Q: Should the depth of the insert into the die be the same as the diameter? How much interference is appropriate?
... Our die-sinking customer has a forging job that’s different from his usual work. It is a commercial-type forging weighing approximately 15 lb, being produced on a No. 23 Chambersburg Die Forger. It has a 2-in.-deep lock sweeping around one side, and is fairly well contoured. The bottom cavity is bowl-shaped, 4-in. deep with some ribs. The top die cavity is a flange 0.25-in. deep, and taller on the lock side. This shallow side has a 3-in.-diam. plug at the base with 10° draft, 4 in. tall, so it leaves a web 0.25 in. thick to be pierced out. After the first production run, the plug is cracked around the base. I suggested inserting a replacement plug, so the die wouldn’t need to be faced-off 4 in. Should the depth of the insert into the die be the same as the diameter? How much interference is appropriate? Also, there is a 0.75- to 1-in. hole drilled through the shank, to drive the plug out. What is your recommendation on the depth of the insert? What type of press would be needed to push the plug out? What would the procedure be for reinserting a new plug on the next re-sink?
A: Generally, using inserted plugs requires care, but there are advantages. The rule of thumb for a shrink fit is 0.015 in. per foot of diameter. For a shrink fit that requires a pressing load, the interference is more like 0.02 in./ft. This is where the die is heated to 400ºF and the plug is frozen to -60 to -100ºF. Koppercote, or some copper powder, added to light grease, will allow press removal with a knockout pin as you described and prevent the galling that often accompanies a shrink fit.
This example shows where an inserted plug is valuable. However, I am puzzled about why they have such a plug in the bottom die. This leads to excessive contact time on the plug that leads to all sorts of problems with die checking, softening, etc., as the hot part sits on the plug.
Having said that, using a shrink fit alone on the top die can be risky unless the plug is inserted from the shank side with a tapered undercut to assure that it does not come out on striking empty blows (common in hammer die setups). In other words, the shrink fit would work fine following the undercut approach for the top die. In this case, the bolster would back up the plug. If your customer continues to keep the plug die in the bottom, then the guidelines discussed are appropriate.
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.
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