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Die-Setting Fasteners—What Every Press Technician Should Know

January 23, 2025
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Die-setting fasteners are subject to cyclical loading due to vibrations from the stamping process and the retorquing required with reuse. Thus, die-clamping systems should incorporate redundant fasteners in the event that one setup should fail.

1-thread-engagejment-length-tooling-By-Design-UlintzTwo methods of die-setting fasteners find widespread use. T-bolts (typically 1-in. dia.) and nuts are the preferred method. The other frequently used system: a bolt and T-nut combination. Because T-nuts must fit inside of the bolster T-slots, the threaded hole in the nut typically measures ¾-in. dia., which offers about 60% of the tensile strength of a 1-in.-dia. bolt of the same grade. For equivalent clamping force, nearly twice as many clamps are required with the bolt and T-nut arrangement. 

When using the bolt and T-nut arrangement, select a bolt that enters the T-nut threads equivalent to the bolt diameter, at minimum. That is, if the bolt diameter measures ¾ in., then the bolt threads must enter into the T-nut threads at least ¾ in. The preferred engagement is 1.5 times the bolt diameter. The proper-length bolt will require no more than two washers, with the bolt threads not extending beyond the bottom of the T-nut. Ensure that the selected bolts or studs are straight and that the threads are not damaged. To avoid wrench slippage, ensure that the hex head does not have rounded corners.

At minimum, stampers should use SAE Grade 5 fasteners for die-setting purposes. These fasteners are alloy steel, heat treated and tempered to industry standards. All U.S.-made specification fasteners feature a manufacturer’s mark or logo on the head in addition to the grade marking. A bolt without a manufacturer’s designation may be imported and fail to meet established specifications. 

Avoid the use of plain-carbon or soft bolts, which provide half the strength of Grade 5 bolts. Never use commercial threaded rod, as there is no way to know its mechanical strength.

Die-Setting Washers

2-75-fastener-stress-Tooling-By-Design-UlintzWhen selecting washers, the ID of the washer should fit freely over the bolt with little play, and the OD should extend beyond the mounting hole or slot.

Common flat washers are not suitable for die clamping. Use only large, thick, carburized-steel washers designed for die setting. Recommendations for a ¾-in.-dia bolt are a ¼-in.-thick washer and a 1-in.-dia. T-bolt, which requires a 9/32-in.-thick washer. 

Nuts used for die-setting applications should be made of the same high-grade, heat-treated materials as the bolts. Nut thickness should be large enough to permit a length of at least 1.5 times the thread diameter. At least two full threads should extend beyond the nut after tightening (Fig. 1).

Fulcrum Blocks

Select a fulcrum with the same height as the die shoe. When using adjustable fulcrum blocks, set them at the same height as the die shoe. Installing a fulcrum that sits too high or too low will tilt the bridge and allow the die to slip.

Position the bridge clamp so that it extends sufficiently over the clamping surface and the fulcrum block. When feasible, position the T-bolt slightly closer to the shoe than the fulcrum. This ensures greater clamp pressure on the die-shoe surface than on the fulcrum block.

Properly Tighten Fasteners

When a fastener (e.g., a bolt) is tightened, it stretches slightly within its elastic limit—the range where the material deforms like a rubber band and returns to its original shape when the force is removed (Fig. 2). This stretching creates tensile stress within the fastener, effectively turning it into a tensioned spring. The stretching generates a clamping force that holds the joined materials tightly together.

3-fasteners-beyond-yield-strength-Tooling-By-Design-UlintzThe materials being fastened compress slightly under this force, enhancing the friction between them and resisting slippage or separation. The bolt threads allow for gradual tightening, enabling precise control over the amount of stretch (tension). Fasteners for die setting perform optimally when tightened to approximately 75% of their yield stress.

Use a torque wrench to ensure proper stretching of the die fasteners. Over tightening, often occurring when setup personnel use pipe extensions on box wrenches or ratcheting wrenches, can stress a fastener beyond its yield stress. This results in permanent elongation, thus substantial weakness, of the fastener (Fig. 3). Permanently deformed fasteners should be discarded and never reused.

Number of Clamps or Fasteners

A generally accepted clamping rule: 20% of the rated machine capacity for clamping force. A 400-ton press, then, would require 400 tons x 0.20 = 80 tons total clamping force—40 tons upper die and 40 tons lower die. When using bridge clamps, only half of this force will be exerted on the die as the other half exerts on the fulcrum block.

The 20% method generally overestimates the required clamping force because it assumes an upper-die weight at the maximum specified by the press manufacturer and the press running at maximum strokes/min. As a result, this method provides the clamping redundancy mentioned earlier. 
Finally, double-check that all fasteners have been properly tightened and torqued before starting the press and opening the die. MF

Industry-Related Terms: Compress, Die, Surface, Tensile Strength, Thickness, Torque
View Glossary of Metalforming Terms

 

See also: Precision Metalforming Association

Technologies: Tooling

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