Machine tools are generally equipped with spindles for locating the job (to provide centering), holding tool or work, rotating the work or the tool and feeding the tool also as in the case of drilling machine. The spindles are made out of hollow steel shaft with a tapered hole at the front end for receiving the centering element (spindle nose).
It is desirable that the axis of the tapered hole and the axis of the spindle rotation be concentric. Machining accuracy depends to a considerable extent upon the rotational accuracy of the spindle which transmits motion to the cutting tool or to the work. Generally machine tool spindles are made up of allow low carbon steel heat treated to give a case hardened surface.
Such a spindle possesses resistance to wear combined with a tough core for strength in torsion. High precision spindles for external grinding machines are made from Nitralloy. Spindles of heavy machine tools are made of manganese steel with subsequent normalisation or hardening followed by high tempering.
In the case of lathe spindle the external screw threads are cut at the nose end to receive the chuck or face plate on it. Further for accurate location, an accurately machined shoulder and journal are also provided. The taper of the centering element is self-locking type having 1 : 20 taper in metric system.
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In the case of milling machines, the slow tapers of the order of 1 : 10 can’t transmit torque under pulsating forces of high magnitude and hence steeper tapers (self-releasing) of the order of 7 : 24 are used for internal centering of the spindle nose.
The nose of milling machine spindle is provided with two slots to locate the keys of the driven tool which is secured to the spindle nose by four screws. The cut-off isometric view of the milling machine spindle with an arbor fitted into the spindle and secured with a draw-in bolt is shown in Fig. 11.23.
The spindles are supported inside the housing by means of more than one pair of bearings. The geometrical accuracy and surface finish of a machined component depends to a great extent on the quality of spindle bearings used. It is necessary to maintain an accurate and suitable location of the axis of the rotating spindle at all speeds and loads.
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The type and number of bearings and distance between them is decided by the forces being experienced and the fact that the end play should be minimum possible. The bearings have to take up both radial loads and axial thrusts. In order that both the types of loads be taken by one bearing, often tapered roller bearings are used; otherwise two separate sets of bearings are needed for taking the two types of loads.
The requirements of spindle supports are guiding accuracy, high stiffness, minimum heating, vibration stability, and ability to perform satisfactorily under varying conditions of spindle operation.
In order to avoid the possibility of buckling, the thrust bearings are located close to the spindle nose. The location of the journal bearings depends upon the permissible bending deflection, pre-loading of the bearing and the amount of radial play.
The anti-friction bearings result in low heat generation, low starting resistance, high load capacity per unit width of bearing, easy maintenance and low consumption of lubricants.
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The desired characteristics in the modern machine using high-performance cutting tool materials are:
i. High static stiffness,
ii. High dynamic stiffness and damping,
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iii. Minimum deflection under varying loads,
iv. Running accuracy, (run out),
v. Axial-load carrying capacity,
vi. Adjustability to obtain minimum radial and axial slackness,
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vii. Thermal stability,
viii. Axial freedom from thermal expansion,
ix. High speed operation,
x. Simple and convenient assembly,
xi. Sufficiently long service.
The spindle must have a fixture which provides quick and reliable centering and clamping of the cutting tool or workpiece. The mating surfaces such as journals, quills must be hardened for high wear resistance. It is common to attain cutting speed as high as 600 to 1800 m/min.
All this is possible due to development in the bearings (which are the heart of the spindle) viz. ball and roller bearings, as also hydrostatic bearing and new innovations like active magnetic bearings. (Preloading eliminates the radial and axial movements which produce errors of roundness and parallelism on the workpiece).
Angular contact ball bearing in which balls are so seated in races as to produce axial force are quite popular. Cylindrical roller bearings are used for radial loads and double-direction angular contact ball bearings for axial thrust.
In the conventional bearing arrangement, the differential thermal expansion causes change in the initial bearing preload and thus reduces the life. Variable pre-load spindles have been designed which employ separate hydraulic system to maintain a constant preload in respect of the thermal conditions.
