G87 – Back Boring

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The G87 Back Boring cycle is used in CNC machining to create precise bores on the back side of a workpiece. This cycle is particularly useful for operations where traditional boring from the front is not possible due to part geometry or setup constraints.

Usage Format

The typical format for a G87 Back Boring cycle is as follows:

G87 X_ Y_ Z_ R_ P_ Q_ F_ L_;

Where:

  • X and Y are the final bore position coordinates.
  • Z is the final bore depth.
  • R is the position above the hole where the boring tool waits before the operation.
  • P is the dwell time at the bottom of the bore.
  • Q is the amount to retract for chip breaking.
  • F is the feed rate.
  • L is the number of repeats for the cycle.

Explanation

The G87 cycle begins by moving the boring tool to the R position above the programmed X and Y coordinates. The tool then advances at the specified feed rate F to the Z depth. If a dwell time P is set, the tool will pause at the bottom of the bore. The tool retracts by the Q amount for chip breaking, then returns to the Z depth to continue the boring process. This cycle repeats L times if specified.

Example

Here’s a text diagram and an example of the G87 cycle in action:

(Starting position)
|
| R
| |
| | Q
| | |
| Z | |
| | | |
| | Q |
| | | |
| R | |
| | | |
+---X---Y

In this example, the tool will bore a hole at coordinates X100, Y50 to a depth of Z-25, with a retract distance of Q2 for chip breaking, and a dwell time of P1 second at the bottom of the bore.

G87 X100 Y50 Z-25 R5 P1 Q2 F10 L2;

This command will execute the back boring cycle twice at the specified location, creating a precise bore on the back side of the workpiece.

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