In reference to the patent: US5786562
Is this the original Arcam EBM patent? if so when does it expire?
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One way to see if this is the original electron beam melting patent is to see if it cites any earlier patents by Arcam. This link shows the cited patents. While there are several earlier patents cited, none are by Arcam so it seems likely this is the first Arcam EBM US patent. It's priority date refers to a Swedish application so there very well might be a earlier international patent, but it hadn't issued at the time of application.
As to patent expiration this Wikipedia article describes the rules. The patent has a priority date of May 12, 1993. Since this is before June 8, 1995 but the patent grant date of July 28, 1998 is after June 8, 1995 the rules state that the expiration date is the later of 20 years from the priority date or 17 years from the grant date. I didn't find any patent term extensions in the US Public Pair site. The latest of these dates is July 28, 2015 so the patent has expired.