In hydrostatic spindle bearings, the bearing shells with their hydrostatic pockets are separated from the rotating spindle sleeve by an oil film. Oil is supplied to the bearings though a throttling system to control pressure and volume. Oil is prevented from leaking out of bearings by lubricated seals and there is no mechanical contact between the spindle and the bearing. Oil temperature is maintained constant by using heat exchanging.
Horizontal Spindle Vs Vertical Spindle:
Horizontal spindle arrangement is best suited for heavy workpieces which are generally housed in a box type fixture for machining purposes. The weight of such parts can be easily positioned in front of the spindle. In this arrangement the chips drop out of the way of the tool during machining, thus preventing the recutting of chips.
The four sides of a part can be machined without refixturing by employing a rotary indexing worktable. In the case of horizontal spindle machine, the table in front of the main column is open and easily accessible and easier to load, and thus automatic part handling systems like FMS (Flexible Manufacturing System) and pallet shuttle can be easily designed and installed.
Vertical spindle machine is best suited for operations like drilling, etc., calling for thrusting the tool into the face of the workpiece, since it has the inherent benefit of absorbing most of the cutting thrust direct into the bed. Flat parts with through holes can often be easily fixture on a vertical spindle machine.
Vertical spindle machine is preferable for machining three-axis work on a single face because the operator standing in front of the machine can easily peer down into the cutting area unobstructed by the column. In large vertical-spindle design the weight of the head held out over the table can cause the head to droop, lose accuracy and chatter. Thus vertical spindle appears to be best for smaller, flat parts that need more operator attention.
Lubrication of Bearings:
Machine tools are properly lubricated with mineral oil of adequate viscosity (low viscosity for high speed and high viscosity for heavily loaded low speed bearings). The spindle oil should have high cooling effect, be non-corrosive and should not readily oxidise.
The coefficient of friction for dry running lies between 0. 15 to 0.4 depending on the materials in contact. In partial lubrication the bearing elements are not completely separated by an oil film but some oil or grease is present.
The coefficient of friction is generally between 0.02 to 0.1. In fully lubricating bearings the bearing surfaces are completely separated by an oil film. In such bearings friction is minimum and wear is almost eliminated.
The selection of bearings is done on the basis of load, i. e., axial, radial, or combination, intensity of load, speed of rotation, thermal stability, and shaft stiffness.
Rigid bearings are used for stiff well aligned shafts and self-aligning bearings are used for shafts subjected to flexure or mis-alignment. For severe conditions and shock loads, roller bearings are used.
Bearing Metals:
Bearing metal should have high compressive strength so that it can resist the bearing pressure, have low coefficient of friction, have high thermal conductivity so that heat produced due to friction is dissipated easily, should be able to resist the temperature without losing strength, be wear resistant, should maintain a continuous oil film, should be able to embed in itself any dirt or grit present in lubricating oil, and should be shock resistant.
The various bearing metals are classified as follows:
(i) Copper base bearing metals (85% Cu, 10% tin, 5% Zn).
These alloys are stronger and harder and are used for heavy loads.
(ii) Tin base bearing metals Babbit metal (85% tin, 10% antimony, 5% copper).
It has low coefficient of friction and is used where bearings are subjected to higher pressure and load.
(iii) Lead base bearing metals (Lead 10—30%, antimony 10—15% and rest copper). It is used for light loads.
(iv) Cadmium base bearing metals.
It contains cadmium 95%; very small amount of iridium. It has more compressive strength and has more favourable properties at higher temperature.
Power Units:
The universally adopted power unit for machine tools is electric motor. There may be only one motor from which other movements are transmitted or there may be a main motor with separate drive motor for individual elements such as coolant pump motor or a feed movement motor.
Transmission Elements:
These are used to transmit and control the essential movement from the motor, and include the drives to the spindle and the feed drives. These may consist of pulleys, belts, gears, screws and nuts. In some machine tools, hydraulic and pneumatic devices are also used.
Work Holding and Tool Holding Elements:
Different machine tools use different types of work holding devices such as three or four jaw chucks on a lathe and vices in drilling and shaping machines. Tool holders are designed to suit a particular type of tool.