The first thing to do when installing a milling machine is to level it. Mills produce the best accuracy when properly levelled. Mills last longer when properly levelled; if a mill is operated while it is not level, moving parts wear unevenly, causing premature malfunctions. Mills have four levelling screws through the base of the machine. The screws are adjusted to compensate for an uneven floor. Each screw seats into a levelling pad, which sits on the floor.
Remove fixtures and clamps from the mill table. Remove oil and shavings from the mill table. If the table has any nicks or dents, stone them flat with a flat machinist's stone.
Position the mill table to the centre of its range of motion in both directions. Lock the slide clamps. As you face the front of the mill, place the level longways, (left-to-right) in the centre of the table.
Adjust the levelling screws on the left or right edges of the base until the bubble in the level is centred. Keep equal pressure on the screws as you adjust them; when you adjust a screw on one edge of the mill, turn the other screw on that edge the same direction until the pressure is equalised on both screws for that edge.
Turn the level 90 degrees. As you face the front of the mill the level will be placed longways toward and away from you.
Adjust the screws in the front and rear edges of the base until the bubble in the level is centred. Keep equal pressure on the screws as you adjust them; when you adjust a screw on one edge of the mill, turn the other screw on that edge the same direction until the pressure is equalised on both screws for that edge.
Rotate the level 90 degrees. Check the bubble. If it does not read zero, repeat steps 2 through 6. Repeat steps 2 through 6 until the level reads within allowable range in both directions.
Tighten the locknuts on the levelling screws if your mill is so equipped.
This will require patience. Each time you level the machine in one direction, you must go back and check the other direction. The adjustments are smaller each time. This job goes faster with two people. One person watches the bubble level and communicates with the person turning the levelling screws. The machine builder specifies an allowable range of accuracy of levelness. The bubble level you use should be capable of reading in the appropriate resolution.
Do not attempt to level your machine on an unstable surface.