Press Forging Die Failures

Q: We press forge a shaft gear to close tolerances and we use a shrink ring assembly for the bottom die. How can we address interference?

Q: We press forge a shaft gear to close tolerances and we use a shrink ring assembly for the bottom die. We have dropped our forging temperature from 2,050ºF to 1,850ºF, and recently we’ve discovered occasional failure in the bottom die.

In your follow-up question, you asked what shrink fit we use, and we found that the interference (as you explained) was about 0.0012 in. per inch of diameter. How should we address that?

A: I suggest that you increase the diameter of the insert, or decrease the ID of the shrink ring sufficiently to achieve an interference fit of about 0.003 in. per inch of diameter. Generally, this will require that you press fit rather than shrink fit, because you cannot get as much interference with a normal shrink fit (unless you heat the ring and deep freeze the insert.) Even with this practice, the resulting interferences likely will be on the order of 0.0015-0.0018 in. per inch of diameter.

As I explained, you will need to use a copper powder mixed with some grease, not only to help assemble the insert into the container but especially to remove the insert by pressout. Grease with zinc power will work too, but not as well as grease and copper powder.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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