Butted is extruded but the centre plug they use is moved closer to the hole they are forcing the hot steel out of to make the tube wall thinner. I think Columbus' website had an animation of it.
@BillB started
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Butted is extruded but the centre plug they use is moved closer to the hole they are forcing the hot steel out of to make the tube wall thinner. I think Columbus' website had an animation of it.