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.