Surface Roughness

[Pjohnny]

Hi, does someone could help me, please?
An Automotive customer ask us for a mechanical part made by turning.
The surface roughness required is 4/Pt12.
Does anybody know what it means?

Thanks.

 

[dgallup]

They need to tell you what standard they are using. Just as a guess, it might be BS 1449. That standard has a number 4 finish, don't know what the Pt12 might be.

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[IRstuff]

I suggest that you RF this posting and ask the moderator to move to the Metal and Metallurgy forum on this site.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[dgallup]

While I don't know that this is the correct forum for this question, it certainly has nothing to do with Metallurgy.

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[IRstuff]

It is a metal part, is it not?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Pjohnny]

Yes it is a metal part.
I don't know the roughness standard they are using, but our customer is a german supplier of a big automotive brand.
On some surface of the piece they require Rz16.

 

[WolfHR]

In Croatian standards*, Rz is used as average height of surface irregularities in um (which can usualy be taken to be correlated to maximum height as Rmax = cca 1.6Rz)... I'll try to check in some German book.

* in many cases based on German DIN standards

 

[WolfHR]

Sorry for following up on my own post, I'm almost certain my guess was correct- but more relevant woul be the formula Rz = cca 4*Ra (and that would be in the ballpark of N8 surface finish, which is Ra=3.2um, N9 being 6.3um).

http://en.wikipedia.org/wiki/Surface_finish

 

[dgallup]

Their drawing should state what standards they use in creating their drawings (yeah, I know, lots of companies, even big one, have lousy drawing formats that leave you guessing). Rz is a surface roughness parameter that has about 4 different definitions depending on which standard you use. You can not with any degree of accuracy convert Rz to an Rmax or an Ra as each roughness parameter is derived from the exact profile being measured. A sine wave profile with the same Ra as a triangle wave profile will have very different Rmax and Rz values.

You could take a look at ISO 1302, it has a conversion between current Ra values and obsolete Roughness Grade Numbers. The obsolete number system always starts with the letter "N". An N4 roughness grade number corresponds to 0.2 Ra (in microns). Still no mention of the /Pt12.

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[WolfHR]

Dgallup, I've given the formulas from (I'd say reputable*) engineering handbook based on this obsolete standard. That should get him in the ballpark figures for Ra- FWIW, if he was to ask them about their tolerancing/surface finish practices, mentioning that he has dug up such arcane formulas might make them more helpful...

* has been in print from 1954, and I believe still is

 

[Pjohnny]

It seems that Pt 12 is the maximun allowable value of Pt.
We don't understand the mean of the number 4.
We checked the roughness of the samples and we found these values:
Ra=1.2 Pt=6.5 measured on 1.75mm lenght.

 

[TVP]

Pjohnny,

ISO 1302 explains the system for indication of surface texture. The numeral 4 that appears before the Pt designation is called the transmission band (ISO 3274) and indicates the short-wave filter or long-wave filter. According to ISO 1302, there should be a hyphen either before or after the value in order to indicate if it is the short-wave filter or long-wave filter:

4- means short-wave
-4 means long-wave

The units are mm.

 

[Pjohnny]

TVP,
you mean the cut-off value I think.
ISO 4288 recommend these cut-off values:
Ra cut-off
<0.02 -> 0.08
0.02 - 0.1 -> 0.25
0.1 - 0.2 -> 0.8
2-10 -> 2.5
10 -> 8
Value 4 means an expected Ra between 2 and 10.
In this case it will not be a critical value because this surface is
made by turning.

 

[TVP]

Pjohnny,

I used the terminology from ISO 1302, Section 6.5. The cut-off values define the limits of the filters, either short-wave or long-wave.