Peter Ulintz
PMA Technical Consultant

August 27, 2020
0

Recent columns addressed horizontal forces (side thrust) in dies and provided methods to manage them. Dies generate other types of forces,―particularly progressive and transfer dies―that can cause the press slide (ram) to tip to one side, resulting in an out-of-parallel condition inside of the die. These forces, called tipping-moments, produce off-center loads that if not properly managed can affect part quality and damage tooling.

Arbitrarily positioning the centerline of a progressive or transfer die with the centerline of the press slide ignores the fact that when two or more die stations perform their work, the forces may not occur simultaneously or be equal in magnitude across all of the working stations. When forces are not distributed evenly across the press slide, the press slide will tip and move laterally in the direction of the greatest force. If this movement occurs while punches are engaged with other die components, excessive wear or die damage may occur. Damage and wear increases maintenance costs and compromises dimensional consistency of the stampings being produced. When off-center loads exceed press capacity, wear and damage to the press also can occur.

Manufacturers design stamping presses to handle some amount of off-center loading. Importantly, the maximum load is not a singular value, but rather a continuum of values relative to the distance from the press centerline. Fig. 1 depicts limits in the left-to-right direction for a hypothetical press. Front-to-back loading would result in a similar graph, but because many progressive and transfer dies produce reasonably balanced loads in the front-to-back direction, for simplicity we will consider only left-to-right loading.

To calculate the tipping moments produced by a die, we multiply the cutting and forming forces by their distances from the center of the press. Using the center of the press as the origin (zero), distances to the left have negative values, with positive values for distances to the right, as in a number line. In a perfect world the sum of all of the moments would total zero, indicating that the center of the load is balanced at the center of the press.

The strip layout in Fig. 2 applies to an eight-station progressive die. Table 1 displays the cutting and forming forces for each station and their distances from the center of the press. The last column in the table provides the resulting tipping moment (tonnage times distance from center) for each die station.

The sum of all of the moments in Table 1, -121.5 in.-tons, indicates that the loads acting on the slide are not balanced about the press centerline. To find the center of the load, divide the tipping moment sum by the total load for all the die stations:

-121.5 in.-tons divided by 87 tons = -1.4 in.

Thus, the center of the 87-ton load locates 1.4 in. to the left (negative value) of the press centerline. To center the load, shift the die 1.4 in. to the right. The goal here is to position the center of the load, not the center of the die,―on the centerline of the press slide.

Regrettably, metal formers usually position dies on the slide centerline simply because it “looks right.” Other times, the die length occupies nearly the entire bed length, forcing the die and press-slide centerlines to coincide. If the die can be shifted, it should. If not, the following steps may be taken.

Determine if the off-center loads fall within the capacity of the press. If we plot the maximum load (87 tons) and the center of load distance (1.4) on the load graph for our imaginary press in Fig. 1, we find that the load at that distance falls within the design limits of the press. However, improvements could be made due to its close proximity to the curve. Keep in mind that the graph assumes a properly maintained press and a correct gib clearance. A press slide with loose connections and gibs will tip more easily. In this case, close proximity to the curve should be cause for concern.

In the design phase, the die designer could divide the cutout station into two separate stations. This reduces the tipping moment in station number two by one-half, and moving the second cutout closer to the die centerline reduces its impact on tipping moments. It also shifts the moment of all of the other die stations to the right, helping to further offset the loads on the left. Of course, this assumes that the length of the die could be increased.

Another option: Strategically apply cutting shear and/or stagger select cutting punches. For example, we could reduce the cutting force in station number two by nearly 50 percent by staggering the punches by approximately one-half of sheet metal thickness or by adding shear. Table 2 shows the resulting reduction in forces and tipping moments.
Also possible: correcting slide-tipping problems inside of an existing progressive die. For example, an artificial load could be introduced somewhere in the die. Artificial loads may consist of an added pressure system (rubber, spring or nitrogen) positioned at the opposite end of the die to counter a tipping moment. Another possibility: Coin an area in the die strip or product trim edge that eventually ends up in the scrap. When all else fails, the process may need to be reassigned to a press with the capacity to handle the off-center loads produced by the die.

Completely balancing the loads may be impossible, especially in progressive dies. Even under ideal conditions where the sum of the moments equal zero, slide tipping may occur because all die forces do not occur at the same time (i.e., at the same slide position). In actuality, the forces generated by drawing and forming processes occur higher in the press stroke as compared to trimming, blanking and punching, where forces occur near the bottom of the stroke.

In the end, the acceptable amount of off-center loading will depend on the condition of the press, the extent of parallelism that the die requires to meet product quality requirements and desired tool life. MF

Industry-Related Terms: , , , , , , , , , , , ,
View Glossary of Metalforming Terms

Technologies: Tooling