Hi guys, this is my first post here.

I have a chinese 6040 cnc since a year and it started to have a strange behavior, it drives me totally crazy.

My problem is the following :

When i press "start" in Mach3, 3 things can happen :

1 - If i'm lucky, the CNC goes from his actual position to the first position in the program and start machining without any trouble.
2 - The CNC goes from his actual position to the first position in the program but has a kind of "freeze" during the travelling. It freezes for approx 0.5sec and eventually the motors loose a few steps. Here is a video of this problem :



3- If i'm unlucky the CNC goes from his actual position to the first position in the program but has some serious issues to travel there, the motors miss a lot of steps and make an awful grinding noise. Video of this problem :

https://www.youtube.com/watch?v=_brAQBL2uRI

These problems only occurs in the very first displacement of the CNC, after i click start. When i jog manually, no problem at all. No problem during machining neither.

1 time out of 4 i get the n°3 situation, which drives me crazy because i often do jobs with 4 or 5 tool change and if it happens i loose my X0Y0 reference point.


So first i thought it could be a mechanical problem like friction in the ball screw for example. So i dismantled the 2 motors and checked the mechanical parts, everything runs smooth. Then i tried to start my programm again with the dismantled motors and had the same problem, so it's definitely an electrical or a software problem.


What i did next was playing with the motor tuning. In the 2 first videos i posted, travel speed was set on 1500mm/min and acceleration 200mm/s/s. The manufacturer suggest 2800 & 300.
I realized that the more i decrease the acceleration the less the problem n°3 occurs. At 50mm/s/s i couldn't make it happen again, eventually only situation n°2 occured.
But 50 is slow acceleration, so it increase manufacturing time a lot..

Then i realized something strange. When my motors are tuned at 1500mm/min for example and when i'm jogging manually, in the feedrate window you can read the feedrate unit/min in real time and once the motors have fully accelerated it max out at 1500mm/min as expected.
But when i run my program, the problem occurs at the first displacement, the corresponding line code is "G0 X19.28 Y130.774" in my case. And during this displacement, the speed is not constant at all and max out at more than 1500. Is it normal ? i read somewhere that G0 speed should not be bigger than the value i put in the motor tuning menu..

Here is a video of what happens exactly, pay attention to the real time feed rate :
https://www.youtube.com/watch?v=aDaEWmhokD8

Finally i tried to change my code line and put : "G1 X19.28 Y130.774 F500." to force my CNC to go at 500mm/min on the first move. But one more time the speed is not constant and faster than 500.. Video proof :

https://www.youtube.com/watch?v=Idbt1_RTSWI


I'm pretty sure that the problem comes from this non constant speed, mach3 is sending bad signals to the motors and when the acceleration is set too high it drives the motor crazy.. That's what i think but i'm a noob so i can't find out myself.

Hopefully somebody here can help me resolve this problem because i don't know what to do... i would like a reliable machine and i'm ready to spend some $$$ to upgrade and fix this if there is no other solution.

Thank you very much