Surface finish specifications 2

[DerbyLoco]

I have been drawing a machined part based on previous designs and one area requires an Ra1.6 surface finish. However the Engineer checking the job has found another drawing which specifies the same area finished to Ra1.2. Having never had cause to give it much thought I have always assumed that the Ra range was specified in the following steps Ra 0.4, 0.8, 1.6, 3.2 and so on. Having looked into it a little I take it you can define whatever finish the job really needs - fair enough. My question is what is the standard range based on and why are those partiular values used?
In this case I suspect the change is just an error copied from one job to another.

 

[dgallup]

While you can specify any value for the roughness you need, there are standard values and a subset of those that are preferred. 1.6 is one of the preferred values, 1.25 is the next lower standard value. However, we have a quite a few parts that use 1.2 and it's never been a problem.

Roughness sampling lengths, on the other hand, can only be a small number of values. 0.8 is the default if you do not specify it.

I don't know who/how the standards were determined but I keep a list of them stuck on the wall for reference.

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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.

 

[CoryPad]

The metric Ra "standard" values are just conversions from old inch "standard" values.

Ra in [µ]in: 2000, 1000, 500, 250, 125, 63, 32...

Ra in [µ]m: 50, 25, 12.5, 6.3, 3.2, 1.6, 0.8...

 

[MintJulep]

The "Standard" values more-or-less correspond to the typical finish expected from various processes

1000 (25.2) = sand casting, torch or saw cut

500 (12.5) = rough filing, disk grinding, heavy cut and coarse feed on mill or lathe

250 (6.3) = forging, extrusion, permanent mold castings

etc.

http://www.engineershandbook.com/Tables/surfaceroughness.htm

 

[DerbyLoco]

Thanks for the replies gentlemen. I think that CoryPad has hit the nail on the head with his answer. In the end we went with Ra1.6 as we could not find a justification for the change to Ra1.2. In my experience off standard changes cost a disproportionate amount of money as manufacturers want to cover the possible extra risk/squeeze a profit out of the job!