Thanks for your reply Neale. I have had issues with my couplers in the past but i have since made solid couplers and ground the threaded bar with a angled slot to prevent it dropping out. I have also made new spring loaded anti backlash nuts. as soon as i can get me another pc i will try that but i do not think that is causing the problem. I have tried the active low in mach 3 which just causes the z axis to go in opposite direction. For example press up it goes down, press down it goes up. I am looking for a reasonable priced shielded cable to redo the wiring. I am however still leaning towards faulty tb6600 drivers considering the alarm light flickering on and off when that axis is changing direction.

cheers neil