Knowing your machine tool's capabilities: lean and beyond

Figure 1: Thermal shift in µm in each axis (thick lines) and temperature changes in Celsius (thin lines)Today, tools are available that make it possible to predict machine tool capabilities, and such predictive abilities allow you take your lean initiatives to the next level.

Once you have calibrated your machine tool – as discussed in “The Key to Lean: Machine Tool Calibration” in the March 2011 issue of Today’s Medical Developments, page 32 – the remaining step is measuring and characterizing the performance of the spindle at operational speeds. That is the only way to really predict the point of machining over changes in spindle speed, temperature, and other factors affecting the machine’s ability to put the cutting point where you want it. This article will discuss what error motions are, what impact they are having on your part quality, and how to measure them.


Spindle Error Motion
In a typical machine tool, the spindle rotates around the Z axis, perpendicular to the X- and Y-axis. Spindles are supposed to rotate in perfect circles in the X-Y plane, and only make programmed moves in the Z-axis; the part programming is based on this assumption. Of course, spindles do not really turn in perfect circles (the axis of rotation is not perfectly stable). Any motion of the spindle in the X- and Y-axis that is not a perfect circle is an error motion. Any unprogrammed motion of the spindle in the Z-axis is an error motion.  Some error motions occur on every rotation,  and others occur over longer periods of time. These error motions are caused by inherent imperfections in spindles and their bearings, spindle/bearing wear, thermal effects, and vibration from drive motors, coolant and chiller pumps, or external vibrations from other nearby machines.
 Figure 2: Spindle performance can be greatly affected by speed.

Without the ability to quantify these errors, every new high-precision part runs the risk of being beyond your machines’ capabilities – and you will only discover that after you make bad parts and are left struggling to understand why. But if you measure the spindle’s error motions, you can predict potential manufacturing problems. When you measure all of your machines, you can choose the best machine for each part based on its unique requirements.


Error motions
Because of error motion’s importance in machine tool efficiency, ANSI (B5.54, B5.57) and ISO (230-3, 230-7) standards list specific types of error motions and call for specific tests to measure them. The test results allow you to characterize each machine tool and empower you to predict each machine’s capability to produce specific parts within required tolerances.


Spindle Position Changes
Figure 3: Spindle Error Analyzer sensor and masterball setupSome changes in spindle position are not rotational errors. The largest error motion on your machine is likely to be movement in the Z-axis due to thermal changes. These can be caused by ambient temperature changes, but more likely by machine generated heat. Temperature changes can also cause distortions in the machine’s structural loop, causing the spindle to tilt – the further the machining point is from the spindle face, the greater the error in cutting tool placement and feature location on the part.

Measuring temperature at several points on the machine while measuring the spindle position enables you to troubleshoot contributors to thermal changes like coolant and chiller pumps, drive motors, and ambient conditions. Another benefit of measuring the thermal characteristics of your spindle is the ability to anticipate required warm up time. Figure 1 shows the measurement of a machine in which the Z-axis position changed 120µm (0.004") over 8°C and almost as much in X and Y as well. How many parts do you manufacture that can stay in tolerance if the machining point moves that far?

Another, perhaps unexpected, contributor to spindle position errors is the spindle speed. A Shift vs. RPM test reveals specific spindle speeds that may cause significant changes in spindle position in X-, Y-, and Z-axis. A Z-axis shift of 60µ" at some speeds, but no shift at 4,000rpm. The magnitude of speed-related spindle position changes could be much higher in machines now using high-speed spindles turning at 40,000rpm or higher.


Rotational Error Motions
Perfectly round holes and surfaces are created by spindles turning in perfect circles. A radial measurement in X and Y (machining centers) or just one axis (turning centers) will indicate your spindle’s ability to create a round hole or surface. Roundness is generally determined by synchronous error motions – error motions that occur at integer multiples of the spindle speed and are often caused by shaft alignment or out-of-round bearing seats. Error motions that are not synchronized with spindle rotation, asynchronous errors, cause a different problem – surface finish.

Surface finish is directly dependent on the asynchronous motion of a spindle. As a spindle turns,  the axis of rotation can move in ways not synchronous with the rotation. These movements may appear random, or could repeat at some frequency related to, but not an integer multiple of spindle rotation. When the error motions of multiple rotations are plotted, the asynchronous error makes the plot look fuzzy, as seen in figure 4. As asynchronous error moves the machining point during cutting, the surface finish is degraded by the apparently random changes in the depth of cut on the part.

Characterizing sweet speeds and sour speeds can help you quickly improve machine tool performance. Due to internal mechanical resonances, balance, and other factors, error motions can be greatly affected by spindle speed. Figure 2 shows the radial error motion plot of the same spindle at three different speeds. The only way to know what your machine tool is doing is to measure it.


Spindle Error Motions
Because today’s spindles are very good and precision parts have demanding specifications, measuring spindle error motions will require more than the basic dial indicator measurements which date back to the 1940s. The current state-of-the-art uses high-performance non-contact sensors to monitor a precision masterball mounted in the spindle. The masterballs are precision ground to a roundness with less than 50nm of error. The sensors measure with resolutions around 10nm.

To take a measurement, the masterball is mounted in the spindle. The sensors’ probes are mounted in a precision probe nest and mounted to the table (or toolholder for turning centers). If spindle tilt measurements are going to be included, a dual master ball will be used to take measurements at two different distances from the spindle face. Figure 3 shows how the probes and masterball are aligned and able to take measurements in all three axes.

Measurements from the sensors are acquired by Spindle Error Analyzer software and analyzed for specific error motions. The results are plotted in linear and polar plots, and critical values are listed on screen. Specific tests described in ANSI and ISO standards are performed by the software, revealing the machine’s capabilities to hold part tolerances. Running the tests at different spindle speeds indicates the best speeds for producing parts and can reveal mechanical resonances that may need to be addressed. Testing over time while monitoring multiple temperatures will clearly show if there is a thermal problem and reveal the time required for the spindle to stabilize after the motor is energized. Deeper understanding and troubleshooting is possible with Fast-Fourier Transform analysis (FFT) to  reveal the frequencies at which error motions are occurring. FFT analysis has led to the discovery of problems with bearing wear, chiller pumps vibrating the spindle, and adjacent machine vibration affecting feature accuracy. An all-too-frequent waste of money is the unnecessary rebuilding of a spindle when the problem was somewhere else.

To control quality, to be as lean as possible, to anticipate maintenance needs, and to know the capabilities of your machine tools, your spindles must be measured. Armed with these tools and a thorough understanding of each of your machine tool’s capabilities, you will be better able to manage and streamline production.


Lion Precision
Shoreview, MN
lionprecision.com