Choosing the correct presses, tooling, and dies can be the key to a more productive and successful roll forming operation. In order to make those choices about the right dies for your roll forming line with confidence, it's important for you to know the advantages and disadvantages of the types of dies available. In this post, we'll compare slug type (also known as blade cut) and slugless (or crop) cut-off dies and the reasons why one might be better for your business or application than the other.
The blade cut type of design is the most common and basic design that has been used since the inception of cut-off dies.
The main advantage of a blade cut die is that there really is not any shape that cannot somehow be cut off with a blade — all the way from closed shapes (tubing) to heavy gauge structural shapes. In most cases, the cut will be decent, at least at first, since the shape is supported on both sides of the blade.
Fragile areas of the piece part can be easily supported for the cut most of the time. Since the blade is sheared to curl the slugs down, the tonnage in fragile die section areas can be cut down tremendously with this shear to keep the die-sections from breaking.
Because of the basic design with two die sections (one incoming and one outgoing) and one blade for simple, similar shapes, the making of replaceable inserts and blades to fit in a common die set is relatively inexpensive.
Another advantage of the blade cut die 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 prepunching operations.
However, there are also many disadvantages to the blade cut die, and for most shapes for simply cutting off, the disadvantages can outweigh the advantages. With the present-day requirements for higher line speeds and less maintenance downtime, we have to take a second look at the capabilities of the blade-cut die.
Let’s observe line speed first. The fact that the blade must be passed through the entire roll formed shape in order to sever it completely means that the die must travel linearly for that entire time. If a part is 4-1/2 inches high, the blade is in contact with the part for over 9 inches of stroke. At 150 FPM, the shape travels 6 inches in one stroke.
If we add to this either an accelerator-booster or a piloting mechanism, we add approximately another 1-1/2 to 2-1/2 inches to the die travel. With a slower press, these figures skyrocket. The lower overall height of the piece part, the less the blade cut die hurts us because travel is cut down.
Die maintenance with a blade cut die is relatively high because of the blade itself. With certain materials and leg heights the die is cutting, the fast gaulling is the only major maintenance problem with a blade cut die. The problems caused by blade gaulling can hurt the appearance of the cut and also cause an expensive line to be shut down quite often just for blade replacement.
Another problem incurred whenever a slug is taken out of metal is slug pulling. Because of its curled up shape, the slug in a cut-off can many times pivot upon blade withdrawal, backup into the die area, and stop the new lead edge from getting through the die, causing a jam-up. It is not always possible to push the slug down through the die opening far enough to prevent slug pulling.
The tonnage factor with high-legged heavy gauge material sometimes becomes critical. The tonnage rating of a crank type press is figured at or near the bottom of the stroke. If cutting a 3-inch high piece with a 4-inch stroke press, we will be getting much less tonnage out of the press when the cut is first started than we will at the bottom of the stroke. If it is too much the press will stall.
The blade cut die is very versatile and should be used, but not all the time and definitely not until all requirements have been carefully weighed.
The crop-off die, which is slugless, is the die most manufacturers tend to use, because of the advantages it brings. There are, of course, disadvantages to the crop-off die in certain cases. It is sometimes more difficult to coordinate prenotching with a slugless cut-off when trying to get radii on the ends or when trying to cut through a prenotched slot, for example.
With certain shapes, a crop-off die necessitates a more fragile construction of the die inserts, but this is too big a risk. Most any shape which does not fall into the above two categories will be cut off slugless.
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. The moveable die section is driven down by the press ram at an angle, which is determined by the shape of 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.
From the short stroke, it can be seen that the wear on the blades in a crop die will only be a fraction of the amount of wear in blade-cut dies. Being that the blade engages the stock at the bottom of the stroke, the crop die supplies another advantage that is very important in this day of high-speed lines — short die travel. The die literally “pops” in and out of the material.
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.
Ninety percent of the time, the crop die is a “self-contained” unit. It is a bump type die not attached to the ram at all, but one that returns its blade by springs. This is a benefit at times when running in an old press with a sloppy ram or in an air press.
If the die has a standard punch holder attached to the ram, and the ram is sloppy, damage can be done to the pins and bushings and eventually to the blades from shearing.
In an air press, if the punch holder is attached to the ram there is more chance for added die maintenance due to the abrupt stopping of the ram on the press stops in each stroke. In the self-contained crop die, we do not get that condition.
You can readily see the reasons for using a crop-off die whenever possible, but only experience can tell you where to draw the line and stick to the conventional blade cut.