CNC machinist screening questions: operator or setup, controls, G-code and tolerances

Good CNC machinist screening questions find out one thing before anything else: whether the candidate runs parts, sets up jobs, or programs them. Ask who set the work offsets on their last job, which machine and control they used, and the tightest tolerance they held on it and how they measured it. Those three answers tell you more than the job titles on the resume.

This bank is for recruiters placing CNC operators, setup machinists and machinist-programmers in job shops and production plants. It covers the level question, knockouts, controls, setup and G-code, inspection and GD&T, and a rubric you can score in the call. It does not cover welding or fabrication (see welder screening questions) or the people who inspect the parts after they leave the machine (see quality inspector screening questions). For general plant roles such as machine tenders and packers, the light industrial screen is the better fit.

Operator, setup machinist or programmer: pin the level first

"CNC machinist" covers four different jobs, and the pay gap between them is large. Get the client to say which one they need, then ask the candidate questions that only the right level can answer.

The fastest single test is the work offset question. A setup machinist answers "I did, with an edge finder or a probe, into G54" without thinking. An operator says "the setup guy did that", which is an honest and perfectly placeable answer, just for a different order.

Knockout questions

Ask these in the first five minutes. A miss on any one usually ends the screen for this order, not for every order.

Machines, controls and software

You do not need to know how to run the machines. You do need the names, because candidates will use them and the client's job order should list them. Ask for the machine, the control and the software separately; a candidate who blurs them together is often describing a shop they worked near rather than in.

• Machine types: vertical machining center (VMC), horizontal machining center (HMC), CNC lathe or turning center, mill-turn, Swiss-type lathe, 5-axis mill.
• Controls: Fanuc and Fanuc-style controls (Haas uses its own control with very similar G-code), Mazak Mazatrol (conversational) and Mazak's G-code mode, Okuma OSP, Siemens Sinumerik, Heidenhain, Hurco.
• CAM software: Mastercam, Autodesk Fusion, Esprit, GibbsCAM, Siemens NX CAM, SolidCAM. Ask whether they programmed in it at work or learned it in school.

Questions with strong answers and red flags

1. "Which control did you use most in the last year, and roughly how many hours a week?"
   • Strong answer: one control named first, then others with honest time on each: "Fanuc on the Doosan lathes daily, Mazatrol on one Mazak for about six months."
   • Red flag: a list of every control brand with equal weight.
2. "On 5-axis work, were you running simultaneous 5-axis or 3+2?"
   • Strong answer: knows the difference. In 3+2, the rotary axes tilt the part to a fixed angle and the cut is 3-axis; in simultaneous work, all five move together, usually from CAM.
   • Red flag: does not recognize the question. Many "5-axis" resumes mean indexing a 4th axis.
3. "On the Swiss, what material and bar size were you running, and how did you handle the guide bushing?"
   • Strong answer: names the bar stock and diameter, talks about fitting the guide bushing to the bar and adjusting it.
   • Red flag: Swiss experience that turns out to be watching a bar feeder.

Setup, offsets and G-code questions

This is where most overstatement lives. Ask the candidate to narrate, and listen for order. A person who has done it describes steps in sequence with small practical details; a person who has watched it describes the result.

1. "Walk me through your last setup, from a clean table to a good first part."
   • Strong answer: read the print and setup sheet, mounted and indicated the vise or fixture, loaded tools and set tool lengths (with a setter, a presetter or a probe), picked up the work offset, dry-ran or single-blocked the program with the rapid override turned down, cut the first part, measured it and adjusted wear offsets.
   • Red flag: skips from "I set it up" to "then I ran it". No mention of checking the first part.
2. "What is G54, and what is G43?"
   • Strong answer: G54 is the first work coordinate system, the zero the program cuts from; G43 applies the tool length offset (with an H number). Bonus if they mention G41 and G42 for cutter compensation.
   • Red flag: none of these codes are familiar, for a setup or programming role. For an operator role, a blank answer is normal.
3. "Tell me about a time a part came out of tolerance mid-run. What did you change?"
   • Strong answer: a specific feature, a cause (tool wear, heat, a loose fixture, a bad insert) and the fix, usually a wear offset or a tool change, then more frequent checks for a while.
   • Red flag: "I told the lead" as the whole story, for a setup role.
4. "When did you last edit a program at the control, and what did you change?"
   • Strong answer: something small and real: a feed rate, a depth of cut, adding a chip-break peck, fixing an approach move.
   • Red flag: claims to program but has only changed offsets.
5. "Tell me about the last crash you had or saw. What happened?"
   • Strong answer: an honest account (a wrong offset, a tool length entered in the wrong register, a clamp in the tool path) and what they now do differently.
   • Red flag: "I have never crashed a machine" from someone with years of setups. It happens to nearly everyone who sets up; what matters is what they learned.

