Leveling Terminology and Notes
Note: Leveling here generally referrs to leveling machines with linear axes that move upon guideways.
Level:
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.
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.
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 main place that the angular errors originate or pivot from, the machine's 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. (The angular error spoken of here referrs to the error that is derived from not being a flat plane. Normally the degree of freedom called Pitch. See "Degrees of Freedom")
Absolute Level: Flat and parallel to the surface of a body of still water.
Absolute Zero: The Level is adjusted to read zero or level to "Absolute Level".
Arc Second: 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.
Comparative Level: Flat and parallel to any plane not necessarily the "Absolute Level" plane.
Comparative Zero: The comparative zero is starting angle used as the reference angle for the plane. The Level would be adjusted to read zero at this point. All other positions on the plane would be relative to, or compared to this zero.
Degrees of Freedom: Click Six Degrees of Freedom then click the browsers Back button to come back here.
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.
Dynamic: Motion as the result of force as opposed to "Static".
Dynamic Leveling: To make a flat plane compensating for the force of an object moved upon it. The force could be the weight of an axis as it moves from one position to another on a machine tool guideway system. The compensation comes from placing the level measuring device on the axis, moving the axis, and adjusting the plane to account for the weight transfer of the moved axis. The effects of "Abbe' Error" is generally reduced the most with this method.
Electronic Level: An instrument which measures angles electronically. Generally more precise and accurate than a Spirit Level but more expensive. Click Precision Electronic Levels then click the browsers Back button to come back here.
Plane: A surface such that a straight line joining any two of its points lies wholly within the surface. Any flat and uncurved surface
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.
Static: Bodies at rest; acting as weight but not moving as opposed to "Dynamic"
Static Leveling: To make a flat plane without placing the Level on the object moved upon the plane. In other words, placing the Level on the guideways. This means the effects of the weight transfer is not measured. The force could be the weight of an axis as it moves from one position to another on a machine tool guideway system. The affects of the weight of a moving axes is commonly reduced by pre-applying a force in the same direction as the weight of the axes through the use of a hold down system when static leveling.
"Dynamic Leveling" usually produces a flatter plane and is an improvement in the machine's accuracy. This does not necessarily mean that dynamic leveling is better than static leveling. Dynamic leveling sometimes requires extra steps and therefore takes longer which means it costs more. If the tolerance of the parts being machined does not require the extra steps, is it better? In other words Bang-for-Buck is an issue to be considered. There are many factors to consider when deciding upon a method. The accuracy and Bang-for-Buck issues are just a few of them. Each different situation requires different considerations.
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