What is Stamping?
Learn Up: Precision Sheet Metal Stamping
Sheet metal stamping is among the most common and necessary of all metalworking technologies. There’d be no body panels or bumpers on your car or truck without stamping. You’d have no coins in your pocket, no electrical connectors in your Xbox, no hinges nor lock on the front door. Metal cookware and utensils, parts for power tools, appliances, and mobile devices—these are just a few of the everyday items produced on stamping presses.
Similar But Different
But how does this vital metalworking technology work? Just as importantly, what do you as a manufacturing person need to know about it? Let’s start with the first question. If you read previous installments of Learn Up, you’re already familiar with turret punch presses and press brakes. Stamping presses share some similarities, and often perform many of the same operations.
We’ll talk more about the various types of stamping machinery and the operations they perform in a moment, but all are equipped with a movable slide or ram, to which the top half of a die is attached. As the ram moves up and down, often at great speed, it forces a punch (or punches) within the die into and sometimes through the sheet of metal below. Forming and coining tools may also be present in the die, as well as tools used to bend, slice, or emboss the metal.
Quantity is King
The metal itself is usually in sheet or coil form, but may also be a pre-cut blank made in a previous metalworking operation. Depending on the type of die and press, the metal might be advanced automatically with each stroke of the ram. For example, a progressive die contains a series of stations, each with various tools that perform discrete operations per stroke. As the material feeds forward, the workpiece could be punched, formed, bent, lanced, and finally, cut free of the coil.
Brief descriptions of each of these operations is forthcoming, but what’s important for now is that progressive dies are generally more expensive than single-station dies, and are therefore found in high volume applications—think millions of parts, although this is not always the case. Transfer die stamping, on the other hand, uses separate dies and sometimes separate machines. With each stroke of the press or presses, the system transfers the in-process workpiece to the next station. Of course, for lower volume work, a human or team of humans can accomplish the same thing, although this would obviously not be called automated transfer stamping.
As you can see, production quantity plays a key role in determining the type of die and stamping strategy. Here’s a brief recap:
- Progressive dies are quite common in high-volume work like that seen in the automotive and consumer goods markets.
- Transfer dies are typically used for larger parts, where the cost or size of a progressive would be prohibitive.
- There are also compound dies, which blank the part (cut its outer shape) and form or pierce its interior features in a single blow.
- Lastly, simple dies are just as their name describes—basic, single operation tools almost always fed by a human or robot.
Whatever the die and however many operations it performs per stroke, all are precision-machined, typically using CNC lathes and machining centers, different types of grinding equipment, and wire or sinker EDM. Hardened tool steel is usually the material of choice for the tools themselves, which are attached to a steel die set, although harder and more expensive carbide might be used on production tooling.
The workpiece material and its final shape may also play a role in all this decision-making. For instance, stamping of hardened steels or superalloys such as Inconel and titanium would almost certainly call for carbide tooling, while a low-volume part made of mild steel could be stamped using regular tool steel. Similarly, small complex parts made in high volumes would mandate a progressive die, just as stamping out a few dozen salad bowl-shaped parts is easily accomplished via a simple die and manual loading.
What Kind of Press?
So what are the different kinds of stamping presses? Here again, the size and quantity of workpiece plays a big role in deciding which one is used. A basic C-frame press is an excellent choice for the salad bowl example just given. A high-speed, straight-sided press able to hit 1000 strokes or more per minute would be used to make electrical connectors and smartphone parts. An open back inclinable (OBI) is a general-purpose press that’s good for blanking and shallow forming, whereas an eccentric geared press is often employed for deep-drawing operations where very long strokes are required. Many variants, brands, and models of each of these exist.
So do the different drive types. Flywheel-driven mechanical presses are the oldest of all and still make up the lion’s share of presses today, and are available with a wide variety of geared transmissions. Hydraulic presses, however, provide greater control over stroke speed and can apply full tonnage throughout the stroke, an attribute to which mechanical presses cannot lay claim.
Over the past few years, electronic servo-driven presses have come on the scene. Though not yet available in the higher tonnages seen with mechanical stampers, servos are said to offer the greatest control and accuracy of all presses, with full power available anywhere in the stroke. This makes them an excellent choice for job shops, where a wide array of materials and thicknesses as well as workpiece shapes are encountered.
A Universe of Operations
Let’s talk about some of the metalworking operations done on a stamping press. This list is not complete, but it should give you a pretty good idea of this centuries-old machine tool’s versatility:
- Blanking: This is often the first operation for many parts and can also be performed on a shear or laser cutter. Simply put, blanking is the act of cutting the outer periphery of a part, or cutting a square or rectangular “blank” from a larger piece of material that will then be placed into a die for additional work.
- Piercing: Where blanking cuts the outer edge of a workpiece, piercing cuts holes in its interior. They don’t necessarily have to be round holes, either—rectangles, oblongs, and cutouts shaped like your company logo are all possible, provided they can be machined into the punch and its mating die below.
- Lancing: Stop a bit short on a piercing operation and you’ve just lanced the part. Lancing drives the punch through the material but not far enough to actually create a complete hole—look at the covers on your furnace ducts at home as an example, or the little locking tabs found on metal enclosures and housings.
- Forming: The body panels listed at the start of this article were formed on a stamping press. So was your favorite frying pan, the lid on the sugar container you used at breakfast, and the little metal clips that secure the brake lines to your car’s undercarriage. As you’ll see, however, forming on a stamping press comes in…well, many forms.
- Stretching: Actually, it’s possible that the body panels just mentioned were stretched, not formed. What’s the difference? In a stretching operation, the edges of the workpiece blank are tightly secured when the die comes down, forcing the material within to stretch and thin as it bends around the die. Forming has no such edge constraints.
- Drawing: Drawing and deep drawing are used to make everything from oil pans to soda cans. It’s quite similar to stretching, except that the blank edges are constrained, although not tightly. This allows the material to flow into the die as the ram comes slowly down, gently nudging it into its final shape but without any of the thinning that occurs in stretching operations.
- Coining: Remember that recent mention of coins in your pocket? It’s appropriate here because the likenesses of presidents Lincoln and Washington were “coined” on a stamping press using a die containing a machined mirror image of old Abe or George. As you might imagine, coining requires significant force and a rigid, powerful press.
- Embossing: Similar to coining, embossing also creates a mirror image of whatever shape is on the die. What’s different is that this shape carries through the workpiece and onto the opposite side. Military dog tags are embossed. So are the supporting ribs on certain mounting brackets, and decorative shapes on architectural hardware.
There’s far more. For example, cutting is the last step in a progressive die, when the completed workpiece is finally removed from the coil. Flanging creates reinforced edges around holes and workpiece perimeters. Trimming removes the wrinkled bit of leftover material around the edge of blanked parts. And, of course, stamping can also perform bending, ironing, slitting, notching, and other operations.
Admittedly, the past few pages have offered but a small taste of the metal stamping pie. Like most metalworking fabricating processes, this is a deep and highly complex technology that takes years of study and experience to master. That said, the information presented here should serve as a good foundation for anyone looking to design or purchase stamped metal parts.
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