Surface finish (Ra)

Look at a turned shaft under a lamp and you can see fine lines. Those lines are the tool marks, and their depth is what a finish callout puts a number on. A print might say 63 Ra on a bearing journal and leave a rough note on everything else, because only some surfaces need to be smooth.

Microinches and micrometers

One micrometer is about 40 microinches, so 32 microinches Ra is roughly 0.8 micrometers. Prints from US customers usually show microinches with a small check-mark symbol, while prints from metric drawings show micrometers. Either way, the number on the print is a ceiling: the surface may be that smooth or smoother.

Ra is an average, so it does not show a single deep scratch. For that reason a seal surface may carry an extra note, such as no spiral lead or no cross-scratches. The older RMS figure averages the same profile a different way and is covered with the other short entries on the glossary page.

Textbook ranges by process

These are general ranges for the processes, taken from standard machining references. They describe what the processes tend to produce in principle. They are education, not a statement of what any one shop holds, and the callout on your print is what sets the finish.

A rough turned or milled surface commonly falls in the low hundreds of microinches. A sharp tool and light feed commonly give tens of microinches. Grinding goes smoother still, and lapping or polishing smoother again. Each step down takes more time and care.

Where finish matters on a part

Finish matters most where one part touches or seals against another: bearing journals, seal diameters, sliding faces and sealing faces. A seal that runs on a surface that is too rough wears quickly, and a surface that is too smooth can fail to hold oil.

When a worn seal area has been rebuilt by metal spray or a sleeve, the surface is machined or ground back to the finish on the print. Put the finish callout on the drawing, or tell us the shaft or bore it goes with, when you send the quote request. A finish tighter than the part needs adds cost without adding function, as the tolerance entry also explains.