Spindle units are highly accurate and extremely compact bearing arrangements.
They are particularly characterised by their very low running noise, suitability for high speeds, easy maintenance and long service life. Spindle units are now an indispensable part of powerful machine tools.
The ever rising requirements on the machining capacities of modern machine tools and the surface qualities of the workpieces to be machined can be met with high-precision spindles.
The user has a range of special design versions to suit every task.
Angular-Contact Ball Bearings
Angular-contact ball bearings are the most common spindle bearing. They are rolling bearings and consist of one or more rows of rolling balls between concentric grooved rings. They are useful for both radial and axial loads in one direction, and their axial load carrying capacity is determined by the angle at which the load contacts the bearing. The greater the angle, the higher the load capacity.
Radial or Deep-Groove Bearings
Popular in industrial machinery, radial bearings are rolling bearings primarily used for load bearing on the radial axis. Like angular-contact bearings, they are composed of an inner and outer ring with rolling balls between them; however, radial bearings can carry loads in both axial directions, making them more versatile than their angular-contact counterparts.
Roller Bearings
Roller bearings enhance motion through the use of rolling cylinders instead of balls. They are used to support primarily radial loads and axial loads parallel to the axis in one direction. They are useful in moderate to high-speed applications to reduce friction and enhance equipment speeds.
Thrust Ball Bearings
Engineered to specifically support heavy, high precision thrust loads, thrust ball bearings offer exceptionally precise axial support parallel to the drive shaft, but little to no radial support. The rolling element may be a ball, roller, or needle, depending on the application. They are particularly useful for applications such as propeller engines, which support free and easy movement of heavy loads parallel to the shaft.
Motors used in spindle unit
The two motors most often used in machine tools are :-
1. Stepper motors
2. Servo motors
Both work to convert a non-linear input from the control into linear movement at the spindle shaft and table by turning a screw-like shaft to move the load.
1. Stepper motors
Stepper motors use equally sized steps positioned around a cylinder. In machine tool applications, there are often 200 steps, or magnetic poles, in a full rotation, i.e. 100 steps for 180 degrees of rotation. Depending on the size of the motor, a known voltage is established and applied to turn one step.
Stepper motors operate on an open loop, meaning that their position is dictated exclusively by the voltage they receive; there are no sensors providing location or relative motion measurements to the control.
2. Servo motors
Servo motors, also known as axis or torque motors.
Servo motor used fewer electromagnetic poles usually fewer than a dozen and rely on closed-loop communication for accuracy and speed control, meaning there is a two-way signal between it and the control. In other words, when powered on, the control receives an exact positioning signal. There’s no need to reestablish zero.
FUNCTIONS OF SPINDLE UNIT AND REQUIREMENTS
Functions :-
The spindle unit of a machine tool performs the following important functions:
1. Centreing the workpiece, e.g., in lathes, turrets, boring machines, etc., or the cutting tool, as in drilling and milling machines.
2. Clamping the workpiece or cutting tool, as the case may be, such that the workpiece or cutting tool is reliably held in position during the machining operation.
3. Imparting rotary motion (e.g., in lathes) or rotary cum translatory motion (e.g., in drilling machines) to the cutting tool or workpiece.
Requirements :-
1. The spindle should rotate with a high degree of accuracy. The accuracy of rotation is determined by the radial and axial run out of the spindle nose, and these must not exceed certain permissible Values which are specified depending upon the required machining accuracy. The rotational accuracy is mainly influenced by Stiffness and accuracy of spindle bearing, particularly the one located at Front end.
2. The spindle unit must have high static stiffness. The stiffness of the unit is made up of the stiffness of the spindle unit proper and the bearing. Machining accuracy is influenced by bending, axial as well as torsional stiffness.
3. The spindle unit must have high dynamic stiffness and damping. Poor dynamic stability of the spindle unit adversely affects the dynamic behaviour of the machine tool as a whole, resulting in poor surface finish and loss of productivity due to restrictions of the limiting underformed width of cut.
4. The mating surfaces that are liable to wear restrict the life of the spindle unit. These surfaces, such as journals, quills (in drilling machines), etc., must be hardened to improve their wear resistance. The spindle bearings must also be selected or designed to retain the initial accuracy during the service life of the machine tool.
5. The deformation of the spindle due to heat transmitted to it by the bearings, cutting tool, work piece, etc., should not be large, as this has an adverse effect on the machining accuracy. In case of spindles running at high rotational speeds, particular care should be taken in selecting or designing the front bearing as it is the major source of heat transmitted to the spindle.
6. The spindle unit must have fixture which provides quick and reliable centring and clamping of the cutting tool or workpiece. The centreing is achieved by means of an external or internal taper at the front end of the spindle. The spindle ends, including the taper have been standardised for the common groups of machine tools and are shown in Table.
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