G61 – Exact Stop Mode

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G61 is a G-code command that instructs the CNC machine to perform an exact stop at the end of each programmed move. This means that the machine will decelerate to a complete halt before starting the next move, ensuring that the tool follows the programmed path precisely. This mode is useful for machining tight corners, contours, or circular arcs that require high accuracy and smoothness.

Usage format

The general format for using G61 is:

G61

This command can be placed anywhere in the program, and it will remain active until another motion mode (such as G64) is specified. G61 can also be combined with other modal commands, such as G01 (linear interpolation) or G02/G03 (circular interpolation).

Explanation

When G61 is active, the CNC machine will execute the programmed moves as follows:

  • The machine will accelerate from zero speed to the programmed feed rate (F) at the beginning of each move.
  • The machine will maintain the programmed feed rate throughout the move, unless it encounters a feed rate override (S) or a spindle speed override (M).
  • The machine will decelerate to zero speed at the end of each move, regardless of the distance or angle to the next move.
  • The machine will wait until the spindle and the axes are completely stopped before starting the next move.

This mode ensures that the tool will not overshoot or undershoot the programmed coordinates, and that the tool path will match the programmed path exactly. However, this mode also has some disadvantages, such as:

  • The machine will take longer to complete the program, as it has to stop and start frequently.
  • The machine will consume more power and generate more heat, as it has to accelerate and decelerate rapidly.
  • The machine will experience more wear and tear, as it has to endure more stress and vibration.
  • The machine will produce more noise, as it has to change speed abruptly.

Therefore, G61 should be used sparingly, and only when the accuracy and smoothness of the tool path are critical.

Example

Here is an example of a CNC program that uses G61 to machine a square with rounded corners:

% (Program start)
O1000 (Program number)
G21 (Units in mm)
G90 (Absolute mode)
G17 (XY plane selection)
G40 (Cutter compensation off)
G80 (Cancel canned cycle)
G54 (Work coordinate system 1)
G61 (Exact stop mode)
S1200 M03 (Spindle speed 1200 rpm, clockwise)
G00 X10 Y10 Z5 (Rapid move to initial position)
G43 H01 Z2 (Tool length compensation with offset 1)
G01 Z-2 F100 (Linear move to cutting depth with feed rate 100 mm/min)
G01 X90 F200 (Linear move to X90 with feed rate 200 mm/min)
G03 X100 Y20 I10 J0 F300 (Circular move to X100 Y20 with center at I10 J0 and feed rate 300 mm/min)
G01 Y80 F200 (Linear move to Y80 with feed rate 200 mm/min)
G03 X90 Y90 I0 J10 F300 (Circular move to X90 Y90 with center at I0 J10 and feed rate 300 mm/min)
G01 X10 F200 (Linear move to X10 with feed rate 200 mm/min)
G03 X0 Y80 I-10 J0 F300 (Circular move to X0 Y80 with center at I-10 J0 and feed rate 300 mm/min)
G01 Y20 F200 (Linear move to Y20 with feed rate 200 mm/min)
G03 X10 Y10 I0 J-10 F300 (Circular move to X10 Y10 with center at I0 J-10 and feed rate 300 mm/min)
G00 Z5 (Rapid move to safe height)
G49 (Cancel tool length compensation)
G64 (Cancel exact stop mode)
M05 (Spindle stop)
G53 G00 X0 Y0 Z0 (Rapid move to machine zero position)
M30 (Program end)
%

Here is a text diagram to illustrate the tool path:

    Y
    ^
    |
100 +-----+-----+
    |     |     |
    |     |     |
 90 +  +--+--+  +
    |  |     |  |
    |  |     |  |
 80 +--+     +--+
    |           |
    |           |
 70 +           +
    |           |
    |           |
 60 +           +
    |           |
    |           |
 50 +           +
    |           |
    |           |
 40 +           +
    |           |
    |           |
 30 +           +
    |           |
    |           |
 20 +--+     +--+
    |  |     |  |
    |  |     |  |
 10 +  +--+--+  +
    |     |     |
    |     |     |
  0 +-----+-----+-----> X
    0    10    20   30

 

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