and hole placement. Progressive dies are a particularly suitable application: Here, sensors are placed at multiple locations within the tool or die to detect bends, short feeds, long feeds, slugs, and missed hits. Elsewhere, in transfer dies, sensors are incorporated into the grippers to detect that panels are in place before being transferred to the next station.

When incorporating sensors for die protection, the first step is to determine the location and type of sensors needed. Several types exist: Some are contact sensors (mechanical devices activated by physical touch) while others are noncontact electrical sensors that use magnetic fields, light, or sound waves to determine position.

Mechanical sensors are less expensive, but eventually wear and fail. In contrast, electrical sensors are more expensive, but last longer — typically only failing when a catastrophic event occurs, and rarely from wear.

More specifically, solid-state proximity sensors are impervious to oil, coolant, and other fluids. Photoelectric sensors are another noncontact, solid-state option, but are more susceptible to the relatively dirty environments of stamping and pressing. Housing styles abound: Flat-pack proximity sensors can be embedded in dies to monitor stripper plates, for determining whether slugs have been pulled into the die. Cylindrical versions can be placed in a spring-loaded lifter to detect whether material has fed properly into position before the die closes; if slugs are deposited after the die stroke, the sensor detects the difference in position.

A final consideration is that sensor cables, wires, and junction boxes (that route signals to the controller) must be protected from scrap metal, forklifts, or other hazards. Manufacturers often cut channels into die shoes to protect cables from damage; some also fill these channels with silicon rubber sealants or use conduit in the channel around the cable or wire.