Stamping - A Cost Effective Process
The cost effectiveness of the metal stamping process stems from its ability to produce material-intensive parts at production rates much higher than are possible using other traditional methods. With material for stamping approximately equaling usage in another process, the prime area for savings is in cutting production time.
Many types of parts being designed to be cast, die cast, forged, machined, or fabricated could easily be redesigned for stamping.
There are many benefits in converting parts to Stampings, which add to the reduction in unit manufacturing costs achieved through higher production rates.
Stamping dies, especially those used for short-run stamping, can cost considerably less than the tooling used in other processes, e.g. molds, forging and casting dies, expendable cutting tools, etc.
Quality, accuracy, function, wear life and appearance can all be dramatically improved by making parts as Stampings. Often, the parts can be made from material which is tougher or harder than that obtainable in other processes.
Frequently, the number of secondary operations needed to finish a part, each requiring handling time as well, can be considerably reduced, and sometimes eliminated for added savings.
Here is a partial list of the types of parts for which stamping should be considered as the first choice production method:
- Friction plates
- Lock tabs
- Spring seats
- Conveyor flights
- Frame hangers
- Balance clamps
- Bushing seats
- Wear and swivel pads
- Swivel plates
- End, driven and butt plates
- Brake flanges
- Gear and sprocket blanks
- Yoke guides
- Wheel or backing rings
- Reinforcement plates
- Base weights
- Flywheel shrouds
- Engine bases
- Upright bars
If you are currently producing any of these in-house or buying them, made by a process other than stamping, you may wish to compare current costs to prices quoted by stampers. Or, if currently designing a new part, but not for stamping, ask some stampers to quote prices for producing it. You have little to lose, and much to gain. All that is usually required is to send out a print of the part as-made, or as-planned, and ask for prices for conversion. In making cost comparisons, include the secondary operations otherwise needed which stamping may eliminate.
If you are responsible for the specifications for parts, whether they will be made in-house or purchased, familiarizing yourself with stamping's capabilities is a first step.
There are basic considerations in designing for stamping; whether only one piece or millions are required. These must be taken into consideration if optimum results through stamping are to be gained.
It is equally important that purchasing, value engineering and other departments become involved with a project while it is still in the design stage, and before materials and specifications have been finalized.
Purchasing's involvement, early on, is especially important if the stampings must be bought. Purchasing must be sure it has competitive, capable sources lined up when the time comes to request quotations. if cost comparisons are planned between stamping and other processes, purchasing must also be ready to request quotes from several sources, and be able to answer questions which may be asked about materials, tolerances, finishes, function, and other parts specifications.
If a comparison is to be made, it is better to design specifically for the originally considered process, plus stamping. If you are short on stamping expertise, call in one or more potential suppliers, in the part design phase, to advise you.
The ultimate quantity of a stamping project, in-house or to be purchased, is a major factor in establishing meaningful cost estimates. Quantity is essential to planning the most suitable tooling and press scheduling for a part.
For example, if you are thinking in terms of an initial quantity of 5,000 Stampings, but your marketing department anticipates that the eventual annual volume will exceed 50,000 pieces, and projects 500,000 pieces over 5 years, how will this affect cost?
As lot size increases, using automated tooling would reduce the labor burden. Obviously, tooling for a 5,000 lot size will cost less because it will not be designed like the tooling required to produce 500,000 pieces. It may be designed for manual feeding, with the higher labor costs acceptable, generally, in exchange for the lower cost of tooling.
The same factors apply to selection of the tooling material and its processing costs. Tool steel versus carbide, for example. Even the type of operations, where a sufficient quantity may dictate use of progressive or transfer dies and automatic or semiautomatic tooling, is dictated by quantity.
Further, lot size affects material costs. Stampers buying material in high volumes may be able to pass along some savings in discounts they receive. Where volume is high enough to warrant developing scrapless, or near-scrapless tooling, even further material savings will be realized.
Material specified also can greatly affect costs. Avoid hidden or unnecessary costs by reviewing the thickness and tolerance specifications. Your stamping department or stamping supplier should be consulted on this.
Unless there is a specific engineering requirement, specify readily available standard mill materials and thicknesses. Unusual thicknesses may require very expensive and time-consuming special rerolling or mill orders.
Many new materials, from high strength alloy steels to super-strong aluminum alloys, are readily available in many standard thicknesses and widths. The pioneering work in processing them has been carried out by progressive stampers and their expertise is available to you when developing applications.
There is a direct correlation between the dimensional tolerances on a given part and the cost of tooling to produce it. It is by no means a straight-line relationship. Therefore, specifications and tolerances should be as tight as the end-product application requires, but no tighter.
If, for example, you specify a tolerance of .002" on a non-critical dimension where .010" is all that is really needed, you add greatly to the cost of tooling, processing, and inspection costs and get no greater return in part performance. Similarly, specifying an overall tolerance, such as "hold all dimensions +/-.005 inches", can add substantially to tooling and manufacturing costs. An overall requirement is wasteful when only one or two dimensions must be tightly controlled.
