Oversize problems

June 25, 2007
Q: What is the industrial norm/optimum die surface needed after cutting required cavities? We consistently have oversizing problems with a ball yoke, despite sinking a total depth of 0.005 in. below max. allowed (0.015 in. plus normal dimension). Our die
A: Regarding die-surface finish, a die surface depends on the sinking method used. If it is EDM finished, there is a polishing cycle on most EDM machines that requires some orbital motion of the ram. The resulting finish is usually on the order of about 2040RMS. This is similar to a fine sandblasted finish. If you are sinking with very high spindle-speed NC controlled machines, then the surface finish is usually quite good, but more likely to have a "striated" surface that can influence metal flow somewhat. These striations may require some minor polishing.

Regarding the oversize issue, recall from the usual guidelines that the thickness of the flash and the depth of the impressions need to be adjusted for a platter, because the centrally located parts are usually thicker than the end forgings. Because this tendency is typical, there is an expected variation in the flash geometry, usually land width, which is usually wider at the end impressions than in the center impressions.

This is done to encourage more flash to flow out of the centrally located impression and to aid in balancing the die fill.

The issue of die impact surface concerns me, because I am not sure your hammer man needs to continue to hit the forgings after the first impact of the impact area. I am guessing that he is trying to get the center impressions down to size. See my comment below.

I am concerned about the quality problem, too. For example, the forging temperature is quite high for 1141 (a free-machining grade), for which the max temperature should be 2,250°F becase sulfur tends to lower the overheating temperature.

Your blow sequence seems normal, but the thickness variations often are the function of the flash thickness and gutter relief at the blocker. It is not uncommon to find that you are actually forging on the (cooler) blocker flash, which can cause variations in thickness along the length of a platter.

A careful review of progressions (samples from each die station) could determine if this is a problem. This condition also may lead to loss of die life, because the die loadings are affected due to beating on the blocker flash rather than on the finished impression. Going back to the buster can also cause similar problems in the blocker die.

The 7° lock draft angle and 0.010-in. clearance (on your print) puzzles me. This can mean that the locks do not fully engage for central location until the dies impact. They should engage at least a 0.5 in. above forging contact. Otherwise, you can experience bad die life or forgings that experience trim tears due to the final die shift that occurs at final impact.

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.