Hello.. We have CNC turning machine with sinumerik 828 Basic control.there r two feed axis in this machine,, X and Z. The problem in machine is related to accuracy. When machine is turned off by pressing emergency button, then Z position display on screen starts changing value and it changes value continuously going up and down by +-1 mm on screen. During this, motor doesn't move. After turning machine ON, physically Z remains at position before pressing emergency button but value on screen changes by 0.7 to 0.8 mm..

Kindly guide to resolve this issue. 

@piscean

X and Z r separate feed axes like in normal CNC turning machine. They r not synchronized.

X or Z axis doesn't have gearbox. Z axis motor is coupled to balls crew through belt. 

Zor X has no linear encoder. Z axis and X axis measuring system is working fine during normal operation. Only when when emergency button is pressed, Z axis reading which is coming from Z axis motor encoder keeps on changing although motor doesn't move. Today I checked encoder cable, it is fine also. We interchanged the cables of Z and spindle axes encoder cables(green), and still Z axis value on screen keeps changing after pressing emergency.

After turning machine ON again, Z axis changes position and doesn't remain same as before turning off. 

Hello

What is the mechanical design of the machine?

Is the Z axis a hanging axis? And does the Z axis motor have a brake?

Z axis is straight.. It's flat bed CNC lathe. There is no hanging or free fall in Z axis.

I don't think so there is brake in Z AXIS motor.. But I have observed Z motor during reading fluctuation on display... There is no movement of Z axis motor or Z axis ballscrew during reading fluctuation on display.. 

@piscean

Today I checked the machine.Even when feed override is set at 0%, the fluctuation is still going on.. 

@piscean2010

Today I checked the drive paramters related to Z axis:

r2(Drive Operating Display) = 45

r20(Speed Setpoint smoothed) = -0.0 rpm

r60(Speed setpoint before the setpoint filter) = 0.00 rpm

r63( Actual speed smoothed) = 0.00,0.46,-0.46,0.92 (continuously changing from 0 to 0.46,then to -0.46, then to 0.92 and then 0 again and this cycle repeats with these values) 

@piscean2010

One thing more which I observed yesterday is that when we press emergency stop, then firstly Z axis moves by 0.03-0.04 mm on display and at that time Z axis motor moves too...then motor and display reading remains stationary for 1 or 2 seconds and then after that Z axis reading on display starts changing continuously as I described earlier..