Crash Course in Machining – Machine Spindles​

The machine tool spindle provides the relative motion between the cutting tool and the workpiece which is necessary to perform a material removal operation. In turning, it is the physical link between the machine tool structure and the workpiece, while in processes like milling, drilling or grinding, it links the structure and the cutting tool. Therefore, the characteristics of the spindle, such as power, speed, stiffness, bearings, drive methods or thermal properties, amongst others, have a huge impact on machine tool performance and the quality of the end product, i.e. your machined parts.

The main elements of a spindle are the tool interface, the drawbar, the spindle shaft, the bearings, the driving system, the cooling system (if any) and the housing. The driving system can be either a motor coupled, directly or indirectly (by belt or gear), to the spindle or an integrated motor.

Spindle bearings

The bearings used in a machine tool have to meet the demands of the spindle in terms of rotational speed, load capacity and life. That is why rolling bearings are the most commonly adopted solution. The choice between taper, roller, deep groove or angular contact ball bearings in a machine tool spindle depends on the application. Angular contact ball bearings combine precision, load carrying capacity and high rotational speed. On the other hand, taper bearings provide higher load carrying capacity and stiffness, but they do not allow high rotational speeds. The raceways of deep groove bearings are of a similar size to the balls that run in them. They can take heavy loads but may present misalignment problems. Roller bearings allow a certain amount of axial movement of the axis with respect to the support, and are suitable for large radial loads.

Spindle warm-up

In order to avoid heat problems and increase the spindle service life, it is necessary to know about the heat sources that affect a spindle to reduce their contribution. The main sources of heat that affect the spindle are the motors, the friction in bearings and the cutting process.

Keeping a stable spindle temperature is more important than simply reducing the temperature. The idea of a warm-up is to achieve a stable operation temperature during the entire machining process in order to reduce the effects of thermal expansion caused by temperature variations. The warm-up cycle should be carried out in conditions similar to the operating conditions, with the same cutting tools, if possible. Nowadays, many machine tools have automatic spindle start-up and warm-up systems that bring the machine to the correct temperature ready for the operator to begin work as required.

Spindle runout

The term “runout” in machining describes the state of a spindle and tool when they have a rotational axis that differs from their geometrical axis. The total indicated runout, TIR, is the total distance measured from the maximum position in radial direction to the minimum position in the same direction. This factor limits productivity, as it negatively affects the dynamic balance, the chip load distribution, the part finish and the tool life. Runout can arise from either/both the spindle or the tool.

Eccentricities in the nose of the spindle can arise for various reasons, such as defective elements of the spindle, inaccuracies in the assembly, slack or simply the ageing of the spindle, among other possibilities. Although radial and axial runouts are measured at the nose of spindles, it is the radial runout that is the most significant. Monitoring of spindle runout is a good method for determining the available life in a machine spindle. 

Cutting tool runout is usually measured as a composition of radial and angular runout, and is produced by inaccuracies in the mounting operation. The result is that one tooth supports larger chip loads than the others. This error is due to deficiencies in the tool-collet adjustment, the degree of collet wear, or small impurities of chips or coolant that may have been left on the collet that render the grip of the cutting tool imprecise or less rigid.

Runout can be measured while the spindle is in operation (dynamic measurement) or at rest, using static tests. Dynamic measurement provides more reliable and precise results, although the instruments required are more expensive.

Spindle runout measurement sketch
Checking spindle runout using static tests: (1) at the taper and (2) on a precision test bar

At CNC Proto we specialise in the machining of non-ferrous metals, mostly 6000 and 7000 series aluminium alloys, and engineering plastics. You can see a list of our most used alloys in our Services page. If your parts require a specific aluminium alloy other than the ones listed, feel free to let us know on our Quote Request form!

For more articles on CNC machining, materials and design of machined parts, be sure to check our Resource Library.

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Fernando Duarte Ramos

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With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing and building the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost. Learn more About Us

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