True Position Question

[[ZE...]]

Attached is a drawing that I think happens to be incomplete in regard to the .011 True Position for the Ø .359 Counterbores (also no information about them being counterbores).

The engineer & I have went back and forth on this.

I believe that one of the counterbores should have a basic dimension from the Centerplane of -B-. The 1.030 is just the distance between the two features.

His opinion is that the 1.030 basic implies they have to be symmetric to -B-.

Someone give a professional opinion on this please 🙂


Shawn Hillegas | 03-16-2018 03:10 PM |

[[ZE...]]

Drawing is incomplete, so the rule of common sense supersedes any standard. Bores must be concentric and 1.030 apart.

Kai KendallKKendall | 03-16-2018 04:16 PM |

[[ZE...]]

See attached.

Andreas Binder | 03-17-2018 11:15 AM |

True_Position_4.pdf

[[ZE...]]

The drawing is fine; the location of the leader for Datum B indicates the mid-plane is the Datum, therefore the callout to ABC is valid and your Engineer is correct.

Mr Binder's illustration is (as always) a fine example.

Alan Ryerson | 03-17-2018 02:15 PM |

[[ZE...]]

Sorry fellas but I totally disagree. That True Position example is not the same thing as my drawing. The 2 diameters called out in Andreas's example are implied on centerline of -B-. The two I have called out are not and have no reference dimension to the Centerplane. The problem with his drawing is that if he has no dimension referenced from the center of -B- to at least one of the counterbores, their location can be anywhere to -B-. I haven't read anything officially on this but it seems to me like it's just common sense, like Kai said.

The problem also is that those holes functionally have to be located symmetrically to -B- for assembly.

Shawn Hillegas | 03-17-2018 06:07 PM |

[[ZE...]]

See attached.

Andreas Binder | 03-19-2018 09:16 AM |

True_Position_5.pdf

[[ZE...]]

If an ID & OD are on the same center-line does it have to specify the number 0 for you to know that they are? ASME Y14.5-2009 shows explicitly in several Figures that placing basics over center-lines can imply symmetrical relationships. See Fig. 7-4 & Fig. 7-65.

but...

ASME Y14.2 states in section 2.7 that center-lines may be unbroken when no confusion results with other lines. I believe that the shown center point for Datum B and the (R.655) does cause confusion...

in conclusion, per ASME...

1.) The 1.030 basic is sufficient & legal to shown the 2xØ.359 pattern as being located symmetrically to Datum B.

2.) The center-point/center-line does cause confusion as to whether it applies explicitly to Datum B or the radius and should be re-drawn using the proper center- line format per ASME y14.2

Zachary Sutton | 03-19-2018 07:25 PM |

[[ZE...]]

So two Counterbores can be considered symmetrical features without any description?

Shawn Hillegas | 03-20-2018 12:24 PM |

[[ZE...]]

they would have a description. A position FCF to the specified DRF with a basic between the two, crossing the center-line. I would recommend getting a copy of Y14.5-2009 to check out the above referenced figures. Although that spec doesn't have much written explanation of implications such as that, it does have many excellently drawn figures illustrating such scenarios.

Zachary Sutton | 03-20-2018 06:32 PM |

[[ZE...]]

It's dangerous to infer things strictly from figures in the standard, especially regarding what is permissible to omit on a print. Section 1.1.4 (Figures) says, "In some instances, figures show added detail for emphasis. In other instances, figures are incomplete by intent."

More to the point is Fundamental Rule (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."

In 7-65, the center planes are shown coincident. In 7-4, that is not the case. The basic dimensions stating the basic symmetry are omitted.

That said, it could easily be argued that assuming basic symmetry from the print is pretty obvious. But then, I've seen a third-party lab get wrong results because they made an assumption that seemed just as obvious.


Aaron Gerber | 03-20-2018 07:45 PM |

[[ZE...]]

See attached.

Andreas Binder | 03-21-2018 11:25 AM |

True_Position_6.pdf

[[ZE...]]

Andreas, are these figures something that you've pulled from a standard or are you making them yourself?

Shawn Hillegas | 03-21-2018 12:43 PM |

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All I am publishing here in this forum concerning this thread is the product of my own technical fantasy.
Just my two cents.

Andreas Binder | 03-22-2018 08:44 AM |

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Are they supposed to be suggestions for how Shawn's engineer could dimension their part, or ?

Aaron Gerber | 03-22-2018 02:27 PM |

[[ZE...]]

This is legit. ASME Y14.5-2009 Section 1.4(k) States:
"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..." The standard is littered with examples in various figures that show the exact same situation you have here. I'll give one example which would be Fig. 7-4 which shows both Datum B, and C as center planes, and a pattern of 4 holes in a square pattern surrounding them. Not one basic dimension is drawn to the actual centerlines. The Pattern of features is already implied at basic 0 to the center planes.

You might be missing a basic dimension to datum C, but maybe theres more to this drawing than I can see. Datum C could also use a 2x if its intended to include both sides.


Brett Krienke | 03-22-2018 04:37 PM |

[[ZE...]]

Ahh man, I just realized the C'bores aren't tied to -C- in the Y+ direction. Great.

