GD&T True Position interpretation - True Position FCF on a B/P

Q

QC Rick

I have yet another odd True Position FCF on a B/P (Go figure eh? :rolleyes: )
As follows:
2x Dia. .184-.190
[TP | Dia. .010m | A ]

One of the holes is Datum C (Where the feature dimension is noted) and the other is Datum B.
How does the above True Position control Basic dimensions on both features?
 

Stijloor

Leader
Super Moderator
Re: GD&T: True Position interpretation

I have yet another odd True Position FCF on a B/P (Go figure eh? :rolleyes: )
As follows:
2x Dia. .184-.190
[TP | Dia. .010m | A ]

One of the holes is Datum C (Where the feature dimension is noted) and the other is Datum B.
How does the above True Position control Basic dimensions on both features?

Can you make a sketch, and post it here?

We are "picture people.":D

Stijloor.
 
Q

QC Rick

Re: GD&T: True Position interpretation

Im no artist :cool:
Datum A is the surface of the view
 

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Paul F. Jackson

Quite Involved in Discussions
Re: GD&T: True Position interpretation

The "2X" designation makes the two holes a pattern... which means there are two 0.10 cylindrical tolerance zones... oriented to datum feature |A| and spaced their basic distance apart that the axis of both holes must reside within. Furthermore the size of the tolerance zone is variable with respect to the size of the oriented "actual mating envelope" of each hole.

Basically the note controls the spread between the holes while oriented to |A| and the upper tolerance limit is variable with respect to size.

2 holes with a size of 0.184 , perfectly oriented to |A|, would have a spread that equals 1.80 +/- 0.10.

2 holes with a size of 0.190 , perfectly oriented to |A|, would have a spread that equals 1.80 +/- 0.16.

Paul
 
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D

David DeLong

Re: GD&T: True Position interpretation

I agree with Paul that the 2 holes are a pattern to each other in a RFS (regardless of feature size) and perpendicular to datum A.

Using a CMM, take 3 points on datum A to create a datum surface. Now take a number of points around the ID of 1 of the holes perpendicular to datum A and create a center line. Plug in the basic dimension from hole to hole (CL distance). Now take a number of points around the 2nd hole and your CMM software should reflect how far it is off theoretical centers. It cannot be off any more than diametrically .010 mm or radially .005. If it is not more than .005 mm, the pattern conforms to requirement.

Hope this helps.
 
D

David DeLong

Re: GD&T: True Position interpretation

2 holes with a size of 0.184 , perfectly oriented to |A|, would have a spread that equals 1.80 +/- 0.10.

2 holes with a size of 0.190 , perfectly oriented to |A|, would have a spread that equals 1.80 +/- 0.16.

Paul

Paul:

Are you suggesting that the hole tolerance is at MMC? The information did not state that so wouldn't the tolerance be the same since it does default to RFS? Maybe I didn't read the information correctly??
 

Paul F. Jackson

Quite Involved in Discussions
Re: GD&T: True Position interpretation

Dave: Read the original post again and look at the thumbnail print. I didn't suggest it was MMC, QC Rick posted it that way.

Respectfully, Your GD&T pal Paul
 
D

David DeLong

Re: GD&T: True Position interpretation

Paul:

You are correct that the quick post did reflect the tolerance in MMC. My mistake. (didn't think you would ever hear that did you?)
 
Q

QC Rick

Re: GD&T: True Position interpretation

Thank you for all the replies!
It helps having information backing an interpretation when explaining this to those who are not as savvy with GD&T let alone know what GD&T is. :cool:
 
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