--- title: "Lathe vs Mill G-Code Differences: A Cheat Sheet" description: "The motion core is identical; six differences do the damage: default plane, diameter programming, feed modes, CSS, cycle families, and comp mechanics." url: https://gcodepractice.com/journal/lathe-vs-mill-g-code-differences-cheat-sheet/ canonical: https://gcodepractice.com/journal/lathe-vs-mill-g-code-differences-cheat-sheet/ author: "Lawrence Arya" authorUrl: https://www.linkedin.com/in/vibecoding/ published: 2026-06-04 updated: 2026-06-04 category: "Code reference" tags: ["lathe", "mill", "cheat-sheet", "g-code", "comparison"] lang: en --- # Lathe vs Mill G-Code Differences: A Cheat Sheet > **TL;DR** Lathe and mill G-code share their motion core: G00, G01, arcs, units, positioning, and work offsets behave identically. Six differences matter: lathes default to the G18 plane with I and K arcs, program X as diameter, commonly feed per revolution, add constant surface speed with G96/G97 and the G50 cap, use the G70-G76 turning cycles instead of G81-G89 drilling cycles, and apply the same G41/G42 comp codes to tool-nose radius instead of cutter diameter. Machinists crossing between turning and milling fear a second language course. The real workload is smaller: the motion core is identical on both machines, and the differences fit on one short list. This page is that list, with the traps marked. ## What stays exactly the same? The code that does most of the work. `G00` rapids, `G01` feeds, `G02` and `G03` arcs, `G20`/`G21` units, `G90`/`G91` positioning, the `G54` offset family, and the coolant and spindle M-codes all behave identically, the shared inheritance documented in any [standard reference](https://linuxcnc.org/docs/html/gcode/g-code.html). The M-code side of the crossing, including the tool-change split, has its own map in [how to remember lathe M-codes vs mill M-codes](/journal/how-to-remember-lathe-m-codes-vs-mill-m-codes/); this page covers the G side. ## The cheat sheet: six differences that matter | Difference | Mill | Lathe | | --- | --- | --- | | Default plane | `G17` (XY), arcs use `I`, `J` | `G18` (XZ), arcs use `I`, `K` | | X values mean | Distance | Diameter, usually | | Common feed mode | Per minute (`G94`) | Per revolution | | Speed control | Direct RPM (`S` + `M03`) | `G96` CSS + `G97` + `G50` cap | | Canned cycles | `G81`-`G89` drilling family | `G70`-`G76` turning family | | `G41`/`G42` compensate | Cutter diameter (`D`) | Tool-nose radius | Row one quietly explains a whole class of arc errors: a [lathe](https://en.wikipedia.org/wiki/Lathe) arc wants `I` and `K` because the arc lives in the XZ plane, and `J` there is an out-of-plane word. Row two is the identity anchor from [stopping the X and Z mix-up](/journal/how-to-stop-mixing-up-x-and-z-axis-on-a-lathe/): lathe X is a size. Row four is its own safety topic, the [G50 spindle cap with constant surface speed](/journal/g50-max-spindle-speed-crash-lathe/). Row five's turning side, `G71` and `G72` roughing with `G70` finishing, is unpacked in [the lathe roughing cycles](/journal/lathe-g71-vs-g72-roughing-cycle-practice/). And one family note: the differences above assume the Fanuc-shaped mainstream; controls with their own parallel traditions, like [Okuma's OSP](/journal/okuma-osp-p300-g-code-list/), keep the motion core and re-spell the conventions around it. ## Which trap catches the most people? The codes that exist on both machines with different meanings. The famous pair on Fanuc-style controls: on a lathe, `G98` and `G99` select feed per minute versus feed per revolution; on a mill, the same two codes set canned-cycle return heights, initial level versus R plane. Nothing in the syntax warns you, so the [cheat sheets](https://www.cnccookbook.com/g-code-m-code-cnc-list-cheat-sheet/) that list both meanings side by side earn their keep. The safe habit is contextual reading: before trusting any G-code in the 90s, name the machine type first. A concrete failure from this trap: a mill programmer covering a lathe shift read `G99` in a turning cycle as a return-height word and left it alone while doubling the spindle speed, not registering that `G99` made every feed per-revolution, so the doubled RPM doubled the actual feed too. The insert paid for the lesson. ## How should you actually learn the crossing? As core plus deltas, never as two languages. The shared core is most of every program and is already a compact recall set; the six differences attach to it as a checklist you can rehearse in one sitting. Reading real programs from the other machine type, narrating each block, converts the list into instinct within a couple of weeks, and the coolant and spindle families that behave identically on both, the `M08`/`M09` pattern included, come along free from [the coolant-code family drill](/journal/i-keep-forgetting-which-m-code-turns-off-the-coolant/). ## Bottom line Lathe and mill G-code share their motion core; six deltas carry all the difference: plane and arc words, diameter X, feed per revolution, CSS with its cap, the cycle families, and what `G41`/`G42` compensate. Learn the core once, drill the deltas as a list, and treat any 90s-numbered code as machine-type-dependent until proven otherwise. The recall layer for both machines runs free on the [G-code practice hub](/g-code-practice/). ## Sources - [LinuxCNC G-code reference](https://linuxcnc.org/docs/html/gcode/g-code.html) - [Wikipedia: Lathe](https://en.wikipedia.org/wiki/Lathe) - [CNCCookbook: G-code and M-code cheat sheet](https://www.cnccookbook.com/g-code-m-code-cnc-list-cheat-sheet/) ## Frequently asked questions ### What are the G-code differences between a lathe and a mill? Six matter: default plane (`G18` with `I`/`K` arcs vs `G17` with `I`/`J`), diameter programming for lathe X, feed per revolution on lathes, `G96`/`G97` constant surface speed with the `G50` cap, the `G70`-`G76` turning cycles vs `G81`-`G89` drilling cycles, and tool-nose radius behind the same `G41`/`G42` codes. ### Do G98 and G99 mean the same thing on a lathe and a mill? No. On Fanuc-style lathes they select feed per minute versus per revolution; on mills they set canned-cycle return heights. Same numbers, unrelated meanings. ### Is the motion core really identical on both machines? Yes: rapids, feeds, arcs, units, positioning, and work offsets behave the same, which is why core-plus-deltas beats learning two languages. ### What is the best way to learn lathe and mill G-code differences? Learn the shared core to instant recall, then drill the six deltas as a list. A free app like G-Code Sprint quizzes the everyday codes for both machine types and repeats whichever ones you miss. *G-Code Sprint is a study and practice tool only. Always follow your instructor, employer, machine manual, and shop safety procedures.* --- Source: https://gcodepractice.com/journal/lathe-vs-mill-g-code-differences-cheat-sheet/ Author: Lawrence Arya — https://www.linkedin.com/in/vibecoding/