spring loaded pencil

Hi All,
When I was in shop class, we had a spring loaded pencil that was used in place of a milling cutter to prove out g-code (cnc mill). I'd like to get one of these to use with my cnc router. Any ideas where I can buy one? Thanks.
Eric
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I've seen spring loaded center punches, which may be what you're talking about, but never a spring loaded pencil.
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