What makes one manufacturer’s roll forming line enormously profitable and another manufacturer’s line marginal, even when they are producing similar products on the same model roll former? The key is to have the proper auxiliary systems, tooling, and dies. In this post, we'll discuss the differences between, and the design criteria for, the different types of prenotch roll forming dies and how the different types affect line speeds, part tolerances, and part costs.
Prenotch dies and sophisticated prenotching systems are used because of the economic implications of secondary operations. Even though prenotching operations can slow line speeds down, there is still so much saved by performing all operations in-line that it more than makes up for potentially slower line speeds.
There are three basic prenotching setups, so let's explore the considerations for each of them.
The stationary prepunch die is basically a conventional stamping die, with some modifications for operation in a roll forming line, mounted in a variety of presses (mechanical, hydraulic, or pneumatic) in a variety of ways. The stock is fed through the die and is stopped while the punching is being performed. Contrary to the standard stamping die, because of the roll former, two methods can be used to feed the stock through the die.
With a continuous pattern or a series of repeating patterns running the length of the part, a standard press feed (either an air feed or a mechanically driven roll feed) can be used, mounted on the entrance or exit end of the press. The ram-driven roll feed can be a more consistent performer when compared to an air feed. However, it is also more expensive and less versatile.
Intermittent punching is employed when you have a longer part and the punched openings or patterns are not continuous down the length of the part. Today, this is accomplished by the use of computer controlled servo-feeds where we can program a different feed length for each stroke of the press if required.
It is possible to alter a pattern of punching within a die after each press stroke or after any number of press strokes by the use of air cylinders to “gag” punches in or out. To gag a punch simply means to move a cam or a striker bar, depending on the die design, into position behind a punch (or cluster of punches) so on that particular stroke of the press the punch is driven deeper in order to penetrate the material being run. The gag functions are controlled by predetermined counters, PLC's, or computers.
“Flying” dies are made to move linearly along tracks built into a mechanical, pneumatic, or hydraulic press as opposed to being fixed to the ram and bolster as stationary prenotch dies are. They also can be mounted on tracks mounted on top of bases on which the entire pneumatic or hydraulic press and die assembly moves.
One advantage to the flying die system is that because the stock is run straight into the roll former from the usually unpowered uncoiler and through the prenotch press (the “tight line”), it takes up less floor space in the direction of travel than a stationary die setup with its free loop in and out of the press. Another advantage is that when running heavier gauges of material, we don’t need a more expensive, heavy duty servo-feeder with its’ accompanying powered uncoiler or powered straightener.
A bit more care and time must generally be put into the construction of a medium to high production flying prenotch die than most stationary prenotch dies. There are lateral forces exerted on the die components because of the in-line motion as the die is propelled back and forth along the press tracks. The punch holder wants to keep flying when the die shoe hits the backstop position. Leader pins and bushings in die set were originally designed to keep the punches and die openings aligned, but not to take lateral forces in the process. Also, if a mechanical pickup is being used, and if the pickup finger is releasing before the punches are back out of the material being punched, there is pressure on the punches trying to pull them out of position.
All of this means that the guide pins and bushings must be securely fastened with additional support for the guide pins and additional bearing area in the bushings, and that the punches for some applications may need to be guided through a fixed stripper. However, the additional costs involved in making a properly constructed flying prenotch die are more than offset by the lower down time and longer die life.
Rotary punching means that any of the above operations can be done by using a mating set of rolls, the diameter of which depends on the length of the pattern or multiples of patterns to be punched. The time has now come in the development of rotary punching machinery when many of the required perforated, slotted, tabbed, louvered, punched, or embossed patterns can be done using the rotary technique.
The advantages of rotary punching include increased line speeds because you are not restricted by feed and press speeds and decreased equipment costs because you no longer must pay for high-speed presses with sophisticated feeds. Additionally, rotary punching allows for decreased noise and vibration thanks to the natural shearing motion and less floor space required because of the compact equipment.
Manufacturers are always looking for new ways to increase production, reduce labor cost, and maximize floor space. As die technologies continue to advance, choosing the best dies for your operation becomes even more crucial in order to stay ahead of the competition. Your auxiliary roll forming tooling and dies contribute to the efficiency and profitability of your total system engineering. Whether you're just getting started with roll forming or looking to invest in new dies and equipment to improve your business, be sure to pay close attention to the types of dies you incorporate in your line. When in doubt, always reach out to the OEM for help choosing the right methods and systems for your needs and goals.