REDUCTION OF DIE TRYOUT - The most opportune time to employ the grid analysis is during die tryout. A simple, single blank test will identify critical stampings. Efforts can then be directed toward reducing the peak strain by redistributing the strain more uniformly throughout the stamping.
Many modifications can be made to the die and blank geometry. By numerically comparing strain distributions measured from finely gridded blanks formed before and after the modifications, mush guesswork and opinion would be removed. Sometimes modifications unintentionally increase the peak strain instead of reducing it. This result and other effects of changes may not be observable without grid measurements.
Progress made during die tryout can be quantitatively recorded. Some master mechanics carry portable electrochemical marking units when visiting various die shops in order to evaluate the current conditions of the dies.
Finally, numbers are available to substantiate requests for engineering changes. If the best material available shows a critical level of strain, then lubrication and/or die changes are the only available avenues remaining.
SPECIFICATION OF INITIAL MATERIALS - Analysis of the circular grid is used today to specify properties (or a commercial circular grade of steel) for stampings. First, a trial blank with a grid on the surface is formed into a finished stamping blank. The maximum or peak strain in the formed stamping is measured. If this strain is well below the critical level shown in Fig. 6, the mechanical properties of the trial blank are considered to be the property specifications of the material. These properties, in turn, indicate the grade and quality to be used. If the peak strain in the stampings is at the critical level, a material with a higher tensile-to-yield strength ratio and uniform elongation is suggested, or die and press variables must be changed.
Emphasis must be placed on knowing the properties or quality of the blanks used for circular grid test. Small changes in properties can create widely different press performances: the same property changes also affect the strain distribution. One, therefore, must identify the measured peak strain and strain distribution with a given set of properties. The test blank must be identical to the steel intended for the specific job. A blank with different properties will have a different strain distribution and will respond differently in the press.
Similar terms can evaluate lubricants. Each lubricant is used to form a gridded blank of a standard material. The lubricant which shows a maximum reduction of the peak strain below the critical strain level is the besteconomic factors being equal.
MONITOR PRODUCTION RUNS - Die conditions often change during an extended production run. Optimum die conditions may then vanish, causing the peak strain to approach the critical level. Periodic checks would forewarn of the impending danger. Alternatively, the peak strain may move away from the critical strain level to increase the safety factor.*
*The safety factor is taken to be the difference between the critical strain level and the actual strain level in the stamping. A safety factor of 0 indicates a critical stamping.
Table 1 - Physical Properties of Samples Used in the Instrument Panel Mounting Plate
|Per Cent Breakage||Yield, psi||Tensile psi||TS/YS Ratio||Per Cent Elongation in. 2 in.||Rockwell B||(a) n||(b) r|
|(a) Calculated by the Nelson ý Winlock method|
|(b) Equation Insert by using Microsoft 3.0 Equation|
Sometimes sudden breakage occurs and it is not known whether the material or die has changed. By maintaining a library of standard material, a "check blank" with a grid could be rapidly formed and measured. If the strain distribution remained the same, then the tools most likely did not change and the material would be suspected. If, on the other hand, the strain distribution had peaked to a higher strain, then tool or press variables should be investigated. This would be especially helpful information when resetting tools back into press after removal.
Experimental steels could be rapidly evaluated with only a few blanks by comparing the strain distribution obtained form the experimental steel with that found in the production steel.