Here's another one for you gents:

Since the two counterbores are a diametrical True Position, how do I output the True Position with the given basic dimensions?

I am in agreement with Aaron and the assumption of interpretation with the "Figures" in the ASME standards. Just because the basics are shown however they are shown in all the figures does not apply to every situation, specifically with the pattern figures. Are my C'bores considered a pattern?

We've already made a choice on how that part will be dimensioned and we added a location to one of the C'bores from the -B- centerplane for less confusion and functionality to a mating part.

Shawn Hillegas | 03-22-2018 05:08 PM |

[[ZE...]]

The "2X" before the Ø.359 indicates the tolerance applies to a pattern. This isn't to say that you have to draw the 1.030 basic, as long as you can trace all the features back to the datums weather implicit or explicit-That's what matters. Technicaly, all features that have the same DRF with the same MMB modifiers are subject to a simutanious requirement, and are considered a single pattern.

Calypso allows you to tolerance bore patterns for Position if that is what you're asking.

Brett Krienke | 03-22-2018 05:27 PM |

[[ZE...]]

Not to be a stickler but this has to be taken into consideration when folks are using examples of "Figures" littered throughout the 2009 standard.

Shawn Hillegas | 03-23-2018 02:18 PM |

Capture.PNG

[[ZE...]]

Right, so first those figures exist. 1.1.4 Refers to the "ABSENCE" of a figure does not mean inapplicability. We're not trying to say something is not legal because there's no example of it.

Second, The only thing that 1.1.4 could come close to is that sometimes figures are incomplete by intent. Once again figure 7-4 does not fit this criteria. What would be the intent for omitting basics back to the center-planes when the purpose of the figure is to show an example of how Position is drawn back to datum center-planes? The specific figure pointed out "7-4", is meant to show the exact problem from this thread. It's literally titled "Position Tolerance at MMC Relative to Datum Feature Center Planes". It uses the 0 basic rule I quoted above to show a hole pattern to center planes.

Brett Krienke | 03-29-2018 02:52 PM |

[[ZE...]]

About that 0 basic rule (1.4 k). It applies when axes, center planes, or surfaces are shown coincident. The pattern of holes in 7-4 cannot be related to the center. A "center line" drawn to the middle of a grouping of 4 holes doesn't fit any of those three categories as they are defined in the standard. It's not really the axis of anything. Widths (like datums A and B) have center planes. Patterns of holes do not. And it's definitely not a surface.

Overall, I find application of the 0 basic rule (even when properly applied) to be problematic. I've seen many situations where axes are nearly coincident or a part is nearly symmetrical, and what's on the print is unclear at best. If the center line of a non-concentric diameter is omitted from a print (not a violation of any rule I can find), the unsuspecting print reader might assume that it is simply coincident with another center line that is shown. Or, in other cases, where some are coincident and others are not, it may be unclear which center lines belong to which features. Et cetera ad nauseam.

Aaron Gerber | 04-02-2018 06:25 PM |

[[Me...]]

100% agree Aaron, if I have to look up in the standard about interpretation of basic zero and centerplanes as datums and patterns and blah blah blah just to figure what the Engineer's (In question to begin with) intent was on something this simple and easy...I'd say adding that extra dimension was the right move regardless of what is the actual answer and explanation. The one thing I know without a doubt is that the Engineer responsible for this thread has ZERO training in GD&T and we make thousands of similar parts like this and none were dimensioned the way he did this drawing.

[[Ma...]]

Well, it has been a long time since I have been on the forum. Glad to see that everyone is still alive and kicking (each other in the teeth over dimensions and tolerances. 🙂).

You all figured this one out without me, so good on ya.

Just for additional food for thought, what if I made the original basic distance in question a "bolt circle" (hate that term) diameter. Would you then also feel that a "bolt radius" basic dimension would be required so that you would know that I wanted that basic "bolt circle" to be centered on the referenced datum?

I don't disagree that a better drafting methodology is to place basic dimensions directly from the datums (in this case, the center of B) to the entity that must reside within the specified tolerance zone (in this case, the axes of the counterbores). So the "best" drafting here, IMO, would be to have half the basic dimension that was originally shown as the distance between the two counterbore axes, one coming from datum center plane B to the axis of one of the counterbores, and one coming from the center plane of B to the axis of the other counterbore (or just using "2X" with one of those dimensions shown). Having said that, I still believe that what was there originally was within the standard's definitions and was valid, even if it isn't the "best" practice. It is analogous to baseline versus chain dimensioning with basic dimensions. It technically makes no difference, but one way is much clearer to the reader than the other way.

[[Aa...]]

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Mark, apparently you're not the only one who enjoys seeing people thrash each other on the forum. On this new forum, when you view someone's profile, you have an option to add them as a friend...or as a foe. 😱 Of all the features to add...

[[ZE...]]

There is no need for a half dimension. The features are clearly related back to B and shown centered on B. Rule 1.4(k) supports the implication of a 0 basic dimension between features that are shown centered. It doesn't surprise me that the committee didn't describe every single way that features could be shown centered. If this hole pattern is supposed to be offset from B by 0.1mm then it needs to be shown that way.

John Acosta | 04-05-2018 06:39 PM |