How to check the straightness error of the lathe slide moving in the straight plane?

Check the straightness error of the lathe slide moving in the straight plane: swing the handle, such as the rack to the base line, on the slide close to the tool bar and bed guide rail equally place a level. The slide plate moves to the near headstock and records the reading of the level once. In the future, the slide plate moves 500mm (or less than 500mm) towards the tailstock to record the reading once, and record at least four times on all the trips of the slide plate. Place the readings of the level in sequence and draw the movement curve of the slide plate. The maximum coordinate value between the motion curve on each 1000mm travel of the slide plate and the connecting line between its two ends is the straightness error on the 1000mm travel. Connect the two ends of the motion curve of all trips. The maximum coordinate value of the motion curve to the line is the error of straightness on all trips. Note: before checking the movement accuracy of the machine tool, first adjust the device level of the machine tool. Place the bed inspection deck on the bed guide rail, and place two level gauges on the deck, one parallel to the guide rail, and the other straight with the guide rail. Check whether the machine is level at both ends of the guide rail, and the readings of both level instruments shall not exceed the specified value. The rule for high-precision machine tools is (1000:0.02) mm, and the rule for general precision machine tools is (1000:0.04) mm.