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10 common CNC machinist interview questions along with strong, practical sample answers.

1. How do you set up a Work Coordinate System (WCS) like G54 on a CNC machine?

• Sample Answer: “I clean the table, fixture, and stock to ensure no chips affect positioning. I use a touch probe or edge finder to locate the X and Y datum edges of the stock, accounting for the edge finder radius if using a manual one. For the Z-axis, I establish the zero point—either top of stock or top of table—using a tool setter or precision block. I input these relative coordinates into the G54 register on the control panel, then perform a dry run or single-block air-cut to verify the program home relative to the part.”

2. What is the difference between G00 and G01, and why is the distinction critical?

• Sample Answer: “G00 is rapid linear motion used strictly for positioning the tool in non-cutting space at maximum machine speed. G01 is linear interpolation, which moves the tool at a specified feed rate (F) to execute an active cut. mistaking a G00 for a G01 inside stock will cause a tool crash or damage the workpiece because the tool moves at full speed without cutting physics accounted for.”

3. How do you calculate Spindle Speed (RPM) and Feed Rate for a milling or turning operation?

• Sample Answer: “I start with the recommended Surface Feet per Minute (SFM) for the material and tool combination. For RPM, I use $\text{RPM} = (\text{SFM} \times 3.82) / \text{Tool Diameter}$. Once I have RPM, I calculate Feed Rate using $\text{Feed Rate (IPM)} = \text{RPM} \times \text{Number of Flutes} \times \text{Chip Load per Tooth}$. If I notice chatter or heat buildup during the cut, I dial back RPM or adjust the feed per tooth accordingly.”

4. How do cutter radius compensation (G41/G42) and length compensation (G43) work?

• Sample Answer: “G41 offsets the tool to the left of the programmed path, and G42 offsets it to the right, allowing the control to adjust for tool diameter wear without rewriting the program. G43 calls up tool length compensation relative to a specific offset register (e.g., H01), telling the Z-axis how long the tool extends from the spindle gauge line.”

5. How do you approach reading complex engineering drawings with GD&T callouts?

• Sample Answer: “I review the title block for material, general tolerances, and projection type. I identify the primary, secondary, and tertiary datums establishing the reference frame. Then, I cross-reference GD&T callouts—like true position, runout, or parallelism—against my machining sequence to ensure key features are machined in the correct orientation or same setup where possible to avoid cumulative stack-up errors.”

6. What steps do you take when a tool breaks during a run?

• Sample Answer: “First, I hit Feed Hold or Emergency Stop depending on the severity. I jog the Z-axis clear, inspect the workpiece for damage, and recover all broken carbide pieces from the cut zone. I inspect the tool holder and spindle for damage, replace the insert or tool, re-measure the tool offset using the tool setter, reload the code to a safe lead-in position or line number before the break, and restart with reduced feed to clear any work-hardened material.”

7. How do you prevent and fix chatter during a heavy cutting pass?

• Sample Answer: “Chatter comes from a lack of rigidity or resonant vibration. To fix it, I first reduce stickout length on the tool or workholding. If the setup is fixed, I alter cutting parameters: slightly dropping the RPM or increasing feed per tooth to change the harmonic frequency and force the tool into a stable cut. I also check insert wear, depth of cut (DOC), or switch to an variable-flute end mill if necessary.”

8. What key daily preventive maintenance tasks do you perform on CNC machinery?

• Sample Answer: “At start-of-shift, I check way lube levels, pneumatic pressure gauges, and coolant concentration using a refractometer. I wipe down spindle tapers and tool holders to keep them free of debris, empty chip trays, and verify that way covers move freely without chip buildup. At end-of-shift, I blow out the enclosure, clear swarf from the bed, and leave the machine in a clean, safe state.”

9. How do you handle inspecting a part with tight tolerances (e.g., ±0.0005″)?

• Sample Answer: “I allow the part to stabilize to ambient shop temperature if it got hot during machining. I calibrate and use precision instruments like micrometers, bore gauges, or a CMM rather than standard calipers. I measure the part while still fixture-bound if possible; if re-machining is needed, I apply offset adjustments in the control wear table rather than editing the base code.”

10. How do M00, M01, and M30 differ in a program?

• Sample Answer: “M00 is an unconditional program stop—the machine pauses execution until the operator presses cycle start (used for chip clearing or inserting hardware). M01 is an optional stop that only pauses the machine if the ‘Optional Stop’ button on the control panel is toggled on. M30 marks the end of the main program, stopping spindle and coolant, and rewinding the program back to the top.”