Sounds to me like you didn't center the end of the piece of plastic. The
jaw does center that end to about a thou or so but the far end could be
almost anywhere.
Invest in a nice dial test indicator and stand for it and use it. Used
machinery stores will often have such tools for a lot less than what the
retail is and will do fine for the home hobbiest. Pros often don't go
theere because they can afford the full retail on a new one.

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Why do penguins walk so far to get to their nesting grounds?