CNC Programming: Sequence, Formats, Codes, and Words

 

CNC Programming: Sequence, Formats, Codes, and Words

CNC (Computer Numerical Control) programming is the language used to instruct CNC machines to perform specific operations. It involves a detailed sequence of instructions written in a specific format, utilizing various codes and words to define the machining process.

Sequence of a CNC Program

A typical CNC program follows a logical sequence to ensure safe and accurate machining. While the exact order can vary depending on the complexity of the part and the machine, a general sequence includes:

1. Program Start: Usually indicated by a specific character like "%" or a program number (e.g., O1234).
2. Safety Block: This initial block often includes commands to:
   • Establish the coordinate system (e.g., G90 for absolute positioning, G20/G21 for units).
   • Cancel any active compensation (e.g., G40 for cutter compensation, G49 for tool length compensation).
   • Select the working plane (e.g., G17 for XY plane).
   • Activate safety features.
3. Tool Selection and Spindle Control:
   • Selecting the appropriate tool for the operation (T-code).
   • Starting the spindle with the desired rotation direction (M03 for clockwise, M04 for counter-clockwise) and speed (S-code).
4. Coolant Control: Turning the coolant on (M08) as needed for the machining operation.
5. Workpiece Positioning: Moving the tool to a safe starting position above the workpiece using rapid traverse (G00).
6. Machining Operations: This section contains the core instructions for material removal, including:
   • Defining the type of motion (e.g., G01 for linear interpolation, G02/G03 for circular interpolation).
   • Specifying the target coordinates (X, Y, Z, A, B, C).
   • Setting the feed rate (F-code) for cutting movements.
   • Implementing machining cycles (canned cycles like G81 for drilling, G83 for peck drilling).
   • Applying cutter radius compensation (G41/G42) if necessary.
7. Tool Changes (if required): Bringing the machine to a safe tool change position and executing the tool change command (M06). The sequence then returns to step 3 for the new tool.
8. Coolant Off: Turning the coolant off (M09) after machining is complete.
9. Spindle Stop: Stopping the spindle rotation (M05).
10. Return to Home/Safe Position: Moving the machine axes to a predefined home or safe position (often using G28).
11. Program End: Indicated by a specific M-code (e.g., M02, M30) which may also include a rewind of the program.

CNC Program Formats

CNC programs are typically structured line by line, with each line representing a block of instruction. A block usually contains one or more words. The general format of a CNC block is:

N__ G__ X__ Y__ Z__ A__ B__ C__ F__ S__ T__ M__ ;

Where:

• N: Sequence number (optional but helps in program organization and troubleshooting).
• G: Preparatory function (defines the type of motion or cycle).
• X, Y, Z, A, B, C: Axis coordinates (define the position of the tool).
• F: Feed rate (specifies the cutting speed).
• S: Spindle speed (specifies the spindle rotation speed).
• T: Tool function (selects the tool number).
• M: Miscellaneous function (controls machine operations like spindle, coolant, program flow).
• ;: End of block character.

Not all words are required in every block. The specific words used depend on the desired operation.

Different CNC Codes

CNC programming utilizes various codes to instruct the machine. The most fundamental categories are:

• G-Codes (Preparatory Functions): These codes define the geometry and type of motion the tool will follow. They control the movement of the machine axes. Examples include:

  • G00: Rapid traverse (non-cutting movement).
  • G01: Linear interpolation (straight-line cutting).
  • G02: Circular interpolation clockwise.
  • G03: Circular interpolation counter-clockwise.  
  • G20: Programming in inches.
  • G21: Programming in millimeters.
  • G28: Return to home position.
  • G40: Cancel cutter radius compensation.
  • G81: Drilling cycle.  
  • G90: Absolute programming.
  • G91: Incremental programming.

• M-Codes (Miscellaneous Functions): These codes control auxiliary machine functions and program flow. Examples include:

  • M00: Program stop.
  • M02: Program end.
  • M03: Spindle start clockwise.
  • M04: Spindle start counter-clockwise.
  • M05: Spindle stop.
  • M06: Tool change.
  • M08: Coolant on.  
  • M09: Coolant off.
  • M30: Program end and reset.  

• Other Codes: These codes specify machining parameters and tool information:

  • F-Code (Feed Rate): Controls the speed at which the tool moves during cutting operations (e.g., F100 for a feed rate of 100 units per minute or revolution).
  • S-Code (Spindle Speed): Controls the rotational speed of the spindle (e.g., S1500 for 1500 revolutions per minute).
  • T-Code (Tool Function): Selects the desired tool from the machine's tool magazine (e.g., T01 to select tool number 1). Often accompanied by an M06 for the tool change to occur.
  • D-Code (Cutter Radius Compensation Number): Specifies the offset value for cutter radius compensation.
  • H-Code (Tool Length Compensation Number): Specifies the offset value for tool length compensation.

CNC Words

In CNC programming, a word is a single instruction within a block. It consists of an address (a letter) followed by a numerical value. For example:

• G01: "G" is the address, and "01" is the numerical value. This word instructs linear interpolation.
• X10.5: "X" is the address, and "10.5" is the numerical value. This word specifies the X-coordinate.
• F200: "F" is the address, and "200" is the numerical value. This word sets the feed rate.

A single block can contain multiple words to define a complete action for that step in the machining process.

Understanding the sequence, formats, different codes, and the meaning of words is crucial for writing effective CNC programs that can produce accurate and high-quality parts. CNC programming requires precision and attention to detail to ensure safe and efficient machine operation.