Characteristics of Spindles

Spindles are rotating shafts used to cut metals. Spindles play a vital role in enhancing machine tool performance, thereby increasing manufacturing productivity. Today, manufacturers are coming up with wide variety of spindles having advanced technology and additional features for improved performance. Spindles are essential in all kinds of manufacturing. They are used to perform various tasks in a machine tool like grinding, milling, drilling, turning, engraving and routing.


Characteristics
There are certain characteristics of spindle, especially a high-speed spindles, that allow it to achieve high accuracy and productivity which we are discussing below:

• Motor power: In most spindles, there is a stock frame less motor mounted directly on the shaft and into the spindle housing. By doing this the bulk required to mount the motor on one side is eliminated. The basic drawback of this configuration is that the motor must able to achieve the maximum spindle speed. However, if the motor was mounted on one side, and a pulley and a belt were used to drive the shaft, the motor could spine at significantly less than the spindle speed. A brushless motor is another option for spindles and it requires more expensive as well as complicated driving circuitry. However, maintenance and reliability of the motor is greatly enhanced because there are no brushes to misalign, replace, wear out, and generate carbon dust.
  
  
• Spindle speed: The required speed for the spindle motor is determined by the formula: Spindle Speed [RPM] = Cutting Speed [m/min]/(p * Tool_Diameter [m]).
  
  The following table gives an idea about spindle RPM required to achieve various cutting speeds by Tool Diameter.
  
  244 MPM (800 FPM): The minimum cutting speed for copper and aluminum

  6.35 mm (0.25 inches)	12,200 RPM
  9.525 mm (0.375 inches)	8,150 RPM

  
  305 MPM (1,000 FPM): The maximum cutting speed for materials except copper and aluminum
  

  6.35 mm (0.25 inches)	15,300 RPM
  9.525 mm (0.375 inches)	10,200 RPM

  
  610 MPM (2,000 FPM): The maximum cutting speed for copper and aluminum
  

  6.35 mm (0.25 inches)	30,600 RPM
  9.525 mm (0.375 inches)	20,400 RPM

  Source: pergatory.mit.edu/.../portf/spindle/spindle.
  
  
• Spindle torque: Torque is one of the most important characteristic for determining a spindle's cutting performance. Increasing torque in a spindle ultimately affects not only the the design of the machine but also the choice of the spindle. Power is the product of speed and torque. Thus, a high power spindle motor can use high speed with a low amount of torque at that speed. It can also be that the maximum power might not be available at the maximum speed. In both these cases, the amount of torque that can be achieved is proportional to the size of the motor. Specifying torque is very useful to the spindle designers and manufacturers in the machining center, because it is the torque and not the power that determines the size of the motor.
  
  
• Spindle Bearing Speeds: Bearings are essential components in spindles. Once the spindle's operating speed is determined, bearings are selected and sized. Bearings are properly lubricated using grease or oil. As the size of the bearing increases, maximum RPM decreases. The reason behind this is that for a given RPM the balls are traveling at a faster velocity due to the larger diameter. The materials used in making balls also affects the maximum speed. Ceramic balls, compared to steel balls allow a higher speed but can cost about 30% more. The maximum speed given by bearing manufacturers are usually for a single bearing. When double bearings are used their maximum speed is further reduced. To achieve a spindle speed of 12,000 RPM, bearings should have a maximum speed between 18,000 and 19,000 RPM.

Characteristic of high speed spindles compared to conventional spindles
High speed spindles are designed with the spindle motor flanged to the spindle shaft. While conventional or normal spindles are mostly gear or belt driven. The maximum rotational speed of conventional spindles are limited to about 12000 to 15000 RPM. The greater the speed, the greater the chances of vibrations, which usually result in poor surface finish. That is the reason why most high speed spindles use a directly flanged motor.

Two types of high speed spindles.

• Spindles running in an open loop: This spindle is not always controlled. The speed of this spindle may drop as a matter of load. It is usually used for graphite machining, small cavities in hardened steel or finishing operations.
  
  
• Vector controlled spindles: They run in a closed loop and their exact angular position and speed are always controlled. Vector control spindles are today considered as the underlying technology of high-performance motorized spindles.

Future characteristics
With proper research work and advancement of technology, most spindle designers are of opinion that the ultimate spindle will have the following characteristics:

• Unlimited Speed
• High Power
• Long Life
• Self-Balancing
• Self-Diagnostic
  
  

This might seem unattainable but they will definitely be fulfilled in the future.