Ask the Stamping Expert: How to calculate tonnage and force for swaging

Q: I am quoting a swaging application for a pitman arm part. The material is a bainitic microalloy steel forging with Brinell hardness from HB 229 to 285. We are trying to size the press for the swaging operation. How do I calculate the required tonnage and force?

A: To answer your question, I first need to explain the skiving process, which would be used to produce a pitman arm. Skiving is the process of machining or forming serrations (like gear teeth) on a shaft or a hole to transfer torque between mating parts. An example is the mating between the motor shafts and the hubs of windshield wipers on a car.

If the shaft and the mating hole are cylindrical, it’s called a spline. The shaft is machined with conventional milling, while broaching is used to machine the spline in a hole. If the end of the shaft is tapered, milling is used, and if the mating hole is tapered, the procedure is completed by swaging, and this operation is called skiving.

Skiving is rather complicated, not for the tapered shaft but for the tapered hole, so I will only address skiving a tapered hole, using the pitman arm as an example.

Skiving of the pitman arm usually begins with machining a straight hole with a diameter somewhere between the large and small diameters of the final cone. The swaging punch, which is a mirror image of the skived hole, is pushed into the hole to displace the material from the large diameter to the small diameter of the tapered hole (much like in shaving, when the material is moved from the shear side to the break side of the hole). The starting diameter of the straight hole is chosen so that the volume of the displaced material from the top is equal to the filled material in the bottom (see Figure 1).

Calculating the tonnage required to do the skiving is complicated, and there’s no straightforward formula for it. However, through proper analysis, you can estimate the approximate force required. When calculating the tonnage, keep the following facts in mind:

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