Practical set-up

The measuring system consists of an areal-topographic measuring instrument and its support frame and a rotary table to mount and rotate the workpiece for circumferential measurements. Measuring instruments that commonly come with rotation units for cyl- indrical workpieces are roughness and roundness measuring instruments. Roughness measuring instruments often have their rotation unit aligned horizontally, i.e. parallel to probe scanning direction. Roundness measuring instruments rotate a shaft around its vertically aligned axis. The major difference between the horizontal and vertical align- ment of the shaft is that a vertical shaft can easier be adjusted for eccentricity and tilt than a horizontal shaft. Keeping eccentricity and tilt small is beneficial in this application for two reasons: First, the height axis of the instrument does not need to be adjusted previous to the each measurement around the circumference. And second, the tilt can possibly manifest in a higher frequency deviation (see section 3.1.2).

The set-up resembles a roundness measuring instrument with a vertically aligned shaft. Due to the vertical alignment, the shaft can be adjusted for eccentricity and tilt. The in- strument is tilted 90° from its upright position, so that the optical axis is horizontal, and fixed to a support frame. A picture of the system is shown in fig. 3.1.

The set-up allows for various degrees of freedom of positioning and aligning both the shaft and the instrument. The support frame was designed at MTS and makes position- ing of the instrument in all three translational axes and tilting in the plane incorporating both the shaft axis and the optical axis possible (see fig. 3.2a). It can be clamped to a frame of aluminium profiles for better resistance to vibrations. The rotary table is

support frame measuring instrument rotary table precision probes frame of aluminium profiles adjustment screws

Figure 3.1: Picture of the complete set-up including the rotary table with the adjustment screws, the horizontal measuring instrument, two precision probes, the support frame, and a frame of aluminium profiles.

equipped with four adjustment screws to adjust eccentricity and tilt relative to the axis of rotation. For this purpose roundness measuring instruments are usually equipped with a motorised vertical stage to calibrate radial run-out at several axial positions. Here, the lack of a vertical stage is compensated by two electronic precision probes (see fig. 3.2b) connected to the table manufacturer’s proprietary software. Alternatively, one probe can be replaced by the confocal measuring instrument itself and deviations be adjusted manually.

(a) Sketch of the rotating measuring system with a shaft with positive lead angle and de- grees of freedom according to [6]

(b) Picture of the rotating measuring sys- tem with probes for adjustment and a fine- threaded screw as workpiece

Areal-topographic measuring instrument

The areal-topographic measuring instrument used is the confocal microscope NanoFo- cus µsurf explorer. The confocal principle of preventing out-of-focus light from passing through the pinhole applies only to a point [113, pp. 31 sq.]. In order to measure a topography the surface has to be scanned. Scanning is achieved in this case by a ro- tating multi pinhole disc [100, 144]. The device comes with several objectives. Here, only magnification factors of 20 and 50 are used and the objectives are 20×/0.45, 20×/0.6, and 50×/0.5 with the second number being the NA. Unless stated other- wise the 20×/0.6 objective is applied throughout this work. The instrument’s camera has a resolution of 8 bit gray scale and provides 512 pixel× 512 pixel. The resulting lateral properties are tabulated in table 3.1. Optical resolution is calculated as the min- imal distinguishable distance between two points according the Rayleigh criterion [113, pp. 48 sq.]. The wavelength of the LED light source is given as 505 nm [100]. Posi-

Table 3.1: Objectives and lateral properties of the confocal microscope Objective Field of view Optical resolution Lateral sampling distance 20×/0.45 800 μm × 800 μm 0.685 μm 1.5625 μm× 1.5625 μm 20×/0.6 800 μm× 800 μm 0.513 μm 1.5625 μm× 1.5625 μm 50×/0.5 320 μm× 320 μm 0.616 μm 0.625 μm× 0.625 μm

tioning along the optical axis is achieved by a piezo drive with a range of 250 μm and a stepping motor for longer distances. The sampling distance along the optical axis can be set freely. It commonly varies between 62 nm and 125 nm for the 20×/0.6 objective. The reference file correcting the image field curvature is kept up to date. The camera gain is switched off to decrease noise.

