The mill answers absolute-versus-incremental with a mode switch, G90 or G91, one at a time, whole program regions in one or the other. The system A Fanuc lathe answers it with vocabulary: X and Z always absolute, U and W always incremental, four letters meaning two axes referenced two ways, mixable in a single block. Stranger than the mill’s answer at first sight, and better fitted to how turning actually thinks, which is why the convention has outlived every prediction of its retirement.
A worked contrast makes it concrete
Take a step-down from a 50 mm diameter to 46, then a 20 mm shoulder cut. In pure absolute words: G01 X46.0 F0.2, then Z-20.0. In the mixed idiom a working program actually uses: G01 U-4.0 F0.2, then W-20.0, the U expressing “four off the diameter” exactly as the machinist thinks it, the W expressing “twenty more of cut” without anyone consulting where Z currently stands. Both spellings command identical motion from the same starting point; the difference is which mental arithmetic the words absorb, and turning programs mix the spellings line by line to absorb whichever arithmetic each move would otherwise demand.
The four letters, placed
| Letter | Axis | Reference | A typical block says |
|---|---|---|---|
| X | Cross slide | Absolute: a diameter | X48.0: go to 48 mm diameter |
| Z | Carriage | Absolute: a position from work zero | Z-30.0: thirty back from the face |
| U | Cross slide | Incremental: a diameter change | U-2.0: close the diameter 2 mm |
| W | Carriage | Incremental: a Z change | W-5.0: five more in the feed direction |
Two rules complete the system. U speaks diameter exactly as X does on diameter-mode controls, U-1.0 means the diameter shrinks by one, the tool moving half that, the whole-system convention covering incremental words too, and treating U as per-side depth is the convention’s classic violation, doubling every cut it touches. And mixing is idiom, not sloppiness: X48.0 W-30.0, absolute diameter with incremental length, is everyday turning grammar, the lathe-user references and worked program traditions both full of it.
Why turning earned dedicated letters
The convention fits the work. A turning program alternates constantly between print-language and motion-language: diameters come from the drawing (absolute, X), while grooving steps, peck retreats, and shoulder approaches think in changes (incremental, U and W), frequently within one operation. Mode-switching that alternation through G90/G91 would cost a line per switch and an error per forgotten switch; dedicated letters cost two entries in the alphabet and zero modal state. System A kept this answer; systems B and C adopted mill-style distance modes, which is why one Fanuc lathe family programs three ways and the control’s system declaration, manual page one territory, decides everything downstream, the same per-machine fact-finding every configurable control demands.
The letters’ second job
U and W moonlight: inside G71-family cycle lines, the same letters serve as depth-of-cut and finish-allowance parameters, meanings the cycle defines, positions that moved across the one-line and two-line format generations. Shared alphabet, unrelated job, and the collision is navigated the standard way: motion-context U/W read by the incremental rules above, cycle-context U/W read against the manual’s worked example, never by habit, the discipline the threading cycle’s anatomy already teaches.
The fluency target for all of it is reflex-grade: U closes diameters, W extends cuts, the mixed block reads as naturally as the pure one, cycle letters get the manual. That is recall material, built free in the 60-second rounds on the G-code practice page, and the payoff is reading any system A program, which is most of the world’s lathe legacy, at the speed turning work expects.
Sources
Frequently asked questions
What do U and W mean on a Fanuc lathe?
On system A controls, the incremental spellings of X and Z: X and Z command absolute positions, U and W command changes, U for diameter, W for Z. Absolute and incremental mix freely in one block: X48.0 W-30.0 is normal idiom.
Is U on a lathe a diameter or radius value?
Diameter, on diameter-mode controls: U-1.0 closes the diameter by 1.0, moving the tool half that. Treating U as per-side depth doubles every cut, the incremental cousin of the X diameter rule.
Why do lathes use U and W instead of G91 like mills?
Because turning alternates absolute diameters and incremental moves constantly, often in one block, and dedicated letters express the mix without modal overhead. System A kept the convention; systems B and C moved toward mill-style modes.
Are the U and W in G71 cycles the same incremental words?
Same letters, different job: in cycle lines they are depth and allowance parameters the cycle defines. Motion-context U/W read by the incremental rules; cycle-context U/W read against the manual’s example.