BLANKING is an initial operation involving a punch and die combination for cutting an outside part contour from a piece of stock. It's a single operation on low volume or larger part projects. Tooling is generally master dies with inserts.
PIERCING is the process describing production of inside contours and holes in sheet or plate material. It generally follows blanking, in short run stamping, or can be accomplished at the same time. In progressive stamping work, it is usually done first, with blanking the part as the last step.
COMPOUND DIES stamp both inside and outside contours at the same time in a single station tool. It is the most accurate method for maintaining concentricity and flatness of parts.
PROGRESSIVE DIES are designed for handling high volumes of parts from strip sheet or coil stock. By handling most of the stamping operations at progressive stations in the tooling, while the material is advanced through it, maximum efficiency is realized per press stroke and secondary handling of material is substantially reduced. This is particularly practical for large volume projects.
DRAWING is the process in which a draw punch causes a flat metal blank to flow into a draw die cavity without stretching the material. Die clearances are designed for 1T (one thickness) of material. The more ductile the metal, the easier it is to draw.
EMBOSSING uses a male and female die to produce shallow indentations or raised designs with no change in material thickness. Metal displacement is approximately 30%.
COINING is a closed-die squeezing operation for medium indentations or raised designs. Material displacement is approximately 60%.
SWAGING provides deep indentations of up to 100% material displacement. It is a squeezing operation where part of the metal under compression plastically flows into contours of the die while the remaining unconfined metal flows at an angle to the direction of applied pressure.
SHAVING is a secondary shearing or cutting operation on a previously cut edge for producing a smooth edge with minimum breakaway. Using highly accurate dies with precision clearances, the finished quality is comparable to fineblanking.
BENDING is the shaping of flat steel or strip metal utilizing a bend die and punch, with the restriction that the radius allowed must be 1T of material or larger.
EXTRUDING involves a punch and die combination in which metal is forced to plastically flow through a die opening so that the metal assumes the contour and cross-section of the opening. This process is used on flat stock to form parts that otherwise would have to be produced by machining.
BURR-SIDE/CUT RADII are common characteristics of stamped parts. There is a rough edge or breakaway burr around the periphery on one side of the blank or on pierced holes. The opposite side has a cut radius or "rollover" effect where the blanking or piercing punch enters the metal surface. Either or both of these conditions can be corrected if they prevent the part from meeting application criteria. Burrs can be removed through a choice of deburring processes, using a deburring tool, tumbling, or sanding. Drilling or reaming will make holes straight through the entire part thickness. A shaving station in a progressive die, or several, will also work, eliminating secondary handling operations. Also, using thicker stock than the final size called for and double disc grinding, or single side grinding, as required will remove burrs and rollover.
CONCENTRICITY describes the relative condition between inside and outside contours. It tends to "float" slightly in all types of production stamping other than compound die work. The variance can generally be covered with an allowance in the tolerance specifications.
FLATNESS/BLANKING DISTORTION, a slight bowing of the part, occurs due to peripheral metal stress from blanking pressure. The amount of bowing increases directly with the size of the part surface. While type of material, thickness and part configuration all affect control of flatness, the type of die used has the greatest influence. Again, a compound die produces the flattest parts.
Knowing the criteria of importance to the stamper being asked to quote on parts is often useful to the buyer, helping him/her to establish quantities to be quoted.
If you are in the habit of releasing orders for only a few thousand parts at a time, due to the pressure of inventory turnover requirements and limited storage space, you may be passing up savings of much greater significance.
The longer the run, the greater the economy, due to fewer manufacturing set-ups being required. It is also a fact that tool damage occurs most frequently during set-up, or soon after. Fewer set-ups mean fewer chances of damage and repair costs. Larger runs can also be more efficiently scheduled than frequent short runs. Therefore, initial and repeat orders should be as generous as possible.
Additional "up front" data required by the quoter includes specifics on dimensional limits, raw material specifications, tolerances, etc.
Important factors in the accurate costing of a project are: the type of die to be used, type and size of press to be utilized and analysis of die maintenance and tool cost per thousand pieces run.
These are generally contingent upon the thickness of material being processed, plus its hardness and formability.
Many stampers maintain an inventory of master die sets that accept relatively low cost inserts. Using this approach on short to medium runs can result in a lower investment in tooling. For longer runs, it is generally more economical to invest in a custom die set dedicated to the one part.
In addition to complete company name and address information; and name, title, telephone number of the person able to answer questions, the following should be submitted: part description (material, size, application, function), annual volume required, release quantities planned, whether a single buy or continual buy, percent of the total volume to be considered for quoting and part sketch or prints. Also advise if you wish to have preliminary consultation or feasibility data. By furnishing this information you are assured of the most favorable quotation possible.
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