Topography orientation

Evaluating measurements of anisotropic surfaces with areal-topographic instruments requires agreement on a coordinate system. Throughout this work the (xt, yt, zt)| co- ordinate system of fig. 3.3 is used unless stated otherwise. The coordinates of the confocal microscope and a major evaluation software, however, rely on pixel coordin- ates similar to image coordinates or row-column coordinates of a matrix with the origin in the upper left corner. Lateral indices are mt, nt, and Mtand Nt the respective number

zt xt yt Nt· ∆xt Mt· ∆yt ∆xt ∆yt nt 1 2 3 mt 1 2 3

Figure 3.3: Coordinate system within a topography

nt

mt

xt

yt

ψax

Figure 3.4: Coordinate system of a topography with structures with a positive lead angle (right-handed lead) and their appearance in different coordinate systems

ates and (xt, yt, zt)| coordinates is

xt = nt− Nt+ 1 2 (3.1a) yt =−mt+ Mt+ 1 2 (3.1b) zt = zt . (3.1c)

This relationship is outlined on a shaft measurement in fig. 3.4. It can be seen that structures are swapped around the xtaxis and structure orientation is different, too. The

example of a real measurement of a right-hand threaded screw with fine thread in fig. 3.5 confirms the effect. The right-handed thread in fig. 3.5a corresponds to right-handed lead structures (positive lead angle). However, due to the minus sign in eq. (3.1b), the

surface is flipped over the xt axis exhibiting left-handed lead structures (negative lead

angle) in fig. 3.5b. But this effect can easily be taken care of in the evaluation and all results are presented in the (xt, yt, zt)|coordinate system.

1 256 512 1 256 512 ntin samples m t in samples

(a) Microscopic picture of a right-hand threaded screw with fine thread from fig. 3.2b

-400-200 0 200 400 -400 -200 0 200 400 xt in μm yt in μm 100 200 300 zt in μm (b) Areal-topographic measurement of

fig. 3.5a in (xt, yt, zt)|coordinates

Figure 3.5: Measurement of a right-hand threaded screw measured similar to fig. 3.2 to determine topography orientation

Positioning and repositioning

The support frame enables free positioning of the measuring instrument within a large volume to measure large objects like crank and cam shafts as well as non rotating parts. The system is located on the ground floor of a university building without special atten- tion to vibration damping and several laboratories with test facilities are present on the floor below. Due to the large positioning range and therefore large measuring circuit the instrument exhibits vibrations that require clamping to a frame of stiff aluminium profiles visible in fig. 3.1. An undesirable side effect of additional clamping is that it inevitable wraps the support frame and shifts the instrument from its previous position. While a small shift is tolerable a rotation around the optical axis is not. A test with six successive series of 50 measurements each looks at the effects of several position- ing and clamping operations on the calculated orientation. The experimental standard deviation s(ψt,i)is defined as

s(ψt,i) = v u u t 1 N − 1· N ∑ i=1 (ψt,i− ψt)2 (3.2)

and indicates the variability within a series, which is very low. ∆ψt is the difference

between the means ψ_tof two successive series of measurements with

ψ_t = 1 N · N ∑ i=1 ψt,i . (3.3)

Between each series the support frame was handled in a different way described to- gether with the results in table 3.2. For high sensitivity and minimal measurement pre- cision the measurements were taken on a grating of high quality (see fig. 3.23a). It follows that even though the radial position along the optical axis ranged in a large in-

Table 3.2: Description and results of six series of 50 repeated measurements each after positioning and clamping of the support frame

Series description ψ_t s(ψt,i) ∆ψ_t

1 very lightly clamped −7.90′ 0.08′ —

2 very strong clamping (zt position shif-

ted≈ 360 μm)

−7.36′ 0.03′ 0.54′

3 unclamped, laterally moved, reposi-

tioned, and reclamped (zt position

shifted≈ 800 μm)

−7.50′ 0.04′ 0.14′

4 clamping with additional bracket (zt

position shifted≈ 50 μm)

−7.77′ 0.03′ 0.27′

5 small traverse movement without un-

clamping

−8.23′ 0.04′ 0.46′

6 traverse moved back −7.64′ 0.03′ 0.59′

terval the rotation varied much less. Still, the differences observed between the series are significant larger than the measurement precision within each series. Encouraging is the fact that the variation between two positioning operations with clamping and un- clamping (2 to 3) is rather small. A traverse movement without unclamping should be avoided (4 to 5 to 6) and the clamping should be the same for all measurements (i.e. clamping each time with one bracket or two brackets (3 to 4) and with approximately the same torque (1 to 2)). For practical applications of measuring at different axial po- sitions the case of 2 to 3 will be the most common. It is therefore examined further by another ten series of ten measurements each. Lacking a shaft of high reproducibility in axial direction measurements are taken on the grating again. The support frame was unclamped, the instrument moved down and up approximately 10 mm, and reclamped. Of the ten series only the first eight can be used because evaluation shows a significant shift in the last two, indicating an involuntary movement of the grating. The experimental standard deviation of the eight mean values of each series is 0.17′. This value is in the range of the result (2 to 3) from table 3.2.