Measurement Technology for Precision Machines
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wThe precision of machine tools is the foundation of machining to achieve high precision. Therefore, an investigation on the measurement technology for precision machines is extremely important for precision/ultraprecision machining. This chapter provides an overall introduction to the measurement of precision machines, including the precision along linear axes and rotary axes, e.g., the precision of length, precision of angle, straightness, squareness, parallelism, flatness, and runout. The basic principles, achievable accuracy, application, and characteristics of a wide variety of measurement methods and instruments with precision in the nanoscale or below are discussed. It is found that digitization, automation, and flexibility are the development trends of modern instruments used for the measurement of precision machine tools. Though a number of measurement methods have been developed during these years, the existed methods face challenges to comprehensively characterize the performance of machine tools, especially for ultraprecision machine tools with a precision better than 100 nm. In addition, online measurement still remains to be investigated to improve the performance of precision machining.
KeywordsMeasurement Metrology Online measurement Machine tool Precision machining
Principles of Measurement for Machine Tool Metrology
When measuring the dimensions of an object, one must know its temperature. The temperature differences in the structure will introduce errors due to linear expansion of machine elements, and the magnitude depends on the temperature difference, dimension, and coefficients of expansion.
Avoid the application of force. Noncontact methods of measurement are preferable to contact methods. Contact stresses generated by the contact of two bodies under load will generate deformation due to the elasticity of the bodies, and the magnitude depends on the contact force, the equivalent modulus of elasticity of the two bodies, and the equivalent radius of contact, as revealed by Hertz.
The precision of the measuring instrument must be five or ten times higher than the expected precision of the measured object. This is because the precision of the measuring instrument is one key component of the measurement uncertainty.
Be aware of the behavior of the instrument. It is important for machinist or metrologists to understand the factors that influence the performance of the instrument to use it properly.
Measurement of Dimension
In practical metrological measurements for the inspection and calibration work undertaken on machine tools, the use of equipment such as gauge blocks, length bars, angle gauges, straightedges, and dial gauges together with dial test indicators forms the foundation for machine verification.
The Bessel support points (i.e., after Friedrich William Bessel) are the support positions for an evenly loaded bar, where the droop for both the middle and ends of the bar are at a minimum (i.e., termed its neutral axis). These supporting points (Fig. 5b, bottom), often known as the points of minimum deflection, are not valid for end standards such as length bars but can be utilized more effectively for the support points for flat metrology tables or similar level items.
Dial Gauges, Dial Test Indicators, and Inductive Gauges
Straightness Calibration with Straightedges
Straightness measurement systems consist of a straightness reference and a displacement indicator. In order to measure the straightness errors with a material reference, the straightedge reference is placed in direction of the machine axis. As a straightness reference, a calibrated ruler or a stretched wire (for long axes) can be used. The axis is then moved, while a distance sensor (capacitance gauges, electronic gauges, or dial gauges) measures the lateral displacement.
Once satisfactorily completed, it simply reverses the straightedge by rotating along with its long axis and remounting the indicator so that it is touching the straightedge, but it now has had its direction or sign reversed. This action is important to note, because now when one calculates the indicator output “I2(x),” it is possible to see an apparent reversal of the machine axis; the ostensible lack of change in the straightedge output being despite the fact that this straightedge has been reversed.
Spindle calibration with Cylindrical Precision Mandrels
Squareness Calibration with Precision Squares
Measurement of Angle
Sine bar, angle gauges, polygonal mirrors, serrated tooth circle dividers, and rotary encoders are used for establishing motion through an angle or measuring angles.
Davidson Optical Polygons are available in standard configurations of 8, 9, 12, 18, 24, and 36 facets and may be ordered with any number of facets, up to 72. The flatness of facets is λ/10 and accuracy of polygons is ±5 arc seconds. As a result, its calibration of 1/10 arc second can be achieved.
Autocollimators can be used to measure angle (indexing head accuracy), straightness (machine tool slides in two axes), squareness (column to slideways), parallelism (slideways), etc.
Straightness, Squareness, and Parallelism Calibration
Figure 19a shows the schematic of measuring straightness with an autocollimator. Using a reflector (on a carriage) and a dual axis autocollimator, up to 200 measuring steps can be taken for straightness checking by moving the reflector carriage along the slideway in equidistant steps. Any out-of-straightness in either of the two slideway surfaces X and Y (side and top of the slide) will cause the reflector carriage to change angle with respect to the autocollimator, and it is these changes which are measured and computed to determine the errors in straightness. Measurement of parallelism of two machine tool rails can be achieved using an autocollimator with an optical square and mirror, as shown in Fig. 19c.
Angular Calibration of a Precision Polygon
A complete set of readings “Σ(R2 − R1)” can be established, enabling the value of “T ” to be determined. “T ” can be substituted back into equation, for each face, enabling the angle for each of these faces to be simply determined.
