CNC G Code Explain: How a Part Actually Gets Cut

• 1
  What CNC G Code Actually Describes
• 2
  The Four Motion Codes That Do the Cutting
• 3
  Where the Tool Actually Is: Offsets and Compensation
• 4
  What G Code Cannot Fix, and When It Is the Wrong Tool
• 5
  Common G and M Codes at a Glance

A CNC program is a list of instructions executed in order, one block per line. Each block tells the machine where to move, how fast to move, how fast to spin the tool, and which auxiliary action to take. The control reads the block, computes the path, and drives the servos. Nothing in the file is a suggestion. The machine does exactly what the numbers say, including the mistakes.

The letters in a block are called addresses. G addresses set the mode, such as rapid positioning or a feed move. X, Y and Z carry coordinates. F sets feed rate, S sets spindle speed, T selects a tool, and N is a line number that makes a program easier to restart. M codes handle the non-motion events: spindle on, coolant on, tool change, program end.

Most controls are modal. A G or F word stays active until another word of the same type replaces it. That is why a long finishing pass may carry only coordinates. The mode was set 40 lines earlier and never changed. Reading a program means tracking that state, not reading each line in isolation.

Programs come from CAM software, not from a person typing at the control. The CAM system takes a 3D model, applies toolpath strategy, and posts the result in the dialect of your machine. Fanuc, Siemens, Heidenhain and Haas all share the core G codes but differ in cycles and syntax. A post processor that is wrong by one decimal place will scrap the part.

• 1
  G wordsSet the mode: motion type, plane, units, offsets.
• 2
  Coordinate wordsX, Y, Z and rotary axes carry position.
• 3
  F and SFeed rate and spindle speed, in mm/min and rpm.
• 4
  M wordsSpindle, coolant, tool change, stop, end.

G0 is rapid positioning. The tool moves at the machine's maximum rate to a point where no material is being removed. It is a travel move, not a cutting move. Feed rate is ignored. On a 5-axis machine, a G0 that looks safe in three axes can swing the part into the fixture in the fourth. That is one reason setup simulation matters more than the code itself.

G1 is linear interpolation at a commanded feed rate. Two points, one straight line, material removed along the way. Almost all roughing and most finishing on flat or tapered surfaces is G1. Feed is set in mm/min on a mill and often in mm/rev on a lathe, so the same F value means different things on different machines. Always check the unit mode: G20 is inch, G21 is metric.

G2 and G3 are circular interpolation, clockwise and counterclockwise. They need an endpoint plus either a radius (R) or center offsets (I, J). A full circle cannot be cut as one arc on many controls, because the start and end point are identical and the control cannot resolve the plane. Programmers break it into two arcs or use a helical move.

Feed rate on an arc is the rate at the commanded path, not at the tool edge. On a small internal radius, the inside of the cutter travels much slower than the center, which raises chip load and can break a small end mill. Controls offer feed rate control on arcs for exactly this reason. For a Ø2 mm cutter in a tight corner, dropping the feed by 30 to 50 percent is normal practice.

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  G0Rapid travel, no cutting, maximum machine rate.
• 2
  G1Straight cut at the programmed feed rate.
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  G2 / G3Clockwise and counterclockwise arcs.
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  G81 / G83Drilling and peck drilling cycles.

A program is written against a coordinate system, not against a physical block of metal. Work offsets such as G54 through G59 tell the control where the part origin sits on the table. Change the fixture and you change the offset, not the program. This is what makes one file reusable across a batch of identical vises.

Tool length offsets tell the control how long each tool is. When a tool is replaced, the new length is measured and stored, and the program keeps running. Radius compensation, G41 for left and G42 for right, shifts the path sideways by the cutter radius so the programmer can draw the finished profile and let the control offset for the tool.

Compensation is the part engineers most often misread. A program with G41 does not cut on the line you see in the drawing. It cuts beside it. If the CAM post and the control both apply the offset, the part comes out undersized by roughly one tool diameter. That failure shows up as a slot that is too wide, not too narrow.

On our 5-axis centers, rotary axes add a second layer. The control blends linear and rotary motion, and the offsets must be correct in all five axes or the tool tip will not land where the CAM system expected. We verify the setup with a test cut or probing before the first production part is released.

• 1
  G54–G59Work offsets: where the part sits on the table.
• 2
  H and T wordsTool length and radius data stored in the control.
• 3
  G41 / G42Cutter compensation, left and right of the path.
• 4
  G43Applies tool length offset on a mill.

G code controls the path, not the physics. It cannot stop a long thin wall from deflecting, and it cannot remove heat from a cut. Chatter, taper and poor surface finish are usually rigidity, tooling or fixturing problems. Rewriting the program rarely solves them. If Ra is out of spec, check the setup before editing the code.

Feed and speed come from the material, the tool, and the depth of cut. Aluminium 6061 runs fast and dry or with light mist. Stainless 316 work hardens, so a dwell or a too-light pass will harden the surface and destroy the next tool. Titanium Ti-6Al-4V needs lower surface speed and more coolant. These are cutting-data decisions, not code decisions.

G code becomes the wrong tool when geometry is too complex to program by hand or when the part needs many setups. That is where 5-axis simultaneous machining and mill-turn work earn their place. A part with features on five faces, or one that needs turning and milling in the same setup, is a geometry problem first.

Our tolerance floor is ±0.005 mm (±0.0002 in), and surface finish runs from Ra 0.2–0.8 μm on a fine finish up to Ra 1.6–3.2 μm as machined. Hitting the tight end depends on the machine, the tool and the inspection plan together. The program is one input among several.

For prototypes and low-volume runs, no minimum order quantity applies. We can start production within 24 hours of a released order, and parts typically ship in 3–5 days. Quotation and a free DFM analysis come back within 12 hours.