Nonmandatory Appendices and Surface Roughness

[lmdrafting]

Hello,

Does anyone know what the nonmandatory appendices mean at the end of some of the ASME standards? For example, ASME B46.1-2002 has a nonmandatory appendix B. I'm wanting to implement a general note "SURFACE ROUGHNESS SHALL BE 125 MICROINCHES OR BETTER." But someone has discovered this nonmandatory appendix that states "Surface texture should not be controlled on a drawing or specification unless such control is essential to the functional performance or appearance of the product. Unnecessary restrictions may increase production costs and mitigate the emphasis on specifications for important surfaces." The engineer interprets this as not to callout surface roughness at all.

lm

 

[ctopher]

I do not have the spec here.
Surfaces/faces need to be indicated which roughness is needed, or use a general note. 'How' this is accomplished is not indicated on the drawing.

Chris
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[ewh]

"... OR BETTER" is also a poor choice of words, as depending on the purpose of the part, rougher may actually be "better".

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[KENAT]

I can't completely agree with you or your engineer. If roughness matters, be it functionally or aesthetically, specify it. If not leave it off, which is fundamentally what the spec says. If it's on there, then you are providing an opportunity for parts to 'not be good'. You are also causing an inspection requirement, which costs.

We have a default surface roughness note on most drawings, however we set the value on each drawing depending on function/aeshetics. It's rarely left off completely due to aesthetic concerns but in theory I'd be fine with it.

If you use the symbol from B46.1 then this defines the 'or better' part as less rough and gets over the problem ewh spotted.

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[lmdrafting]

I initially had "SURFACE ROUGHNESS SHALL BE 125 MICROINCHES" because I always thought 'or better' was implied. But I can't find documentation to support that. And Engineers think that if 125 is called out that the part has to meet that at inspection.

Kenat-without the symbol is this note incomplete? I was trying not use the symbol just to keep all the notes together cleanly. Solidworks adds a hard return about the note to allow room for the symbol.

 

[ModulusCT]

If 125 is called out, it does have to be met at inspection. However, I'm unsure what the allowable tolerance on roughness would be.

If you really want to indicate that surface roughness shall be 125 microinches or better, I would suggest you word the note differently. Something along the lines of "Max roughness shall be 125 microinches" thereby allowing lower values to be acceptable.

 

[KENAT]

Agree with Modulus that stating "...maximum roughness 125... RMS per ASME B46.1" avoids the problem ewh saw. I'm not at work at the moment but we don't use the symbol in our note, I think we say something like "SURFACE ROUGHNESS XXX RMS MAX PER ASME B46.1" or something like that, I'll check Monday if I remember. Explicitly referencing the standard is a good idea in my opinion.

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[dgallup]

All surface roughness callouts are single ended dimensions (maximums) unless both upper & lower limits are specified. Yes it is possible to be too smooth for certain applications but they are rare & the designer should be applying a special callout in such a case. It is redundant & incorrect to add the word MAX after a general surface finish specification as it could be interpreted as an Rmax requirement instead of the default Ra requirement. Adding the RMS qualifier as Kenat did gets around this, but I believe RMS is obsolete & should be replaced by Rq according to ASME B46.1-2002. Obviously, too tight a general surface roughness tolerance will add cost but this is true for all general tolerances.

 

[ewh]

The occasions I was refering to weren't that rare, but were somewhat specific in that they had to do with sealing surfaces. Since we are referring to a general note which may be (sometimes blindly) applied to a drawing as a standard note, I still feel that it is poor practice to use the term "BETTER" (a relative term at best) but agree that in the situations I refer to something more specific should clarify what finish is required and is acceptable.

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[KENAT]

We also sometimes want a minimum surface roughness for bonding or optical reasons so I'm not sure rare is the right term. Interesting what you say about RMS dgallup.

Maybe I didn't make it clear, while we have a 'standard note' for default surface tol, the actual value is decided on an individual basis, there is no default value.

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[KENAT]

"6. SURFACE ROUGHNESS TO BE ____ RMS PER ASME B46.1. "

So, looks like my memory was playing up and I may need to look at changing our note. Oh well.

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[CheckerRon]

The term "OR BETTER" is clearly redundant, as is MAX. It has been eluded to above, most clearly by dgallup, that a stated roughness average (Ra) is: (per Figure 5 of ASME Y14.36)"The specified roughness average shall indicate the maximum value and any lesser value shall be acceptable."
The only other option to this is a specified range, and yes, RMS has been replaced by Ra.

 

[KENAT]

Thanks Ron & Dgallup, another template change on my to do list;-).

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[lmdrafting]

Thank you all! So what do the nonmandatory appendices at the end of some of the ASME standards mean? In my original post I had mentioned that an Eng found a statement in B46.1 (App B, first sentenance) and so now argues per that statement that surface roughness should not be called out.

 

[KENAT]

lmdrafting, I thought we'd been over this, if the surface roughness matters, be it from functional or aesthetic point of view, it should be specified. If it doesn't matter, don't specify it.

That's what I'd take that specifific appendix to mean.

In your case does the surface finish matter?

Or if by general note you mean it will go on all drawings then I'd say it should be customisable based on individual drawing. The value should be set based on the requirements of each part, and if there are none on a specific part it should be deleted.

If you have a more general question about nonmandatory appendices, well, I think the description says it all. They are non mandatory, which means you can choose whether or not to invoke them. Who gets to make the decision of if you invoke it or not is up to your company.

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[CheckerRon]

B46.1 Appendix B1 Specification says this:
"(a) Surface texture should not be controlled on a drawing or specification unless such control is essential
to the functional performance or appearance of the product.
Unnecessary restrictions may increase production
costs and mitigate the emphasis on specifications for
important surfaces."

This is clearly boiler plate that does not say surface finish should not be called out on drawings, it just warns against over usage for non-essential applications. Hence the word Non-Mandatory.

 

[MechNorth]

Without a surface finish callout, the maximum roughness on any surface is equal to the maximum form tolerance on on the feature; surface finish is a refinement of form when you get right down into it.

Avoid jargon by using the ASME symbology; there's a default to RMS as I recall, and the value stated is the MAX RMS value then. The finish can be 'perfect' and be acceptable, or can be at the MAX RMS value and be acceptable. Note though that because it is an averaged limit and averaged measurement, you may have locally poorer finishes.

Jim Sykes, P.Eng, GDTP-S
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