sams25•3mo ago

DAE see people mixing up true position and concentricity on drawings all the time?

In my experience, I've caught three different junior drafters in the last month calling for concentricity on a simple bore when true position with a diameter symbol is the correct, and way simpler, callout for the machinist to actually work with.

3 comments

3 Comments

rileygonzalez1mo ago

Whoa hold up, saying true position is simpler for the machinist isn't quite right. True position and concentricity work completely differently. Concentricity lines up the center points of two features, which is almost impossible to check on a shop floor without a CMM. True position with MMC or LMC actually lets the machinist use functional gages or simple indicators to check it. That's way faster and more practical for them. So yeah, mixing them up makes the drawing harder to work with, not easier.

calebscott3mo ago

Ever wonder why that mix-up keeps happening? Is it just a bad habit taught in some drafting schools, or are the old GD&T charts in the office just that unclear? Like, do they see a round feature and their brain just auto-fills 'concentricity' without thinking about how it's actually measured?

max9633mo ago

Oh man, I bet it's because concentricity sounds right for circles! But runout or position are usually what they actually need.