Tolerances, GD&T and inspection

Tolerances are stated in thousandths of an inch ("thou") or, in tighter work, ten-thousandths ("tenths"). Metric shops talk in hundredths of a millimeter. Ask what they held, on which feature, and with what gauge. A tolerance with no gauge attached is a number from a print, not a skill.

Geometric dimensioning and tolerancing (GD&T) is the symbol language on the print for position, flatness, profile and the rest. In the United States it is defined by ASME Y14.5, the ASME standard for dimensioning and tolerancing (as of September 2026). Aerospace and medical shops lean on it heavily; general job shops less so.

1. "Which measuring tools do you use every day, and which do you use only sometimes?"
   • Strong answer: calipers and micrometers daily; bore gauges, thread gauges or pins, height gauge on a surface plate, dial test indicators as needed; CMM or comparator results from the quality team.
   • Red flag: calipers only, for a role holding tenths. Calipers are not the right tool for that.
2. "On a print, what does a true position callout with a datum reference tell you?"
   • Strong answer: the hole or feature has to sit within a tolerance zone around its exact location, measured from the listed datums, and the order of datums matters for how you set up and measure.
   • Red flag: for an aerospace or medical order, no recognition of datums at all.
3. "Who checked the first article on your last job, and what happened if it failed?"
   • Strong answer: knows the handoff: they measured, quality or a lead signed off, and the job stayed on hold until it passed.
   • Red flag: no idea how first parts were approved.
4. "What materials have you cut the most, and which one gave you the most trouble?"
   • Strong answer: specific alloys (6061 aluminum, 304 or 316 stainless, 4140, titanium, Inconel, plastics like Delrin or PEEK) and a real reason one was hard: heat, work hardening, chip control, holding size.
   • Red flag: "all materials".

Credentials, shifts, tools and pay

Credentials

Most shops hire machinists on shown skill, not certificates. The credential you will see most is from the National Institute for Metalworking Skills (NIMS). As of September 2026, NIMS says it offers multiple Level I machining credentials that can be earned in any combination and order (NIMS FAQ), and its credentials pair a theory exam with a hands-on performance evaluation, such as machining a part to print (NIMS credentialing). Ask which specific credential they hold, since an operator credential and a programming and setup credential mean different things. A registered apprenticeship certificate is also worth writing down.

Shifts and schedule

• Which shift, and is it fixed or rotating? On second and third shift a setup machinist often works without a lead or engineer nearby, so ask whether they have set up alone at night.
• Overtime: how much in a normal week, and is Saturday expected?
• Lights-out or unattended running: will they be expected to set up jobs to run overnight?

Tools and pay

• Does the candidate own their measuring tools, and does the client require them? Some shops supply everything; others expect a full box.
• Current hourly rate, shift differential, and overtime pattern. Compare base to base: a candidate on a 60-hour week will quote weekly earnings that a 40-hour role will not match. See salary expectation questions for wording.
• If the client does defense or export-controlled work, ask the client exactly how they want eligibility questions handled and use their wording. Do not improvise questions about citizenship; the illegal interview questions guide covers why.

How CNC experience gets overstated

• "Setup" means loading a saved program into a fixture that never left the machine. Ask whether they indicated the fixture and set the offsets, or whether the job was a repeat already dialed in.
• "Programming" means editing offsets. Ask for the last program they wrote from a print, what software, and how long the program ran.
• "5-axis" means an indexing fourth axis. Ask the simultaneous or 3+2 question.
• "Held tenths" was on a ground part, not their machine. Ask which operation and which gauge.
• Years of experience include deburring and inspection time. Ask how many years they have been setting up, specifically.
• CAM experience is a school course. Nothing wrong with that for a junior role, but say so in the submittal.

Scoring rubric and the note to the shop

Send the shop a note it can use at the trial, not only a resume. The format below takes two minutes to fill in from your notes.

Candidate: [name] — [operator / setup / machinist-programmer]
Last 12 months: [machine and control], [hours a week], [shop type]
Setup: [what they described, in order, in one line]
Programming: [none / edits at control / CAM: software, last program]
Tightest tolerance: [value] on [feature], checked with [gauge]
Materials: [top two]
Probe at the trial: [the one thing you could not confirm on the phone]
Shift and overtime: [confirmed]   Tools: [own box / needs tools]
Rate: [current base, differential], expecting [range]

If you screen a lot of machinists, keep the same questions in the same order for every call so the scores compare. Interview Signal can build that question guide from the job order and tick off each question as it gets covered, which helps on the calls where the candidate talks about their last crash for ten minutes and the tolerance question never gets asked.