The entry currently reads, "A bangy, squeaky, marvellous red-hot metal machine. For the mutation of Plain Metal into Metal Rods, Metal Rods into Metal Bars and Metal Bars into Girders." In fact, you can't make bars from rods, or girders from bars; all three types need to be built from ingots.
Apparently, the metal machine was once planned to work as the wood machine does, but got changed. I like the variety introduced by the change.