pattern positioning

[dnobie]

I am trying to determine if it is legal within ASME Y14.5M to locate a pattern of holes with conventional dimensioning (no datums) from an outside profile and then use GD&T to control the spacing of the pattern to itself. We are reviewing an old (80's) drawing package in which this was done extensively and are trying to determine if this is/was legal. The holes within the pattern are located with basic dimensions. The feature control frame does not include any datum references. Thank you in advance.

 

[PeterStock]

I would not consider it strictly legal. What I would interpret the callout to mean would be that there is an implied primary datum (the surface where the holes are located). The holes are located with reference to the implied primary and to each other. There is no requirement that anything in the pattern is locate in reference to anything else, except that the conventional dims must be met. If you are updating the drawings, add the needed datums. If you are working to the drawings, determine what is needed as if you are creating the drawings from scratch.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[axym]

dnobie,

Was ANSI Y14.5M-1982 referenced on the drawings?

Locating a pattern of holes with plus/minus tolerances and using a geometric tolerance to control the pattern spacing is legal.

I don't think it was legal in '82 to have a Position feature control frame with no datum references though. Especially with a regular hole pattern - they've only recently brought in the idea for coaxial holes.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dnobie]

axym,

The drawings only call out ANSI Y14.5 (no year). Thank you for the feedback. This was also my understanding, however I am having a hard time finding any reference (in '94 standard). I will do some more digging.

 

[ringman1]

I believe the method you describe was allowed prior to 1982. I also believe that it was 1982 Y14.5 that departed from the rectangular pattern locating tolerance zone to allow only the diametric. (I have been of the opinion that that was not well thought out.)

Without a date for the Standard on your drawing you are faced with a dilemma. I think that it was only in the 1982 that the requirement to add a note stating specifically the date was added. FWIW.

Im not sure just how you go about applying portions of 1994 Std to a pre-existing Std.

 

[fcsuper]

Looks like a highjacked post thread. I also iterate that the year of the ASME Y14.5 needs to be specified. No other ASME standard requires the release year, only Y14.5. This is because each release changes how one should read a drawing. Even though 1994 and 2009 are similar, there are fundamental differences. Both are certainly different from 1982.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

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

 

[pmarc]

Evan,

You said that locating a pattern of holes with plus/minus tolerance and using geometric tolerances to control pattern spacing is legal.
Could you please give me any example from Y14.5 (1994 or 2009) standard that shows this method of specifying position tolerance? Sorry to answer, but I couldn't find such.

 

[pmarc]

Of course there should be "Sorry to ask, but I couldn't find such"

 

[drawoh]

Evan,

You said that locating a pattern of holes with plus/minus tolerance and using geometric tolerances to control pattern spacing is legal.

He said it was legal. He did not say it was recommended or explicitly shown in the standard.

Even in the 2009 standard, it is legal. It is strongly recommended that you use positional tolerances, but ± tolerances are defined, and have meaning on a drawing.

JHG

 

[mfgenggear]

some companies have an internal policy the Y14.5 applies on the inception of the drawing. government contracts are normally the date of design unless other wise specified or
updated contract.

nfgenggear

 

[Belanger]

Even if legal, realize that it is ambiguous because the implied datums will not have clear precedence (one over the other).

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

 

[drawoh]

Even if legal, realize that it is ambiguous because the implied datums will not have clear precedence (one over the other).

The ± dimensions apply from the edges. Datums are not even called up. I do not see ambiguity here.

A more serious problem is that the edges you are measuring from are controlled in perpendicularity by the angle tolerance note on the title block. No fabricator in his right mind is going to submit parts with a 0.5° or 1° error, but he is within his rights to do so. He has every right to be paid.

Like I said, legal, interpretable, but not recommended.

JHG

 

[Belanger]

I agree that it's legal, interpretable, and not recommended. But realize that when you say that the ± dimensions apply from the edges, that automatically makes those edges "implied datums."

The problem is that the inspector won't know if the vertical edge should be primary and the horizontal secondary, or vice versa (or what about the face of the part?). Even if the 90º angle between those two edges is off by a tiny fraction of a degree, the shop now has a 50/50 chance of making the part as the designer intended!



