some update...the printer seem to be working correctly now! many thanks to all!
i have to fix the last problem. i need to mantain the 4th axis 30mm over the current position after the homing, cause i have insufficient lenght as i mentioned before. and i have no possibility to move it up.
is it possible to do it via firmware?
at the moment i've made in that way and it seems to work properly.
; homedelta.g ; called to home all towers on a delta printer ; ; generated by RepRapFirmware Configuration Tool v2.1.8 on Sun Jan 19 2020 19:28:23 GMT+0100 (Ora standard dell’Europa centrale) G91 ; relative positioning ;*** Slow homing has been configured. Change F300 to F3000 below when your configuration is working G1 H1 X325 Y325 Z325 U325 F1000 ; move all towers to the high end stopping at the endstops (first pass) G1 H2 X-3 Y-3 Z-3 F600 ; go down a few mm ;*** Slow homing has been configured. Change F300 to F360 below when your configuration is working G1 H1 X10 Y10 Z10 F200 ; move all towers up once more (second pass) G1 Z-30 F1000 ; move down a few mm so that the nozzle can be centred G1 H2 U30 F1000 ; 4th tower clearance of 30mm G90 ; absolute positioning G1 X0 Y0 F600 ; move X+Y to the centre