hole patern used as Datum call out

[Jgonzalez1]

I am a Designer not Engineering and I would like to go back and let engineering know they are not correct. Am I the one that does not understand the standards? Can the center line of a hole patern be used as a datum call out? In this case Dia 4.750 was used as Datum B.

 

[KENAT]

I'd be skeptical about how they've done it, however in principle a pattern of features can be used as a datum, see secion 4.5.8 of ASME Y14.5M-1994 & figure 4-22 for example.

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[Jgonzalez1]

In figure 4-22 of the ASME Y14.5M-1994 Datum call out B is the upper right hole. In my case they use the diameter of the hole patern. The diameter 4.75 that is a referance dimention. should i request for one of the holes to be used as the datum? like in figure 4-22 of the ASME Y14.5M-1994.

 

[KENAT]

No, in figure 4-22 the pattern of 4 holes is the datum feature. The datum symbol is attached to the FCF for the pattern of 4 holes.

Read the paragraph don't just look at the figure.

You may be able to extrapolate from paragraph 4.5.8 how it might possibly be used on a bold circle, I'm not smart enough to do such things however;-).

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[SteveMartin]

Actually, while the dimension points to the upper right hole in the figure you both reference, the axis of the pattern is the meaning of datum B, not the axis of that one hole. The 'means this' section of the figure makes that pretty clear I think.

Having said that, I agree with Kenat about being skeptical of exactly how this has been called out. I'd prefer to see something that matched the spec example more closely (just because it's easier to point at and say 'this is what I/it means'), but I don't think the print you give is wrong relative to the standard.

All this assumes that you (and your engineers) are working to the Y14.5M-1994 standard - which is just a guess on my part, since I don't see a callout on the print.

 

[Jgonzalez1]

Thank you I have a better understanding now..

Yes we are working to the Y14.5M-1994 standards, just some misunderstand them and do not reason till proven wrong.

 

[axym]

Jgonzalez1,

KENAT is right, in Fig. 4-22 the pattern of 4 holes is the datum feature.

A fundamental concept of datum referencing in Y14.5 is that datums are defined in precise holding devices like plates and pins. In this case, the part would be held using a plate with 4 perfect pins that are on a perfect 4.750 circle. Datum axis B is at the center of the 4 pins.

A common approximation is to measure a best-fit circle on the 4 holes, find its center, and call that Datum B. This will get you close, but it doesn't follow the Y14.5 principle.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

Steve, I didn't mean to imply otherwise.

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[pmarc]

Assuming the drawing follows ASME Y14.5-1994 standard, the way it is specified on the drawing is not in accordance with the standard. As you said, Jgonzalez, attaching datum feature symbol to a centerline is not allowed per ASME Y14.5. In paragraph 3.3.2 you can find that:

"The datum feature symbol is applied to the feature surface outline, extension line, dimension line or feature control frame"

If the design intent is to make pattern of holes as a datum feature B, the most common way to place datum feature symbol (which is also very often presented in the standard) would be to attached it to FCF which is actually missing on this drawing.

 

[NomLaser]

The way that I read it, the datum feature should be called out on the pattern dimensions and not the actual centerlines (Bolt Circle). I went back to some old drawings that I did in the past with similar circular patterns and we called angles and BC diameters all with basic tolerances. We then used positional GD&T tolerances to locate the holes within a circular tolerance. We then used typical features to locate the parts and never used BC's or centerlines as datums.

ex. |(?)|Ø.007|A|B|

Sorry that was the only symbol that i could find that was close to a pos tolerance.

 

[fcsuper]

Agreed. The drawing is not correct. You can use the hole pattern itself as a datum, but you must associate the datum with a hole callout, not with the B.C. centerline. My suggestion, let the engineer know you understand what they mean and you will apply the correct callout method per ASME Y14.5. There is actually an example of this in the standard if you need reference.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[KENAT]

Fcsuper, where is the example of a circular pattern of holes being the datum? I tried extrapolating from 4.5.8 of ASME Y14.5M-1994 & figure 4-22 but it made my brain ache;-).

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[ewh]

The example along with 4.5.8 could be interpreted as either a circular pattern or a square pattern of holes. Either way, I think the explanation would be the same. The same example is used in the new version.

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