For certain applications, the A axis is programmed as being 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 uses the A axis, the A position will have to be rewound back to zero before starting a new run. Jogging (physical rotation) to “0” from a large A position left over from the previous job may take a considerable 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 losing the proper A-axis orientation which may then require homing. Therefore, the best way to keep the A-axis orientation, but still save time by foregoing the physical rewind back to zero, is to reset all complete rotation turns of the A axis (each 360 degrees), and then physically jog to the A-axis “0”, retaining the previous A-axis orientation.
This can be done through macro programming, using the M309 macro by default. 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 code provided below.
In the macro we will:
G90G0Z[#7020] (Lift to a safe height) #10=#5044 (Store A coordinate in register #10) N10 IF [#10 LT 180] 20 (Go to line 20 if the loop is complete) #10=#10-360 JUMP 10 N20 IF [#10 GT -180] 50 (Go to line 50 if the loop is complete) #10=#10+360 JUMP 20 N50 G91 G0 A [-1*#10] G90 G10 L70 P0 A0 (resets machine coordinates) G90 G10 L70 P#5220 A0 (resets program coordinates)
Below is a video which illustrates the way the M309 macro functions:
The main screen of the myCNC software already has a “Move A to 0” button which uses the M309 macro by default, as shown in the screenshot at the top of this page. The process 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.
In cases where it is necessary to lift the tool to the top instead of safe height (global variable #7020), we can edit the first line to use Global Variable #5433, which is our maximum Z-axis limit, or use #5433 - 1, which will be 1 mm lower than the maximum allowable limit to prevent the machine from reaching the very top of the allowable Z-axis travel. The code in this case will look the following way:
G90G0Z[#5433 - 1] #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