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"half" dimensions

"half" dimensions

"half" dimensions


What fundamental rules suggest that ‘half” dimensions are implied, as specified in the attached document?
Might be multiple rules combined and I would like to know them.
Not disagreeing, but I would like to get the details.

RE: "half" dimensions


The drawing looks symmetric, and there is a centre-line implying symmetry. Also, note how the secondary datum is a feature of size. It defines the centre. This is the only possible interpretation of the drawing.


RE: "half" dimensions


Thank you for your answer.

Just to resolve my “work argument” what are the written ASME Y14.5 rules to reflect what you just said (symmetry)?

Thank you for your help

RE: "half" dimensions

Fundamental rule 1.4(k) in Y14.5-2009. Is this what you are looking for?

RE: "half" dimensions

Thank you pmarc.
Is that the only Y14.5 rule needed to put the argument to rest forever, or I might need more "hidden" (not so obvious/ subtle) rules?

RE: "half" dimensions

I don't think you need something else (besides proper understanding of the rule, which in reality quite often is not well understood).

Do you have any specific scenario in which you feel this "implied basic zero dimension" rule does not fully work for you?

RE: "half" dimensions

10+/-0.05 is not the only "half" dimension missing from the drawing.

I have nothing against FOS missing a half dimension. In fact, that's the only way to do it.

But locating dimension is different from size dimension. Implied zero is the real thing, while the implied symmetry is not.

Could anyone provide the reference, what are the fundamental rules behind the way dimension 60 is applied?

(See enclosed picture as well)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: "half" dimensions


Fundamental rule 1.4(k) in Y14.5-2009 does not state anything about symmetry.

(j) A 90° basic angle applies where center lines of features in a pattern or surfaces shown at right angles on a 2D orthographic drawing are located or defined by basic dimensions and no angle is specified. See para.

(k) A zero basic dimension applies where axes, center planes, or surfaces are shown coincident on a drawing, and geometric tolerances establish the relationship among the features. See para. Implied 90° or 0° Basic Angle. Where center lines and surfaces are depicted on 2D orthographic engineering drawings intersecting at right angles or parallel to each other and basic dimensions or geometric tolerances have been specified, implied 90° or 0° basic angles are understood to apply. The tolerance on the feature associated with these implied 90° or 0° basic angles is provided by feature control frames that govern the location, orientation, profile, or runout of features. See paras. 1.4(j) and (k).

John H. Dunten, CD
Certified Drafter

RE: "half" dimensions

You pointed out a sort of inconsistency between wording of para. 1.4(k) and para. Please notice that 1.4(k) does not say about "implied basic ANGULAR zero dimension", but "implied zero basic dimension", which may actually include both, angular and linear 0 basic dimensions, even though the paragraph refers to para., where only basic angular dimensions are discussed. (FYI, it has been slightly improved in the draft of the future version of Y14.5).

If that does not convince you, I would like to ask you (or someone else) following question:
If zero basic linear dimension is not implied per 1.4(k), then why is this dimension not shown in fig. 7-56 in Y14.5-2009 (and in many other figures in the standard)?

I am looking at fig. 4-33 in Y14.5-2009, and I would say the if on your sketch design intent is to have both slots disposed symmetrically on both sides of the datum center plane A, you do not need the red dimension. With that being said, in order to avoid confusion I would use two basic half dimensions instead of one.

RE: "half" dimensions

Thank you pmarc,

I am glad we both agree that possibility for confusion still there. Standard provides the picture but not strong textual support.

It is interesting that even when center plane is a datum, you rarely see it used as origin for dimensioning.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: "half" dimensions

I have a few "guidelines" I use when I teach intro courses. One is "The drawing pictorially shows the perfect part. For example: If you SEE (8) holes in a circular pattern and the hole-to-hole spacing visually the same, there is a BASIC 45 degree spacing. If it was something else it would be dimensioned.

Also, many of the higher level concepts are supported by more basic fundamental definitions. Datum center planes established from width features used to define symmetrical relationships can be confusing for this reason. You have to understand several basic concepts to build into the "whole" understanding.

Certified Sr. GD&T Professional

RE: "half" dimensions

Is this situation one of the primary reasons fundamental rule 1.4(k) in Y14.5-2009 was added to the standard - it does not appear in the 1994 version? Or was this covered in a different area in the 1994 version?

Curious, Diego

RE: "half" dimensions

Yes is was added in 1994 and was not in 1982 version.

Certified Sr. GD&T Professional

RE: "half" dimensions

Yes, the rule was not explicitly spelled out in the '94 edition of the standard, even though it was widely used throughout industry (again, there are many figures in the '94 book where it is understood that implied basic zero linear dimension applies).

This could be one of the reasons why the committee decided to add this new rule to the '09 edition. Unfortunately, in my opinion, they did not do the job 100% correctly. There should have been 3 fundamental rules for implied basic dimensions. Two for angular dimensions - 0 and 90 degrees, and one for linear 0 dimension.

RE: "half" dimensions

mkcski - Never heard of an implied basic 45 degree dimension, IMHO that should be dimensioned.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

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