Since many high-precision XY tables are commercially available as standard items, they can be referred to as an established technology in terms of the production technology. In this example, the following requirements are added to the commercially available items in order to realize the high-speed driving and high-precision machining on a large-sized unit (long stroke):
- Large yet lightweight movable body is supported
- Made of materials that are insensitive to the long-term environmental fluctuation (such as temperature changes and vibration)
- Materials and structure are suitable for high-precision machining
To meet these requirements, we developed our own model using granite stones, rather than adopting a commercially available XY table made of steel-based materials.
Vital Points in Designing Large XY Table
The designer of the large automation device should consider design processes by acknowledging the following points:
- It is important to select a structural material that is insensitive to thermal displacement and vibration while the deterioration with age is minimal.
Description: Granite stones are those used in the surface plate of precision measuring equipment image below and have the characteristics listed previously.
- Highly rigid structure (minimum deflection by the self-weight)
- Structural design allowing effortless precision adjustment during assembly
Description: After fine-polishing the granite stone, adjust the flatness, perpendicularity, and parallelism by ultra-precision machining.
The second image below illustrates the sectional shape accuracy of 1.4m-long linear guide rails. The parallelism of the bottom reference plane against the top side is 0.003; the parallelism of the side reference plane against the lateral face of the rail is 0.008.
- Selecting inexpensive components that are compatible with large equipment design
Linear guide rails for a large table design.
Characteristics of Granite
While the granite stones adopted for the large XY table have the following advantages, they also have the weakness in the bending rigidity, since the material strength is smaller than that of metal. Therefore, we needed to develop a technology of enhancing the rigidity in the structure using granite stones.
- Minimum degradation with age
- Low thermal expansion rate
- Vibration absorption properties
- Low material strength
High Rigidity Structure
Figure 1 shows the structure diagram of a linear guide mechanism adopted for an XY table designed in the general machining dimensions (machining area is equivalent to A4 to A3 size.) Since the pneumatic hydrostatic bearing is adopted, it has the bearing shape as shown in Fig.1.
Figure 1: Compact drive table with dual rail guide.
If you increase the equipment size without modifying the structure, the load will cause a considerable deformation of the table top between the two bearings. As a result, the following problems will occur:
- Error in linear motion
- Bending of workpiece caused by deformation of the table top
- Vibration caused by the unstable pneumatic bearings resulting from deformation of the table top
In this case, the deformation volume (deflection) in the center of the drive table increases in proportional to the cube of distance between the two guide rails (Fig. 2).
Maximum deflection at critical points:
y = WL3/48EI
L: Length of beam;
E: Young’s modulus; and
I: Second moment of area.
Figure 2: Relationship between load and deflection for both ends support beam model.
- In this case example, an auxiliary rail is installed between the two sliding guides to minimize deflection caused by the table weight (Fig. 3).
- The auxiliary guide in the middle does not work as a bearing for linear motion. It simply supports and reinforces the moving table from the bottom.
- For large automation devices, notice that the flatness of the base plate or slight inclination will influence the assembly precision.
LaFigure 3: Large drive table having three-rail guide structure with auxiliary rail.
In short, there are a variety of design techniques that can be used for large automation designs.