Hello everyone !
I'm Maxime JOURDREN, a student in electronic in France for 4 years now, and pretty new in RepRap Forum. I have tried to find a solution for my problem in RepRap Wiki and Forum, and I don't find something (Maybe I'm not the best to find something on this forum, sorry ! Also, sorry for the not perfect english.)

So ! One of my project is the next one :

I have to realise a 6 axis robot arm in order to have a 3D printer, with 6 axis. We gonna add a extruder at the end of this arm after, but actually, this is not my problem.

The fact is that we have actually 5 usable axis, and we are trying to have the final one.
We are using an Arduino MEGA2560 with a RAMPS_1.4, with a triple stepper expander.
Also, we use Marlin for the software !

Actually, I know that we have to change the number of movement in Configuration.h at this point :

"
//// MOVEMENT SETTINGS
#define NUM_AXIS 4 // The axis order in all axis related arrays is X, Y, Z, E
"
Moreover, in pins.h, we have to add more pins for the final axis, and that is probably my problem. Do I have to use non assignied pins ? Or use the pins wasn't used by the extruder or the bed that we don't use ?

Also, in Marlin.h I know that I have to add one more axis at this line :

"enum AxisEnum {X_AXIS=0, Y_AXIS=1, Z_AXIS=2, E_AXIS=3, X_HEAD=4, Y_HEAD=5};"

But, for the rest, we are pretty lost !

It's a pretty new topic in RepRap if I research nicely, so... I hope you gonna be able to help me !
Thank a lot !

The triple stepper expander will dictate the pins you have to define in pins.h. Except it connects to RAMPS by a bunch of jumper wires.
But when you have 6 axis defined, the actual work begins. You have to implement your own kinematic model. A good starting point is the SCARA printer.

Thanks for this quick answer.

So, before the real work, I juste have to define the 6th axis in the pins.h ?

I gonna look at this printer. Thanks !