Machine Tool Leveling Basics: Terminology
1. A flat and even surface parallel to the surface of a body of still water (the earth's gravitational field), a line or plane wholly at a right angle to vertical, absolute level.

2. A flat and even surface not necessarily parallel to the absolute level plane (the earth's gravitational field).

3. To make flat. To bring to a common state or condition, leveling. In the case of machine tools, normally two guideways made flat and parallel to each other vertically.

4. A device or instrument which measures angles. There are two main types of Levels, Spirit Levels and Electronic Levels. Henceforth, the word "Level" with a capital L will refer to the level measuring instrument.

Absolute Level: Flat and parallel to the surface of a body of still water.

Absolute Zero: The instrument used to measure level is adjusted to read zero when it is at Absolute Level.

Comparative Level: Flat and parallel to any plane not necessarily the Absolute Level plane.

Comparative Zero: The instrument used to measure level isn't adjusted to read zero at Absolute Level. It is zeroed at a angle, a reference angle. All other positions on the plane would be relative to, or compared to that zero.

Abbe' Error: Also referred to as Abbe' Offset Error. One of the objectives of leveling precision machine tools is to reduce Abbe' Error. This error is detrimental to Linear Positioning. This is because the workzone where the parts are being machined is offset from the area that the angular errors originate or pivot from, closer to the guideway system, the very same area we are concerned with when we level. The greater the angular error and the greater the offset the greater Abbe' Offset Error will be.

Arc Second: An angular unit of measure. The 1,296,000th part of a circle, the 3,600th part of a degree. If one end of an inch long object were raised .000005" (.000004848") of an inch, that raised end would be one arc second higher than the other end. Another way of describing one arc second is when one end of a 20 inch object is .0001 of an inch higher than the other end.

Plane: A surface such that a straight line joining any two of its points lies wholly within the surface. Any flat and uncurved surface.

Electronic Level: An instrument which measures angles electronically. Generally more precise and accurate than a Spirit Level but more expensive. Programable electronic levels commonly have resolutions as fine as 0.000006” per foot, or 0.1 Arc Second.

Spirit Level: A device measures angles with a bubble in a liquid filled slightly curved vial. A common inch standard Spirit Level for leveling machines has a resolution of .0005" per foot.

Dynamic: Motion as the result of force as opposed to "Static".

Dynamic Leveling: To make a flat plane compensating for the weight of the axes moved upon it. The compensation comes from placing the measuring instrumentation on the axis, moving the axis at pretermined intervals then adjusting the plane from data collected that includes the effect of the weight on the plane. The greater the weight relative to the strength of the leveling system the greater the reduction of Abbe' Error as opposed to Static Leveling.

Differential Leveling: A dynamic leveling method which increases the accuracy of the measured angular errors by discounting the angular errors that do not matter and by adding the errors that do matter. An example is, a Level would be placed on a spindle housing and another on an axis, they would also be placed parallel to each other, as the axis is moved, the Level on the spindle housing would be observed along with the Level on the axis. If the spindle tilts the same amount and in the same direction as the axis being measured, the angular change would have no effect on a part being machined. Therefore, this tilt is an angular change but not an angular error and would not be considered an error with this method.

Static: Bodies at rest; acting as weight but not moving as opposed to "Dynamic"

Static Leveling: To make a flat plane without compensating for the weight of the axes moved upon it. This method is different than the Dynamic Leveling method in that the measuring instrumentation is not placed on the axis and moved, they are placed on the guideways and moved on the guideways. Static Leveling is common and appropriate in circumstances it is used in.

Also consider Six Degrees of Freedom