Surface Roughness General Coverage Specification

[KENAT]

Our notes are headed:

NOTES: UNLESS OTHERWISE SPECIFIED

We have a general surface roughness note the value of which gets set as appropriate for each drawing. (I've just added the italics here for clarity - they aren't on drawings.)

SURFACE ROUGHNESS SHALL BE 125 Ra PER ASME B46.1.

We've been using this note since 2005, always believing it to be clear that it was a maximum roughness value, and I don't think we've ever had machine shops complain or be unclear on this. We don't use the symbol as part of the note because these notes are done in an inserted Word document as the text editing abilities of our CAD software are a bit weak and it would be time consuming/error prone to manually position the symbol etc.

We apparently have a new shop as part of a vendor consolidation effort (replacing several smaller previous suppliers) and they seem to be confused by this.

On one print where I'd separately indicated a certain bore to have min & max roughness requirements (125-250) using standard symbol they actually asked for then general roughness to be reduced to 63 Ra from 125. I debated this with purchasing but eventually gave in making some allowance for my (125-250) call-out perhaps being a bit unusual and causing confusion.

However, now they are asking for me to change another print where I have 125 in the general notes and no weird 'minimum roughness' call outs on the drawing. I'm refusing to tighten the roughness for no functional reason (another shop made this part previously with no issues) but have added 'MAX' after the value just to clarify.

I've looked in Global & Genium DRM's but don't see any mention without the symbol and I don't have B46.1. Machinery's does say "It is considered good practice to always specify some maximum value, either specifically or by default" but doesn't really say if a value given in a note is assumed to be maximum.

So to the questions

Q1: Per B46.1 does our standard note clearly specify maximum roughness, or because we aren't using the standard symbol do we need to explicitly say 'MAX' to clarify?

Q2: Am I right to be concerned about how good this shop is if this note that we've been using for years is causing this much confusion and they're actually asking me to tighten requirements?

Thanks,

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[edwidder]

In my previous post, I mentioned ASME B36 - meant ANSI Y14.36. Freudian Slip. I'll re-post with correction.

 

[edwidder]

This question is not at all trivial, especially in the international environment. The two standards documents are ANSI Y14.36 and ISO 1302. In Y14.36, specification of a single value is implied as a maximum value. (NOT a target value!). However, in ISO 1302 (& ISO 4287), the simple specification applies the "16% Rule" as a default. Per this standard, if one wants the value to be an absolute maximum specification, then "MAX" must be added to the parameter designation ("Ra Max"). [Thankfully, Rmax is not recognized within ISO, but is within ASME B46.1.] This 16% Rule is a quirky blending of an acceptance criteria within the parameter definition document. There is an opinion that quality acceptance criteria are properly contained within the measurement plan for the part and not on the part drawing - or indirectly through the parameter definition. There is some uncertainty in surface roughness values - but there is uncertainty present in ALL measurement activities. Why single out surface roughness? One of the problems with the 16% Rule is that it would allow 16% of the measured values to be outside of the specified value, but doesn’t specify how far out any single measurement value can be. On some critical surfaces, outlier areas may be functionally inappropriate.

 

[CheckerRon]

As Jim Sykes says, don't confuse Roughness Max and Roughness average Max.
ASME Y14.36 para 4.3 "Roughness Average (Ra)" states "The principle parameter specified for roughness average, Ra, defined in ASME B46.1. It is the value shown in position "a" of the surface texture symbol (i.e. directly above the V of the surface texture symbol)."
Also Figure 5 of ASME Y14.36, which shows samples of surface texture symbols staes in the primary example that "The specification of only one rating for roughness average shall indicate the maximum value, and any lesser value shall be acceptable."
For this reason, in accordance with the standard, maximum roughness average the interpretation, and adding the word MAXIMUM to the drawing only muddies the waters, IMHO. Note that ASME Y14.36 SURFACE TEXTURE SYMBOLS, is specified in ASME B46.1 as the drawing standard for surface texture symbology. Having a copy of B46.1 is probably not necesary, but you should get a copy of the 8 page ASME Y14.36 if you can.

 

[KENAT]

I'm not confusing Roughness Max and Roughness average Max.

My issue is just that it's difficult/error prone to have the roughness symbol embedded in my general notes the way we do them.

So then we just have the text statement per my OP.

However, it seems the relevant ASME std only talks about the value in context of the symbol.

When I remember I've been changing the note to:

SURFACE ROUGHNESS SHALL BE 125 Ra (MAX) PER ASME B46.1.

I haven't had any more complaints, but I'm hesitant to change the default template in case it does get some other vendors confused between Ra & Rmax. Given I explicitly state 'Ra' and the 'Max' is in parenthesis I'd sure hope not but then I wouldn't have expected any machine shop to complain about the note as is so...

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[CheckerHater]

Maybe you should try to say something like “Ra 125 OR BETTER” to get rid of “max” altogether?

 

[KENAT]

Maybe, or 'UPPER LIMIT' or something, cheers Ron - good to see you on here again.

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[CheckerRon]

Since KENAT is checker and I am a checker, can we trust the comments of "CheckerHater"? .
Nice to be back. A former client dragged this old job shopper out of retirement and brought him back to check again, for a while. We may be a dying breed, but not just yet.

 

[KENAT]

CH and I have had that conversation before Ron!

I'm not really checking anymore though (since Jon got let go in 2009) so I guess I get to go back to the dark side!;-)

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