Unpopular opinion: learning to read chip color saved my butt more than any measuring tool
I spent my first two years at the shop in Wichita glued to my calipers, checking every part three times because I didn't trust myself. Then one night shift about six months ago, this old timer named Ray stopped at my mill and watched me mike a part for the fourth time. He asked me why I didn't just look at the chips. I kind of laughed at him, but he pulled a handful out of the pan and said thin silvery chips mean you're rubbing, thicker curled ones mean you're cutting, and if they're turning blue you're cooking the tool. I started watching them on a run of 4140 steel the next week, and sure enough I could hear and see when the insert was going bad way before my calipers ever caught a size drift. Now I still mike my parts, but the chips tell me what's happening while it's happening instead of after. Anybody else learn to trust the sound and color over the numbers, or is that just an old guy thing?
Ray's trick works till you switch to aluminum or run dry. Thin silvery chips on 6061 are normal, not rubbing, and blue chips on a roughing pass in mild steel just mean you're moving metal. Chip color is a guess based on one material and one setup, so trusting it over a mike on a tolerance call is how you scrap a $400 block.