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Combining hot and cold forging.

The combination of hot and cold forging can significantly improve part quality and tolerances over hot forging alone. Hot forging is ideally suited for high-volume production, but seldom produces parts ready for assembly because of temperature influences on tolerances. Hot-forged parts are routinely coined at ambient temperature to improve their dimensional accuracy in certain areas. However, this coining is usually limited by high forming pressures to hitting relatively small surface areas. By combining hot and cold forging procedures, it is possible to produce parts that are either partially or completely "assembly ready."

The choices

The degree of deformation is the natural logarithm of the ratio between the initial height/area/ diameter and the final height/ area/diameter of the subsequent cold-forging operation. It is the determining factor in forging and always pertains to the largest single deformation ratio of a part. If, for example, a shaft is reduced by a 0.3 ratio and subsequently the flange by a 0.1 ratio, shaft deformation will be the determining factor.

The possible variations are listed in the table. In the first case-hot forgings that are coined-the main deformation is at forging temperature, and the coining is limited to sizing only selected areas. In the second case, a hot-forged preform is cold forged (combinations of reducing, coining, or ironing) that can affect the entire part. In the third, hot forging produces a blank, for example, a material that would work harden severely if cold forged; and subsequent cold forging can include any common cold-forging method.

Cold forging of hot preforms

Coining hot forged parts is not new. However, the other two methods were developed primarily over the past decade and require a facility capable of hot and cold forging. Cold forging of blanks is used mainly to produce a blank shape advantageous for subsequent cold forging. The goal is to produce a cold-forged part without the need for intermediate annealing and surface coating. In the cold forging of parts extensively hot preformed, the degree of deformation is high. Because certain sections of the hot-forged part are finished-formed, the part has to be able to transmit the required cold-forging force (in reducing operations in the case of solid parts, or ironing for hollow parts).

Obviously, the more sections of the preform that can be subsequently cold formed, the closer the tolerances of the finished product. Although absolute tolerances are size-dependent, tolerance-improvement ratios of ten to one are not uncommon. At least one length and diameter dimension must be permitted to vary sufficiently to allow for mass variations.

The advantages of combination

The drive shaft for a fuel-injection pump (shown in photo) can be made in three possible ways. 1. Hot forged, followed by shearing to length, centering, and by rough machining. 2. Cold forged in three steps, with an additional annealing operation. 3. Combination forged.

In this example, the first method was used only to make sample parts and was subsequently abandoned because of cost. The second method, similar in cost to the first, was also abandoned because of unsatisfactory tool life. The third method was chosen for favorable cost, minimal material removal, and excellent tool life.

The first method started with a sheared billet and was a fully automated operation on a screw press with three steps (preforming, finish forming, and trimming). Due to shank tolerances, the resultant part was heavier than that with the third method. The part required subsequent isothermal annealing and shot blasting.

The second method started with annealed, drawn, and coated wire. The parts were sheared, preformed, and finish formed on a multistation cold former, followed by an intermediate anneal and surface coating. Forming the projections required a larger head volume compared to the combination method for technical cold-forging reasons. The result was a higher finished-part weight and additional machining. To machine the severely work hardened head required annealing under a protective atmosphere to avoid scale formation.

The third method, hot and cold forging, starts with hot-rolled bar stock. The first three forming stages are done on an automated screw press. After isothermal annealing (without protective atmosphere), shot blasting, and phosphate coating and lubrication, the part is finished by cold forging in two additional stages on a multistation press at rates of 60 pcs/min. The part is then complete-the small amount of workhardening requires no further annealing.

The photo shows the hot-forged preform and the finished cold-forged shaft. The decisive elements with this part are final formed length, minimal stock removal (0.020" per side), centers in both ends, and assuring a high degree of process reliability. The hot-forged preform must maintain weight accuracy in the shaft portions to be cold forged. This means the hot-forge tooling tolerances must be held closer than usual. Large transition radii caused by wear must also be avoided because they can create folds during cold forging. Finally, a uniform as-annealed tensile strength must be maintained to assure uniform press forces and part length.

By Henry VanDommelen and Manfred Hirschvogel Hirschvogel Inc Columbus, OH TABULAR DATA OMITTED
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Dommelen, Henry Van; Hirschvogel, Manfred
Publication:Tooling & Production
Date:Dec 1, 1991
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