Guideways can be distinguished according to the kind of force transmission between the moving partners as follows:
•
Sliding guideways (slideways)
•
Roller guideways (rollerways)
•
Hydrostatic guideways
Slideways work in the mixed friction domain: partly solid friction and partly fluid friction takes place. Normally, the traveling speed is not so high that a pure hydrodynamic state can be reached. For ceramic grinding machines, it is important that the travel of the wheel against the workpiece is exactly performed as dictated by the control and the drive of the machine. In addition, the guiding system must not show stick-slip behavior (Figure 4.62), otherwise small engagements will not be brought through from the driving system to the working point. The precondition to avoid stick-slip is that the friction coefficients of movement and standstill are equal, which can be achieved by polymer sliders, such as reinforced poly-tetra-fluoro-ethylene (PTFE). These layers on one side of the guideways solve the stick-slip problem for slideways. Because of the continual low pressure between the sliding surfaces—the weight of the workpiece and the process loads are normally of minor importance—guideways are often designed according to the sliding principle. This solution is also fairly inexpensive. The slideways may be built in an open form as prism-flat or prism-prism versions (Figure 4.63). The prism-flat design is considered as statically determined, though slideways consist of larger matching planes. The friction characteristics of the two guideways are different because of the wedge behavior of the prism. This situation is avoided by two prisms, although the design is statically overdetermined and can be overcome by giving a floating freedom to one of the prisms.
Figure 4.62.
Stick-slip model
Figure options
Figure 4.63.
Prismatic-flat slideways. (a) Prismatic guideway, (b) Double prismatic guideway, (c) Double prismatic/flat guideway
Figure options
A sophisticated technical solution for linear guiding systems is hydrostatic guideways. Oil is pressed by an external source into the gap between the moving partners. The guiding surface is divided into several pockets that must be hydraulically decoupled to enable the system to carry eccentric loads. Often, capillary hydraulic throttles are used to obtain this decoupling effect. Stiffness and transversal damping abilities of hydrostatic guideways may be extremely high. The fluid friction force is almost negligible. The damping in the moving direction, therefore, may be critical when using path or position control loops. The systems are especially sensitive to pollution of the hydraulic medium. Hydrostatic guideways must be thoroughly protected against swarf and particles, especially when grinding ceramics. The system is dependent on the reliability of the hydraulic oil supply and is expensive compared to slideways or rollerways.
Rollerways contain balls or rolls as rolling elements. Their systems return the elements like roller chain, roller shoes, and others in which the rolling elements move against the guided table or slide. An important advantage of rollerways is that they are standardized and can be purchased from special suppliers. The stiffness is dependent on the number and diameter of rolling elements and especially of the preload. Prestressed guideways gain considerably in stiffness but also show higher friction. The damping ability is low compared to slideways or hydrostatic guiding systems. As mentioned earlier, the protection against swarf and particles is critical, especially in grinding ceramics.
Spindles
Generally, spindles are the most important components of a machine tool. They are located in the flow of force and determine the static and dynamic stiffness; thus they determine the accuracy and productivity of a machine tool to a great extent.
In grinding machines for ceramics, the design of the wheel-carrying spindle is of special interest. The high spindle speeds, high radial forces, and the required extremely low radial runout set specific demands. High dynamic stiffness is necessary to suppress self-excited vibrations as well as to minimize the vibration amplitudes in the case of enforced vibrations. Besides a high static stiffness, high damping values are necessary. These properties are mainly dependent on the bearing principles: hydrodynamic, hydrostatic, aerostatic, roller, or magnetic bearings (Figure 4.64).
Figure 4.64.
Spindle-bearing principles. (a) Roller bearing (b) Hydrodynamic bearing, (c) Hydrostatic bearing, (d) Magnetic bearing
Figure options
Hydrodynamic bearings are fed by nonpressurized oil. The pressure distribution and its resultant force direction are dependent on the spindle speed. Thus, the bearing capacity, the stiffness, and the position of the spindle axis change with the rotational speed. For stationary conditions, the bearing shows a favorable behavior.
Hydrodynamic radial or axial bearings are normally designed with at least three pockets on the circumference to develop the necessary pressure fields. In the past, this type of bearing was typical for grinding machines because the spindle speed was mainly kept constant. Today, hydrodynamic bearings are still in use, especially for large spindle diameters. They have design features that enable pure fluid friction with low speeds.
A fluid layer of oil or air is generated between the rotating spindle and an outer ring. The behavior of these bearings is not dependent on the speed. In hydrostatic bearings, the oil temperature is critical for the performance because of the high shearing velocity in the fluid.
The oil should be temperature controlled to avoid thermal influences on spindle performance. Hydrostatic bearings are sensitive to pollution, so the spindle must be protected against the intrusion of chips and coolant, especially in machining ceramics where small abrasive chips are generated. It is obvious the costs for producing a grinding machine with a hydrostatic spindle are high. On the other hand, hydrostatic spindles show good performance concerning the radial deviation, maximum load characteristics, and damping characteristics. The radial and axial runout of such a bearing is principally low.
Aerostatic bearings are based on the same principle as hydrostatic ones but use air as a medium. This aspect limits the maximum pressure and the damping characteristics of the bearing. The advantage of gas as a medium is the reduction of friction (compared to oil), which enables high rotational speeds. Aerostatic spindles have low radial deviations below 0.01 μm. It is the best value of all spindle types. Because air is compressible, the maximum load that can be applied to the bearing is lowered, limiting the acceptable forces on the spindle. The damping rate of air is lower than oil. A lower damping rate, along with the density characteristics of gas, is the reason for the sensitivity of aerostatic bearings to self-excited vibrations. This spindle type is used for precision grinding operations at low loads. Typical examples include inner diameter cut-off grinding machines for slicing single crystalline silicon wafers and internal grinding machines.
The most frequently applied spindle bearings are ball bearings. As standardized parts, they are available in many different modifications at reasonable prices compared to other bearings. So-called antifriction bearings for spindles in machine tools can either be roller or ball bearing types. Roller elements have linear contact areas that are larger than the contact zones of balls with spherical contact. Ball bearings experience decreased friction compared to roller bearings because of their reduced contact area. Therefore, for higher velocities, ball bearings are applied with a reduced maximum load capacity. Figure 4.65 shows the ranges of rotational speed for different grinding operations, and the maximum speed for the different bearing principles. For the majority of grinding machines, angular ball bearings also capable of carrying axial loads are used. The load in a ball bearing is transmitted by a thin lubricant layer between the rings and the roller elements. A breakdown of this layer causes fatal damages to the spindle bearing. The lubricant of roller bearings can be realized by grease, which under normal conditions lasts the whole life cycle, by oil injection, and by oil mist for high speeds. High frequency spindles for small grinding wheel diameters generally use an oil mist because of its lower friction and decreased losses. Ball spindle bearings are regularly assembled with a preload to achieve close tolerances. Because of preloading and the thin lubricant layer, ball bearings have a good stiffness at low damping rates. The roundness deviation tolerances of these bearings depend on the quality of manufacturing and assembly of rings and rolling elements. For precision bearings, roundness deviations below 3 μm are realized. For higher speeds, so-called hybrid ball bearings can be applied. The rings of these elements are conventional bearing steel—for instance 52,100 according to SAE—and the balls are made from ceramics with lower density and thus lower centrifugal forces and favorable tribological properties. For the grinding of ceramics, an air pressure protection can be used to avoid the intrusion of dust and chips.
Figure 4.65.
Speed ranges and spindle bearings for grinding machines