1. Introduction
When working with air bearings, we often describe the quality of motion as one of the key factors in selecting an air bearing; specifically, the straightness and flatness of travel of linear bearings and stages. Just how straight and flat? What levels of precision are achievable?Get more news about Improve Flatness,you can vist our website!

In this technical note, we will discuss several aspects: the error motion over full travel, impacts on error motion, the repeatability of the error, and the short-term errors.
2. Definitions: what is straightness and flatness?
Straightness error motion (“straightness”) is defined as error motion in the Y direction. As the stage travels along the X axis, the motion deviates from the perfect line by some amount.

Flatness error motion (“flatness”) is defined as error motion in the Z direction. As the stage travels along the X axis, the motion deviates from the perfect line by some amount.
3. Error motion over full travel
The first specification we discuss is the total error motion over full travel. This measurement is usually specified in “microns TIR.” TIR means Total Indicator Reading. When TIR is used, we are specifying the peak-to-peak measurement of error motion. We do not assume symmetry about a zero reference. In Figure 3, we see the straightness error plotted over 220mm of travel. The TIR reading is 0.3 µm. Air bearing stages can typically achieve better than 1 µm flatness and straightness TIR for every 200mm of travel. See detailed product spec sheets for actual specifications.
4. Error motion impacts
Besides the inherent quality of the stage or bearing itself, straightness and flatness error motions of air bearings can be affected by several outside influences.
4.1 Quality of the mounting surface
Air bearing stages require a very rigid, clean, flat surface for mounting. Any irregularity in the surface, debris, burrs, or dirt, can have a noticeable impact on the stage’s error motion, particularly flatness. In Figure 4, we see the difference in flatness for a stage mounted to granite surface plate of high quality vs. mounted to an optical breadboard. The breadboard mounting adds a full micron of flatness error motion to the stage.
4.2 Length of travel
Since air bearing precision is in many ways a function of machining and grinding tolerances, which are inherently harder to hold over larger surfaces, longer travel stages will often have larger straightness and flatness error motions than shorter travel stages.

4.3 Size of stage
Stages with a larger footprint or with a larger separation between bearing surfaces will usually have smaller straightness and flatness error motions than a smaller-sized stage. Greater separation between bearing surfaces allows angular errors to translate into smaller lateral errors. Larger bearing surfaces increase the averaging effect (see Figure 5) of air bearings and reduce the error motions.