February 27, 2003
Today more than ever, the metal forming industry is economically challenged. Everyone is scrambling to find new ways to lower process costs without sacrificing quality. In the case of metal formability, a lot is at stake. With metal prices and operating overhead continuing to rise, any downtime or wasted material significantly impacts the bottom line.
By looking at the relationship between metal formability and lubricant, we can make some significant discoveries. Both the metal and lubricant used influence the formability outcome. If the part requires a given N value of steel, but the lubricant fails to provide consistent metal flow over critical radii, the part may thin and fracture. On the other end of the spectrum, if the lubricant can enhance the formability of the metal, a lower grade of steel may be used.
The N value represents the most direct correlation of sheet metal to formability. However, N value comes at a price, often represented by the metal grades for forming, for example, a high N value represents draw-quality interstitial-free (DQIF) cold-rolled steel, while a lower N value represents draw-quality hot-rolled (DQHR) steel.
The chart in Figure 1 shows the combined cost per ton of four steel and lubricant grades with the most expensive steel matched up with the lowest-cost lubricant at one end ($504 per ton) and the lowest-cost steel matched with the most expensive lubricant at the other end ($367 per ton). If you select a better grade of metal, the effect of the lubricant is lessened. The opposite is also true; if you select a higher grade of lubricant, you may be able to substitute a lower grade of metal. As Figure 1 illustrates, the metal selection represents 98 to 99 percent of the total lubricant-metal investment. Lowering metal grade has a far greater savings impact than lowering the lubricant grade.
A recent study at a major lawn and garden equipment manufacturer determined that a drawn lawn mower deck could be produced at current quality levels with lower-cost DQHR if a high-performance dry film was used in place of a lower-cost-extreme pressure fluid. This change gave the company an annual steel savings of more than $1 million.
A one-percent change in scrap rate has a significant impact on the bottom line. A company pays for scrap in two ways: wasted metal and wasted time. The time spent making bad parts can quickly add up, and the value of the scrapped metal never can be recovered.
Field evidence has shown that skimping on lubricant can have a direct impact on scrap levels. Figure 2 shows the metal cost impact of a 1 to 5 percent reduction in scrap for a 25,000-ton annual user of DQCR. The savings realized by scrap reduction reinforces the necessity of making sure that the metal-lubricant relationship is properly understood and optimized.
Any major process or material change must be thoroughly studied before it can be properly considered, especially if it impacts formability or part quality. The selected evaluation method must be able to establish a current process benchmark to quantify any subsequent change in a press shop environment.
One method is in-press strain deformation mapping, a process that uses circle grid and peak strain analysis. Critical areas of current production metal are electrochemically gridded, formed, and then analyzed. The resultant strain data is then mapped to record the metal thinning at critical areas. This information serves as a baseline and a record of the current process's part capability. Alternative metal and lubricant combinations then can be studied using this data as a baseline.
Figure 3 shows the mapping of a drawn section of a DQHR part, highlighting the critical areas to monitor when evaluating a metal or lubricant change. Areas with a lower forming safety factor (potential for local thinning) are depicted in red. More tolerant areas are represented in green.
Lubricant and metal quality significantly impact the productivity and profitability of any metal forming business. A thorough consideration of a metal-lubricant change or at least a benchmarking study to verify that the current metal-lubricant selection is optimal is critical. These exercises can provide a valuable insight into how these interrelated factors affect your business's bottom line.
Metal cost data was provided by Larry Dalsin of Ferro Business Solutions e-mail: firstname.lastname@example.org.
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