Adding 4th/Rotary Axis to XYYZ CNC Router. A or C Axis ?

Hello Guys, 
i have a XYYZ cnc router (2 motors on Y axis) where i want to add a Rotary 4th Axis, but i am having a problem.
I want to use the 4th axis in vertical position to the spindle, so like a Vertical Lathe, but also in standard position as a standard 4th axis.
Now the problem is, that if i want to create a milling/drilling operation in Fusion 360, i have to use a e.g. the Haas universal post processor so that i get checkboxes on the fusion post processor screen where i can choose to use "A axis", "B axis" or "C Axis".

When the rotary axis is in vertical position and i use milling operation, i need to use "C-Axis", oherwise i cannot generate the G-Code if i want to do e.g. milling or drilling operations. (i have tried to use A axis, but its not working due to some coordination problem - which i understand, because A axis seem to be always in horizontal position to the cutting spindle)

But how do i configure the Axis in Linuxcnc? I am trying to get some help from the stepconf wizard, but it does allow me to use only the A Axis.
This already makes think that i dont manage to get it done by myself alone.
Does it mean that if i want use the 4th axis in standard position e.g. horizontal to the spindle, then i need a seperate Linuxcnc configuration?
So, 3 seperate configuration:
1. my standard XYYZ config
2, XYYZ with A axis
3. XYYZ with C axis
Is this the way to do it?

I will attach all my files of my CNC router in a .RAR file.
I hope you can help me out with that. I have still a couple days left until i get the harmonic drive with 80:1 reduction, but until then i would like to get the configuration done.
Thanks in advance.
 

Thank you Andy.
So, i will not try to get the commin.ini done, because i will just loose time for that due to the fact that i am not that good with Linuxcnc.

I am ready to create 2-3 configs, depending on how the rotary axis is set up, but do i understand it correctly, the C axis will have exactly the same settings in the Ini and the Hal file, just the name is different. So, C axis instead of A axis?
Or i ask differently: Is it enough to get it done for the A axis, then duplicate the INI and just from AXIS_A to AXIC_C?
Are this changes correct:
[KINS]
Joints = 5 (instead of 4, i currently have 4 due to dual motors on Y)
KINEMATICS = trivkins coordinates = "XYYZA" or "XYYZC" (instead of XYYZ)

[TRAJ]
COORDINATES = X Y Z A (or X Y Z C)

I will of course do my own tests, but not sure if i wil find time on today, but latest on tomorrow i will try it out.
Thanks in advance.

Hello,
i have found a bit of time today to test it out. I have a second parport setup as "out". The second BOB is the cheap standard chinese one.
The Stepper is connected to P2 and P3. Enable pin is not connected.
No homing sensor for the rotary axis.

I changed and added what i thought should be working and i am getting no errors when starting linuxcnc, but strangly when i want to home the machine with one button click, its homing the X axis (its moving), the Y axis (dual motors - also moving) but the Z axis is not moving, but the homing signs are shown for all 3 axis, like all have been homed. BUT, when i want to move the A axis, the Z-axis is moving. When i want to move the X, Y or Z axis, i get a error message: "HOMING is REQUIRED to jog requested coordinates because joint (1) home sequence is Synchronized (-1).
So, when trying to move the rotary axis, the Z axis moves. When trying to move all standard axis, its not working due to homing error.

I attached my updated ini and hal file.
I will now continue to try out some things, but actually i dont understand why the Z axis is moving when jogging A axis.

UPDATE: it seems that its working now. I had to add the "home sequence" command to the A axis as well.

But, i am confused why before when there was not home sequence command for the A axis, the A axis was the Z. Strange.
I will now soon test a g code for 4 axis milling and on tomorrow i will try the C axis setup. (the harmonic drive arrived already, so i can soon do some maching tests with the rotary axis.

New problem.
I wanted to test a g code. As post processor i choose the Haas Next Generation post processor. Activated "A Axis".
I am getting "G code out of range" error. 
I attached the G code file.
Unfortunately i cannot get it fixed. I found this post "forum.linuxcnc.org/38-general-linuxcnc-q...ut-of-range?start=10", but it seems that it has not been solved.
It would be great to get a working Post processor for A axis and C axis.

Hello,
@andypugh:
could you share the post processor that you are using for the A-axis or C-axis if you have one available? I am getting headache already. Nothing works. I have tried already 6 or 7 different post processors.

UPDATE: It seems that its working. I created a A axis config and a C axis config. The postprocessor that i modified a bit seems to work also.
Tomorrow i might get the 4th axis mounted on the table and then i will do some machining tests. Hope it will work properly:)

Hello,
i got the scale correct, so the axis is moving the distance it should but i am having some other problems, which might is not a problem, but maybe i just dont understand it correctly:
I can move my rotary axis only in one direction. The motor reacts only if its jogged to one direction.
If i jog to the other direction, the motor does not move, but DRO changes, like the motor would move. This is strange for me. If the A axis can/should be able to move only one direction then why the DRO changes like the motor would turn to the other direction?

Edit: I forgot to mention that its not only when jogging. Its always like that.