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

 

[axym]

pmarc,

I don't know of any examples from Y14.5 that show it, but I still think that it's legal. drawoh is right, positional tolerancing is the preferred method for locating features of size. So the standard doesn't show an example of the non-preferred method.

John-Paul,

I didn't see the problem with implied datums. I was envisioning the hole pattern as a datum feature (probably secondary) and the edges of the part being inconsequential. It would be more clear to apply a surface profile tolerance to the edges, but if the tolerance was large then plus/minus might be just as workable.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dingy2]

I agree that locating the pattern using +/- is legal. Does it not depend upon the function and relationship of the pattern? In some cases, the FRTZF is more important that the pattern location and I don't see anything wrong with locating the pattern with +/-. It does reflect the design intent.

As far as and Inspector not knowing the implied datum set up, well, I was a layout Inspector at one time. Believe it or not, we did have a datum structure using a primary, secondary and tertiary datums and any experienced layout Inspector or CMM Operator should be able to figure it out.

Dave D.

http://www.qmsi.ca

 

[drawoh]

I agree that it's legal, interpretable, and not recommended. But realize that when you say that the ± dimensions apply from the edges, that automatically makes those edges "implied datums."

This is the question I asked in thread1103-261904.

If I specify the bottom of the plate as the primary datum, and the edges as secondary and tertiary, then the inspector picks up two points on the seconary edge, and one point on the tertiary.

I believe that this all is irrelevant if I do not specify datums, and I use ± dimensions. The inspector picks up the edges the dimensions come from. If the edges are not perpendicular, I am in trouble.

JHG

 

[Belanger]

I'm confused. EVERY measurement has a datum. If you're measuring a simple length, then it doesn't matter which side is the datum, but there is still a datum. Now we are talking about position, and that involves TWO directions of space (ignoring the perpendicularity to the face for right now).

So JHG, what do you mean why you say that "this all is irrelevant if I do not specify datums, and I use ± dimensions"?

I think I agree with your last statement: "If the edges are not perpendicular, I am in trouble." That's why I maintain that the datums must be specified in an order of precedence; otherwise you are saying that the part is always made at exactly90º -- good luck!

Dave,
What do you mean that your inspection shop had "a datum structure using a primary, secondary and tertiary datums"? Was it an in-house rule of thumb? I think I would want the print to tell me what the datum precedence is. Just wondering...

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

 

[drawoh]

Belanger,

I suppose that the nominally straight edge that my hole is dimensioned from can be considered a datum. I think you are assuming that the datums are perpendicular as per ASME Y14.5. I think that only applies if I call up ASME Y14.5, and explicitly call up the datums.

JHG

 

[dingy2]

JP:

You stated "Dave,
What do you mean that your inspection shop had "a datum structure using a primary, secondary and tertiary datums"? Was it an in-house rule of thumb? I think I would want the print to tell me what the datum precedence is. Just wondering."

This was many years ago at GM but we would review how the part was machined and simulated the datum structure from machining. The primary datum was the mounting surface (3 point set up) while the secondary was a side (2 point set up) and the tertiary was also a surface (1 point set up). We did not use holes as secondary or tertiary datums but, at that time, I was in the casting plant.

Today I would use the same datum structure that is already shown on the part. We could have a pattern shown in positional while another, on the same plane, is shown with +/-. Guess what? Same datum structure.

I can see situations where the pattern location is not important while inside the pattern is vital to its function. Let's say we have a cover that is to assemble onto a machined casting. The hole pattern on the cover and the mating part are vital to its function so the mating part could have the FRTZF in positional while the pattern position is shown in +/-. This truly reflects the design intent.

If we have a composite FCF, the shop floor would end up developing and using a checking fixture for the pattern location when it is important to its function. This is a waste of $$ and time. Of course, inside the pattern (FRTZF) certainly is vital and a checking fixture should be used to simulate assembly.


Dave D.

http://www.qmsi.ca

 

[KENAT]

Don't basic dimension have to be tied back to an explicit datum?

Don't basic dimensions have to be used with position tol?

However, I'll admit I know little about the 82 standard.

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