Assumed Straigtness or Flatness 1

[Creech]

Hello, I'm having a debate with others to interpret the assumed flatness or straightness on a thickness or "bow" callout. The spec is Ansi 14.5M-1982. The thickness callout is .2787-.2745. Do I get RFS and the measured "bow" or do I have to place the part down and the entire form has to fit within the maximum of the tolerance. Thanks, Rich

 

[ctopher]

I'm not clear what you mean "bow" callout.
Is it a flat part, or curved into a "bow" shape?

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[Creech]

yes it has a .002 curve along length of part

 

[ewh]

If all you have to go on is a thickness dimension, I don't think you can assume anything about flatness or straightness without a specific callout, especially if the measured thickness including "bow" meets the dimension.

 

[Creech]

People are telling me that my flatness is assumed to be the total tolerance zone eg. .2787-.2745= .0042. That would be fine, even though I don't know if that is even correct, but they take it a little farther and state that if I run at the high end of the tolerance I have .0000 allowable flatness tolerance and on the other hand if I run at the low end of the tolerance I get the full .0042.

 

[ewh]

Without a specific callout, I'd have to agree with them. All you have to go on is your dimension and its tolerance.

 

[Creech]

People are telling me that the assumed flatness callout would br the total tolerance of the thickness callout eg. .2787-.2745=.0042. Even if I agreed with them about that, which I don't, they take it another step and say that if I run to the max of my tolerance (.2787) I will have .0000 tolerance for flatness. And on the other hand if I run at the low end I will have .0042 tolerance for flatness. I have Interpretation of Geometrical Dimensioning and Tolerancing by Daniel E Puncochar and I don't see anything about it in there.

 

[drawoh]

Creech,

I brought that specific question up when I took my GD&T course. Your assumption makes a lot of sense, but it is wrong as per ASME Y14.5M-1994. I am not 100% certain about the 1982 version of the specification.

According to ASME Y14.5M-1994, the thickness specification is the thickness at the point measured. It has nothing to do with flatness. If you want flatness, you must specify it.

JHG

 

[ewh]

I agree, but from a practical standpoint it may not be measured at two opposing points, but from a flat surface upon which the part lies.

 

[ctopher]

I agree with the others.
I suggest upgrading to ASME Y14.5M-1994.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[Creech]

I think I found it.

For anyone who is interested. In Modern Geometrical Dimensioning And Tolerancing by Lowell W Foster along with the National Tooling and Machining Association or NTMA, the following was stated:

Rule 1 - Limits of Size Rule. Where only a tolerance of size is specified, the limits of size of the individual feature prescribe the extent to which variations in its geometric form as well as size are allowed.

It continues later in the same rule: The surface, or surfaces, of a feature shall not extend beyond a boudary (envelope) of perfect form at MMC. This boundary is the true geometric for represented by the drawing. No variation is permitted if the feature is produced at its MMC limit of size.

Chris We're a machining job shop and the prints states the specification. We cannot change it unfortunatly

Thank you all

 

[ctopher]

Good news. Congrat's!
Also (should be another thread) I'll bet the dwg states ANSI Y14.5M-1982, but all GD&T is per ASME Y14.5M-1994. Very common.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[ringman]

Where is it written that all drawings are to be interpreted
by the 1994 version, even if prepared and stated to the 1982 version?

 

[drawoh]

ringman,

I referred to the 1994 standard because that is what I keep lying around. If drawings are prepared to the 1982 standard, they should be interpreted to the 1982 standard.

JHG

 

[ctopher]

Some differences between the two.
My experience with military design, whatever is called out, you follow that standard. It will be checked.
I am looking to see if/where it is written.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[ringman]

There are minor but nontheless significant differences in the callouts between the different versions. This can and does understandably lead to some misinterpretation. The standard that is called for on the drawing MUST be the one applied for the interpretation of callouts.

 

[dingy2]

Rule 1 applies to form as well of thickness since this is a feature of size. Make sure that the drawing stated "complies with ANSI Y14.5-82" or ASME Y14.5-94" but people on the shop floor don't really know this rule.

Most people on the shop floor will just measure the thickness and that is it - nothing else unless you specify straightness or flatness, etc.

To confirm that the thickness does not exceed the form of .2787 (MMC) , one would need 2 parallel planes of .2787 apart which is MMC. Measure the thickness of the part (micrometer) and then make sure that it goes between the 2 parallel planes.

Hope this helps.

 

[ringman]

Creech,

Just curious, but when was the drawing being discussed prepared? Do you have confidence in it being in conformance with the stated version of Y14.5?

 

[Creech]

ringman

I have no idea when this drawing was prepared. From what I understand we have been making this part for a couple of years and to dingy2's point we have not considered this up until this time. The only reason it even came up is that there was gross curvature on a part (rocking on the surface plate) and it raised some flags. We dug in to it and found we haven't been checking this feature correctly all this time. Functionally I think we're OK. Perhaps the designer didn't consider it either or we just lucked out.