If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
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Bill Graves RKBA!