G90 Absolute Positioning in CNC Programming Explained

G90 in CNC programming sets the machine to use absolute positioning, meaning all coordinates are measured from a fixed zero point. This tells the CNC machine to move to exact positions based on the program's origin rather than relative moves from the current position.

How It Works

Imagine you are giving directions to a friend in a city. Using absolute positioning is like telling them to go to a specific address, such as "123 Main Street," every time. The address is fixed and does not change based on where your friend currently is.

In CNC programming, G90 sets the machine to absolute mode. This means every coordinate in the program refers to a fixed point called the origin or zero point. The machine moves directly to those exact coordinates, no matter where it currently is.

This is different from relative positioning, where moves are made based on the current location, like saying "walk 5 steps forward" instead of "go to 123 Main Street." Absolute positioning helps ensure precise and repeatable movements.

Example

This example shows how G90 commands the CNC machine to move to absolute positions on the X and Y axes.

gcode

G90 ; Set to absolute positioning mode
G00 X10 Y20 ; Rapid move to X=10, Y=20 from origin
G01 X30 Y40 F100 ; Linear move to X=30, Y=40 at feed rate 100

Output

The machine moves first to coordinate (10, 20) from the origin, then moves in a straight line to coordinate (30, 40) at the specified speed.

When to Use

Use G90 absolute positioning when you want precise control over the tool path based on a fixed reference point. It is ideal for machining parts where exact dimensions and repeatability are critical.

For example, when drilling holes in a metal plate, you want each hole at a specific coordinate relative to the plate's edge. Absolute positioning ensures the tool moves to the exact spot every time, avoiding cumulative errors that can happen with relative moves.

It is also commonly used in CNC milling, cutting, and engraving where the design is based on a fixed coordinate system.

Key Points

• G90 sets CNC machines to absolute positioning mode.
• Coordinates are measured from a fixed zero point (origin).
• Ensures precise, repeatable tool movements.
• Useful for machining parts with exact dimensions.
• Different from G91 which uses relative positioning.

Key Takeaways

G90 commands CNC machines to use absolute positioning from a fixed origin.

Absolute positioning moves the tool to exact coordinates regardless of current location.

It is essential for precision machining and repeatable part production.

Use G90 when your program coordinates are based on a fixed reference point.

Remember G91 is the alternative for relative positioning.