What is the key that 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 and tooling such as presses, dies, and measuring systems.
It is the differences between, and the design criteria for, the different types of dies (both cut-off and pre-notch), the different types of presses, and the different types of measuring systems, and how the different types affect line speeds, part tolerances, and part costs, that this series of posts will cover. The subject of today’s post is the pre-notch die design.
Pre-notch dies and sophisticated pre-notching systems are coming increasingly into use because of the economic implications of secondary operations. The following are the three basic pre-notching setups: stationary dies, “fly” dies, and rotary punching.
The stationary pre-punch 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.
“Flying” dies are dies that 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 pre-notch dies are.
Rotary punching allows for perforated, slotted, tabbed, punched, or embossed patterns to 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 blade cut die is the most common and basic die design. The blade cut design is effective for everything from closed shapes (tubing) to heavy-gauge structural shapes.
The making of replaceable inserts and blades to fit in a common die set is relatively inexpensive.
Another advantage of the blade cut design is the ease with which punching or forming can be combined with it in the same die set, and the ease with which it can be coordinated with pre-punching operations.
With the present-day requirements for higher-line speeds and less maintenance downtime, we have to take a good second look at the capabilities of the blade cut design.
Maintenance with a blade cut die is relatively high because of the blade itself. The problems caused by blade gaulling can hurt the appearance of the cut. A simple blade replacement can often shut down an expensive production line.
The blade cut die is very versatile and should be used, but not all the time, and certainly not until all requirements have been carefully weighed.
There are, of course, disadvantages to the crop-off die in certain cases. It is sometimes more difficult to coordinate pre-notching with a slugless cut-off when trying to get radii on the ends or when trying to cut through a pre-notched slot.
The basic action of a crop-off die is a scissor type action; the ingoing blade is stationary and the outgoing blade slides down to close off the die opening to cut off the part. There are many advantages to the crop-off die, and below are a few of them.
Since we just have to close up the die opening to cut, only a short stroke is necessary. We only use, at most, the bottom 5/8 inches of stroke, and in most cases, only the bottom 1/4 inches. Due to the short stroke, there is very little wear on a crop die compared to other die designs.
On high-speed lines running continuously and producing short-length parts, the scrap savings from not taking a slug out can be substantial by the end of a year.
The crop die is almost as versatile as a blade-cut die in that different sets of die inserts can be interchanged in a common holder as long as the cut angle is the same.
From the discussion above, you can readily see the reasons for using a crop-off die whenever possible. Only experience can tell you where to draw the line and stick to the conventional blade cut.
