Threaded Hole as a Datum Feature

[randy64]

I have always been told it is not good practice to use a threaded hole as a datum feature, but occasionally it is necessary. When we have done this, we place the datum symbol on the shoulder of the threaded hole callout and the words "MINOR DIA" next to the symbol, indicating that the minor diameter of the thread is the feature to be used as the datum feature. The FCF for the datum hole is RFS as we do for all threaded holes.

Checker is now telling me that if the threaded hole is going to be a datum feature, the notation "MINOR DIA" goes under the FCF and we are to add MMC to the FCF.

Which way is correct? I really have no documentation to say my way is correct, other than it seems cleaerer to me to place the "MINOR DAI" notation next to the datum symbol (since that's what's being defined), and the threaded hole can be a datum regardless of feature size - I don't see where making it MMC buys you anything.

Checker says he has documentation about this from a class he took, but hasn't produced it yet.

Thanks in advance for you help.

 

[pmarc]

The minor diameter of a threaded hole has its own "size" tolerance, so actually one has to specify at which boundary condition the datum feature is simulated:
1. MINOR DIA is simulated at RMB (RFS per Y14.5M-1994) -- in that case MINOR DIA goes next to datum feature symbol and any feature control frame referencing to that datum feature is not followed by any modifier. Datum feature simulator is expandable pin of diameter equal to size of maximum possible inscribed cylinder.
2. MINOR DIA is simulated at MMB (MMC per Y14.5M-1994) -- in that case MINOR DIA goes next to datum feature symbol and any feature control frame referencing to that datum feature is followed by (M) modifier. Datum feature simulator is fixed in size pin of diameter equal to the minimum size of minor diameter as specified in an appropriate standard or spec.

All said above is true if threaded hole is assigned as primary datum feature. If it is defined as secondary or tertiary datum feature, proper geometrical tolerances between the hole and the higher precedence datums must be taken into account.

 

[randy64]

pmarc,

We are using the threaded hole as a tertiary (clocking) datum. How would this change your comments above?

Am I to understand that the MINOR DIA notation is to go next to the datum feature symbol? Can you point me to some documentation on this e.g., what part of Y14.5M-1994?

Thanks

 

[pmarc]

Y14.5 is not too generous as for pictorial examples for threaded datum features, but you can find one in fig. 5-62 and read more in para. 2.9.

Before I answer your second (or as a matter of fact the first) question, I would need to know how are you going to reference the threaded tertiary datum feature - at RFS or at MMC (if we are using '94 terminology)?

 

[randy64]

Para. 2.9 says that MINOR DIA can go below the FCF or next to the datum symbol. What would determine the difference in location?

I talked with our checker and showed the para. 2.9 to him and we came up with this: If there is no FCF as part of the hole callout, the MINOR DIA goes next to the datum symbol (as shown in Fig. 5-62). If the hole callout has a FCF then the MINOR DIA goes beneath the FCF. Would that be a proper interpretation?

 

[pmarc]

The location of MINOR DIA (below or next to) has no impact on meaning of the callout.

I think now I picture your situation. The threaded hole is controlled by a FCF for its relationship to other datums, for example position FCF wrt A|B and you are interested in controlling minor diameter of that thread which then is going to be assigned as tertiary datum feature. In that case I would suggest following:
1. If the datum feature symbol is attached to a leader, place MINOR DIA next to it.
2. Place MINOR DIA beneath the FCF.
3. Any other FCF that uses threaded hole as tertiary datum feature should specifically define at which material condition the threaded hole is simulated. If at RFS, place nothing after appropriate datum letter. If at MMC place (M) modifier after the datum letter.

What you and your checker figured out in case of presence of a FCF controlling the minor diameter of the thread is incomplete specification in my opinion. One MINOR DIA (next to datum feature symbol) would be missing.

 

[randy64]

Your picture of the situation is correct.

So you're saying MINOR DIA should be at two locations in the hole callout - next to (or beneath) the datum symbol AND below the FCF? Para. 2.9 says "or", not "and." Why would it be needed in both places?

Thanks for your help.

 

[fsincox]

There is rarely just one way. The people who write the NAS, SAE, AN and MS standards (and are not beholden to machine shop practices) use the pitch diameter as a datum all of the time, they are primarily concerned with functional requirement definition and not ease of gauging.
Frank

 

[pmarc]

If you omit MINOR DIA next to or beneath datum feature symbol, and leave it beneath the FCF that would mean that the FCF controls minor diameter of the threaded hole, BUT the tertiary datum axis is derived from pitch cylinder of the threaded hole. If this is your intention and requirement, you can go with it - no problem. However, if the intent of the FCF is to control minor diameter of the threaded hole AND to define tertiary datum axis as derived from minor diameter of the thread, two MINOR DIA's are needed.

 

[Belanger]

If you want the datum to be simulated based on its minor diameter, then you (the OP) are correct and not the checker: the note goes wherever the datum feature symbol is. This is taken from paragraph 2.9 of the standard, which says that MINOR DIA "shall be stated beneath the feature control frame, or beneath or adjacent to the datum feature symbol, as applicable" (emphasis added).

Well, since you want it to apply to the datum feature, then having it beneath the datum feature symbol is the "applicable" way to do it. Placing it beneath the feature control frame only speaks to the minor diameter of the feature being toleranced with that FCF.

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

 

[SeasonLee]

pmarc

You mentioned expandable pin to simulate the datum feature, may I ask where can we order this special pin?
Thanks

Season

 

[pmarc]

SeasonLee,
I do not think there is anything special in this pin. This could be an expanding collet, for example, perfectly (within gage tolerance) located and oriented to primary and secondary datum feature simulators.

Example:

http://www.dunhamtool.com/expanding_collet.html#drawings

 

[SeasonLee]

Thanks. pmarc

Season