Calibration of a Rotary Table
The alignment telescope decomposes the autocollimator into two optical tools, the collimator and the telescope. Similar to the autocollimator, micro-alignment telescope is used for checking and setting, for example, alignment (series of bores or bearings); squareness (column to a base); parallelism (series of rollers); level/flatness (machine bed foundation); and straightness (rails or guideways).
The setting or checking of squareness (shown in Fig. 28b) utilizes a lamphouse to illuminate the crosslines, with the telescope being focused to infinity. Accordingly, from this telescope the rays of light are collimated and are successively reflected back along their own path from a mirror set square to this line-of-sight, forming a reverse image of the crosslines, on the actual original crosslines. A specific feature of the crossline patterns is two pairs of short heavy lines which produce a bold reverse image for precise setting. In consequence, if the mirror is slightly tilted, this crossline image will be displaced. It is worth to emphasize the point that when the telescope is focused at infinity, the micrometers are then ineffective; they cannot be employed in measurement process; no graticule or target other than the crosslines are utilized for autocollimation. A representative micro-alignment telescope system offers the following advantages when either inspecting or calibrating machines: optical and mechanical axes alignment to ≤3 arc seconds and concentric within 6 μm, an achievable accuracy and precision to ≤0.05 mm at 30 m, and an extensive field of view of which ranges from 50 mm at 2 m to 600 mm at 30 m. In addition to the performance of the autocollimators and alignment telescope themselves, the earth curvature and atmospheric refraction will also influence their calibration precision. The effective radius of the earth curvature becomes more noticeable over much longer linear distances, especially when a line-of-sight configuration with the telescope is compared with a level surface setup by a gravity-controlled instrument. The line-of-sight deviates from a straight line owing to refraction within the same atmosphere due to the density gradient in the atmosphere resulting from differential heating/cooling of the layers of air at varying heights above the earth surface.
The Taylor Hobson Talyvel electronic levels and a range of precision clinometers are used for level and flatness measurement (surface plates and granite tables), straightness and twist (roll) measurement (machine slides), squareness measurement (of machine columns), angle measurement (remote monitoring of movement of structures), parallelism measurement, etc.
Straightness and Parallelism Calibration
Interferometry is most widely used for displacement/length measurement today as well as some form measurement. It can also be used for measurement of positioning accuracy and straightness of the moving axis, positional accuracy of a rotary axis, surface form of a surface plate, perpendicularity of two nominally orthogonal axes, etc. It is a measure of relative movement (measurement from an initial position) rather than an absolute measurement (measurement of a specific position). Different selections of optics pass the laser beam through different paths, allowing a variety of measurement modes (e.g., linear, angular, straightness) to be taken from a single laser unit. Without reliable and accurate wavelength compensation, linear measurement errors of 20 ppm (parts per million) would be common in typical conditions. These errors can be reduced to ±0.5 ppm by applying precise environmental compensation, which is better than many other methods including tracker laser system or ballbar testing.
Achievable specifications of XL-80 laser measurement system. (Courtesy of Renishaw plc)
0.01 arc sec
±0.0002A ±0.1 ±0.007F arc sec
0.1–4.0 m (short range)
0.01 μm (short range)
±0.005A ±0.5 ±0.15 M2 μm (short range)
1–30 m (long range)
0.1 μm (long range)
±0.025A ±5 ± 0.015 M2 μm (long range)
Up to 25 revolutions
±0.002A ±0.02 M2 μm
±0.005A ±2.5 ±0.8 M μrad (short range)
±0.025A ±2.5 ±0.08 M μrad (long range)
Rotary Axis Measurement
Flatness measurement analyzes the form of a surface. This enables a 3D picture to be built up and documents the deviations from a perfectly flat surface. The flatness kit contains two flatness mirrors and three flatness bases to suit the size of the surface (Fig. 39b). The flatness mirrors not only rotate horizontally but also tilt vertically. This allows horizontal and vertical adjustment of the laser beam. In addition, angular measurement optics are required for flatness measurements. Two standard methods of conducting flatness measurements are supported by the laser software, i.e., the Moody method in which measurement is restricted to eight predefined lines and the grid method in which any number of lines may be taken in two orthogonal directions across the surface.
3D/6D Laser Interferometer
Cosine error. This will be present when the machine tool’s travel is not identical to that of the interferometer’s beam.
Abbé offset error. This is when the position of the laser measurement is shifted away (i.e., offset) from the position of the actual axis cutter travel.
Dead path errors. These are influenced by ambient conditions in the same manner as the displacement part of the beam. So, the interferometer reasons that the path length is the distance moved by the mirror, but in actuality this path length is affected by ambient conditions, the distance from the interferometer block to the matched mirror.
Normally, material dead path errors are potentially much more significant than its associated air dead path errors. The optimum methodology to lessen these undesirable conditions is to apply good metrology practices.
The material dead path – this should be made by situating the setup of the laser as closely and directly as possible to the point of metrological interest.
Any changes in material temperature during measurement – this should be made by initially stabilizing the temperature and/or promptly completing the measurements.
The optics separation (when the system is datumed) – this should be made either by utilizing a preset reading or by employing a beam splitter as the moving optic.
