Definition
Surface roughness is a measure of the fine irregularity of a machined surface. Ra is the average of the deviations from the mean line. Rz looks at the difference between the highest peaks and the deepest valleys. Both are given in micrometres.
What it means for the calculation
The roughness requirement decides the last operation. A coarse value is met by a normal milling pass. A fine value forces an extra finishing pass, a slower feed, or a separate machine such as a grinder. That is often a whole extra operation in the routing, not a few seconds of cycle time. A fine requirement on a large surface counts twice: that time scales with the area.
Where it appears on the drawing
As a tick-shaped symbol on a face or dimension line, with the value beside it, for example Ra 1.6. Sometimes a single general roughness requirement sits in or next to the title block, applying to all surfaces that are not called out separately.
Common mistakes
Mixing up Ra and Rz: on the same surface Rz is considerably larger than Ra, so a number without its letter cannot be priced. Also overlooking the general roughness requirement next to the title block, which still forces a finishing pass on the surfaces that are not called out.
The roughness requirement decides the last operation. A coarse value is met by a normal milling pass. A fine value forces an extra finishing pass, a slower feed, or a separate machine such as a grinder. That is often a whole extra operation in the routing, not a few seconds of cycle time. A fine requirement on a large surface counts twice: that time scales with the area.
See how Blake reviews a partFrequently asked questions
Is Rz just Ra times a fixed factor?
No, the ratio depends on the process; a reliable fixed conversion does not exist.
Does roughness also set the size tolerance?
No, they are two separate requirements; a surface can be smooth and still be out of size.
Why is a fine roughness expensive?
Because it usually needs an extra, slower finishing pass or a second machine.