The Science of Forming
Let Your Supplier Help
Top-notch suppliers bring expertise and data to the table. Many have forming specialists on staff; some have virtual forming ability, to analyze parts early in the design process; and others have created and thoroughly studied new metal alloys or available forming techniques.
In contrast, some supplier/customer relationships remind one of the cold wars. Three examples of these relationships (or lack thereof) are:
• Bad: Adversarial—“My supplier causes all of my problems.”
• Worse: Protectionist—“I don’t trust my suppliers because they might give all of my secrets to my competitors.”
• Worst: Ignorance—“I don’t know who my suppliers are because my end customer purchases the lowest-cost steel for me.”
Know thy Steel—Well
|Fig. 1—Mill capability shows the property range expected for a specific steel type and grade.|
One must be careful to use the correct distribution data. Obviously, steel produced for simple highway guard rails will exhibit different properties than will steel produced for a complex-shaped oil pan. Therefore, do not use a distribution curve developed for all AKDQ steel from a given mill for all customer products. Instead, steel mills can provide a series of process codes that details the exact composition and processing parameters used to produce the steel for forming different types of parts. You need the property range for the process code that closely matches the forming requirements of your specific application. If you are going to order steel directly from the mill over an extended length of time, some mills will create special process codes just for your difficult parts.
Another curve can be constructed that shows n-value limits that will allow forming of an acceptable part (Fig. 2), typically referred to as the engineering window. If n-value is too low the part will neck and tear, and
|Fig. 2—Engineering window shows the steel property range required to produce a satisfactory part.|
The Final Step
Next we overlay Fig. 1 and Fig. 2 to create Fig. 3—the proper AKDQ steel-product match exists when the engineering window encompasses the entire mill capability. This indicates that all properly made steel should produce acceptable parts.
Unfortunately, this did not happen with the actual case study illustrated in Fig. 3. Here
|Fig. 3—Overlaying the engineering window and mill capability reveals potential production problems.|
Follow this Procedure
A simple press-shop procedure could have avoided the problem described above:
1) The press shop orders and receives a coil of production steel, and has an independent firm tensile-test the steel to ensure that its mechanical properties are near the mean properties of the mill capability. Don’t assume that coils are shipped with the required properties.
2) The press shop sends half of one coil and a container of the production lubricant to the tool-build and tryout vender, even if it is another department within your own company. Use these coil and lubricant samples during die tryout and buyoff.
3) At final buyoff, perform circle-grid and forming-limit-curve analyses to document that the safety margin is in the green zone. Good parts may be produced, but the process cannot be on the edge of the deformation cliff. This procedure often is performed by the steel supplier at tool vendors making dies for automotive-OEM press shops.
4) When the tool finally arrives at the customer’s press shop, use the other half of the test coil and the same lubricant to check the safety margin of tooling run in the intended home press line—the tool, steel and lubricant were checked out at die buyoff, but the press was different. Also, consider testing the tool in a backup press line. Run this check well before the first production is scheduled. A few press shops have stored coils with the highest and lowest expected n-values, to run extended tryouts. MF
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