The original laser tracker was patented in the USA in 1987. These now very popular trackers can be considered as polar coordinate measuring systems, being capable of high-accuracy measurements over quite long linear distances. For example, in one of the systems now being utilized by NPL, it can measure at distances of up to 40 m to within discrete accuracies of measurement of ±60 μm. The current laser trackers can make measurements to ~10 μm accuracy without any special geometry or data processing. By choosing advantageous geometry, by calibrating repeating errors, and by averaging random errors, the instrument’s tracking allows it to measure to even tighter levels of <1 μm.
Laser trackers equipped with IFMs split their lasers in two: one beam travels directly to the interferometer, while the other travels to a target device in contact with the component being measured. The two lasers interfere with one another inside the interferometer (hence the name), resulting in a cyclic change equal to one quarter of the laser’s wavelength – approximately 0.0158 μm. Each time the target device changes its distance from the tracker. By counting these cyclic changes, what’s known as fringe counting, the laser tracker can determine the distance the laser has travelled. ADMs use infrared light from a semiconductor laser which bounces off the target device and reenters the tracker. The infrared light is then converted into an electrical signal for time-of-flight analysis.
Component dimensional and finish defects may result from bad tooling, worn spindles, or workpiece clamping, but the major causes of defects can usually be attributed to positioning errors in the machine tool itself. In the past the workpiece accuracy may have been achieved by machining test or “master” parts and then inspecting them. However, this was time-consuming and gave limited confidence when machining parts with geometries different to the master part. Ballbar testing first developed by Bryan provides the quickest (typically 10 min), easiest, and most effective way to monitor machine tool condition and quickly diagnose problems that may require maintenance and the error sources that produce them.
In an archetypal ballbar test, the ballbar might be assembled to sweep, say, a 150 mm radius, but for other tests the ballbar’s radii can be changed, ranging from 50 mm to that a larger radius of >1000 mm becomes possible. A longer radial ballbar length could be employed to increase sensitivity to geometry errors such as squareness; conversely a very small ballbar radius might be utilized to highlight any dynamic errors such as servo mismatch.
A Closer Examination of Machine Tool Inaccuracies
Since it was designed for only simple three-axis milling centers and three-axis lathe centers with ball screws and feed drives, the expert system aiding inference about the causes of errors is a series limitation of the above method. Consequently, only simple circular interpolation tests can be carried out for other machines, both conventional ones and with parallel kinematics. Another drawback of the ballbar device is that measurements are taken only in a selected part of the machine tool workspace. Despite the above drawbacks, the ballbar enables machine tool diagnostics and does not necessitate long breaks in the manufacturing process, mainly owing to measurement simplicity and speed.
Variations of Ballbar
Similar to the magnetic ballbar, the laser ballbar (LBB) (Schmitz et al. 2001; Ziegert and Mize 1994) uses the telescoping assembly to maintain beam alignment with the moving retroreflector and features a much longer range of motion than does Bryan’s with relative displacements measured via laser interferometry. It consists of a two-stage telescoping tube with a precision sphere mounted at each end. A heterodyne displacement measuring interferometer is aligned inside the tube and measures the relative displacement between the two sphere centers.
The main limitation of the LBB and 3D LBB devices is the minimum length of the telescoping tube, which determines the dead part of the workspace and the maximum size of the workspace in which measurements can be made. The state-of-the-art tracking laser systems have no such limitations.
Measurement of Spindle Error
The performance of precision spindles is a significant contributor to the overall accuracy of a precision machine or instrument. For years, the error motion of the highest precision spindles was essentially too small to reliably quantify because the measurement uncertainty exceeded the measurand (a few nanometers for the best spindles). However, improvements in sensors, instrumentation, data acquisition, signal processing, and structural design are closing this gap and enabling low uncertainty spindle metrology.
Principle and Instrument
A conventional method for the calibration of rotary axis is described in ISO 230-1 (ISO 230-1 1996) which proposes the use of a dial gauge to measure the radial and axial runout deviation at the center hole of the rotary axis. If the gauge cannot be applied at the center hole, it can be used in combination with precise manufactured check gauges which are mounted on the rotary axis. Another possibility is the use of capacitive (Chapman 1985; Marsh et al. 2006) or inductive (Frennberg and Sacconi 1996) sensors, which measure contactless and can be used at much higher rotational speed. Capacitance gauging has several advantages over the alternative Eddy current gauging or inductive sensing, which most notably is that capacitance: insensitive to metal alloy variations, not affected by variations in the surface, allows measuring equipment to be readily calibrated in an approved calibration laboratory, and can be traceable to an international standard.
Averaging the multistep measurements separates the error motion of the spindle from the artifact form, except at frequencies that are at integer harmonics of the number of steps. Therefore, caution should be used when interpreting the results. However, when a sufficient number of steps (e.g., 20) are used, the first distorted harmonic occurs at a relatively high frequency. For high quality lapped artifacts, the amplitude of artifact error occurring at these higher harmonics is relatively small (normally a few nanometers) (Marsh et al. 2006).
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