Positional confusion 1

[RedNova1]

you guys really help me out here, and am always performing searches to find answers
this time i have looked and cannot seam to find a thread with what im looking for.

i have the attached part come into quality assurance today & the inspector is asking me if he can hold it between centers & use a dial indicator on exterior Ø to measure the true position of the .569 hole.
weather he holds the exterior Ø in a v-block and runs indicator down the hole, or uses centers and runs the indicator over the exterior Ø does it matter?

 

[CheckerHater]

How about the third option - putting OD into some sort of chuck?

 

[fsincox]

Looks like first angle projection, ISO?
Frank

 

[fsincox]

Sorry, you are OK I am all screwed up.
Frank

 

[Belanger]

Ideally you are supposed to grab onto the entire OD. That is not always practical, but the centers and the dial indiator method are not the best. This is because the real datum A is the axis of the "actual mating envelope" of the OD. The chuck idea might work.

Also, FYI, datum B adds no value at all to this tolerance, although it's not wrong to have it there.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[DanStro]

If you are holding on the ID aren't you covering up parts of the surface that you are interested in? And since it is tool wander it won't necessarily be a constant slope to the surface and may be pretty localized.

It seems like the only was to mount this without potentially skewing the results is to mount on the OD.

Han primo incensus

 

[dgallup]

Depending on the production methods, the ID could wander a lot more the the OD. If they drill from both ends there could be a lot of positional error in the middle. So putting the part on centers and checking the OD could pass a lot of bad parts.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MechNorth]

As posted by several others now, the results will be different. Best to find a way to chuck up on the outside (perhaps a couple of circumferential line contacts or several area contacts along the length if you can't handle a 6" part). The addition of the tool wander limit should be included in the position tolerance. Also consider using a (L) modifier in the position tolerance; this will allow you to check for minimum wall thickness, which seems to be the overriding concern based on the callout.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Do you guys think the envelope principle applies to the ID & OD here?
My guess is, yes
Frank

 

[CheckerHater]

Do you guys think the envelope principle applies to the ID & OD here?

No, it's an urban legend.
Possibly at some point it was automatic, but today note like "PERFECT COAXIALITY AT MMC REQUIRED" is recommended.

 

[pmarc]

CH,
What has coaxiality to do with envelope principle? Am I missing something?

 

[CheckerHater]

There was an old argument (that's why I called it "urban legend"), that in some "special" cases envelope requirement may actually control relationship, like between OD and ID in parts looking like donuts / bushings.
I couldn't find it in any modern standard, but some book suggested to use the note if you want to imply relationship between OD and ID without using GD&T symbology.
Not like I cared much about using such note, but Franc asked about OD and ID being affected by envelope requirement, so here you go.

 

[fsincox]

CH,
Sorry for the confusion, I mean each of those features separately.
Frank

 

[CheckerHater]

No problem,
Just brought up another interesting topic.

 

[pmarc]

This method is actually mentioned in 2.7.4(c) of Y14.5-2009, and it is there to show that limits of size (so Rule #1) do not control relationship between individual features unless such note is specified.

 

[CheckerHater]

The way it's shown in 2.7.4, in context and together with "zero at MMC" requirements it makes much more sense.
Thanks!

 

[fsincox]

Aren't there enought note on the drawing already? To me the unit straightness of the ID could be done with a GD&T callout for better definition.
Frank

 

[fsincox]

There are so many good threads in the forum right now that to me exemplify problems I see with the standard.
First to the OP, I am not an inspector only a theorist, but my answer would, naturally, be the opposite of the others. I have been waiting for them to fill you in with the good relevant data before I took off. There doesn’t appear to be much they can offer, but, make a gage or in essence the same response. How do they think we all get our work done without gages anyway and it goes to my issue with the standard and gages that I have started elsewhere?
Naturally, it is always best to check as directed but if you can’t you’re going to do the best you can with what you have. To me the answer is you need to check many other things if you are not going to check what is actually asked directly to make certain to the best of your ability you have achieved the requirement stated on the drawing. Your proposed method could then be part of a system of checks to establish that you feel this is the case. I do not know what you can do and what equipment who have, but, I think the flavor of the answers is wrong.
No one can even say wheither we have to follow the envelope priniple on the ID & OD as part of our calculations, except me, I said yes, how is the CH.
Frank

 

[CheckerHater]

Envelope requirements?
Absolutely.
The great irony though, to check envelope requirements you will have to perform several INDEPENDENT measurements.

 

[fsincox]

CH,
In almost every post I wish I could take back some spelling error, I get too excited to respond, quick while at work. Like the "How is that". Let’s not even start with my grammar mistakes.
Frank