Zero planes

[aniiben]

4.10.3/2009 states:

Depending on the number of planes established by higher precedence datums, secondary and tertiary datum axes may
establish zero, one, or two theoretical planes.



My question is: which figure shows Zero theoretical datum planes (if an example of such is shown)

If not, can someone describe/ show an example for proper understanding.

 

[chez311]

aniiben,

Seems to me the key is that it refers to secondary/tertiary datums. There are plenty of cases where a secondary/tertiary datum only serves to orient the datum planes established by higher precedence datums. See Fig 4-6/2009 - tertiary datum C does not establish any planes but orients the 2x planes established by datum B. If B were primary and C secondary, the same would still be true.

 

[aniiben]

I am taking about datum axes. Tertiary C is not an axis.

 

[chez311]

Apologies - I missed that the secondary/tertiary datums you referred to were axes. Refer then to fig 4-9/2009, my statements still stand for datums B and C of that figure.

 

[AM Engineer]

chez311

Any example in the standard where secondary datum axis establishes one or zero planes?

 

[aniiben]

Should I understand that in fig 4.9, B will establish two theoretical planes and C will establish zero?
If yes, then any examples establishing one?

 

[chez311]

AM Engineer,
After a quick page through the standard I did not see any. Just imagine as I said Fig 4-9 replacing reference A|B|C in one of the FCFs to B|C instead. The secondary datum C will establish zero in that case.

aniiben,

Should I understand that in fig 4.9, B will establish two theoretical planes and C will establish zero

Yes.

I do not see any establishing one either - not surprising as that is probably rather rarely encountered. I think you could accomplish this in Fig. 4-5/4-6 with the references to B and C reversed - ie: place A|C|B in the FCF. In that case C will establish one and B will establish the remaining one perpendicular to it, passing through the axis of datum feature B.

 

[Belanger]

Look at Fig. 4-16, using option (c). Datum D doesn't really establish a datum plane because it simply acts to orient the existing planes created by datum B (the higher-prededence datum). Perhaps that's what they mean by saying that the tertiary datum creates zero planes.

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

 

[chez311]

JP,

Even though 4-16(c) violates datum precedence, is it that different from 4-9 solely in regards to the number of datum planes created by the tertiary datum axis?

 

[Belanger]

No different -- my example duplicates your comment.

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