datum target dimensions

[dho]

ASME Y14.5, 4.6.2 datum target dimensions "the location .., where applicable, of datum targets are defined with either BASIC or TOLERANCED dimensions. if defined with basic dimension, established tooling or gaging tolerances apply..."
1) what are tooling and gaging tolerances?
2) if defined NOT basic and "not" toleranced, the UOS block linear tolerance applies?
thanks.

 

[Belanger]

That's always been a fun statement in ASME to interpret. Most prints that I work with use basic dims for datum targets, and paragraph 4.6.2 seems to mandate that there BE some establishment of such "tooling or gaging tolerances." In any case, I would NOT use the title block tolerances; those are for product dimensions and datum targets are not defining the actual product, but rather the holding device or gage.

Sometimes you'll hear about a 10% rule of thumb, but that still leaves the question of what to take 10% of...

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

 

[dho]

the distance between one target to another is 18.24". not basic. no +/- next to it. i dare not use 10% rule if from 18.24".
thanks.

 

[PeterStock]

The 10% "rule" referes to 10% of the tolerance not 10% of the dimension. In general gages are manufactured with much smaller tolerances than parts. In general a gage should be built such that it accepts no bad parts and rejects as few good parts as possible.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[Belanger]

Right Peter -- yes, I meant 10% of the tolerance. Of course, that still doesn't help with datum targets (unless you take 10% of the title block tolerance). So I say that ASME necessarily implies that there be some sort of "tooling or gaging tolerance" established.

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

 

[dho]

thanks.
18.24, two decimals, the block says +/-.03, use 10%, +/.003?
weird Y14.5 !?

 

[SeasonLee]

The 10% Rule is a guide line that has been used for years. It is not a part of Y14.5, you may find the 10:1 rule from The ANSI/ASME STD B89.7.3.1-2001 (or ASME B89.1.5:1998 p 4.1) “Guidelines for Decision Rules : Consideration Measurement Uncertainty in Determining Conformance to Specifications “.You may also find it from ASME Y14.43-2003 Dimensioning and Tolerancing Principles for Gages and Fixtures.

If the tolerance is ±.03,
The whole product tolerance range is .06
10% gage tolerance is .006
But, you need to divide the full gage tolerance in half to find the gage member tolerance.

This is the right way to calculate the gage tolerance, for example:
Hole size Ø.375+.006-.000
.006 X 10% = .0006
.0006 /2 = .0003
GO = .375 + .0003
NO-GO = .381 - .0003

The link below is a good source to understand datum target.

http://www.tec-ease.com/gdt-tips-view.php?q=133

SeasonLee