wear due to usage and manufacturing imperfections. It is highly uneconomical to replace the whole guideway. Instead clearance between mating surfaces may be controlled periodically by means of gibs or strip lining.
These guide ways cannot be used directly in CNC machines, as the demand is for better characteristics because of rapid movements of slides and higher utilization of machine. The solution to this is “antifriction and frictionless guide ways”.
Stick - Slip Phenomenon.
In the case of friction guide ways before the commencement of motion of the slide there exist static friction between the guide ways and the slide, and once the motion has commenced it will be Kinetic friction which is a function of velocity of the slide. The static friction which is high acts on the drive mechanism such as screw and deform it elastically. This results in storing of energy in the screw along with the applied force and cause the slide to slip or move with a jerk. As more and more force in applied to overcome static friction and when the force applied exceeds the static friction, the slider tends to move faster than intended leading to improper positioning of the slide and consequently resulting in jerky motion. This is known as Stick - Slip phenomenon. Fig 4.7 shows graphically the relationship between coefficient of friction and velocity of the slide for friction guide ways.
constant co-efficient of friction is maintained between moving surfaces is contact. This can be achieved by fixing strips of materials having low and constant coefficient of friction, such as poly ten a fluro ethylene (PTEF) or turcite liners on the guide way surfaces. Turcite is a special type of plastic embedded with graphite particles. These graphite particles also act as solid lubricant.
Antifriction guide ways and slides
Even though friction can be minimized in friction guide ways still they are not used in CNC machines. Antifriction guide ways a special type with very low co-efficient of friction are used are in CNC machines. In this case the surface contacts between the guide ways and the slide is prevented by using an intermediate rolling elements such as spherical ball or cylindrical roller between them. These intermediate elements make point contact or line contact (Point contact by spherical ball and line contact by cylindrical roller between the surfaces of the guide ways and the slide ways resulting in considerable reduction in friction. Fig 4.8 illustrates the basic concept of antifriction guide ways. These guide ways ensures precision movements of the slide at uniform speed and have long service life.

These guide ways are employed in CNC machines for the following reasons.
a. Reduced friction, heat generation and wear.
b. Improve smoothness of movement of slides
c. Uniform speed of motion.
d. Long service life.
There are several types of antifriction guide ways that are employed. The most common of them used in CNC machines are,
a. Linear bearing with balls
b. Linear bearing with rollers.
Recirculating balls bearing
In the case of conventional ball and roller bearings, the rolling elements are confined to their pockets, provided in between the inner and outer races as shows in fig 4.9. These are called bush or non-recirculating ball and roller bearings. These functions properly when they are used in the horizontal plane as in the case of vertical spindles of machine tool, where in the load acting on the rolling elements will be uniform. But when used in any other plane as in the case of horizontal spindle of machine tool, there is every possibility of nonuniform wear of the pockets and the rolling elements. This reduces the performance of the bearing. The above problem can be avoided by recirculating the rolling elements, so that all the rolling elements are uniformly loaded. Fig 4.10 illustrates the concept of recirculation of rolling element. The balls in addition to revolving about its own axis also revolve around in the continued race similar to planets. The antifriction bearings listed earlier make use of this concept.