CNC Surface Finishing & Post-Processing 1 — Questions and Answers
Question 1: What does the surface finish symbol 'Ra' represent in machining drawings?
- Maximum peak-to-valley roughness height
- Arithmetic average roughness of a surface profile (Correct answer)
- Root mean square of surface deviations
- The total range of surface texture values
Correct answer: Arithmetic average roughness of a surface profile
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?
- 125 Ra (micro-inches)
- 63 Ra (micro-inches)
- 32 Ra (micro-inches)
- 16 Ra (micro-inches) (Correct answer)
Correct answer: 16 Ra (micro-inches)
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?
- Rough boring
- Reaming or fine boring (Correct answer)
- Drilling
- Counter-boring
Correct answer: Reaming or fine boring
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?
- Face milling with a large shell mill
- Peripheral (side) milling (Correct answer)
- Ball nose contouring
- Boring with a single-point tool
Correct answer: Peripheral (side) milling
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?
- Hand-filing to remove burrs
- Cylindrical or surface grinding to achieve fine tolerance and finish (Correct answer)
- Sandblasting for texture
- Anodizing before machining
Correct answer: Cylindrical or surface grinding to achieve fine tolerance and finish
A 'ground' callout means the surface is finished by grinding, achieving tolerances and finishes beyond what turning or milling alone can produce.
Question 6: What is the primary purpose of a 'spring pass' or 'cleanup pass' at the end of a CNC turning sequence?
- To rough out excess material quickly
- To remove tool deflection error by repeating the final cut with no additional infeed (Correct answer)
- To change the spindle speed automatically
- To check the part with the probe cycle
Correct answer: To remove tool deflection error by repeating the final cut with no additional infeed
A spring pass re-runs the final tool path without additional depth, allowing the tool to spring back to its true position and clean up any deflection-caused material.
CNC Surface Finishing & Post-Processing 1
What does the surface finish symbol 'Ra' represent in machining drawings?