In the roll forming sphere, design principles for roll forming processes depend on an array of variables in both the materials being formed as well as the limitations of specific types of machinery and tooling. In particular, metal formers must pay close attention when selecting the right types of dies for their application. Knowing what constraints each type of die can handle is critical to an efficient operation. Here are the design principles that metal roll forming professionals need to know about the most common types of cut-off dies.
It goes almost without saying that die weight is a very important factor in flying dies — in some cases, downright critical. The problem is always hitting a happy medium between die weight and die strength. Only experience (usually a bad experience) will teach a person this in a hurry.
The following can reduce weight:
The following can increase strength:
Except for heavy structural-type shapes, a good die fit around the contour of the part is essential for a good cut. The contour should sometimes be ground in according to the part print and sometimes should be ground to fit an approved sample, depending on the part tolerances. If a radius is ground in the die according to a print, and the finished radius turns out larger, a gap will occur. When the die cuts off, a burr and distorted radius in that area will result.
Another reason for grinding the shape in after heat treating the steel is to achieve maximum life between sharpening. Hardening steel produces a thin layer of softer oxidized steel at the surface. If this layer is not ground off, the cut edge will become dull much quicker than it would if the surface was ground. This is a more expensive method of making the die, but it has proven to be absolutely essential.
If the part tolerances and roller die accuracy warrant it, the blade contour can be wire EDM’d to finished sizes (with running clearance and cut timing).
The cut-off blade should always be guided in flexing into the die sections and shearing. The stripper should be relieved in that area up and bind in the stripper. Slug pulling can be a serious problem in a flying die. The die sections should always be relieved 1/32 inch deep to within 1/16 inches to 1/8 inches of the cut edge, depending on the thickness to be cut. Therefore, if the slug is pushed beyond this lip, each slug will drop completely through the die and not run any risk of pivoting into the die opening.
Most of the time, many different sized shapes are run through the same line and press. Many times, we must cut the part with a blade, and yet there is not enough room between the slides to let the slug drop. This is solved easily enough by breaking the slug so that two or more piece can fall through the die where one could not.
To avoid, at least in part, the biggest problem with blade dies — blade gaulling — one can sometimes “shuttle” the blade sideways to sever a high leg cleanly without running down the full length of the leg. This can be done with cam action and with the blade being shuttled while traveling down through the part. The die is more expensive initially, but die downtime due to blade wear is only a fraction of blade action.
Another method to minimize gaulling is to surface treat the blade — Titanium Nitride, Titanium Carbide, Nitride and Graphitic Chrome — or retreat with cryogenics. In addition, lubrication is critical to long blade life.
For tubing dies, there are both conventional straight up and down blade dies and swing blade dies. If a tube is hit correctly in the proper location, it will be somewhat self-supporting and collapse only a small amount. The die sections should be cammed-in tightly around any tubing to keep it from flaring out to the die walls. After the cut, the die sections spread only about 1/32 inches to 1/6 inches and the tube passes freely through.
With square, rectangular, and lockseam tubing, a “swing blade” is sometimes incorporated to hit the tube in a self-supporting area. If hit properly, the tube will tend to collapse much less.
To help the blade gaulling somewhat, try to go a full 10-15 percent per side die cutting clearance. This takes some of the tremendous side pressure off the blade.
If no distortion is required on the tube you may have to:
Four basic types of blade construction are commonly used today in crop-off dies. One method is to make the entire blade, both moveable and stationary, out of tool steel. The solid blades then slide in the gibs on this same tool steel, needing no inserts for wear plates.
A second method is to make the actual cutting inserts thinner and smaller overall than the solid tool steel type of blade, and then insert these into a prehardened or hardened holder. This holder then slides up and down in the side gibs.
The third method involves using a more standard post-type die set with four leader pins and guide bushings in a die shoe and punch holder, making the blade contour the same as the piece part. Since only an incoming set of die sections is used, no slug is taken out.
Fourth is a self contained hydraulic pocket type die that takes the second method but now we mount a hydraulic cylinder at the desired angle. With this option there is no need for a press since a hydraulic power unit will cycle the cylinder/die.
In conclusion, understanding the key design principles for the most common types of cut-off dies is crucial for achieving optimal performance and precision in metal forming applications. By carefully considering factors such as material type, thickness, and desired cut quality, as well as selecting the appropriate die design—whether it be a shear, notching, or slitting die—manufacturers can minimize waste, reduce downtime, and improve overall productivity. Proper maintenance and alignment of dies also play a vital role in extending tool life and ensuring consistent, high-quality cuts. By integrating these design principles into the manufacturing process, companies can enhance efficiency, maintain competitive advantages, and meet the ever-evolving demands of the metal forming industry.