Motion Scenarios: Stamping and pressing

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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.

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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.

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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.

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Case in point: Turret punch

Controls are also used to boost punching-machine productivity. Consider the case of Specialty Fabricating Co., Omaha. The company purchased a used turret punch, and its mechanical components were in fine condition — but the controls were lacking. After determining that pricing to connect wiring was costly, Specialty Fabricating decided to replace the parts themselves. Owner Art Venteicher now uses components and software from Baldor Electric Co., Fort Smith, Ark., for positioning sheet-gauge work material and turret-mounted tooling. Baldor BSM 90 brushless servomotors drive X and Y axes, which move sheet metal on a flat, horizontal plane into a punching position.

Baldor 23H servo control amplifies the controller signal; Baldor's ISA bus, DSP motion NextMove/PC controller (loaded with the MINT programming language) instructs the motors to move from point to point. MINT receives standard RS-274 (G-Code) commands, and converts them into X-Y movements and punching actions. The program also helps with statistical process control functions by keeping track of total punches per tool, so the operator may change tooling after a designated number of punches. Explains Venteicher, “We've nearly doubled our productivity — from about 100 hits per minute to 200 on a given move.”

Other sheet considerations

Initially, before being fed to a press, sheets are separated. Suitable here are cylinders from Festo Corp., Hauppauge, N.Y., which have piston diameters to 125 mm and strokes from 10 to 2,000 mm. Sheets to be pressed are also handled with vacuum grippers in flat, bell, or bellows configurations — in conjunction with semi-custom connectors.

Appropriate vacuum valves smooth actuation, so for this portion of the operation, Festo offers multi-function vacuum-lifting cylinders tailored to confined workspaces. An oval piston, hollow piston rod, and endcap vacuum port create lifting vacuum; vacuum cup attachments serve to transport metal and protect workpieces against rotation.

Vibration mitigation

Pressing and stamping machines are often installed on custom-designed flooring and slabs, because the force and shock they produce is so great. Dampers are used in conjunction with these foundations, or sometimes to compensate for convertible designs.

One such product is heavy-gauge polymer and rubber supports: Kellett Enterprises Inc. Greenville, S.C., sells vibration-isolation pads that reduce transmitted shock and vibration in overly loud screw presses; injection molders that would otherwise tend to “walk” from vibratory movement; die casters that need protection; and metal stamping presses.

More specifically, Kellett's LP-13 custom-cut Shake Absorbers are sold as area pads, bumpers, and neoprene shims to stabilize (and sometimes level) machinery.

Another option for damping is friction springs. Friction springs consist of separate inner and outer mating tapered rings that stack together to form a column. Under compressive loading, the wedging action of the tapered rings expands the outer rings and contracts the inner rings allowing for axial deflection, thus absorbing the compressive load.

Friction springs are useful in stamping and punching, as they can be installed at various locations to absorb the huge dynamic forces of these applications. More specifically, friction springs manufactured by Ringfeder Power Transmission USA Corp., Westwood, N.J., also called RING springs, absorb forces from 0.1 to 10,000 kN and energy from 1 J to more than 100 kJ. They can be installed to damp motion on the normal machine axis in either the negative or positive direction — in other words, on the up or down stroke. “Friction springs with high capacity and low terminal force best protect stamping-machine structures,” explains Carl Fenstermacher, president of Ringfeder PT USA. Terminal force is that which the damping device exerts on the structure towards its stroke end.

A RING spring has a linear force-travel diagram, which means that the forces exerted on the machine structure are controlled and predictable during its entire stroke, unlike buffers of a different design. Larger impact forces are accommodated, and stroke length is a direct function of the number of rings in the stack height; more rings make for more stroke, and vice versa.

One caveat: “After a stamping machine completes the working portion of a cycle, it tends to recoil,” warns Fenstermacher. “Because friction springs damp two thirds of the force transmitted to them, approximately one-third of the compressive force is recoiled back into the system. Proper sizing of the spring is the key to protecting stamping machinery and supports.”

In contrast to velocity-dependent spring systems, friction springs also provide full spring work and damping, even when the load is applied quickly or very slowly.

FABTECH 2010 in Atlanta

FABTECH 2010 will be held November 2 to 4 at the Georgia World Congress Center, as the southeast is seeing expansion in metal fabricating and manufacturing. FABTECH 2010 will showcase metal forming, stamping, finishing, and welding equipment and technology. The event is expected to draw 22,000 attendees; 2,000 pieces of equipment in action; and more than 500 product debuts. Presentations and educational sessions abound; a Finishing Pavilion allows manufacturers to review metal manufacturing processes from beginning to end. The American Welding Society, Fabricators and Manufacturers Association International, Society of Manufacturing Engineers, and Precision Metalforming Association cosponsor the show. Registration is now open through fabtechexpo.com.