When I look at the one presented by Gerhard and then the one in the link from John, add that with this description :
lines for all the 0 10 etc., medium length on all the 5 ,15 etc and then short ones inbetween
I figure that you set the spike on the top so that it enters the deepest hole in the wheel on the top of the tool till it stops, then set your workpiece so that the cutter extends, lets say 6mm (just an increment, use a 1/4" if you feel happier?) set it to just touch the work piece then retract the cutter (push handle forwards) and then set a very small depth increase, say 0.1mm? and when the lever is pulled backwards the cutter shoots out and scores a groove into the metal workpiece.
Then you move the tool or workpiece (lets say 1mm) and turn the dial on the top of the tool to the shallowest hole say 2mm and pull the handle again, This would have scored a much shorter line onto the workpiece next to the longer one you just made because the pin has hit the bottom of the shallow hole in the wheel on the top of the tool. Similarly there could be a medium (half way between the long and short settings) line setting for (as Gerhard said) the "medium" say 4mm line.
That's what I can make out from all the above. Am I correct?
Hope that's not too over figured? or too incorrect!!! (If it's incorrect... Then I think it will work like I said?

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I've never seen or even thought of using such a tool before Gerhards post.... I always thought you painstakingly used your cross slide or Z feed on the mill to move a sharp tool accross the workpiece?! .... Now I'm thinking this could be used not only for it's intended purpose but even the posibility of adding patterns, flair or dare I say it.... bling! To your work!?
Couple this with the pencil die grinder and either a (dressed) stone or a carbide burr and you could have a very interesting tool for making very accurate lined patterns or accurate indents of all manner of shapes in your metal work?!
My mind is a glow, with whirling transient nodes of thought, careening through a cosmic vapour of invention

.....(love that Quote

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Well, the cog's are turning anyway, You've got to be happy when things come out of others posts that you'd not thought of before!
Ralph.