OK chaps this query actually comes from the plastic modelling side of my hobby but I'm sure you blokes are the ones to solve my problem as usual.
OK I have a cylinder about 4" in diameter and 18" long. Imagine a piece of plastic drainpipe with a hemispherical cap on each end.
I need to draw or (lightly) scribe a centre line onto this cylinder such that it goes along its length, around the end, back along the underside (so to speak) parallel with the one on the top then back around the other end to join up where it started.
How do I do this?