Sometimes the line must be slowed down to accommodate these extra operations, and the resultant speed must be weighed against the savings from the elimination of secondary operations.
The advantages of performing as many operations in the cutoff as possible overdoing them in a prenotch die are substantial:
Assuming that a given shape can be cut off using either a blade cut or a crop cut (slugless), there are many definite economic advantages to using a slugless die.
Take a simple 1/8-inch thick “CEE” channel that is 3 inches wide and has legs that are both 2 inches high, for example. If a blade-cut die is used, it takes at least 2-1/4 inches of stroke to sever the part and push the slug down. Assuming the press is a 275 R.P.M. press with a 3-inch stroke, and the line is traveling at 100 F.P.M., the die would travel 3-1/4 inches out during each cut-off stroke.
If a crop die is used instead, we need only the bottom 1/4 inch of the stroke, and we would use 3/8 inche of die travel. We can see how the blade-cut die could keep line speed down if there was not much bolster length for die travel, if a positive stop is used (which adds to the die travel), or if the press had a slower R.P.M. In short-bed medium R.P.M. presses, a person could almost double the line speed just by using a slugless cutoff in some cases.
With the price of steel today, when heavier gauge parts are being run, the 3/16 to 3/8-inch thick slug, which is taken out each time a cut is made, quickly starts adding up to a potential cost saver.
When taking out a slug, not only the obvious direct cost of the material is involved, but also the cost of handling the constant flow of slugs from the press to the scrap collector.
There is also the problem of slug pulling, which can over-travel the die, jam the line, and therefore, cause downtime. To alleviate the problem, I sometimes lower the shut height in order to push the slug lower. This has the effect of increasing die travel, which in some situations could cause the line to slow down.
Die maintenance is a big factor in the economic considerations of a roll forming line. If there is no backup tooling, any time the dies are out for sharpening in the middle of a run, the entire line is down.
There is a tremendous difference between a blade cut and a crop cut die in total die life and length of run between sharpenings. The pressure buildup and resultant blade face gaulling from the blade traveling down the leg of a piece part is much greater than in a crop die where the blade is only traveling across the thickness of the part and not down the part. If the blade deteriorates, so does the quality of the cut.
In some lines, the blade must be changed or sharpened every few hours. In cases like these, it is much cheaper if replacement blades are kept on hand so downtime is kept to a minimum.
Not only does this continued maintenance affect line downtime directly, but it also affects overhead costs in that an extra maintenance man could possibly be eliminated simply by eliminating the extra maintenance required because of running blade cut dies.
