Can a Derived Median Line be a Datum? GD&T 

Thanks for the great explanation!

I like the video, Dean! It is a recurring theme in GD&T that we are made to straddle two universes. One is a realm of perfect, "basic" geometry, which is imaginary and not observable - the best we can do is to simulate it. The other is the universe that we can actually observe and in which we have real, physical parts that we can measure. The GD&T concepts typically exist in one universe or the other. Drawing views, centerlines, centermarks, basic dimensions, datums, and tolerance zones all exist in the first, basic universe. A derived median line is an observed phenomenon that we can only find (derive) once we have a real part to inspect. Like you say at the end, a derived median line isn't even straight, which is another clue that it cannot be a datum. Hey by the way, if you're going to do millimeters, don't forget to put a zero to the left of the decimal on dimensions that are less than 1!

Greatly appreciating the metric units, even if I'm forced into imperial for my work

For some number of years I have passed review on other's work. Now I find myself reverse engineering and I must say looking at a blank slate is far more interesting than passing judgement. Your thought process is greatly appreciated.

I really appreciate your channel

Hi Dean ! Thanks for all your videos so far, I've been following your RU-vid channel ! Can you please make a series of videos on 1D, 2D & 3D Tolerance stack up. You've already made 1D basic stackup videos, but can you make a series ?

Thank you

Sir Thanks to you for using matric units

What effect does the choice of the coat line as a datum have compared to the center line?

Could it be dumbed down as datum A is C/L of 30MM with cylindric of 2MM and the 60MM, max R/O of .03 to A? Or am I incorrect?

You may confuse people here b/c you have applied much larger tolerances to the datum feature A than to the features tied to Datum A. If Datum A is allowed to wobble within 2 mm, you cannot have a feature using this datum at a tolerance of .05mm, things would just not work. Datum A would need to be tied down much tighter in reality....good discussion thou...

Position and all orientation tolerances drives the center plane or axis and flatness, straightness , stmmetricity and concentricity gives derived medium plane or line But , what about curvylarity, cylindricuty ,and runouts what does they means center or derived line ? Or simply we cant apply these 4 to a pripary feature of size

thx for using metric

Collapsing cylinder ? What do you mean?

Axis is a reference line

Wouldn't coaxiality be the more correct way to tolerance this? than position? my understanding is that positional tolerance is more of use when you have Your datum and/or references not in line with the toleranced feature

this is way too much info it is great i hope someday i will get there