Datum interpretation 4

[SeasonLee]

Attached print is a fiber glass reinforced resin molded part, my question is:

1.The pivot hole is datum B, section B-B defined the datum, but I can’t image how to set up the datum, it’s a datum point, datum line or datum area?

2.There is a position callout on the pivot hole, can we use the pinot hole itself as a 2nd reference datum in the DRF?

Thanks in advance for the replies.

SaesonLee

 

[pmarc]

1. If I had to guess the intent was to define two datum target points, though technically this has not been done in a correct way.
2. Shortly saying in my opinion this is illegal callout.

 

[SeasonLee]

1.It could be better to use the whole pivot hole as a datum.

2.I guess the designer intended to control the pivot hole axis parallel to datum A. What is the best way to call out this ?

SeasonLee

 

[powerhound]

Yes, it will be better to use the entire hole as a datum as I cannot imagine the functional advantage of using two points within the hole.

If the desire is to control the axis of the hole tighter than a diameter of .020 parallel to datum A, then just make the position FCF a composite and place a tighter tolerance than .020 with respect to datum A in the lower frame. This should solve the problem.

You still have the problem of constraining the hole in the vertical axis. You can use coordinate tolerancing to locate the 10-24 holes from the hole in question, but not the other way around and still use position with a cylindrical tolerance zone. As it stands the hole is located .025 from datum A, perpendicular to datum C but it is nothing says it has to be centered in the protruding feature. Wherever it falls, the 10-24 holes will follow.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
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SSG, U.S. Army
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[fsincox]

A composite tolerance should not be used on a single hole feature to refine for orientation use the orientation tolerance.
Frank

 

[powerhound]

Oops, I agree Frank. My bad. Instead, refine the tolerance with parallelism applied to the feature of size with respect to datum A.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[SeasonLee]

Is it technically illegal if only datum A referenced in FCF on the position callout to define datum B.
|POS|Ø.020|A|

SeasonLee

 

[fsincox]

I wouldn't see why not, you have to get started somehow. This framework on this part actually makes good functional sense to me, the mounting face, the hinge pivot, and the hinge slot.
Frank

 

[SeasonLee]

Frank

Do you think its legal on the pivot hole position callout ? The pivot hole is assigned as datum B, can we use the pivot hole itself to define the position ?

SeasonLee

 

[dingy2]

Going to put my 2 cents in here.

I think that the datum targets are trying to show a line on each end rather than a point especially since a point does not assist in the set up in any manner but the full diameter certainly does assist. The method on the drawing was not quite right though.

Fig. 4-52, page 89 of the 2009 version of the standard reflects the attached sketch on a cylindrical feature while fig. 4-32 of the 94 standard has the same approach for a line but on a flat surface.



Dave D.
www.qmsi.ca

 

[fsincox]

Dave,
I suspect you are exactly correct. I tend to assume they spent the time doing this for some reason, that is how I would choose to interpert it, personally. I am not saying it is shown correctly. How about B1 & B2?
Frank

 

[dingy2]

Frank:

Right on. It should read B1 & B2. Copy and paste sometimes messes up unless you read it again. Unfortunately, I did not.

Dave D.

http://www.qmsi.ca

 

[fsincox]

You all put up with all my spelling errors so I am not one to be critical, I just wanted a clarification, can you actually do that (B1 & B1)? I guess I don't see why not, probably no more important than this line vs a point issue. It seems at least Dave and I would have interpreted the drawing in the same way.
Frank

 

[dingy2]

No you can't Frank. One needs 2 datum targets to create an axis. B1 & B1 is wrong.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

I suspect that the intent was as Dave indicates; two datum target lines (B1 & B2). Dave's drawing, however, has a problem (beyond two B1's) which is that the line must stop at the diameter of the bore, not go right thru the entire part. Otherwise, good.

No, you can't have two datum targets labeled the same (e.g. B1).

There is nothing illegal with the position fcf on the thru hole relating back to datum B. It may not be terribly useful if the entire bore were the datum feature, but it is legal (and yes, a buggar to set up for inspection ... but doable). It is somewhat more useful given the datum targets establishing the datum axis.
People are often confused by a datum callout attached to a fcf which seems to reference itself. It doesn't. The datum is something theoretical (lots of discussions on this one already) and the datum feature is something real. Controlling something real to something theoretical is what GD&T does. There are lots of times that this is practical and necessary.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

I agree, Jim. The chain lines could be interpreted as either OD or ID. Having the lines on only the ID is appropriate.

Dave D.

http://www.qmsi.ca

 

[SeasonLee]

Jim. Thanks for your valuable interpretation on the datum reference, very interesting. Anyone who can give more examples with datum callout attached to a FCF and looks like to reference to itself, this really confused me.

SeasonLee

 

[GDTcoach]

Here is a Tip I did on datum features "referencing themselves". Like Jim said, you have to distinguish between the datum feature on the part and the datum that is established by that datum feature.

http://www.tec-ease.com/gdt-tips-view.php?q=191

Before anyone responds with "how can this be inspected?", A good friend and an excellent machinist made a video illustrating the set up and inspection of this part. The video is on our Premium Site. The part is the Output Shaft on our Inter-Mec Air Clutch/Brake. It is very inspectable.

 

[SeasonLee]

Thanks Don.
A good example.

SeasonLee

 

[dtmbiz]

Just a guess...

Cant determine for sure or what material it is but if this were a cast or molded part, then I have seen this type of attempted dimensioning to deal with draft. Just the nature of the part makes me suspect this.

Agreed that the datums are not ideally defined.