Machining centres are the most versatile machinery, NC controlled, multifunction, tool changer equipped horizontal or vertical spindle machines. NC Machining centres being faster and more accurate are replacing many-single-purpose machines.
Structural elements of machining centres are of cast iron so as to provide excellent vibration dampening and rigidity. Axis slide ways are of rectangular cross-section, hardened and finish ground in place for maximum alignment accuracy.
TFE (a bronze fitted Teflon material) with excellent static and dynamic coefficients of friction is used on female slide ways, permitting to produce minute motions at controlled velocities (which is very essential for contour machining). Positioning of all axes is accomplished through DC electric servomotors and preloaded, precision-ground ball screws. Sensing devices to feedback information about position of axes are provided.
Direct position feedback systems utilising linear digital scales are used. In CNCs, software takes care of lead screw errors. AC or DC motors are used as spindle’s prime power source. The spindle drive is most reliable one and has high response for tapping, wide-infinitely variable speed range. These are equipped with NC system with paper tape as program memory.
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Machining centres have tool changers which take all shapes and size starting with the turret type. Tool can be transported from tool storage magazine to the spindle automatically. In order to reduce tool change time, some machines have two transfer arms, so that second tool is in a ready position. Tool storage varies from 6 to 60 in number.
Machining centres are usually equipped with a computerised numerical control (CNC) that is generally microprocessor based. CRT displays present program and diagnostic information to reduce downtime to minimum. Interface between machine and control system is provided.
This interface controls the machine sequence of operation and converts low power logic in the CNC to higher levels to operate solenoids, relays, etc. It is possible to program a part of the machine itself. Features like adaptive control, worn tool substitution, and inspection probes for in-process gauging are incorporated.
Fig. 34.16 shows a schematic diagram of a machining centre.
