After fixing the sliding seat on the heavy duty double column milling machine, put the milling head body downward to descend 50 mm. Place a surveying indicator on a bed plate (level datum plane) to start correcting precision. First, place the surveying indicating needle on the surface of the spindle and touch the bed plate with the indicator. Note, the reading on the indicator should be back to zero. Then rotate the spindle with a hand and read the figures at A. B. C. D on the indicator, as Fig 4. A and B are left and right correcting points. The readings at the two points should be the same when correcting. Loosen the 12 bolts on the sliding seat (as shown on Fig 4) and adjust, if needed, until reaching the needed value, then screw up the bolts tightly. C and D are front and rear correcting points. The reading on these points should be adjusted, according to working situation. Second, put the head body downward and descend it until 300 mm. Then correct it by following the previous way. It can be used after completing the correcting positions.