I have a HP 7221 (7225?) that I have used many times to plot an etch
resist pattern on copper clad board. I took a standard HP plotter pen,
removed everything except the body, then bored it out in a lathe to fit
a permanent ink pen. (You can probably do this by hand reaming with an
appropriate set of drills or careful sanding and filing.) I found that a
Sanford ultra fine point Sharpie worked the best. You need to adjust you
pen speed and width settings to get the best line while still moving
fast enough so that the pen tip does not dry out. Width setting is
important so that you don't overlap what has already been plotted as the
partially dried ink will clog up the pen tip. Likewise, don't let the
pen dry out in the holder between plots. I used drafting tape to hold
the board in place. I successfully did a design once with pads for a
flat pack IC that came out useable.

I also experimented with other brands of pens (and pencils!). I never
was able to get good results with a Berol Fine Point Permanent (which
is, BTW, the "etch resist" pen that GC electronics, Kepro, and Radio
Shack used to sell for more than a whole box of the same pen at a office
supply store). I never could keep the tips from drying out during the
plot. I have also found that for extremely fine line drawings on paper
or vellum (not film), a Pilot Precise roller ball gave excellent
results. Text smaller than 1/16 inch was clearly readable!


Frank Richterkessing

FRANK.RICHTERKESSING@APPL.GE.COM