Always turn OFF and disconnect all the power to the CNC lathe before any work.
Follow the steps in above pictures, one can remove the sheet metal cover for the X-axis servo motor first in order to have the direct access to the timing belt between the small pulley of the X-motor (at lower position) and the large pulley of the X-axis ball screw (at top position).
When a new CNC lathe is delivered, the tension of the Timing-Belts is properly adjusted in such a way that the timing belt for axis servo motor has a very slight deflection, say, about 1/16” (or around 2 mm) when you hold the belts at the center position or half way between the top and bottom pulley and push towards the center.
In other words, it should be very rigid without visible flexibility at all.
To tighten the timing belt, first loosen four locking bolts at each corner of the large flange mounting plate for about half turn each. Then, use a soft hammer to tap down the flange plate until the timing belt is under full tension. Then lock these large bolts on the flange plate again. Then, reverse the steps above to install the sheet metal cover plate for X-axis servo motor before further testing.