CNC Surface Finishing & Post-Processing

Question 1

What does the surface finish symbol 'Ra' represent in machining drawings?

Accepted Answer

Arithmetic average roughness of a surface profile

Answer

Ra is the arithmetic mean of the absolute deviations of the surface profile from the mean line, the most widely used surface roughness parameter.

Question 2

Which of the following surface finish values represents the smoothest machined surface?

Accepted Answer

16 Ra (micro-inches)

Answer

Lower Ra values indicate smoother surfaces; 16 Ra is finer than 32, 63, or 125 Ra.

Question 3

What machining operation is typically used to achieve a surface finish of 8–16 Ra micro-inches on a bore?

Accepted Answer

Reaming or fine boring

Answer

Reaming or fine boring with sharp tooling and proper feeds produces 8–16 Ra finishes, which are required for precision fits.

Question 4

Which CNC operation produces lay marks parallel to the direction of cutter travel?

Accepted Answer

Peripheral (side) milling

Answer

Peripheral milling cuts along the edge of the end mill, leaving lay marks aligned with the cutter feed direction.

Question 5

When a drawing calls for a 'ground' surface, which post-processing step is implied?

Accepted Answer

Cylindrical or surface grinding to achieve fine tolerance and finish

Answer

A 'ground' callout means the surface is finished by grinding, achieving tolerances and finishes beyond what turning or milling alone can produce.

(CNC) Computer Numerical Control Machinist Technical Practice Test