Hole pattern 1

[nrostrander]

All,

I was reviewing a drawing and came across a pattern callout as shown in my attached example. Is this right? I have not seen it done to this format prior to today that I can recall.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[KENAT]

No, as far as I'm aware assuming ASME Y14.5M this is not legal.

They are defining both datums twice.

I guess the intent is to indicate that the hole pattern is centered in the part but this isn't how to go about it.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[nrostrander]

Kenat,

Thanks. I was sure it wasen't. What section of the ASME would call this out? The part being created, must have the holes centered on the plate.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[KENAT]

3.3.2 of the 94 version kind of addresses not having multiple datums with the same letter.

As to 'implied centering' or symmetry. This is an area of some debate. Look at figure 5-4 for an example in the standard. However, this example isn't explicitly demonstrating 'implied centering' so some people doubt it's applicability to the question.

To my mind, if it's obvious the featurs are centered, and the tolerance FCF reference appropriate datums you don't need centering dimensions or similar, they just clutter the drawing.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[nrostrander]

Kenat,

This is what I beleve it should look like. Comments?

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[KENAT]

I meant to reference thread1103-240068 on the centering issue.

Looking at your re-post. I don't think it's right. I'm not familiar enough with correct application of symetry control to go into detail without doing research I don't have time for right now.

I would leave off the symetry FCF and just add centerlines to the part to make it obvious the hole pattern is nominally centered, much like figure 5-4.

To me the reference to B & C in the position FCF, combined with showing nominally centered (aided by Center Lines) is perfectly clear.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[dingy2]

This revision is still incorrect. The symmetry feature control frames just don't make any sense at all.

Take the symmetry FCFs out of the design and then you would have the face as datum A, overall width as datum B (would end up the centre line of the width) and overall length as datum C (again, the centre line of the length). The positional feature control frame is now correct as long as there is a tolerance on the hole size.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[JLang17]

Don't datums B and C need a condition modifier in the FCF? Due to datum shift.

 

[KENAT]

JLang17, that would have to be decided based on function. You don't 'have' to have the modifier.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[SeasonLee]

Besides all problems highlighted by Kenat and Dave, here is another mistake on the re-post:
Material modifier is not allowed on Symmetry callout.

SeasonLee

 

[dingy2]

SeasonLee:

You are correct about the modifier not allowed on symmetry but the symmetry FCF is all wrong in this application so it should be deleted.

Dave D.

http://www.qmsi.ca

 

[SeasonLee]

Dave: Yes, the symmetry callout is definitely wrong here, I meant for all symmetry application, material modifier is not allowed.

SeasonLee

 

[nrostrander]

So this is how I now understand it if there were two differnt patterns. Comments?

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[SeasonLee]

1. Vertical basic dimension missed for the four big holes.
2. Where are the basic dimensions for the four small holes?
3. Datum D is illegal here, datum C is good enough as the tertiary.

SeasonLee

 

[nrostrander]

Season,

I see the missing spacing dims. I should have stated that I was wondering if perhaps datum D was good or not. I don't understand as to how the centering pattern of the smaller holes could be made using C. Unless would it be understood that the 4 smaller holes are centered between C and the edge of the part. I suppose for this, I would be better off just basic dimming the smaller holes from the center of the larger holes or the edge. This is why I was wondering if the same concept used for symmetrical of a slot (fig 5-61), could be used for a hole pattern alignment.

P.S. - Spacing dims added.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[SeasonLee]

Try to use the 4 big holes as a pattern and assign as datum D, add the necessary basic dimensions wrt datum D on the 4 small holes, then the 4 small holes' positional tolerance can be called out as |TP|Ø.001|A|D|.

Attached is an example (from Geo-Metric III page 270) for your reference.

SeasonLee

 

[looslib]

You cannot assume anything such as holes being cenetered just because they have GD&T to the center of the part. You have to dimension one hole from the edges of the part or the implied part center.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[SeasonLee]

Ben

Please ref to the original post, its a symmetrical part.

SeasonLee

 

[nrostrander]

I got this answered by a 40+ year ME with a PHD and a professional GD&T certification. Having family and friends in the same field as myself is VERY helpfuly. I thank everyone for this answer, especially him!

Attached is the answer to this ever-growing debate as to if the figure in the ASME is missing dims or not. See attached document for full details. I was not supprized as to the outcome personnaly.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc. (

http://www.compass-sys-inc.com/

)

 

[powerhound]

Actually, the example that you have marked out as wrong and incomplete is the correct example. A pattern absolutely CAN be understood as centered, especially when centerlines are used as you have shown. When a feature or pattern of features is shown centered, it is implied centered unless there is a dimension to define it otherwise. This is a commonly understood principle. It's in Geometrics IIIm by Lowell Foster and in Fundamentals of Geometric Dimensioning and Tolerancing by Alex Krulikowski. Also see this tip and take note of where it says that many people would add a half dimension from center the to edge of a feature that is shown centered and they would be incorrect to do so.

http://www.tec-ease.com/tips/mar-08.htm

The example you are showing as correct and preferred is incorrect because datums B and C are the centerplanes of the part yet you are showing a basic dimension from the edge of the part. This is not good. What if the outside dimension of the part is big to the top of the tolerance? Let's assume .030" is the top. Do you check the holes WRT to the center of the part or WRT to the edge of the part? According to your FCF, you would check it to the center and ignore the .500 basic dimension thus giving it the status of a reference dimension. The example shown as "right but not preferred" would be better labeled as "acceptable but not checkable".

Are you sure your 40+ year ME has a GDTP certification?

Powerhound, GDTP T-0419
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