Peter Ulintz Peter Ulintz
Technical Director

Managing Horizontal Forces in Stamping Dies—Part 1

April 30, 2020

A well-constructed, symmetrical blanking die, with no shear and a well-aligned press ram, will result in minimum horizontal loads during the cutting process. In many dies the horizontal thrusts due to punch-to-die clearances are the only side forces present. Dies that experience significant side loads often have multiple causes, including poor alignment of die components during die construction; misalignment resulting from a miss-hit or die crash; angular contact between surfaces, such as angular form steels; nonsymmetrical forms or draws where the punch and die are loaded off-center at initial contact; the use of shear or angular cutting faces to reduce cutting forces; and cutoff, trim, bending and flanging operations where forces act on only one side of the die steel. 

Guide Pins

Guide pins and bushings guide the upper and lower die halves. For precision or master diesets, the pin-to-bushing clearance measures less than 0.0005 in. dia.; commercial-dieset clearances measure less than 0.001 in. dia. A guide pin primarily aids in die construction, maintenance and setup. Guide pins are not intended to absorb horizontal forces (side thrusts) or realign the ram in a poorly maintained press. 

Since alignment is a matter of maintaining clearance, the running clearance of the guiding elements must be closer than the design clearance between components in the die. The following example, adapted from Techniques of Pressworking Sheet Metal (Prentice-Hall 1974 pp.424), illustrates this concept:

Fig. 1The cutting steels act along one edge of a formed stamping (Fig. 1). This type of die can produce an increase in cutting clearance due to side thrusts generated by the cutting action. 

At least four components in the die assembly can cause an increase in cutting clearance: two guide pins, the punch steel and the die steel. For simplicity, assume that the riser remains rigid and does not bend under load.

Step 1—Determine the allowable clearance variation:

Sheet metal thickness, t = 0.035 in.  

Cutting clearance = 10 percent of t = 0.0035 in.  

Allowable increase in cutting clearance under load = 10 percent 

Allowable increase = 0.0035 in. x 10 percent = 0.00035 in.  

Step 2—Divide the allowable clearance increase between the die details:

Guide pins 0.00015 in.

Punch steel 0.00010 in.

Die steel 0.00010 in.

Total 0.00035 in.

Fig. 2Step 3—Determine allowable guide-pin deflection:

Take-up clearance between guide pin and bushing = 0.00005 in.

Allowable pin deflection = 0.00015 - 0.00005 = 0.00010 in.  

How to divide the clearance variation between the die components is a matter of choice. More deflection usually is allowed for the guide pins, as they are much longer and less rigid than the punch and die steels. After establishing the allowable pin deflection, guide-pin diameters can be determined based on the loading conditions of the die assembly.

Guide-Pin Heel Blocks

Two philosophies cover the purpose of using guide-pin heel blocks:

1. The heel-block wear plates engage first, aligning the upper die to the lower, before the guide pins enter the bushings to provide final alignment.

On a properly maintained press with the running clearance in the gibs within specification, this theory works by sequentially reducing the running clearances between the individual guide systems. For example, the specified running clearance between the ram and gibs equals 0.004 in.; the running clearance between the heel-block wear plates equals 0.0015 in.; and the running clearance between the guide pin and bushing = 0.00075 in. This arrangement also allows the guide pins to be a short as possible to resist deflection due to internal die forces, and allows engagement of the pin and bushing to reside in the same plane as the work—the strongest design. 

In an improperly maintained press where the running clearances in the gibs exceed manufacturer specifications, the heel-block system will prove inadequate. Die-mounted heel blocks cannot shift the press ram into position reliably and repeatably. If they could, presses would not require complex ram-guiding systems.

2. The guide pins engage the bushings before the wear plates make contact. The guide pins provide initial alignment of the dieset, and the wear plates absorb lateral forces during the working portion of the die cycle. This may sound reasonable, but it works only if the running clearance between the wear plates are tighter than the guide-pin and bushing clearance. Otherwise, the guide pins will bend before the wear plates contact. Heel blocks intended to counteract side thrusts should be positioned on the die near the source of the lateral loads (Fig. 2). MF

Next month: prevention of die-component tipping and shifting caused by horizontal die forces.

Industry-Related Terms: Blanking, Die, Edge, Bending, Form, Ram, Thickness
View Glossary of Metalforming Terms

Technologies: In-Die Operations


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