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

I discuss the effect of a derived median line straightness on a primary datum feature. Then I show the same thing with cylindricity.
#asme #engineering #drafting #gdt
‪@BrianWickhamMFG‬
‪@asme_youtube‬

[@gdtguychuck]

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!

[@alfaradbean]

Thanks for the great explanation!

[@RDeanOdell]

Glad it was helpful!

[@timothyobrien1850]

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.

[@RDeanOdell]

Thanks!

[@Trustachio]

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

[@ariel59864470]

Thank you

[@aniketwarake4432]

Sir
Thanks to you for using matric units

[@RDeanOdell]

You’re welcome!

[@nitishpande377]

Hi Dean ! Thanks for all your videos so far, I've been following your CZcams 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 ?

[@wordsretainpower6250]

I really appreciate your channel

[@RDeanOdell]

I appreciate that!

[@SANJAYSHARMA-bk5jk]

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

[@riptidecnc6697]

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...

[@sc2epyon]

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

[@ronconovoa7566]

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

[@RDeanOdell]

I have lots of other videos that explain the fundamentals, this one is more detailed and does assume some knowledge of GD&T.

[@ManishGupta-gv4kk]

thx for using metric

[@RDeanOdell]

Welcome!

[@Martin-ef4xh]

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?

[@asaholey]

The runout tolerance shoulbe be equal or larger than the cylindricity tolerance? I think yes

[@Martin-ef4xh]

@@asaholey Have you ever dealt with defense type engineers? "They can make the impossible happen"! LOL I've seen them put out tapped holes as datums and other times, tapped holes (again) with true position of .001. Absolutely no BS.

[@asaholey]

@@Martin-ef4xh if they can make part without deviation

[@PeterGustafsson-w2s]

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

[@RDeanOdell]

I may not have stated in the video, but I’m using Y14.5. Position can be used for coaxiality control, usually on parts that are stationary in function.

[@PeterGustafsson-w2s]

@@RDeanOdell I assume that is ASME? I'm from Europe (Sweden) so I'm measuring using ISO and had never seen it done like that before, thanks for the quick reply

[@stillcad]

Collapsing cylinder ? What do you mean?

[@RDeanOdell]

It’s also called the true geometric counterpart (TGC) or datum simulator. I address it in this video:
czcams.com/video/MxvJ9aWbiY8/video.htmlsi=qq9wLSIj1JUxCE46