Ra and Rz don't say the same thing
Ra is the mean roughness parameter derived from the absolute values of profile deviations of evaluation length. Rz presents a different assessment of hills and pits; its definition should be read according to the standard used. It is not a reliable acceptance method to substitute these two values with a constant coefficient.
Specify the relevant parameter and limit instead of just saying "very smooth surface" to a supplier. Explain this also if there is visual expectation. cosmetic sample photo does not assume the same task as the roughness requirement in the technical drawing.
Connect the requirement to the surface's task
In a valve body, the screw female body, connection female and sealing zone have different needs. Determine which contact is important before applying the surface condition to the whole part in the same way. The method of evaluation of a desired surface for friction, wear, sealing or just appearance may not be the same.
The question of design here is: "What do I expect from this surface?" The question of production is "By what method of operation and control will I confirm this expectation?" The two questions should correspond on the same drawing.
Before or after the final procedure?
If a coating, sandblasting or any other finishing is required, write down the stage at which the measurement and surface conditions apply. The report of the processed surface does not describe the final state of the finished piece alone. In particular, masking and final gauge conditions should be discussed together in co-operating areas.
It is not possible to extract Ra value from a photo. Likewise, a dimensional CMM report does not always have a roughness result. The required device and report scope In your measurement request Specify as separate title.
Sentence to add to bid request
"On the marked surface, the roughness requirement specified in the technical drawing will be provided after the final operation; the relevant result will be shown in the measurement report." Such an explicit statement is more useful than a "make the surface good" note. The current standard, measurement direction and assessment settings should also be defined when necessary.
the FactorAB In demand for production to identify critical surfaces. Machining and any required external processes are assessed for the individual part; a surface requirement should not be assumed to be included in a standard service package.
Sources and further reading
This article is an original narrative that deals with technical principles in resources through the process of parts and bids. Examples are for disclosure purposes; it does not mean partnership with resource organizations or their approval of FactorAB.