Can a feature be called Concentric to a Datum that is established by a Bolt Circle?

[randy64]

Here's what the drawing shows:

Datum is a bolt circle pattern of holes (actually called out on the bolt circle diameter dimension - not on the hole callout).

Datum is then referenced for Concentricity of an outside diameter of the same part.

I believe the intent is to establish a centerline datum using the bolt circle diameter, then make a diamater surface concentric to that centerline datum.

Doesn't seem right to me. I'm checking the drawing and need to present a coherent argument - assuming this is wrong and I'm right. If doing it the way I've described above is correct, please explain to me how it works so I can gain understanding.

Thanks.

 

[randy64]

Let me also say, I don't believe a centerline datum can be established by a bolt circle of holes, and that is where the root of the problem lies.

Am I right about that?

 

[pmarc]

Bolt circle is not a physical feature of the part therefore can't be used as datum feature to establish datum axis.

The other side of the story is usage of concentricity tolerance, which in most cases is not according to design intent.

 

[drawoh]

randy64,

As pmarc noted, datums must be a real feature that you can fixture to. A pitch circle does not accomplish this, especially if you are trying to control concentricity.

The concentricity is a fairly limted spec. nowadays. If you are trying to centre things, you should use a positional tolerance, or a profile tolerance.

--
JHG

 

[randy64]

Thanks all. You've confirmed my suspicions.

 

[pmarc]

drawoh,
My apologies for nit-picking, but DATUMS do not have to be real features. In fact they are never real. DATUM FEATURES are.

 

[pylfrm]

randy64,

I agree with the previous replies that a datum feature symbol attached to the bolt circle diameter dimension is invalid. However, the intention may have been to use the pattern of holes as the datum feature, which is allowed if done correctly. I should note that this is probably a bad idea, especially when combined with concentricity.

- pylfrm

 

[Belanger]

Randy -- you say that you need a coherent argument to support your hunch. One ace-in-the-hole would be to see whether the drawing mentions a standard.
If the notes or title block say something about tolerances per ASME or ISO, then we can help give you more ammo. But if the drawing doesn't have any note like that then there's an immediate problem.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[randy64]

Belanger, yes, ASME Y14.5 is the standard that this drawing fall under.

Pylfrm, does making the hole pattern a datum mean that the centerline of the bolt circle is a datum? I don't think so, but I believe that is what the belief was when it was created that way on this drawing.

 

[Belanger]

In a good GD&T scheme, the holes that make up that datum bolt circle should have their own position tolerance among each other (and probably back to a datum plane to include the perpendicularity aspect). Then the datum symbol should have been placed on that callout, not on the bolt circle diameter.

Then, when referencing that datum in a subsequent callout, the datum would usually be modified with a circled M (called MMB in the ASME standard). In that case you could say that the true datum is the center of the perfect bolt circle, because MMB means that the hole pattern would be simulated by fixed-size pins which are anchored around a precise center axis. (In the standard check out Fig. 4-26.)

But I suggest that your biggest issue is the fact that they are using the concentricity symbol. This is why I asked if it's ASME or ISO.
Concentricity must reference a datum with no modifiers. Thus, what I just wrote in the preceding paragraph wouldn't be allowed -- no circled M allowed. Then you're talking about something called RMB, or "regardless of material boundary," and I would be really surprised if that's what they intended.
If you change the symbol from concentricity to position and add MMB then most of your headaches about the drawing will go away.

If you have a copy of the ASME standard, check out paragraphs 4.12.3, 4.12.4 (especially Fig. 4-26), and 7.6.4.1.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pylfrm]

randy64,

Belanger has a much better explanation of what I was getting at, and I am in full agreement.

If you are working to ASME Y14.5-1994 instead of -2009, the same concepts are described in paras. 4.5.8 and 5.12.1.



- pylfrm