CNC Machinist Interview Questions
& Model Answers, 2026

G54 (through G59) is the work coordinate offset, it tells the machine where the part zero is on the table. G43 applies tool length compensation from the tool's height offset so the Z position is correct for that tool. G41 and G42 are cutter compensation, left and right of the programmed path, which lets you adjust the size of a feature by changing the wear offset instead of re-editing the program. Say that last part explicitly, using wear offsets rather than editing code is what a setup machinist does.

That is a half-thou window, so process, not luck. Rough it, leave finish stock, let the part get to temperature, and finish with a sharp tool and a rigid setup. Verify with a bore gauge set to a gauge block or a ring gauge, not a caliper, and account for temperature, an aluminium part warm off the machine will read differently at the CMM. Then say how you would keep it in tolerance across the lot: check frequency, and adjusting the wear offset as the tool wears.

Run one part, then stop the machine, do not run the lot. Check every dimension on the print against the FAI or control-plan requirement, using the right instrument for each (micrometer, bore gauge, gauge pins, CMM for anything positional), record the actual measured values rather than 'in tolerance,' and get the sign-off your shop requires before you press cycle start again. Add that you check the datum scheme and any GD&T callouts, true position gets missed most often.

Before it scraps a part, not after. The signals: dimensions drifting in one direction across parts (the classic tell), surface finish degrading, chip colour and shape changing, sound and load meter rising, burrs appearing at the exit. In production you set a tool life count based on actual data and index on schedule. Say that you would rather index early and lose an insert than lose a $400 casting on the last operation.

Read the print first and understand the datums and the critical features before touching the machine. Then: load and indicate the fixture, set the work offset (G54) and prove the part zero, load and touch off tools with their length and diameter offsets, dry-run the program above the part with a Z offset, then run the first cut at reduced rapid and feed override with a finger on the feed hold. First article, verify, then release the lot.

A coordinate dimension boxes a feature into a square tolerance zone; true position gives it a round one relative to the datums, which is both larger where it matters and correctly referenced to how the part actually assembles. Mention maximum material condition (MMC) and the bonus tolerance you get when a hole is made larger than its MMC size, that is the answer that tells a shop you can read an aerospace or medical print rather than just a shop sketch.

Everyone has. Use STAR and be exact: what you scrapped, what the root cause was (a wrong offset, an insert you should have indexed, a fixture that was not indicated in), what you did the moment you found it (stopped the machine and told the lead immediately, rather than letting the rest of the lot run and hoping), and the check you added afterwards. What they are scoring is whether you report scrap or hide it in the chip bin.

Diagnose before you touch the code: is it the program, the tooling, the fixture, or thermal growth? Fix what you own (an offset, an insert, a setup) and take the rest to the programmer with data, the measured values and what you have already ruled out. Say clearly whether your shop lets machinists edit programs at the control and whether edits get pushed back to the master; a machinist who quietly edits a program and never tells the programmer creates a landmine for the next run.

Sixty to ninety seconds: your years, your controls (Haas, FANUC, Okuma, Mazak), mill or lathe or both, your tightest routine tolerance, whether you set up or only run, any CAM (Mastercam, Fusion 360), and your NIMS credentials. Say 'I do my own setups' early if it is true, that single sentence changes the pay band you are interviewing for.

Systems, not willpower: measure to the check frequency on the control plan whether or not you feel confident, record the readings so a drift is visible on paper before it is visible in scrap, and never change two variables at once when you are tired. Add that you flag it to the lead rather than pushing through if you are too fatigued to hold a tight setup, that answer earns trust in shops that run nights.

Anchor to data and to what you actually do: BLS OEWS May 2025 puts the machinist median at $58,750/yr ($28.24/hr), with the top 10% above $80,010, and that top band is setup machinists, 5-axis and programmers. State a range for your level, and be explicit about which level that is, because 'CNC operator' and 'setup machinist' are two different pay bands in the same shop.

Ask, because in machining they are a large part of the cheque. A second or third shift differential is often two to six dollars an hour, and production shops frequently run mandatory overtime or four-tens. Ask how much overtime is typical, whether it is mandatory, and how the differential is applied. A nights offer at the same base as a days offer is not the same offer.

Yes, and it compounds. Ask the shop to pay for NIMS credential testing, control or CAM training (Mastercam or Fusion 360), and 5-axis exposure, because those are exactly what move you from the median toward the $80,010 top decile. If the base is capped, paid training that raises your ceiling next year is worth more than a fifty-cent bump this year.