When the chuck was made the backlash was taken out by chucking at that spot. Try chucking a piece of stock at the other spots and run an indicator on it. You’ll see.
Thanks: I tried it with a 1 in. round bar stock (not a test bar) and it had .002" run out when using the "O" pinion and about twice that with either of the other 2.
An then you see machinists tightening all 3 in sequence. The reason the center shifts when you tighten the other pinions is that it shifts the scroll with respect to the chuck body. The scroll needs clearance to rotate inside the chuck body. When you tighten the "0" pinion you are always shifting the scroll the same way in its clearance space.
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