Position tolerance of circular pattern - some questions

[RobinMB]

Hi all

I have some questions about position tolerance of a circular pattern.

Consider below drawing

I realize there are numerous mistakes here, but please bear with me.

For the purpose of positioning tolerance of the holes, if I do the callout on a section (of two opposite holes), would the tolerance still apply on the other 10 holes in the remaining drawing?
Or does this generally fall under below statement?

Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified for a given feature on one level of drawing (e.g., a detail drawing) is not mandatory for that feature at any other
level (e.g., an assembly drawing).
I am confused by the example of the assembly drawing, as it is still the same part (not part of an assembly).
If this is not case, and it is allowed / has sufficient meaning. Where would you apply the datum?
On the plan view or the section view?


Then a second question I had. Looking at the 'proper' example in the standard:

What part of the callout refers to the angular position of the holes, A or B or something else?
What controls the global angularity of the holes? Why can't they all be shifted 1° (from north) but still be 6x60° from each other?


Thanks in advance.

EDIT
Another question, is it allowed to detail the diameter of the holes like below?
Would the position tolerance even be valid for the indicated centerline of the diameter.
Would the tolerance of the diameter (60+- 0.10) be valid for the rest of the holes in the plan view drawing?

 

[Burunduk]

RobinMB,
It's not the view in which you show the tolerance that drives the interpretation you ask about, but the feature(s) you apply the tolerance to. If the position should apply to the twelve holes, there should be a "12X" indication preceding the size dimension of the holes. On a drawing, the position callout should be attached under the size dimension, except for rare occasions such as position of coaxial holes of different sizes. Once the position and size callouts are attached, and the "12X" precedes the size dimension, the holes are grouped into a pattern and are mutually located (creating twelve tolerance zones, spaced per the basic dimensions).
I would recommend doing that in the view where all twelve holes are visible, but that's not a rule.

The statement you quoted differentiates between assembly drawings and "detail"=(part) drawings. It doesn't pertain to detail views versus main views.

"Where would you apply the datum?
On the plan view or the section view?"

I would need to know what the datum feature is first. Again, it is not about the view, more about the feature.

"What part of the callout refers to the angular position of the holes, A or B or something else?
What controls the global angularity of the holes? Why can't they all be shifted 1° (from north) but still be 6x60° from each other?"

Neither A nor B locks the rotation of the pattern clockwise/counterclockwise. The part has rotational symmetry and there is no need in locking that degree of freedom.

 

[RobinMB]

Hi Burunduk

Thank you for your response. Very helpful.

The datum feature would be the position/centerline of the hole. So I take it from your response that it is incorrect to apply the tolerance to the centerline of the hole but it should in fact be below the Ø60+- 0.1 feature or as shown in the figure from the standard?

How would you lock the rotational freedom of the part?
Let's say the flange is marked with 0 & 180° markings and it is imperative that the (two) holes align with the markings.

BR

 

[Burunduk]

The datum feature would be the position/centerline of the hole.

That's not very clear to me. You say "hole", so you do you mean that one of the holes in the pattern should be the datum feature to derive a datum from?
I don't know how this part functions - what locates this part in assembly - the 60+/-0.1 holes or the OD, but it looks like one of those hole patterns where every hole has equal importance as the others. Maybe you want to use the entire pattern as a datum feature? You have an example for that in Figure 7-18 in the standard you are using (ASME Y14.5-2018).

So I take it from your response that it is incorrect to apply the tolerance to the centerline of the hole but it should in fact be below the Ø60+- 0.1 feature or as shown in the figure from the standard?

Yes, that's correct. And the representation as shown in the standard is preferred. The most important thing is not forgetting to specify "12X" if you want to control the entire pattern.

How would you lock the rotational freedom of the part?

Any datum feature that is physically capable of locking that rotation (clocking), such as a keyslot, can be used for this purpose. Another option is the pattern of holes itself. If your figure shows the complete part and there aren't any additional features you chose not to show, then the pattern of holes (or a single hole within that pattern) is the only feature(s) that can be a clocking datum feature. But if there aren't any other features, there is also nothing to control with reference to that clocking datum either. If all you need is to make the scale marking, simply make sure that the marking is aligned with the relevant holes.

 

[drawoh]

RobinMB,

Your two views with positional tolerances contain a significant error. Do not apply Feature Control Frames (FCFs) to centrelines or any other construction geometry. Apply them to features.

You can attach quantities to FCFs, explicitly defining how many features the tolerance applies to...

12[×][ ][box][⌖][/box][box][⌀].010[/box][box]A[/box]

Better yet, attach the FCF directly to the hole specification. After all, the diameter, tolerance, and position of the hole all are part of your requirement. This stuff should not be scattered randomly around your drawing. Note how the quantity attached to your diameter is attached to the FCF as well.

The positional tolerance specifies a circle into which the centre of the hole must fall. This accounts for a radial and an angular error. This is the Right Way to call up hole positions. I have worked out how to specify pitch circles with [±][ ]tolerances.

Pitch Circles

Take a good look at this and see why it is a bad idea.