Your problem is you don't understand the difference between a servo signal (RC pulse) and PWM . The servo signal has a center frequency of 1500uS and only varies */- 500 uS.. You can't send negative PWM values .
// MAP IT FROM 1000,2000 TO -255,255 (1000,2000 MAY VARY AFTER CALIBRATION)
pwm_M1 = map(chThr,minPulseWidthThr,maxPulseWidthThr,-minMaxPWM,minMaxPWM);
Your PWM range must be 0 to 255. It doesn't accept negative values.
In other words, with Arduino's PWM frequency at about 500Hz, the green lines would measure 2 milliseconds each. A call to analogWrite() is on a scale of 0 - 255, such that analogWrite(255) requests a 100% duty cycle (always on), and analogWrite(127) is a 50% duty cycle (on half the time) for example.