The G81 drilling cycle is a fundamental function that allows machinists to perform drilling operations efficiently. When combined with G91 incremental programming, it enables the execution of multiple drilling operations at different positions with ease. This article will delve into the usage and benefits of combining G81 with G91 for repeat drilling.

Usage Format

The basic format for a G81 drilling cycle in conjunction with G91 incremental mode is as follows:

G91 G28 Z0
G81 R[clearance] Z[depth] F[feedrate]
X[increment] Y[increment]
X[increment] Y[increment]
...
G80
G90

Explanation

G91 sets the machine to incremental mode, meaning that each move is relative to the last position. G28 sends the tool to the home position before starting the cycle. G81 is the drilling cycle command, where R defines the clearance level above the workpiece, Z specifies the drilling depth, and F sets the feed rate. After the G81 command, the X and Y coordinates are given in increments, indicating the distance from the last drilled hole. G80 cancels the drilling cycle, and G90 switches back to absolute mode.

Example

Let’s consider a scenario where we need to drill four holes in a row, each 25mm apart on the X-axis.

% 
O1000
G17 G20 G40 G80 G90
G91 G28 Z0
M06 T2
M03 S1200
G81 R0.1 Z-0.5 F5.0
X1.0 Y0
X1.0 Y0
X1.0 Y0
X1.0 Y0
G80
G90
M30
%

Text Diagram:

|---25mm---|---25mm---|---25mm---|
[1st Hole]   [2nd Hole]   [3rd Hole]   [4th Hole]

In this program:

• O1000 is the program number.
• G17 G20 G40 G80 G90 sets the initial conditions (plane selection, inch mode, cutter compensation off, cancel drilling cycle, and absolute mode).
• G91 switches to incremental mode, and G28 Z0 returns the tool to the home position.
• M06 T2 changes to tool number 2, and M03 S1200 starts the spindle at 1200 RPM.
• The G81 cycle is initiated with a clearance of 0.1 inches, a depth of 0.5 inches, and a feed rate of 5.0 inches per minute.
• The X-axis increments by 1.0 inch (25mm) for each subsequent hole.
• G80 cancels the cycle, and G90 returns to absolute mode.
• M30 ends the program.

This example demonstrates how G81 and G91 can be used together to perform repeat drilling operations with precision and efficiency.