Differentiating between surface of shaft/hole and axis

[happystamps]

Morning all-

I'm unsure how to differentiate between indicating a centreline of a hole or shaft and indicating the surface of that part. I've attached a screenshot to show what I mean- from what I remember one uses the datum in line with a dimension, and one doesn't. But I'm not sure.

This is according to BS 8888, but you get bonus points for ASME too

Thank you in advance

Simon

 

[BiPolarMoment]

Apologies because I'm unfamiliar with BS 8888 but using ASME interpretation aren't all 3 instances the same thing? Now I do take some issue with the clarity on whether it's intended that the datum be established with one or both (there are two?) diameters of the same size. In the case of "A" I would assume it's only the first while B and C are more ambiguous.

As far as your first question--I have a hard time touching a centerline, but it's pretty easy to touch a surface.

 

[CheckerHater]

From ISO 1101:2012 Clause 9 Datums:

9.3 The datum triangle with the datum letter shall be placed:
…
• in 2D annotation, on the outline of the feature or anextension of the outline (but clearly separated from the dimension line), when the datum is the line or surface shown (see the example in Figure 31); the datum triangle may be placed on a reference line using a leader line to point to the surface (see the example in Figure 32);
…
• as an extension of the dimension line, when the datum is the axis or median plane or a point defined by the feature so dimensioned [see the examples in Figures 33 a) to 35 a) for 2D annotation and Figures 33 b) to 35 b) for 3D annotation], if there is insufficient space for two arrowheads, one of them may be replaced by the datum triangle [see the examples in Figures 34 a) and 35 a) for 2D annotation and Figures 34 b) and 35 b) for 3D annotation].

One has to mention that standard also has “NOTE” saying “At the next revision of this International Standard, this clause will be moved to ISO 5459”, so newer updated version is in the making.

ASME Y14.5-2009 is trying to take similar approach, but because of half-baked wording several people believe that datum symbol always refers to derived axis, regardless of where it is placed.


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

 

[fsincox]

In essence, I believe it can be stated: parts do not have centerlines, features have axes derived from surface features.
Frank

 

[happystamps]

Correct, but that wasn't my question ;-) but to take tubing as an example-

I have an idea that some people use axes as datums as they'll be measuring a tube with a CMM taking 5 or more points around the surface of the tube at two or more points and determining the central axis. This is because a lot of OEM auto drawings (erroneously- at least I think so) control tube profile by applying a line profile tolerance to the centreline and relying on the ovality and OD of the tube to give the surface profile. So, I'd like to avoid doing that in my own drawings, hence the question.
It seems (as I thought) there is a separate method for doing either, as per Checkerhater's answer. Looks like we've all learned something today, hooray!

Thanks guys, I've been wondering about that for ages.

Si

 

[3DDave]

Datums are an idealization of a feature. Datum simulators are the practical realization of those datums. Datum features are those surfaces used to interface between the part and the datum simulators.

For the establishment of datums, one always is looking at using the idealization as the basis for inspection, not the actual part surfaces. However, in the case of datums at MMC/MMB/whatever, this idealization can be within the gauge and is not determined for the part datum features.

For the inspection of part features there is a case to be made for using either, depending on conditions. That is - if one expects a clearance based on a position tolerance, it's possible to have a case where the feature axis is within tolerance, but the feature surface encroaches on the allowed feature boundary. For these cases Y14.5 give precedence to the surface interpretation. This is also in keeping with the use of fixed gauging, where there is no means to establish feature axes.

In your example, none of them. Parts don't have datums associated to centerlines. Only a feature can, and only the third [C] unambiguously identifies a related feature. [A] and could both be tangent to the cylindrical surface, and is ambiguous as to which surface it applies to.

For formed tubing, there are usually other specification documents that account for the specialized nature of tube forming. There should be a document that specifies how the tubing will be measured given the tolerances on the drawing.

 

[drawoh]

happystamps,

ASME Y14.5 does not permit datums on centrelines.

You apply your datum to a feature, and that defines a centreline. On your figure, I count eight features that could define a centre. You have to pick one. Your figure shows three valid ways of selecting the 16.4mm[ ]diameter as the datum.

--
JHG

 

[Tenkan]

Happystamps,

The correct callout in your original attachment is Datum C witch properly establishes the datum axis.

ASME Y14.5-2009 Fig 3-4 (pg 38-40)shows the correct placement of datum feature symbols. It doesn't explicitly state that it cant be placed on the surface or leader line but in all examples shown in the figure that match your attachment it must be attached to the dimension or in line with the dimension.

If you place the datum symbol on the circular surface it could be mistaken for ISO 5460 5.3.1 and 9.3 (IIRC) which does allow this as a datum that is a line element of the circular surface not the axis.





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