boring bar springs, hole is bigger at the front

Boring a 1 inch hole, 2 inches deep, in a sprocket hub. Bar is 1/2 inch sticking out about 2.5. Hole mics bigger at the mouth by a couple thou. I can feel the bar complain. Farm shop, so the bar is the bar I have.

That's spring. Shorter bar, bigger bar, or a lighter finish pass that isn't trying to correct the roughing pass in one hero move. A 1/2 bar out 2.5 inches will lie to you every time.

Also check the bar is on center. A bar below center springs down and out of the cut at the mouth and back in as the load builds. You get a bellmouth either way. Center it, then shorten it.

It was low. Raised it, stuck it out less, took the last two thou in two passes instead of one. Mouth and bottom agree within what my mic can swear to. Good enough for a sprocket that has never known a tenth.