Features on common centerlines

[[Me...]]

Hi all,
Settle the bet for me.

Specs.... ANSI Y14.5M 1982 & MIL-STD-100F

I have a drawing with no GD&T, only linear dimensions. I have 3 holes on a common centerline that are dimensioned radially from the center of a round part (Y axis), but not laterally (X axis). What is the tolerance for their location in the X axis and how is it determined?

Ok... ready... go!!!!!

Robert

[[Br...]]

There's no tolerance if nothing is specified. There's also no support in the Y14.5 standards for locating features of size with +/- tolerances. So interpreting something like this is completely subjective.

Now if i understand you correctly you have a few holes in a circular bolt pattern dimensioned with cartesian coordinates, where the center of the bolt pattern is zero? So you have a few features that lie on this "Zero" line and there is no dimensions showing them to be at the zero location. There is an implied zero rule, but I cant remember if it's in the '82 standard or not. Regardless you still need a tolerance on that zero. If there isn't one then the drawing is incomplete.

[[Me...]]

Hi Brett,
Thanks for the response. We're on the same page. The argument I'm getting is the "implied centerline" argument and that the zero tolerance is controlled by the block tolerances. I disagree, but I don't have a copy of the standard. I'm curious to learn about the implied zero rule.

I frequently get the centerline interpretation discussion from engineers. My response is always... "the centerline of what? The holes? The outside of the part? The slot? How do you know what this magical centerline is referencing?" They don't like my answer.

Fortunately this only happens on legacy drawings so it's not a lot, but it got me thinking.

Do you have any standard references for the implied zero rule?

Thanks again,
Robert

[[Jo...]]

I have the "implied centerline" deal with several parts. The general note is all features with common axis to have T.I.R of .015".
Problem is, they don't define what feature determines centerline. It does not have a Datum structure. Pragmatically, I use the longest cylinder, but what is the correct method ?

[[Br...]]

Sure. ASME Y14.5-2009 para. 1.4(k) states that a zero basic dimension applies where axes, center planes or surfaces are shown coincident on a drawing. Remember, Y14.5 doesn't support plus or minus tolerances for locating features. A geometric tolerance is required. That's why they make the distinction that it is an implied basic dimension. I think this rule was always assumed prior to the 2009 standard but not formally written in.

It's also important to make the distinction between dimensioning and tolerancing. The zero value that is implied by features visibly sharing the same "Center line", is only a dimension. You still need a tolerance to control the limits of that dimension. I guess one could argue that the title block tolerances apply to that, but the problem with most general title block tolerances is they are decimal based(which is also incompatible with Y14.5 regarding metric drawings; but we wont get into that.). So if the "zero" isnt physicaly written on the drawing, how many decimal places does the zero have? Also, what is the zero in reference to? Is it every possible combination of the different features to one another? How are the irregularities of the features accounted for? Do I pick one feature and treat it like a Datum, then measure back to the other? Is it every possible combination of treating one like a datum back to the other or maybe equal weight is given to both features. The questions could go on and on when you have drawings that are not linked back to a standard that governs all the nuances of the possible geometry.

[[Br...]]

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John that's a great question. TIR is not a tolerance. It's a method of measurement using an indicator that uses the total range of movement of the needle as the final result. (ASME has since dropped TIR for whats now called FIM or Full indicator movement.)

You are absolutely correct to ask questions like "what is the datum", or even "what does the tolerance zone look like?". Are these circular runout tolerances, total runout tolerances, or maybe Concentricity tolerances. So in that regard getting clarification on what is meant by this can be helpful. I think your approach is probably fine.

[[Me...]]

Thanks, Brett. Much appreciated!

[[Ja...]]

I am having a hard time picturing this part. 3 holes on a common center line, located "radially"? Radially would not require separate x, y distance. It would be an XY distance and angle. Can you share a sketch of this?

[[Me...]]

Hi Jason,
See crude sketch attached