G09 is a G-code command that tells the CNC machine to make an exact stop at the end of each programmed move. This means that the machine will not overshoot or undershoot the target position, and will not round off the corners or arcs. G09 is useful for ensuring high accuracy and precision in machining operations, especially when working with small or intricate features.

Usage Format

The format of the G09 command is:

G09

or

G09 X Y Z

The first format tells the machine to make an exact stop at the end of the current move, regardless of the coordinates. The second format tells the machine to make an exact stop at the specified X, Y, and Z coordinates. The second format can be used to override the programmed coordinates of the current move.

Explanation

When the CNC machine executes a move, it usually does not stop exactly at the target position. Instead, it may overshoot or undershoot the position by a small amount, depending on the speed, acceleration, and deceleration of the machine. This is called position error. Position error can affect the quality and accuracy of the machined part, especially if the part has tight tolerances or complex geometries.

To reduce or eliminate position error, the G09 command can be used. G09 tells the machine to decelerate and stop exactly at the end of each move, without any overshooting or undershooting. This ensures that the machine follows the programmed path precisely, and does not deviate from the desired position.

G09 can also be used to prevent the machine from rounding off the corners or arcs of the programmed path. When the machine executes a move that involves a change in direction, it usually does not stop and start at the exact corner or arc point. Instead, it may smooth out the transition by making a small curve or radius. This is called corner rounding. Corner rounding can affect the shape and dimensions of the machined part, especially if the part has sharp or curved features.

To prevent or minimize corner rounding, the G09 command can be used. G09 tells the machine to stop and start at the exact corner or arc point, without any smoothing or curving. This ensures that the machine follows the programmed shape precisely, and does not alter the geometry of the part.

Example

Here is an example of using the G09 command in a CNC program. The program is for machining a square with a hole in the center, as shown in the text diagram below.

+-----+
|     |
|  O  |
|     |
+-----+

The program is:

N1 G90 G00 X0 Y0 Z5 ; Move to the origin and set the Z-axis to 5
N2 G01 Z-1 F100 ; Move down to Z-1 with a feed rate of 100
N3 G01 X10 F200 ; Move to X10 with a feed rate of 200
N4 G09 ; Make an exact stop at X10
N5 G01 Y10 ; Move to Y10
N6 G09 ; Make an exact stop at Y10
N7 G01 X0 ; Move to X0
N8 G09 ; Make an exact stop at X0
N9 G01 Y0 ; Move to Y0
N10 G09 ; Make an exact stop at Y0
N11 G00 Z5 ; Move up to Z5
N12 G00 X5 Y5 ; Move to the center of the square
N13 G01 Z-1 F100 ; Move down to Z-1
N14 G02 X5.5 Y5 I0.5 J0 F200 ; Make a clockwise arc with a radius of 0.5
N15 G09 X5 Y5 ; Make an exact stop at the center of the arc
N16 G00 Z5 ; Move up to Z5
N17 M30 ; End of program

The G09 commands in the program ensure that the machine stops exactly at each corner of the square, and at the center of the hole. This results in a precise and accurate machined part, as shown in the text diagram below.

+-----+
|     |
|  O  |
|     |
+-----+