The success of a roll forming line directly correlates to using the correct cutoff system. As a critical component of roll forming operations, the cutoff die impacts the speed and efficiency of each line, which ultimately determines the profitability of a line. This article discusses cutoff tooling and press design options and how they affect the economic performance of a roll forming line.
Profitability considerations
Assuming that a given shape can be cut off using either a blade cut or a crop cut (slugless), there are many economic advantages to using a slugless die. For example, if a blade cut die is used on a simple 1/8-in.-thick C channel that is 3 in. wide and has legs that are both 2 in. high, it takes at least 2 1/4 in. of stroke to sever the part and push the slug lower. Assuming the press is a 275-rpm press with a 3-in. stroke and the roll forming line is traveling at 100 ft. per min., the die travels 3 1/4 in. out during each cutoff stroke.
If a crop cut die is used instead, only the bottom 1/4 in. of the stroke is needed and 3/8 in. of die travel is used. The blade cut die can keep line speeds down if there is not much bolster length for die travel, if a positive stop is used (which adds to the die travel) or if the press has a slower rpm. In some cases, for short-bed, medium-rpm presses, the line speed could be almost doubled just by using a slugless cutoff instead.
With the expense of steel today and an increase in heavier gauge parts being run, the 3/16- to 3/8-in.-thick slug, which is taken out each time a cut is made, quickly adds costs. When taking out a slug, not only is the obvious direct cost of the material 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, one option is to lower the shut height in order to push the slug lower. However, this has the effect of increasing die travel, which in some situations, could cause the line to slow down.
Maintenance and labor
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 terms of total die life and length of run between sharpenings. With a blade cut die, the pressure buildup and resultant blade face galling from the blade traveling down the leg of a C channel is much greater than in a crop cut die where the blade only travels across the thickness of the part and not down the part. If the blade deteriorates, so does the quality of the cut.
In some roll forming lines, the blade must be changed or sharpened every few hours. In this case, it is much cheaper to keep replacement blades on hand so downtime is kept to a minimum. Not only does this continued maintenance affect line downtime directly, it also affects overhead costs. For instance, an extra maintenance worker could be reassigned if the blades required less maintenance.
Combination dies
The savings involved, directly and indirectly, in running combination dies can be numerous. If the die can punch and form while it is cutting off, the time savings can determine if the product is very profitable or just marginal. Sometimes, the roll forming line must be slowed down to accommodate these extra operations, and the slowdown must be weighed against the savings from the elimination of secondary operations.
Performing more operations in the cutoff press than a pre-notch press offers significant advantages, including:
- There is always some growth between holes in the roll forming process. If all holes are punched in the cutoff, there is no growth.
- Typically, the line can be run faster with no pre-notching operations, even when performing additional operations in the cutoff.
- Because the cutoff press is already in the line, as long as the tonnage is enough, extra operations are “free” in a sense.
- This also frees up the pre-notch press for use on other lines.
Without being able to automatically gag punches in or out on any stroke of the press, the pre-notching setup for some lines would have to be changed. In many cases, the auto-gagging of punches could not be done at all except in secondary operations with much more expense, much lower line speeds and possibly additional presses required.
In a roll forming line, the dies are tightly confined. Pulling a die out to change punching configurations can be time-consuming, especially when large dies are involved. To minimize changeover time, which can sometimes take hours, manual gag or ball-lock punches should be used whenever possible. If changeover time is converted into potential production, the direct loss could be substantial. This can also mean tying up a line that could be running other items.
By choosing the correct cutoff dies and roll forming line setup, fabricators have the power to boost efficiency and profitability in the shop. To maximize the ROI in cutoff methods and equipment, it is critical to consider the advantages of slug versus slugless dies, die maintenance requirements and the benefits of combination dies in roll forming. Equipment providers can be instrumental in helping to choose the correct cutoff tooling for any shop’s needs.
