Chapter 3 Typical components of NC machine tools,3.1 Spindle system 3.2 Feeding system 3.3 Bed & column 3.4 Guide rails 3.5 Position sensor 3.6 Automatic chip remover,3.1 Spindle system of NC machine tools,3.1.1 The requirements of spindle system As one of the forming motions of a machine tool, spindle motion precision determines machining accuracy. To provide high efficiency, spindle system has these requirements:,Wider RPM range and continuous transmission; High precision and rigidity; smooth transmission and silent; Good vibration resistance and thermal stability; C-axis control for turning center; Precise orientation for machining center; Constant linear velocity control;,Transmission configurations (a) gear shifting; (b) belt derive; (c) double motor drive; (d) variable speed motor,3.1.2 Transmission mode of NC machine tool To provide a wide RPM range, spindle transmission can be generally in four configurations:,(a) Gear shifting Usually applied on large size machine tools for adequate torque at low RPM. Shifting is executed by hydraulic forks or hydraulic cylinders.,Working principle of 3-position hydraulic fork,1 & 5 cylinder 2 piston bar 3 fork 4 bush,Friction disc clutch,1 - gear 2 - connector 3 - bush 4 - outer friction disc 5 - inner friction disc 6 - block ring 7 - rolling bearing 8 - solenoid 9 - iron core 10 - contactor 11 - screw,Gearing electromagnetic clutch,1 solenoid 2 contactor 3 screw 4 spring 5 locating ring 6 spacing ring 7 bolt 8 rotating collector 9 magnetic yoke,Types of belts (a) V-belt (b) Cog belt,(b) Belt drive Applied on small size machine tools with high RPM but narrow RPM range and low torque. Speed shifting can be realized without gear thus reducing vibration and noise. V-belts and cog belts are in common use.,V-belt selection chart,Construction and transmission of cog belt,Cog belt selection chart,Compare to flat belts and V-belts, cog belts has advantages (1) Transmission efficiency reaches 98%; (2) No slipping thus accurate reduction ratio; (3) Smooth and silent; (4) Versatile. Linear velocity up to 50m/s; reduction ratio up to 10:1; power transmission from watts to kilowatt; (5) Convenient maintenance; lubrication is unnecessary; Disadvantage (6) Accurate center distance; (7) High cost for belt and pulley.,(c) Double motor drive The spindle can be driven through belt drive with one motor at high RPM/low torque, or through gear drive with another motor at low RPM/high torque.,The configuration sufficiently utilizes motor power and expands constant power range. However, it is wasteful since only one motor is working at the same time.,(d) Direct drive by variable speed motor Spindle is direct connected to the motor. The configuration simplified structure and improved spindle rigidity, but output torque is limited, and motor heating affects accuracy.,Power and torque properties of spindle,Electric spindle components,1 rear bearing 2 stator 3 rotor 4 front bearing 5 spindle,Spindle driven by built-in motor has been developed recently. The compact structure reduced system inertia thus improving start/stop respond (up to 180000rpm), also reducing vibration and noise. Since heat during working may result in thermal deformation, temperature control and cooling are the key technology.,Constructions of high speed spindles,3.1.3 Spindle components Spindle components include spindle support and transmission parts. 1End structure of spindle Spindle end will carry cutting tools or clamp. The design requirements includes: Precise positioning; Reliable clamping; Firm connection; Convenient fixing Be able to transmit enough torque Spindle end structures have been standardized.,2Spindle bearings Rolling bearings Advantages Low friction Convenient purchase and maintenance However Noisy Unstable rigidity,Mounting of angular contact ball bearing a) Back to back b) Face to face c) Same direction,2) Static pressure bearing Commonly used in NC machine tools. oil film pressure is independent to spindle rpm. Advantage: Stable loadability Friction free Disadvantage Complex hydraulic system,Expensive,Accurate,3. Spindle bearing configurations The configuration usually in three types, depending on rpm and rigidity requirements.,(1) Angular contact ball bearing and cylindrical roller bearing for front support; Radial thrust ball bearing for rear support. The configuration reinforces spindle rigidity to withstand large cutting force. It is applied on most machine tools.,(2) A group of radial thrust ball bearings allow the spindle runs at high rpm (up to 4000rpm). The configuration is applied on where high speed, light loading and high accuracy are required.,(3) Tapered roller bearings for both front and rear supports allow the spindle to withstand heavy load, especially shock load. However, the configuration limits rpm and accuracy. Applied on where moderate accuracy, low speed and heavy load are required.,4. Clearance and preload Purpose: incr