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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[MintJulep]

I don't recall anything about Ra being a Max in the definition.

So if you mean Max state Max.

Shop might be on the ball. They might be thinking "the process I'm going to use will leave a much smoother surface. Do they really want a secondary operation to make it rough? "

 

[KENAT]

When it first came up my suggestion was just that I could add the 'MAX' to clarify, but purchasing made it sound like this wouldn't address the shops concerns and they actually wanted it tighter.

It may be purchasing muddling things up, and could be that the shop is actually more competent than anyone else we've used. I asked if they wanted me to speak to the shop and they didn't, whatever.

Thing is on the first drawing, I changed the value to 63 from 125, didn't add 'MAX' and got copied on an email from then vendor where they said they liked it, which is what's making me think it may not just be purchasing.

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[MintJulep]

As you clearly know, you need to get the middle man out of the loop to figure out what's up.

 

[MechNorth]

Kenat, your note specifies an exact average value, not a MAX, so I can see a knowledgeable supplier having an issue with it. The symbology, when just one value is provided, typically indicates a MAX Average value by default, though an absolute MAX is also available depending on how the symbol is "filled in". Symbology also allows upper and lower limits by average or absolute.
If you, as the designer, can live with a rougher surface then that is the appropriate value to put on the drawing. The supplier may be playing a game to get the spec updated to their typical capabilities which may be tighter than most other shops, thus effectively limiting their competition.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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[KENAT]

So it sounds like I do need to add the 'MAX' then - the note was originated by a very experienced checker so I'm a bit surprised he would have missed it but I suppose everyone makes the odd mistake. I'm a bit surprised it's never come up before though - some of our shops are pretty good.

The thing that makes me doubt the 'knowledgeable supplier' part is that they were happy when I changed it to '63' from '125' on the first print without adding MAX.

As to your theory about tightening specs to limit competition, I'm pretty sure we already have one other shop that does this, so it's definitely possible.

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[MechNorth]

A very common mistake for all readers (engineering prints or plain old written words) is that we often read-in things that we expect to see. I do it too frequently. I suspect that's why nobody mentioned it before; they assumed/read-in the MAX without it being there.
Personally I like dealing with small shops on a personal level; I get the best out of them, and we both know where we stand. Lots of tales from the trenches I could share over a pint/shot some day.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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[metalonis]

Machinery's Handbook 27 edition, table 6 "Application of Surface Texture Values to Symbol" states:

"The specification of only one [roughness average] rating shall indicate the maximum value and any lesser value shall be acceptable."

 

[metalonis]

Should have mentioned that's table 6 in the Surface Texture chapter.

 

[fsincox]

I too, think sufrace finish is a maximum value, I think you are caught in the typical it is always engineerings fault if there is ever a question from anyone, anywhere, zone.
Frank

 

[KENAT]

Metalonis, sadly that table is talking about use with the symbol.

For reasons mentioned above we state ours in note form without the symbol.

Thanks for looking though.

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[dgallup]

You can get fonts that include symbols such as this, there may even be something built into Windoze / MS orifice. Wing/web/dingbats or something.

However, I don't think that has anything to do with the issue. A single surface roughness value is always an upper limit. I would stay away from using "max" in this case because Rmax is another surface finish parameter that is completely different from Ra. Stating Ra max. is just redundant and confusing.

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[KENAT]

I talked to the Manufacturing Engineer that reviewed this shop and he was a bit surprised. His take was that our standard note specifies the maximum Ra and I think he was going to follow up.

I'll hold off changing our standard note for now, as based on this thread (and the vendor) it appears there's room for doubt but that just saying 'max' could also confuse.

On the print I just sent out I manually overlaid the symbol on the notes just to be sure!

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[ctopher]

I don't see surface texture/roughness as a max. There is a texture (wave) that is measured.
See this form:

http://www.nist.gov/calibrations/upload/89-4088.pdf

I also do not use "surface finish". It can lead to confusion as a final finish such as paint, etc. Another topic...

Chris
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[MintJulep]

Much information about surface finish:

http://www.nist.gov/calibrations/upload/89-4088.pdf

(although on quick read it doesn't answer the question asked here)

 

[KENAT]

That doc says pretty much the same as Global, Genium & Machinery's around page 37.

That is if using the symbol, then if a single value is stated that is the max Ra roughness.

Sadly it doesn't say anything about use of the term in text.

(Funnily enough, our sister site makes the measurement equipment they talk about & we make the AFM they mention around page 99. So you might think we'd be experts but apparently not on the specification aspect!)


Chris - that's why the note says 'surface roughness';-)

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[dgallup]

Ctopher - go to page 37 of your link. In the paragraph below DRAWING TERMINOLOGY:

"The allowed value of surface roughness .... can be specified either as a maximum value, which may not be exceeded, or as a range of values ...."

So a single value is always an upper limit (with zero as a lower limit since negative values are impossible). I don't think the presence of a surface roughness symbol is necessary if you spell out surface roughness. Symbols are just a convenient shorthand for when you don't want to explicitly write things in text.

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[MechNorth]

Be cautious here; Max & Ra Max are two different things. MAX means absolute MAX roughness whereas Ra MAX means that the maximum "average" is the limit; actual local roughness may exceed the Ra spec.

Using the symbol, "Ra MAX" is the default with a single value; an absolute or range of Ra values can be specified as needed.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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TecEase, Inc.

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[ctopher]

Jim,
I agree.

Chris
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[edwidder]

This question is not at all trivial, especially in the international environment. The two standards documents are ASME B36 and ISO 1302. In B36, 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.