... The question concerns a 10,000-lb double-frame open-die hammer. Does one install a large, wide dovetail and have a metal-to-metal (zero clearance) fit between the bottom of the dovetail on the die to the top surface of the anvil, thereby relieving the wings of the die? Or, does one install a narrow dovetail with a small amount of clearance — say 0.015 in, — between the bottom of the die dovetail to the top surface of the anvil, thereby allowing the die wings to support the load on the top face of the anvil?
A: This is a common issue in open die shops. If you are doing mostly dead-center work, like upsetting and drawing out, the fitted surface is usually best at the shank or dovetail. On the other hand, if you do a lot of off-center work or left-to-right work (say through the windows), then the practice of fitting to 0.012 to 0.015 in. at the shank is acceptable because you are working more on the sides of the dies. I believe that the majority of people will use tight-fitted shanks over the tight-fitted wings to avoid fatigue cracking at the wing-to-shank radius.
I knew a chief engineer at a leading forging firm who took a slightly different tack. He used hardened “spacers” (thick shims) to fill up the gaps between the details on an open die hammer. He also left an intentionally wider gap of about 0.030 in. for a distance of at least 1.5 in. from the undercut fillet radius to allow the die to “breath” during cross-die working. I do not remember much about any improvements in die life, however.
I argued that the mating surfaces should be “flat,” and, if not, reground until flat. Die failures were traced partly to “wallowed” out depressions in the sow block and ram cap that came from changes in die sizes.
One of the worst cases of die failure was experienced with forging upsets on octagonal dies followed by flat die forging on rectangular dies. The octagonal dies actually Brinnelled into the ram cap and sow block leaving the rectangular dies to hang upon the corners, leading to the fractures. This was a failure waiting to happen. We all learned from that.
H. James Henning answers forgers' technical and operational questions. For more than 40 years he held key technical positions in the forging industry, most recently as director of technology for the Forging Industry Association. He is president of Henning Education Services, Columbus, OH, specializing in customized education and training in forging technologies. Send questions to ASK JIMc/o FORGING1300 E. 9th St.Cleveland, OH 44114-1503 Or e-mail: [email protected]