Neil,
Because a pen plotter is usually limited bythe fact it has a simple up / down for a Z axis you might be better going the diamond drag engraving route which is idea for a plotter as you only need up / down and the spring loading does the rest. no spindle motor.
This is an example of diamond drag engraving done on stainless, as a guide the OD is about 2"

That text in the centre is TTF and has an inner and outer and is 0.5mm high
Edited By John Stevenson on 01/06/2013 10:52:31