Metal stamping involves cutting and forming sheet metal with the aid of a punch press. Punch presses vary extensively in bed size, speed and tonnage. Regardless of the size of the press, tooling must be made to produce a part to a specific configuration. Although there are other specialty stamping processes used in industry such as hydro forming, multi-slide stamping, and transfer press stamping, our focus here is on the two most commonly used in industry, progressive die stamping as well as short run or small lot stamping.
Progressive Die stamping – As the name implies a progressive die performs operations on a part as the metal progresses or moves through stations in a die. Each station in a progressive die performs a unique operation on the strip of material. Each stroke of the press results in a part regardless of how many operations or stations are in the die. Material is typically feed through a progressive die in coil form and once the press is started the operation does not require much operator interface.
Short run stamping – Short run stamping differs from progressive die stamping in the method in which the tooling is constructed. Tools are engineered to fit into standard holders eliminating the need for a stand alone die set for each part. Short run stamping companies maintain an inventory of most standard punch sizes and form tools. In many cases the only unique tooling produced is the banking die. Material is sheared to width and then hand fed through the blanking tool cutting the part out. Subsequent operations such as forming are performed at another press with the parts being placed into and removed from the tools by hand. Because of the simplicity of the tooling scenario initial cost and lead time is generally a fraction of building a progressive die.
Metal stamping design guidelines – The guidelines listed below will work for either short run or progressive die stampings. In most instances progressive die stampings can be held to tighter tolerances than listed below with the exception of material thickness tolerance.
Material Thickness Tolerance – The chart below lists standard tolerance for cold rolled coil and sheet steel. Tighter tolerance can be achieved through secondary rolling although this adds to cost, lead time and results in larger minimum material buys.
Minimum Hole Diameter – The general guideline is that the minimum hole diameter should be 1.2 times the material thickness. The guideline changes when dealing with higher strength material. Material with an ultimate tensile strength of 50,000 (PSI) the minimum hole diameter should be 1.5 times thickness. For material with an ultimate tensile strength of 95,000(PSI) the minimum hole diameter should be specified at 2 times the material thickness.
Hole Taper – Because of the clearance between the punch and the die, and the manner in which the material breaks stamped holes do not have a constant diameter throughout the material thickness. Stamped holes should be specified and measured from the punch side.
Edge to Hole – An allowance of 2 times the material thickness should be allowed from the edge of a part to the edge of a hole.
Hole to Form – Leave an allowance of 2.5 times the 4 times material thickness plus the bend radius
Slot to Form – Allow for 4 times material thickness plus the bend radius between long slots and forms.
External notches and Tabs – These features should have a minimum width of 1.5 times the material thickness.
Corner radii – Corner radii should be at least 1/2 times the material thickness. Tighter corner radii can be achieved but often come at the expense of a larger burr as well as additional tool maintenance or shorter tool life.
Forms – Angular tolerance of plus or minus one degree is common for formed features. This is because forms are generally bent beyond the specified angle in order to accommodate the spring back which is inherent to all metals. Inside bend radii should be at least one material thickness.
Flatness – It is recommended that parts be measured in unrestrained condition.
|Surface Length||Flatness Tolerance|
|From 0” to 1”||.005 T.I.R.|
|From 1” to 4”||.005” T.I.R/inch|
|Over 4”||.020 plus .004/inch T.I.R|
Burrs – Burrs are a normal part of any stamping process. Burrs can vary in size and can become more prominent as tooling wears. Burrs of up to 10% of the material thickness are considered normal. Burrs are routinely removed by automated sanding, tumbling or even hand operations in some cases. It is important to specify the degree of deburring that is expected when specifying stampings. Some examples….
- Do not deburr
- Remove sharp edges
- No visible projections to the naked eye
- No visible projections to “X” magnification. Specify degree of magnification
- Break edges to “X” by “Y” minimum
- Round edges to between “X” and “Y” radii
Purchasing Considerations – Most short run stamping tooling is billed as an engineering charge because in most instances, only part of the tooling built is unique to a particular part. Die holders are built to fit a specific press, and in most cases would be of no use to another short run stamping producer. Progressive dies are generally considered to be the customers property. In this case, as with any customer owned tooling it is important that these tools be insured. In the event a transfer be necessary, it is important to know the type of press the tooling was designed to operate on.
How we can help – There is considerable crossover between the various stamping processes. There are also often applications where modern sheet metal fabrication techniques can enter into the equation. Part configuration, function and product life are important factors which we take into consideration when recommending a metal stamping solution. Request a quote and put us to work on your unique requirements today.