November 15, 2001
The article briefly covers stamping terms and discusses press considerations for those purchasing a hydraulic blanking, stamping, or deep drawing press.
Stamping means different things to different people depending, to some extent, on their generation. Some consider stamping to be synonymous with blanking. Years ago, a stamping or blanking press was a mechanical press and vice versa. Today, that has changed, and these presses often are considered candidates for hydraulic power.
In the late 1930s, hydraulic presses experienced a coming of age. Thousands have been used since then in assembly and secondary operations such as forming, hole punching, staking, riveting, broaching, trimming, crimping, coining, and swaging. Many also have been used for blanking and stamping.
To help to decide whether a hydraulic press is a good option, definitions of stamping, blanking, and deep drawing as they currently are understood are needed.
Blanking is a cutting, shearing, or punching operation on a piece of material to form a net shape or part. The material can be a flat sheet or a continuously fed strip of material.
Stamping might be thought of as blanking but also includes additional forming, bending, folding, or piercing operations. With additional features added to the blanked part, the press now can be called a forming press.
Deep drawing is forming a part from a blank or strip that is secured in the die by draw clamp rings or holders that allow controlled flow of the metal as the punch pushes the form into a cavity. A generally accepted definition of deep drawing is that the drawn part or cup length is greater than half its diameter.
In general, high production rates with fewer setups and simple blanking and forming operations may favor mechanical presses. However, considerations such as moderate- to lower-production-rate jobs, JIT requirements, workcell concepts, frequent setups, precise speed, force or position control of the slide, and built-to-order structure configurations favor the hydraulic approach.
Hydraulic presses now are accepted for blanking operations requiring lower production rates and small to medium-size parts. Rates from 20 to 40 pieces per minute are not unusual. Press structure design such as gap frame, four post, or straight side depends on part size, press loading (centered or off-center), and tooling or transfer design. The structure must withstand the shock of snapthrough coincident with a blanking operation.
The gap-frame press deflects in an angular mode when developing force. A four-post design generally deflects along a single axis opposite the force it is exerting. The straight-side press normally is a rigid structure that offers minimum deflection and is well-suited to off-center loading.
Blanking presses often are designed to be twice as heavy as those used in standard assembly or light, secondary operations. As a result, they achieve additional stiffness and lessen the effect of the snapthrough action.
A hydraulic press is suitable for blanking operations with these requirements:
1. Lower production rates. Hydraulic presses can be used to produce smaller parts (1 to 2 inches) at up to 60 strokes per minute (SPM) and larger parts (3 to 15 inches) at up to 30 or 40 SPM.
2. Difficult setup procedures. For varying types and levels of complexity, difficult setup procedures may be necessary. A hydraulic press is easy to adjust.
3. JIT part frequency changeover. This requirement favors the quick setup found in a hydraulic press.
4. Setup difficulty. To decide whether a hydraulic press is appropriate, you need to evaluate passline adjustment and decide whether force or distance limiting of the slide or ram is a requirement. Force or distance limiting favors a hydraulic press when the tooling design sets the distance limit. The hydraulic press also can be set easily to reverse on a preset force, either against the workpiece or against positive stops built into the tooling.
5. Possibility of press damage resulting from part jams. Hydraulic presses have a built-in overload protection. A part jam can occur if more than one part is fed into the die or if the ejection system fails to eject a completed part fully. A mechanical press could be damaged, while the hydraulic is force-limited.
Hydraulic presses in all configurations now are accepted by many manufacturers for stamping operations. The most commonly used press structures include gap frame (20 to 250 tons for smaller jobs), four post (50 to 1,000 tons for medium-size and larger parts), and straight side (for large parts requiring transfer stations or parts with severe off-center loading). Force levels to 5,000 tons are not unusual for straight-side presses.
Stamping jobs with these requirements favor hydraulic presses:
1. Small to large complex parts. To stamp or form small to large parts as described previously, a hydraulic press might be acceptable.
2. Frequent die changes and setups and JIT requirements. Hydraulic presses are suitable for short runs with frequent tooling changes and setups because hydraulic strokes and forces are easy to set up and often can be made in seconds.
3. Press feeding. Is the part hand-loaded or fed automatically? How? By linear feed, indexer, or robot? The type of feed and parts handling depends on the application and volume. Short runs or small-volume requirements (less than 100,000 per year) would be hand-fed, while large-volume parts would be partially or fully fed automatically.
4. Off-center loading. Jobs that require off-center loading can be done on straight-side hydraulic presses.
5. Material holding. Are cushions required for material holding or part ejection? If so, you'll need a hydraulic press.
6. Control over the slide or press ram. A hydraulic press should be considered when a job requires variable speeds, position control, position accuracy, or long power strokes.
7. Installation flexibility. Does the press need to be retooled easily for a new project? For instance, if the passline changes, a hydraulic press is easy to set up because full force is available in any part of the stroke.
A hydraulic press usually is recommended for deep drawing because it offers precise control of ram speeds, which allows metal to flow without tearing. Reverse-acting forces have time to coordinate with main ram action. Lower-production, frequently retooled deep draw jobs go into hydraulic presses as do those requiring varying speeds, variable cushioning, or force adjustments during the work stroke. Here are some other deep drawing job requirements for which a hydraulic press is recommended:
1. Complex designs and easy setup. A hydraulic press is recommended for complex designs or difficult adjustments or setup procedures because of the ease of force and distance setups.
2. Press structures. Can the job be completed in a single pass, or are multiple draws and finishing operations required? Hydraulic presses can be configured easily to suit the application. They are well-suited to single-pass and long-draw/reverse-draw applications. Again, the distance and force limiting features of the hydraulic press design make them suitable for finishing and final operations.
3. Full-force requirements. A hydraulic press offers full force through the entire work or draw stroke, probably resulting in a lower-tonnage machine.
When deciding whether your press requirements favor a hydraulic press — in addition to reviewing the factors given in this article and evaluating safety, repairability, uptime, and overall cost — you must provide the press manufacturer with as much information as possible. Too much information always is better than too little. Critical to the manufacturer are production rates, budget, tooling designs, part details, numbers of die changes, physical size of the structure, and expectations for the project.
Technology changes and improvements dictate that, to make an informed choice, you must stay in close communication with the various press vendors or manufacturers and be willing to share information and expectations. Only then will you select the right press and get the most out of it.
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