What is inside a spindle?

A part by part breakdown of a spindle

Primary function – accurate rotation

The primary function of a spindle is to rotate so all spindles contain bearings, these are typically ball bearings but can also be roller bearings, cage bearings, solid bearings, air bearings, or a mixture of multiple bearing types.

Simple Spindle

A straightforward spindle will typically contain the following key components:

1. Bearings (a minimum of two, typically pairs or sets of bearings) – to provide precision rotation.
2. Housing – to hold the assembly together.
3. Shaft – to interface between the motor and tool interface.
4. Drive or motor – to provide rotary power.
5. Tool interface – to hold the cutting tool.
6. Sealing system – to prevent ingress of cutting fluids, swarf or dirt.

Modern spindle with complex design

In addition to the key components above a more complex spindle may contain many of the following components:

1. Tool drawbar (clamping unit) with hydraulic actuation – to allow automated quick change of tools.
2. Hydraulic brake or Hurth coupling – to prevent rotation and lock a spindle in a specific angular orientation.
3. Rotary coupling – for through spindle air, or coolant, supply directly to the tool.
4. Air-purge cleaning system or labyrinth seals- to clean tools or prevent ingress of dirt into the bearings.
5. Multiple bearings, often ceramic or with exotic coatings – to maximum bearing life and accuracy.
6. Integrated motor drive – for compact direct drive minimising belt or coupling losses.
7. Integrated cooling system – to maximise motor output while minimising thermal expansion.
8. Electronics package, including
   1. Encoder – for accurate angular position and speed
   2. Motor/bearing thermistors – for accurate temperature and early detection of failure
   3. Vibration sensors – to provide data on cutting loads.
   4. Draw-bar position sensors – to verify correct operation of tool clamping.

Spindle speeds and bearing configurations

As well as spinning quickly the spindle should also provide precise rotary motion, as the accuracy of the spindle will contribute to the accuracy of the machined component. The faster a spindle can rotate and feed the quicker the tool can cut, resulting in higher removal rates and faster production. However this introduces higher loads and the potential of deflection.

• Spindles designed for high removal rate will use bigger bearings for the high loads, the surface speed dictates how fast a spindle can rotate and so bigger diameter bearings may have a lower maximum RPM speed. Typical uses are CNC milling, grinding, polishing and sawing.
• Spindles designed for high RPM speed will use smaller bearings. Typical uses are precision CNC milling, engraving, bore grinding.

Checking spindle accuracy

Spindle accuracy is normally defined and measured using run-out (↗) or TRO (total run out ↗↗).

To do this a high accuracy Dial Test Indicator (DTI or ‘clock’) is used to measure the spindle on the vertical and horizontal tool interfaces as the spindle rotates. The DTI converts tiny inaccuracies (normally single microns) into a measurement that can be read.

Spindle refurbishment challenges

Many components within the spindle are designed for assembly with specialist tools and methods, which make disassembly and repair a specialist task:

• High strength adhesives.
• Locknut tools.
• Interference and press fits, which require precise cooling or heating to shrink or expand components.
• Springs under high compression loads and specific pre-loads.
• Hydraulic and cooling fluids which require new seals and pressure testing.
• High voltage electronics.
• Encoders and low voltage electronics that require protection from electrostatic discharge (ESD).
• Precision bearings and clearance fits that require measurement and re-setting, typically to less than 0.005 mm.
• Contaminate free greases and lubricants that need clean assembly areas.
• Precision balancing to ensure vibration free high speed operation.
• Testing and run-in periods whilst checking for thermal loads
• Verifying drawbar pull/clamping forces for safe tool holding

Fortunately we are experts in this field, so feel free to ask for advice if you have a spindle that requires assessment or repair.