American Made - Case Studies
Metal Stamping Design
Metal stamping companies use a variety of sheet-metal forming manufacturing processes using a machine press or stamping press, the processes including punching, blanking, embossing, bending, forming, drawing, flanging, and coining. This could be a single stage operation where every stroke of the press create the desired form on the sheet metal part, or could occur through a series of stages. Stamping is an economical way to form metal components with variety of characteristics including strength, durability, and wear resistance. Also they will have good conductive properties and stability. The purpose of this design guideline is to provide some basic design concepts which could optimize all the features that a metal stamping company offers.
Metal Stamping Material Selection
Over-specifying a steel grade and blank thickness are major factors to drive up the cost of metal stamping. There are many choices of sheet and strip materials that will respond well to metal stamping and forming processes. However, the price and availability can vary in a wide range, so it has significant impact on the cost and delivery of production. The following are some key factors that should take into consideration when selecting the material and specifying physical characteristics of the material.
Metal Stamp Material Properties
There are many different ferrous and non-ferrous alloys available with different stock thicknesses and tolerances. The non-common alloys will be custom-produced by the steel mills, and they will only be available in the large quantities. It is possible to find someone who is using the material with the same specification for another application, but it would be a hit-or-miss, and it will have impact to the delivery schedules if missed. The quality of steel products has been improved greatly in the recent years. Continuous casting yields a very consistent and homogenous alloy mix. Today’s metal materials are more ductile and much more consistent, so the savings can be found from stock warehoused alloys instead of the more specified materials.
Metal Stamping Processes
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 in making high volume part of one 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.
Stamping dies are classified by the type of construction of the dies are compound die, combination die and progressive die.
Compound dies produce very accurate parts, but their production rate is quite slow. These dies consist of a single station where the part is most often blanked out and either formed, embossed, pierced, or otherwise adjusted in a single stroke of the press. No progression of the strip is involved, as each strokes of the press produces a single complete part. Combination dies combine at least two operations during each stroke of the press. Some compound dies are used just for trimming others are specialized for blanking.
Combination die combine at least two operations during each stroke of the press. Some shops, however, are making a distinction between the two types calling any cutting and forming die a combination die, while the compound die is considered only a cutting die.
A progressive die performs a series of fundamental sheet metal operations at two or more stations during each press stroke in order to develop a work piece as the strip stock moves through the die. The work piece on progressive dies travels from one station to another, with separate operations being performed at each station. Usually the work piece is retained in the stroke until it reaches the final station, which cuts off the finished piece. All stations work simultaneously at different points along the work strip, which advances on station at each stroke of ram. Thus a complete part is produced with each stroke. Progressive dies generally include blanking and piercing operations but a complicated progressive die can do the operation of bending, forming, curling and heading also. Each workstation performs one or more distinct die operation, but the strip must move from the first through each succeeding station to fabricate a complete part
Tool steels are used to construct the die components in a variety of press working operations which is subject to wear. If it is heat-treated these steels develop high hardness level and abrasion resistance.
Modern Tooling Design
Tooling design in the United States can be divided into two groups, design and fabrication. Tooling also has recurring and nonrecurring aspects. Tooling design refers to the special tools and equipment unique to a particular system.
General-purpose tooling design is usable for different products normally accounted for in the factory overhead category. Nonrecurring tooling design refers to the initial tool design and in-house fabrication or purchases, as well as subsequent buys for replacement or to increase manufacturing rates. Recurring tooling captures costs for maintenance and repair of tooling unique to manufacturing a particular system as well as wear parts, such as drill bits.
Note that lean principles hold engineering and tooling to be intrinsically related. Designing for lean manufacturing involves careful attention to minimizing all tooling costs. For example, parts that self-locate minimize the need for jigs and other tools that hold parts in the proper position for assembly. Flexible tooling that can be used for more than one part decreases the overall investment in tools.
Advances in Tooling Design
One mechanism to reduce tooling is to design and fabricate parts with devices that properly align them in the next higher assembly or to adjoining parts. For example, small tabs or tongue-and-groove features in adjoining parts can help locate them in the proper position during final joining, whether using fasteners, adhesives, or some other assembly method. This can dramatically reduce the number of dedicated assembly tools required to hold different parts and subassemblies in place as they are joined.
Another technological advance that contributes to lean is flexible tooling. Flexible tools can be used in the fabrication or assembly of multiple parts rather than being dedicated to a particular part or a small family of parts. Ideally, they should also have very low setup times. Tools that can make many different parts can be used to fill in and reduce bottlenecks by allowing for the manufacturing of whatever subassemblies are needed to continue the flow of aircraft through the plant.
Sheet Metal Stamping
Sheet metal stamping presses and stamping dies are tools used to produce high volume sheet metal parts. The press provides the force to close the stamping dies where they shape and cut the sheet metal into finished parts.
Production stamping is generally performed using sheet metal materials .020” to .080” thick, but the process also can be applied to foils as thin as .001” or to plate stock with thickness' approaching 1.000”. Formability is the primary attribute of sheet metal material.
Formability is further defined as the materials ability to be:
The metallurgical term for these qualities is “ductility”. Ductility is the materials ability to deform and elongate without fracture. The extent to which a stamping is subjected to such deformation is directly related to the part’s overall shape and geometry.
Other factors also influence the material’s formability. They include:
• the die design
• the press
• the press speed
• sheet metal feeding mechanisms
• monitoring and control systems
Sheet Metal Stamping Dies
The word “die” is a generic term used to describe the tooling used to produce stamped parts. A die set assembly consisting of a male and female component is the actual tool that produces the shaped stamping. The male and female components work in opposition to both form and punch holes in the stock. The upper half of the die set, which may be either the male or female, is mounted on the press ram and delivers the stroke action. The lower half is attached to an intermediate bolster plate which in turn is secured to the press bed. Guide pins are used to insure alignment between the upper and lower halves of the die set.
The most common types of dies perform cutting and forming. Cutting dies are used to shear sheet material into what is called a blank. These blanks are then exposed to blanking dies which cut the entire perimeter of the part, or to forming dies where the blank is stamped into a part.
Punching is another function of cutting dies. Punching is the cutting of a slug from the sheet metal stock to produce a hole or slot. Cutting dies are also used to trim excess metal from around a formed part. Hole punching and other cutting operations require specific and carefully maintained clearances between the punch (male component) and the die (female component).
The setting of the required clearances is determined by both the stock thickness and temper. In general, die clearances increase as the stock thickness increases. The depth of punch penetration into the sheet metal stock will also increase as softer stock is used.
Forming is a general term used to describe a stamped part whose shape and contour is reproduced directly from the shape and contour of a die set.
The main forming operations accomplished with press mounted dies are: