Implied center for diameters - Y14.5 3

[awol]

In ANSI Y14.5, I found the implied 90 degrees rule for center lines and lines shown at right angles (2.1.1.2)
I am asking if anyone knows of a similar guideline concerning diameters shown on the same centerline as being "implied" to be concentric within a certain tolerance (such as the block tolerances).

I don't think it's there anywhere but I wanted to get this discussion group's feedback.

If it isn't there, I plan on adding a general note to the effect of " Unless otherwise specified, all diameters shown to be on the same center line are to be concentric within .010"

The purpose of this note is to provide a type of "block tolerance" for diameters that the designers are somewhat concerned about but don't see the need to specifically address their location (as in non-mating features)

Any comments or suggestions?

---awol

 

[swertel]

Funny thing about Y14.5 is that it is Geometric Dimensioning and Tolerancing. People always seem to forget about the dimensioning part and focus only on the tolerancing.

Assuming you have the decimal place tolerance shown somewhere on your drawing, commonly the title block (i.e. .XX=+/-.015 .XXX=+/-.005), then if you dimension to the center line, both diameters are dimensioned and therefore have to be concentric withing the specified default tolerances, or block tolerances as I understand how you refer to them. Of course, that doesn't account for circularity or diameter tolerances, which at MMC or LMC, may seem to put the diameters out of concentric.

--Scott

 

[GDTGUY]

ASME Y14.5M-1994, paragraph 2.7.3 defines the default interpretation for features shown on center. It states:

"2.7.3 Relationship Between Individual Features. The limits of size do not control the orientation or location relationship between individual features. Features shown perpendicular, coaxial, or symmetrical to each other must be controlled for location or orientation to avoid incomplete drawing requirements. These controls may be specified by one of the methods given in Sections 5 and 6."

Your idea about adding a general note as a default for features implied to be on center is better than nothing at all. But you still run the risk of letting the drawing user make the choice of how the "zero" is established for making the measurements. Five different inspectors might have five different ways of doing it, yeilding five different answers.

I feel it is always better to define the datum and use an appropriate geometric tolerance to define the desired relationship. That way, there is no question as to the requirement.

GDT_GUY

 

[gearguru]

There is the ISO standards covering the subject:
ISO 2768-1:1989 General tolerances; Part 1: Tolerances for linear and angular dimensions without individual tolerance indications
and

ISO 2768-2:1989 General tolerances; Part 2: Geometrical tolerances for features without individual tolerance indications

I did not see yet any drawing (metric or inch based) where the GD&T would be specified by these standards.
Even the best standard is worthless if the people working with the drawings do not know it. It is safer to put all requirements on the drawings.

gearguru