The surface finish symbol on a drawing is a small structure with several fields, and most drawings use two of them. The symbol itself says whether material removal is required, forbidden or optional; the number above or beside it gives the roughness limit; and additional fields cover lay direction, machining allowance and the process. Reading it correctly matters because a misread doubles a part's cost.
The symbol and its variants
The basic symbol is a tick with a longer arm. A bar across the vee means material removal is required. A circle in the vee means material removal is prohibited, which is how a cast, moulded or forged surface is specified. A horizontal extension carries process notes. Getting these confused is how a drawing accidentally requires a moulded face to be machined.
Reading the numbers
The primary number is the maximum roughness, usually Ra. Two numbers mean a range with a minimum as well as a maximum, which appears where a surface must be rough enough to hold lubricant or take a coating. Values on US drawings are frequently in microinches and elsewhere in micrometres, so the unit has to be established before the number means anything.
What 63 and Ra 3.2 actually are
A callout of 63 on a US drawing is 63 microinches Ra, a normal finish from a good finishing cut in turning or milling, and it is one of the most common values in general engineering. Ra 3.2 micrometres is the metric equivalent of essentially the same surface, which is why the two callouts appear on drawings of the same parts made either side of the Atlantic.
Which process reaches which value
Rough machining is coarse. A finishing cut in turning or milling reaches the common commercial range without extra work. Grinding improves it substantially. Honing, lapping and polishing reach the fine values and are separate operations. Specifying a value one step finer than a finishing cut produces adds an operation to every part, which is where the money goes.
Questions people ask about surface finish symbols
What does a 63 finish mean?
63 microinches Ra, which is a normal surface from a good finishing cut in turning or milling and one of the most common callouts on US engineering drawings.
What is Ra 3.2?
3.2 micrometres Ra, the metric expression of essentially the same surface as a 63 microinch callout. The unit is what changes, not the requirement.
What does the circle in the symbol mean?
That material removal is prohibited, which is how a cast, forged or moulded surface is specified. A bar instead means material removal is required.
Should I put a finish callout on every surface?
No. Only on surfaces where finish does work, such as seals, bearing surfaces and cosmetic faces. A blanket fine finish adds an operation to every part.