Tuesday, 22 December 2020

FUNCTION AND REQUIREMENT OF SPINDLE UNIT

Spindle unit 

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.



spindle is a rotating mechanical element, and an important component in manufacturing because it directly affects the quality and productivity of manufacturing processes. Since the power is transmitted to machine tools through spindles, static and dynamic forces are constantly applied in the rolling elements.


spindle unit generally consists of six main components: spindle house, spindle shaft, front bearings, rear bearings, motor, and clamping unit.

Types of Bearings for Machine Tool Application

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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