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What is stamping

The operations associated with stamping are blanking, piercing, forming, and drawing.
These operations are done with dedicated tooling also known as hard tooling. This type of tooling is used to make high volume parts of one configuration of part design. (By contrast, soft tooling is used in processes such as CNC turret presses, laser profilers and press brakes). All these operations can be done either at a single die station or multiple die stations — performing a progression of operations, known as a progressive die.

Equipment Types
The equipments of stamping can be categorized to two types: mechanical presses and hydraulic presses.
Mechanical Presses: Mechanical presses has a mechanical flywheel to store the energy, transfer it to the punch and to the workpiece. They range in size from 20 tons up to 6000 tons. Strokes range from 5 to 500 mm (0.2 to 20 in) and speeds from 20 to 1500 strokes per minute. Mechanical presses are well suited for high-speed blanking, shallow drawing and for making precision parts.
Hydraulic Presses: Hydraulic Presses use hydraulics to deliver a controlled force. Tonnage can vary from 20 tons to a 10,000 tons. Strokes can vary from 10 mm to 800 mm (0.4 to 32 in). Hydraulic presses can deliver the full power at any point in the stroke; variable tonnage with overload protection; and adjustable stroke and speed. Hydraulic presses are suitable for deep-drawing, compound die action as in blanking with forming or coining, low speed high tonnage blanking, and force type of forming rather than displacement type of forming.

Bending is a common metalworking technique to process sheet metal. It is usually done by hand on a box and pan brake, or industrially on a brake press or machine brake. Typical products that are made like this are boxes such as electrical enclosures, rectangular ductwork, and some firearm parts such as the receiver of the AKM AK-47 variant.

Usually bending has to overcome both tensile stresses as well as compressive stresses. When bending is done, the residual stresses make it spring back towards its original position, so we have to overbend the sheet metal keeping in mind the residual stresses.

When sheet metal is bent, it stretches in length. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside. A bend has a radius. The term bend radius refers to the inside radius. The bend radius depends upon the dies used, the metal properties, and the metal thickness.

Many software packages refer to the K-factor for bending sheet metal. K-factor is a ratio that represents the location of the neutral sheet with respect to the inside thickness of the sheet metal part. The bend allowance is the length of the arc of the neutral axis between the tangent points of a bend in any material.
BD = 2*(R + T) - BA
BA = π*(R + K*T)*A/180
K = (180 * BA)/(π*A*T) - R/T
where:

  • BA = bend allowance
  • R = inside bend radius
  • K = K-Factor, which is t / T
  • T = material thickness
  • t = distance from inside face to neutral sheet
  • A = bend angle in degrees (the angle through which the material is bent)

Blanking is a shearing process where a punch and die are used to create a "blank" from sheet metal or a plate. The process andmachinery are usually the same as that used in piercing, except that the piece being punched out in the piercing process is scrap.
Fine blanking

Typical fine blanking press cross section
Fine blanking is a specialized form of blanking where there is no fracture zone when shearing. This is achieved by compressing the whole part and then an upper and lower punch extract the blank. This allows the process to hold very tight tolerances, and perhaps eliminate secondary operations.

Materials that can be fine blanked include aluminium, brass, copper, and carbon, alloy and stainless steels.

Fine blanking presses are similar to other metal stamping presses, but they have a few critical additional parts. A typical compound fine blanking press includes a hardened die punch (male), the hardened blanking die (female), and a guide plate of similar shape/size to the blanking die. The guide plate is the first applied to the material, impinging the material with a sharp protrusion or stinger around the perimeter of the die opening. Next a counter pressure is applied opposite the punch, and finally the die punch forces the material through the die opening. Since the guide plate holds the material so tightly, and since the counter pressure is applied, the material is cut in a manner more like extrusion than typical punching. Mechanical properties of the cut benefit similarly with a hardened layer at the cut edge from the cold working of the part. Because the material is so tightly held and controlled in this setup, part flatness remains very true, distortion is nearly eliminated, and edge burr is minimal. Clearances between the die and punch are generally around 1% of the cut material thickness, which typically varies between 0.5–13 mm (0.020–0.51 in). Currently parts as thick as 19 mm (0.75 in) can be cut using fine blanking. Tolerances between ±0.0003–0.002 in (0.0076–0.051 mm) are possible based on material thickness & tensile strength, and part layout.

With standard compound fine blanking processes, multiple parts can often be completed in a single operation. Parts can be pierced, partially pierced, offset (up to 75°), embossed, or coined, often in a single operation. Some combinations may require progressive fine blanking operations, in which multiple operations are performed at the same pressing station.

Advantages & disadvantages

The advantages of fine blanking are:

  • excellent dimensional control, accuracy, and repeatability through a production run.
  • excellent part flatness is retained.
  • straight, superior finished edges to other metal stamping processes.
  • smaller holes possible relative to thickness of material.
  • little need to machine details.
  • multiple features can be added simultaneously in 1 operation.
  • more economical for large production runs than traditional operations when additional machining cost and time are factored in (1000–20000 parts minimum, depending on secondary machining operations)

The disadvantages are:

  • slightly higher tooling cost when compared to traditional punching operations.
  • slightly slower than traditional punching operations.

Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.
Process
The total drawing load consists of the ideal forming load and an additional component to compensate for friction in the contacting areas of the flange region and bending forces at the die radius. The forming load is transferred from the punch radius through the drawn part wall into the deformation region (sheet metal flange). Due to tensile forces acting in the part wall, wall thinning is prominent and results in an uneven part wall thickness. It can be observed that the part wall thickness is lowest at the point where the part wall loses contact with the punch, i.e. at the punch radius. The thinnest part thickness determines the maximum stress that can be transferred to the deformation zone. Due to material volume constancy, the flange thickens and results in blank holder contact at the outer boundary rather than on the entire surface. The maximum stress that can be safely transferred from the punch to the blank sets a limit on the maximum blank size (initial blank diameter in the case of rotationally symmetrical blanks). An indicator of material formability is the limiting drawing ratio (LDR), defined as the ratio of the maximum blank diameter that can be safely drawn into a cup without flange to the punch diameter. Determination of the LDR for complex components is difficult and hence the part is inspected for critical areas for which an approximation is possible.
Commercial applications of this metal shaping process often involve complex geometries with straight sides and radii. In such a case, the term stamping is used in order to distinguish between the deep drawing (radial tension-tangential compression) and stretch-and-bend (along the straight sides) components.

Variations
Deep drawing has been classified into conventional and unconventional deep drawing. The main aim of any unconventional deep drawing process is to extend the formability limits of the process. Some of the unconventional processes include hydromechanical deep drawing, Hydroform process, Aquadraw process, Guerin process, Marform process and the hydraulic deep drawing process to name a few.

The Marform process, for example, operates using the principle of rubber pad forming techniques. Deep-recessed parts with either vertical or slopped walls can be formed. In this type of forming, the die rig employs a rubber pad as one tool half and a solid tool half, similar to the die in a conventional die set, to form a component into its final shape. Dies are made of cast light alloys and the rubber pad is 1.5-2 times thicker than the component to be formed. For Marforming, single-action presses are equipped with die cushions and blank holders. The blank is held against the rubber pad by a blank holder, through which a punch is acting as in conventional deep drawing. It is a double-acting apparatus: at first the ram slides down, then the blank holder moves: this feature allows it to perform deep drawings (30-40% tr ansverse dimension) with no wrinkles.

Industrial uses of deep drawing processes include automotive body and structural parts, aircraft components, utensils and white goods. Complex parts are normally formed using progressive dies in a single forming press or by using a press line.

Workpiece Materials and Power Requirements. Softer materials are much easier to deform and therefore require less force to draw. The following is a table demonstrating the Draw force (lbs) to percent reduction of commonly used materials.

Drawing force(lbs)


Percent Reduction

39%

43%

47%

50%

=Material=

 

 

 

 

Aluminum

19,800

22,600

25,400

28,300

Brass

26,400

30,200

34,000

37,700

Cold-Rolled
steel

28,600

32,700

36,800

40,800

Stainless steel

37,400

42,700

48,100

53,400

Tool Materials

Punches and Dies are typically made of tool steel, however carbon steel is cheaper, but not as hard and is therefore used in less severe applications, it is also common to see Cemented carbides used where high wear and abrasive resistance is present. Alloy steels are normally used for the ejector system to kick the part out and in durable and heat resistant blank holders

Progressive stamping


Progressive (punch and blanking) die with strip and punching
Progressive stamping is a metalworking method that can encompass punching, coining, bending and several other ways of modifying metal raw material, combined with an automatic feeding system.
The feeding system pushes a strip of metal (as it unrolls from a coil) through all of the stations of a progressive stamping die. Each station performs one or more operations until a finished part is made. The final station is a cutoff operation, which separates the finished part from the carrying web. The carrying web, along with metal that is punched away in previous operations, is treated as scrap metal.

The progressive stamping die is placed into a reciprocating stamping press. As the press moves up, the top die moves with it, which allows the material to feed. When the press moves down, the die closes and performs the stamping operation. With each stroke of the press, a completed part is removed from the die.

Since additional work is done in each "station" of the die, it is important that the strip be advanced very precisely so that it aligns within a few thousandths of an inch as it moves from station to station. Bullet shaped or conical "pilots" enter previously pierced round holes in the strip to assure this alignment since the feeding mechanism usually cannot provide the necessary precision in feed length.

The dies are usually made of tool steel to withstand the high shock loading involved, retain the necessary sharp cutting edge, and resist the abrasive forces involved.

The cost is determined by the amount of features, which determine what tooling will need to be used. It is advised to keep the features as simple as possible to keep the cost of tooling to a minimum. Features that are close together produce a problem because it may not provide enough clearance for the punch, which could result in another station. It can also be problematic to have narrow cuts and protrusions.

Applications
An excellent example of the product of a progressive die is the lid of a beverage can. The pull tab is made in one progressive stamping process and the lid is made in another

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