A waterjet cuts anything from foam to six-inch titanium with a supersonic stream of water and garnet, and its programs read like mill programs that lost a dimension and gained a plumbing system. The differences are few, systematic, and worth knowing precisely, because two of them carry the part’s accuracy.
What stays the same?
The motion core, wholesale. Lines, arcs, work offsets, absolute and incremental modes, the whole standard vocabulary drives the gantry exactly as it would a router, mostly in X and Y. A machinist who reads mill code reads ninety percent of a waterjet program on sight, which is the recurring lesson of every specialized machine, from dental mills to the robot cells at the far end of the spectrum covered in Fanuc teach pendants vs G-code: niche hardware, shared skeleton, and stone-cutting machines prove it again with diamond tooling.
What replaces the spindle?
Plumbing, switched by M-codes that vary by builder:
| Mill concept | Waterjet equivalent | Code reality |
|---|---|---|
| Spindle on/off | High-pressure pump / cutting head valve | Builder-specific M-codes |
| Coolant | Abrasive feed on/off | Separate output, own M-codes |
| Plunge into the cut | Pierce: dwell on a lead-in | G04 dwells or pierce routines |
| Tool length / Z depths | Standoff height control | Head-managed, little Z in the file |
| Tool diameter comp | Kerf compensation | G41/G42 carry the sizing |
The builder-specific rows deserve the standard caution: the on-off codes for pump and abrasive are the waterjet’s version of vendor M-codes, learned from the machine’s manual and never assumed across brands.
Why is kerf compensation the heart of it?
Because the stream is the tool, and it has a width that changes. Kerf, typically around a millimeter with abrasive, varies with the orifice, material, thickness, and feed, and the finished edge tapers when the stream lags. G41 and G42 offset the programmed contour by half that kerf exactly as they would offset for an end mill’s radius, per the standard compensation rules, and the practical discipline follows: kerf gets measured on scrap of the same material and thickness before a toleranced job, and the offset value, not the geometry, absorbs the difference. A part cutting consistently oversize by half a kerf is the classic signature of compensation set to the wrong side or skipped.
Why does feed rate double as a quality setting?
Because the stream bends. Through thick material the jet exits later than it enters, and the faster the head travels, the more the lag scores and tapers the edge. Slow the same contour down and the edge cleans up. Waterjet CAM packages expose this physics as named quality levels, which are honestly just feed-rate schedules per material and thickness, and hand edits to F on a waterjet change finish first and cycle time second. The pierce is the other tactile difference: drilling through six-inch stock with water takes a programmed dwell and a lead-in placed where the pierce scar cannot hurt the part, which is why waterjet contours start away from the finished edge and arc in.
A concrete check worth stealing: before a batch in new material, cut a 25 mm test square at the planned quality, measure it, and adjust the kerf value until the square measures true. Two minutes of garnet against a scrapped first part.
Bottom line
Waterjet code keeps the motion core and swaps the tool-shaped edges: pump and abrasive M-codes for the spindle words, pierce dwells for plunges, standoff for Z, and kerf compensation carrying the accuracy. Learn the shared core once, the deltas as a list, and respect F as the finish knob. The transferable layer drills in spare minutes on the G-code practice hub.
Sources
Frequently asked questions
How is waterjet G-code different from mill G-code?
The motion core is identical; the deltas: builder-specific pump and abrasive M-codes instead of a spindle, pierce dwells with lead-ins instead of plunges, head-managed standoff instead of Z work, and G41/G42 kerf compensation carrying the sizing.
What does kerf compensation do on a waterjet?
Offsets the path by half the stream’s effective width so parts measure to print, with kerf values tested per material and thickness because the stream’s width and taper change.
Why does feed rate matter so much on a waterjet?
Speed is the finish: the jet lags through thickness, so faster travel scores and tapers the edge while slower travel cleans it. Quality levels in waterjet CAM are feed schedules.
What is the best way to learn the G-code a waterjet uses?
Drill the shared motion core, then add the waterjet deltas as a short list. A free app like G-Code Sprint quizzes the everyday codes 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.