Surface finish is quoted as a roughness average, and the confusion is almost always about units and about which parameter. Ra in microinches and Ra in micrometres differ by a factor that has to be applied consistently, and RMS values are close to Ra but not identical. A conversion chart is only useful once you know which parameter the drawing is asking for.
What Ra actually measures
Ra is the arithmetic mean deviation of the surface profile from its mean line over a sampling length. It is a single number that describes the average roughness and says nothing about the shape of the peaks and valleys, which is why two surfaces with identical Ra can behave very differently in a seal or a bearing. Where profile shape matters, a second parameter is specified alongside it.
Units and the conversions that matter
Ra is quoted in microinches on US drawings and micrometres elsewhere, and the conversion between them is fixed. RMS, the root mean square roughness, runs slightly higher than Ra for a typical machined surface and the two are frequently confused on older drawings. Always read which parameter and which unit the number carries before comparing two specifications.
Which process gives which finish
Rough milling and turning give a coarse finish. Finish turning and milling with a light cut give a considerably better one and cover most commercial requirements without any extra operation. Grinding improves it again. Lapping, honing and polishing reach the finest values and are separate operations with separate costs. Cast, moulded and printed surfaces have their own ranges entirely.
Specifying it without wasting money
Put a finish callout only on surfaces where it does work: seals, bearing journals, sliding faces, cosmetic surfaces. Leave the rest as machined. Every callout tighter than what the finishing cut produces adds an operation to every part, and a blanket fine finish across a whole drawing is one of the most expensive habits in mechanical design.
Questions people ask about surface finish conversion chart
What is Ra?
The arithmetic average roughness: the mean deviation of the measured profile from its mean line. It is the most commonly specified surface finish parameter.
Is RMS the same as Ra?
No, but they are close. RMS reads slightly higher than Ra on a typical machined surface. Which one applies depends on what the drawing specifies, so read the callout rather than assuming.
What finish do I get without asking?
A normal finishing cut in turning or milling produces a surface that suits most commercial applications. Specify a value only where a function depends on it.
Does a better finish always cost more?
Past the point that a finishing cut produces, yes, because it means an additional operation such as grinding, honing or polishing on every part.