GD&T: Composite CFC interpretation 1

Rick:

Here is how a composite feature control frame works.

The top section relates to the postion of the pattern relative to the primary, secondary and tertiary datums. This is called the Pattern-Locating Tolerance Zone Framework (PLTZF). This should be the larger of the tolerances.

The bottom section is inside the pattern (maybe hole to hole) and if it references datum A, as an example, it would mean the positional tolerances inside a pattern and oriented (maybe perpendicularity) to datum A. This is called the Feature-Relating Tolerance Zone Framework (FLTZF). The tolernances should be smaller than the top section.

I sure hope that you have MMC application. Then you could develop an checking fixture otherwise, it is go measure using a CMM.

Hope this helps.

Dave D.

Thank you for your input! I do understand the PLTZF and FLTZF workings however I do not quite understand the layout of the features and True Positioning used, hence my query.

I have a particular concern for the interpretation of the FLTZF, is there a relationship to the called features as shown? Simply put, does the FLTZF imply/state a Coaxial existence between the Dia.'s .25000 and .238 as shown? (This is the exact layout on my B/P).

Regards

Rick Stanich
QC
Advanced Technology MFG
Tempe AZ

Rick:

This depends upon how it is shown on the drawing. You have 2 diameters that are co-axial.

If the drawing shows the hole(s) size and below there is a CB size and the feature control frame is below both, it means that the postional tolerance applies to both the hole and also to the counter bore. This is shown on page 137 of ASME Y14.5M-94 standard.

Sometimes the positional tolerance is separated for both the counter bore and also the hole. We would have one positional tolerance for each and usually they are different.

Hope this helps you.

Dave D.

If the drawing shows the hole(s) size and below there is a CB size and the feature control frame is below both, it means that the postional tolerance applies to both the hole and also to the counter bore.

Click to expand...

If the Composite FCF applies to both features, what is the TP|dia. .0005 of the first feature Dia. .25000? What exactly does this mean? (It just doesn't make sense), it does make sense in coaxial to another feature but to a single feature I do not see an answer.
Also, what makes the FLTZF define that one feature must be coaxial to the other feature (in this example).

The coaxial relationship is information I have been given for the meaning of the FCF.

This particular FCF is new to me, never seen one this way. I can not recall seeing an FCF (composite) without a Datum in the FLTZF before (utilizing TP), thus the confusion on my part.

Regards

Rick Stanich
QC
Advanced Technology MFG
Tempe AZ

The top section of the composite FCF is for both the hole and the counter bore.

The bottom section of the FCF is within the pattern. The holes to themselves and the counter bores within their own pattern. This does not make much sense but that is what it means.

I would rather see the composite FCF apply to the holes only and have the counter bores relative to their respective hole. This is done by creating each hole as a datum and then we would see a positional FCF with MMC in the tolerance section and also in the datum section. One would only need to have a step gauge that would go into each hole to show that the counter bore is relatively central.

I see that you get into some messy things here. If you would like to post the drawing, I could comments more.

I have been involved in measuring for years and am heavily involved in GD & T and how to measure the symbols.

Hope that I am helping you.

Dave D.

If only one FCF is applied and is placed below both the counterbore and hole callouts then there are two separate requirements. Think of the counterbore as a separate hole pattern from the holes. The position of the counterbore pattern does not depend on the position of the hole pattern. It takes a repeated local datum to tie the counterbore to the hole (to control coaxiality among others).



Tunalover

Tunalover:

You are right on here. There are two sets of requirements for both the holes and the counterbores. This also applies to the lower section of the composite feature control frame (FRTZF).

Dave D.

Thanks to all who helped!

Regards

Rick Stanich
QC
Advanced Technology MFG
Tempe AZ