CNC G Code Basics: How a Program Becomes a Cut

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  Key takeaways
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  What a single block of CNC G code basics actually contains
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  Why modal groups are the core of CNC G code basics
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  Coordinate frames and work offsets in CNC machining G code
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  Feed, speed and cutter compensation in CNC G code programming
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  Where the standard breaks: control dialects and machine limits
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  How to read an unfamiliar program in five minutes
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  Command groups and what happens if you get them wrong

A program is a stack of blocks, and each block is one line the control executes before moving to the next. A typical block looks like N120 G01 X45.0 Y-12.5 F250.0. The N number is a sequence label, useful for restarts and searches. G01 tells the machine to move in a straight line at a controlled feed. X and Y give the destination. F sets the feed rate in mm/min or in/min depending on the control setting.

Order matters. The control parses the block left to right and applies the values as it reads them. If you put F after the coordinates, the feed still applies to that move on most modern controls, but older Fanuc-style logic can treat it differently. Writing F before the axes is the safer habit and matches what most post-processors output.

Words are letter-plus-number pairs, and the letters have fixed meanings: G for preparatory commands, M for machine functions, X Y Z for linear axes, I J K for arc centers, S for spindle speed, T for tool selection. Two G words in the same block are fine only if they belong to different modal groups. G01 and G41 can share a block. G01 and G02 cannot, because both set the motion mode.

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  NSequence number. Optional on most controls, required for M99 subprogram returns.
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  GPreparatory command. Sets motion mode, plane, units, offsets.
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  MMachine function. Spindle on/off, coolant, tool change, program end.
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  F / SFeed rate and spindle speed. Units depend on G20/G21 and the control.

A modal command stays active after its block ends. Issue G01 once and every following move is a feed move until something changes it. This is why a program can have twenty coordinate lines with no G word at all: the motion mode from earlier is still running. G00, G01, G02 and G03 sit in the same modal group, so any one of them cancels the others.

The same logic applies to G90 and G91. Absolute mode means every X value is a distance from the work origin. Incremental mode means every X value is a distance from the current position. A program written in G91 will look almost identical to one written in G90, which is exactly why this is a common source of scrap. Check the mode before you judge a coordinate.

Non-modal commands run once and disappear. G04 dwell is a good example: it pauses for the programmed time and then has no further effect. G28 machine-home return is another. Mixing these up with modal commands leads to programs that behave correctly on line 40 and wrongly on line 400.

Units are modal too. G20 sets inches, G21 sets millimeters. A program that never states the unit falls back to the control default, and a 25.4× error on every axis is hard to catch in a dry run that only moves a few millimeters.

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  Group 1: motionG00 rapid, G01 feed, G02 CW arc, G03 CCW arc. One active at a time.
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  Group 3: planeG17 XY, G18 XZ, G19 YZ. Sets which plane arcs and comp use.
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  Group 5: distanceG90 absolute, G91 incremental.
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  Group 6: unitsG20 inch, G21 metric.

Every coordinate in a program resolves against a frame. The machine has a zero point set by the builder, usually at a limit or a reference mark. G53 addresses that machine frame directly. Programs almost never use G53 for cutting moves; it is reserved for safe positions like a tool change point.

The work frame is what the operator sets. G54 through G59 hold stored offsets that shift the program origin to a corner or center of the fixtured part. When the operator picks up the corner of a vise jaw and presses a button, that value lands in G54. The program then says X0 Y0 and the control adds the offset behind the scenes.

A second frame can be stacked on top. G54 with a G52 local offset lets a subprogram run at several positions on a plate without rewriting coordinates. This is useful for multi-part fixtures and for families of holes. It is also a place where bugs hide, because a G52 offset that is never cleared with G52 X0 Y0 Z0 will silently shift everything that follows.

On 5-axis work, frames get another layer. A rotary axis changes where the tool tip actually is, so the control needs a tool center point management function, commonly G43.4 or G43.5 depending on the builder, to keep the tip on the programmed path as the table tilts. Without it, the posted coordinates are only correct at one rotary position.

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  G53Machine frame. One-shot. Use for safe retract only.
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  G54–G59Work offsets stored in the control and set by the operator.
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  G52Local offset added on top of the active work frame.
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  G43.4 / G43.5Tool center point control for simultaneous 5-axis motion.

Feed rate F and spindle speed S are the two numbers that decide whether a cut sounds right. Feed is in mm/min under G94 and in mm/rev under G95. A finishing pass on aluminum might run 1,200 mm/min at 8,000 rpm with a 10 mm end mill; the same tool in 316 stainless drops to roughly 300 mm/min and 2,500 rpm. Those are starting points, not rules. Rigidity, tool stickout and coolant all move the numbers.

G94 is the default on most mills and G95 on many lathes. Confusing the two means a feed value of 250 is either a normal cut or a near-zero creep that rubs the tool. Check the mode before you blame the tool.

Cutter compensation is where the program stops describing the part and starts describing the tool path. G41 offsets the tool to the left of the direction of travel, G42 to the right, and G40 cancels it. The offset value can come from the control's tool table or from a D word in the block. On a part with a ±0.005 mm tolerance, comp is how an operator dials in size without editing the CAM file.

Comp needs a lead-in move that is longer than the tool radius. If the first G41 block is also the first contact with the wall, the control has no room to build the offset and you get a gouge or an alarm. A straight or arc lead-in of at least one tool diameter is the normal fix.

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  G40Cancel cutter compensation.
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  G41Comp left of the travel direction. Climb milling on a standard right-hand cutter.
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  G42Comp right of the travel direction.
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  D wordSelects which offset register holds the radius value.

There is no single G code standard that every machine obeys. ISO 6983 covers the common core, but builders add, drop and rename commands. A canned cycle that runs on one control may need a different R word or return plane on another. The same program posted for a Fanuc mill and a Heidenhain control will not be interchangeable without a post-processor change.

Machine travel is the other hard boundary. A part that fits in a 500 × 500 × 450 mm envelope cannot be cut on a machine with a smaller one, no matter how the program is written. On our floor, the largest travel is 4,000 × 400 × 150 mm, and the compact machines run 500 × 500 × 450 mm. Long parts in one setup usually mean a different machine class, not a different program.

Spindle taper, tool holder length and rotary table size all leak into what the program can do. A Ø400 mm rotary table sets the maximum swing for 4-axis work. A CAT40 holder with 150 mm of gage length limits how deep a pocket tool can reach before it chatters.

This is the practical boundary of CNC G code basics: the language is portable, the machine is not. Read the program together with the machine spec sheet, not on its own.

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  Canned cyclesG81–G89 differ in return plane and R word between builders.
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  Rigid tappingG84 needs synchronized spindle and feed; not all controls sync the same way.
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  High-speed modesG05, G08, G61.1 and M-code look-ahead all vary by builder.