myCNC Online Documentation

For certain applications, the A axis is programmed as endlessly rotating. Therefore, the A position (in both machine and program coordinates) may be specified as several thousand degrees after the G-code has completed running. An example of such behaviour can be seen in the screenshot below:

At the start of a new job which utilizes the A axis, the A position has to be equal to zero, therefore the axis will have to rotate to zero before starting a new job. Jogging (physical rotation) to “0” may take a long time (can be hundreds of complete turn to run). In some cases it's possible to simply reset the A axis position to zero at the end of the job run, indicating that whatever point the A axis has stopped at will be the next zero, thus saving time on the rewind process. This, however, runs into the problem of a lost orientation for the A axis, which may then require homing. Therefore, the best way to still keep the A-axis orientation, but still save time is to reset all complete rotation turns of the A axis (360 degree), and then physically jog to the A-axis “0”, retaining the previous orientation.

This can be done through macro programming, using the M309 macro. All recent myCNC profiles should have the M309 macro in their Macro List. If your profile does not have the M309 macro, it is possible to add it using the macro code provided below.

A-axis program position value is mapped to the global variable #5044. In the macro we will:

• Check if our A-position is positive
• Add a code loop
• Check the current coordinate and subtract 360 degree if current coordinate is more than 360 degree (more than 1 full turn)
• Store the resulting value as the current A-axis position
• Move A-axis to “0”.
• Add a similar branch for a situation in which we have a negative A-coordinate value.

M309

G90G0Z[#7020]
#10=#5044 (Store A coordinate in register #10)

N10
IF [#10 LT 180]  20
#10=#10-360
JUMP 10
 
N20
IF [#10 GT -180]  50
#10=#10+360
JUMP 20
 
N50
G91 G0 A [-1*#10]
G90 G10 L70 P0 A0
G90 G10 L70 P#5220 A0

Below is a video which illustrates the way the M309 macro functions:

The main screen of the myCNC software has a “Move A to 0” button by default, as shown in the screenshot at the top of this page. The way to add such a button to the screen is described here: Add a "Move A to 0" button example, and more information on adding buttons and on-screen elements can be found in the MyCNC Screen Configuration manual.