Crop-off (or slugless) type dies are popular among today's roll forming operations thanks to its versatility and efficiency. Not only do crop-off dies suffer less wear and save more scrap material than blade-cut dies, but it can form most any shape you might need. However, in order to use crop-off dies successfully, there are elements you must consider during the design of the line. Here are the design principles you should keep in mind when designing a general construction crop-off die.
The first thing that must be done in designing a crop-off die is to determine the angle of the cut. In some cases, such as a part with all 90-degree bends, the cut angle is not critical, but it is best at 45 degrees. A 45-degree angle will tear the least amount in the corner radii. Some companies request a 60-degree angle for less wear and tear on the die caused by shock. Under most conditions, the cut will be good enough.
To determine the best-cut angle, take the average angle of all the legs, add experience, and use this as the cut angle.
The timing and fitting of the cut is the key to making a slugless die work well with little or no distortion. The speed at which a certain area of the contoured opening is closed up relative to the area around it depends entirely on the angle at which the blade is coming through that area. If a part is distorted when cut with a crop die, it is distorted because the fit and/or the timing is wrong — not simply because it is cutting slugless.
This points out another good reason to section the blades and grind in after hardening. To develop the timing, a person usually has to regrind certain edges. The best way to do this is to first make a test cut to see where the deformation is, if any. Then take the blades apart and grind out where necessary.
Another one of the key things involved in making a successful crop die is to eliminate or keep to a minimum the blade deflection. Proper gusseting of the moveable front plate and the upright is usually enough. In many cases, gusseting provides a stronger backup than just making a front plate or upright plate thicker.
If breakage of center crop inserts is a problem because of a narrow opening through the part, and you still want to try to cut the part slugless, a slightly different approach can be taken — a “floating” center insert can be used. We developed this design originally for a rack beam which was 5-inch high x 2-144-inch wide x .075-inch thick with 3/4-inch return legs at the top, leaving us with about a 5/8-inch center post. With a standard design, this center insert would have broken immediately. With a floater, the insert is already broken off through the center post in the design and cannot be broken in usage again.
For parts over 1-foot wide, usually the die length (length in direction of line travel) does not increase significantly. This means that besides the part being wider, which naturally compounds any guidance error we might have, proportionately the die rails are now shorter when related to the width of the part to be cut. This means that our squareness error will be exaggerated even further.
To solve this problem, we mount rollers under the die shoe, two on the incoming end and two on the outgoing end. A third rail is mounted on the bolster (keyed), and our rollers ride on both sides of this rail with about .004-inch clearance total. Proportionately, the longer and narrower the guide is, the more the resulting movement of any side play is minimized.
As more and more demands are put on dies today for high-speed, long-run operations, the materials that the dies are made of are more important than ever.
The best all-around blade material for crop-off blades is D-2 (12 percent high-carbon, high-chrome content) hardened R.C. 60-62. However, when running aluminum, lighter gauges of steel or less abrasive steel, A-2 (5-percent chrome) hardened R.C. 60-62 often does just as well at less expense. When the rolled section are such that there is a slight chance of breakage, it is best to back off on die life a bit and use A-2 instead of D-2. It is a little tougher and can take more bending forces without breaking.
Whenever there is a high risk of breakage, it is best to drop down even further in die life and use S-7 hardened 56/58 R.C. Unlike most shock steels, S-7 stands up well when used for cutting. In fragile areas, it is wise to make a separate insert and to make it thicker than the rest of the blade.
And if the above tool steels do not work or you are looking for more tool life, there are a wide variety of CPM materials that can be used in almost any situation, however, at a higher material cost.
The maximum benefits and multiple profits are achieved when dies are not only used to cut off a part but at the same time are used to punch and/or form the part to eliminate all secondary operations. Much of this depends on the placement of the holes or forms in the part contour and the distance back from the cut edge.
With the solid-tool steel-blade construction, the stationary blade is backed up by a solid upright. The moveable blade is screwed and doweled to a front plate/spring bar, which holds the split sections together and backs up the blade too.
In this day of faster changeovers necessitated by high labor costs and the need or high production, the solid blade construction has an advantage. Inserts can be changed quickly by simply taking the blade stops off and sliding the blades out the top of the die. In some cases, the stationary blade must be detached from the upright before it can be reworked.