Details of experimental Set-up

The experimental set up are presented in Figure 5.5. The experimental setup comprises four main groups of component: stylus holder, force sensing system, stylus movement, and monitoring and alignment. The outputs from all of these groups are well integrated and captured by the control software. This setup is the base setup for both the stylus stiffness measurement and maximum safe tip force measurement. The differences between these two experiments are in the aspect of the experimental procedures and the analysis methods, as will be explained in detail in section 5.5, section 5.6 and section 5.7.

Figure 5. 5: Schematic diagram of overall base experimental setup

In this setup, the stylus is attached to the stylus holder while stylus holder is mounted at a manual stage and micro translational stage. The stylus is positioned in the horizontal axis and its intended movement is in vertical axis. In the general experimental procedure of both testing regimes, the stylus is moved into contact with test workpiece made from cooper sheet in the nominally vertical direction by a precision micro translation stage. The test workpiece is located on a precision mass balance. The precision mass balance is used as a force sensor and records results during the contact between stylus and cooper sheet. A chromatic confocal point displacement sensor records the movement of the precision translation stage. For stiffness testing, during contacts, the stylus moves a few micrometres toward the test workpiece and then returns to its initial position before contact. In contrast, for the maximum safe tip force testing, the downward movement of the stylus is only stopped after the stylus breaks. The anti-vibration table is used in this setup while the environmental conditions of the laboratory are control, with a temperature gradient of better than 0.1°C per hour, and a humidity of between 40%RH and 65%RH. The construction of this experimental setup will be explained in section 5.3.1 to section 5.3.5.

131 5.3.1. Design of stylus holder

It is very important to ensure that the stylus is securely mounted in the experimental setup to avoid gross movements and vibrations of the stylus that might confound measurements of its bending deflection. As described previously in section 5.2, each stylus has two parts to its stylus shaft with different diameters. The upper part of stylus shaft is very important to act as an interface to the holder, which would normally be part of the probe. To attach the stylus to the experiment setup, special holder has been designed and manufactured as shown in Figure 5.6. This stylus holder will securely mount the stylus by holding the upper part of stylus shaft securely tight by using a small M2 screw. With this design, the holder is expected to minimise the vibration error. There are also two parts to this stylus holder, an extension part and holder part. These two parts are joined together using a thin stainless sheet. The extension part of the holder (shown on the left in Figure 5.6) is attached to the precision translation stage (and the manual XY stage for coarse alignment) for stylus motion as will be explained in detail in the next section.

Figure 5. 6: Engineering drawing of stylus holder 5.3.2. Force sensing mechanism

A precision mass balance and a lightweight test workpiece are used in sensing the force during the experiments. The test workpiece functions as a representative measured workpiece and is placed on the mass balance as shown in Figure 5.7. The precision mass balance records the results when the stylus touches the test workpiece. Later, in data analysis, by using gravitational acceleration constant, the results reported in in mass units will be converted to forces. The copper sheet is used as a test workpiece. The precision mass balance used in this experiment is manufactured from Mettle Toledo (model AT20) which has the resolution of 2 µg and weighing capacity up 22 g [135].

132 Figure 5. 7: test workpiece is placed on the precision mass balance to function as force sensing

mechanism 5.3.3. Motion mechanism for the stylus

The stylus is required to be moved in the vertical direction toward the test workpiece to make contacts. For this purpose, the stylus holder which was introduced in section 5.3.1 is attached to a precision translation stage. The required motion of the stylus must resolve well into sub-micrometre range and provide close control over sub-tens-micrometres ranges, based on theoretical values of maximum deflection given in Table 5.1. Also, the stylus must be able to move over wide vertical travel ranges to facilitate the setting up and to model CMM approaches. The precision translation stage must have capability to fulfil all the requirements. In this experiment, a model M-110.1 micro translation stage from Physik Instrumente (PI) is selected. Its travel range is 5 mm, with a resolution of 7 nm and minimum incremental motion of 50 nm [136]. The translation stage is connected and controlled by the control software (latter will be explained in section 5.3.5). To record the displacement of the stylus motion, a traceable chromatic confocal point sensor is employed. The chromatic confocal point sensor focused on the micro translation stage to measure the displacement of the stylus motion during the experiment as shown in Figure 5.7. The working range of this chromatic confocal point sensor is up to 250 µm with resolution of 10 nm.

133 5.3.4. Positioning, alignment and Monitoring components

Before starting the experiment, the tip sphere of the stylus has to be positioned accurately toward the top of the test workpiece without contacting it. This can be achieved by using a manual translation stage which functions as coarse alignment in vertical direction for the stylus motion. This manual translation stage is placed between the stylus holder and the precision micro translation stage. For horizontal positioning, another manual translation stage is also attached to the stylus. A CCD camera are also employed to assist positioning the stylus during the alignment process. Furthermore, the CCD camera are also important in monitoring the stylus condition during the entire experiment.

5.3.5. Control software

An interface that controls and gathers all experimental results has been developed in LabVIEW software [137]. The data from the precision mass balance, chromatic confocal point sensor are input to this control software. Furthermore, it also controls the movement of the precision micro translation stage, with each movement being recorded. The movement step of this stage will be explained latter in section 5.6.2.