Surface Max / Min Callout meaning

[KKKing]

We have received a print from a customer that has a surface finish callout that is calling for a Ra with a max of 0.2 and a Min of 0.8. I have not seen where the min callout is greater then the max callout. How do you interrupt the callout and where in any standards does it describe how the min and max variables are used?

 

[djhurayt]

My suspicions is that they have those backwards.

There are indeed cases (hydraulic cylinders) where to smooth of a finish is not desirable.

 

[greenimi]

There are indeed cases (hydraulic cylinders) where to smooth of a finish is not desirable.

Yes, I got one of those where the oil / lubricant did not stick onto the surface due to the surface finish (to smooth)

 

[ewh]

There are indeed cases (hydraulic cylinders) where to smooth of a finish is not desirable.

Which is why "...or better" should not be used in a finish callout.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[chez311]

I got one of those where the oil / lubricant did not stick onto the surface due to the surface finish (to smooth)

Absolutely - I believe what often happens is the oil is essentially squeezed out between the mating parts and becomes unable to form an oil film. Where I've seen it most commonly is on radial lip seals. Too smooth a finish will cause the seal lip to burn up due to direct contact with the rubber.

 

[Belanger]

Which is why "...or better" should not be used in a finish callout.

I gently disagree. Think about it: the most common type of surface finish is where a single number is given. It's the maximum allowed, which automatically means that anything better (smoother) is perfectly acceptable (maybe even desirable).

 

[ewh]

At the extreme that you mention, I agree. However, the vast majority of the time the value is something other than the extreme, and then you are requiring the fabricator to decide whether smoother is better, instead of the designer. It is preferable to be explicit and instead of "better", use "smoother", if that is acceptable to the design. A good part definition limits ambiguities such as this.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

Edit: typo

 

[dgallup]

Ra with a max of 0.2 and a Min of 0.8 makes no sense. There are lots of reasons to have a min/max requirement but there has to be a range of allowable values. Nothing could meet a Ra with a max of 0.2 and a Min of 0.8 specification.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Belanger]

...you are requiring the fabricator to decide whether smoother is better, instead of the designer.

If a single number is given -- which must represent the maximum allowable roughness -- why would anyone think that more rough is "better"? I suspect that you're overthinking this.
If I put a perpendicularity callout on a surface, it's not possible for someone to think that a "better" part would be further away from perfect perpendicularity!


edit: I suppose it's **possible** -- but it would not any sense for someone to say that

 

[ewh]

Removing this ambiguity costs nothing.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Belanger]

ewh -- I think I see what you were saying in your first post. The addition of "or better" is not a good idea, and I certainly agree.
I now see that we were coming at it from different perspectives: My comment was that it's not needed because anything "better" is always OK, and your comment was that the word "better" is subjective. My apologies that we were talking past each other.

 

[ewh]

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV