Operational principle of test-rig method

The test-rig method has been developed, and comprises of a Talysurf 5 system, traversal unit, motorised stand, standard current source (Knick model), Heindenhain gauge, probe from roundness instrument (Talor Hobson Talyrond), digital voltmeter, xyz-adjustable table, and small hallow cylinder for mounting the sample. The important characteristic of test-rig method is that when a small known force is applied on the hub of the triskelion sample and it measure the defection. The maximum force that can be applied is 1 N, and the maximum defection is 1 mm. The scale of the device used requires sub-µm resolution.

The apparatus was set up as shown in the figure 6.2 and 6.3. The force was applied to triskelion sample vertically alongz-axis and deflection occurs in the same vertical direction. An optical probe (Heindenhain) with 1µresolution was used to monitor the vertical motion of the table. The probe from the roundness instrument (Taylor Hobson Talyround) was a side acting inductive gauge and was mounted on a cradle, which also carried a solenoid coil. A small clamp is used to carry a magnet on the probe arm, which engages the coil to

way, a deflection was measured. For fine vertical control, the whole cradle was bolted to the traversal unit of a Talysurf 5 (Taylor Hobson) profiling instrument. Further, the internal electronic setup system of the Talysurf 5 was used to drive the gauge and to provide a wide range with its own setup conditions.

The Talymin side-acting inductive gauge used on may Talyrond (Taylor Hubson) roundness measuring instruments is larger and more robust but electronically compatible with a Talysuf 5 sensor. It is well able to carry the more massive magnet needed to achieve the forces up to about 1 N without excessive heat generation in the coil. Also, it has interchangeable probe-arms, making it easy to tune the displacement range to a specific application. This suggest that a simple swapping of the probe system would suffice. How- ever, the more robust probe bearing and bias system could lead to a significant but poorly characterised variation of force with displacement. Also, the displacement measurement must be highly linear if the test-rig is to be used to detect the onset of non-linearity in the specimen stiffness. Simple tests of the gauge with an operationally convenient size of the probe arm indicated that it would be unwise to use it outside of the central ± 200µm of its operating range.

Most of these disadvantages could be overcome by using a quite basic null-measurement technique. The specimen is placed horizontally on the table of a stiff z-axis adjuster. As the downwards force is imposed by the probe increases, the specimen deflects downwards but the table raised to keep the Talymin gauge output signal as near as possible constant (i.e. a null condition). A long-range, highly linearity displacement sensor (while could not be physically loaded directly against the specimen) can report the change in table hight, which reflects the specimen deflection to the precision of the null. The inductive gauge can then be used at relatively high gain to get good null sensitively and with almost no reliance on its inherent linearity. Also, since the probe arm in principle is kept in the same position there is minimal effect of force variation from the bias springs.

The practical implementation of this system is shown in figures 6.2 and 6.3. The inductive gauge is mounted on a ’rigid’ cradle, which also carries the solenoid coil for the

Figure 6.2: New Metho d: Sc hema of T est-rig Metho d to meas u re the stiffness of trisk elion force artefacts.

Figure 6.3: New Metho d:Picture of T est-rig Metho d to measure the stiffness of trisk elion force artefacts

force actuator. A small aluminium structure is clamped by a set-screw to the probe arm and has a flat top surface to which the magnet can be glued and underside thread for mounting commercially miniature probe tips. The cradle is mounted by a bracket to the Talysurf traverse unit so that it sits, in effect, parallel to the normal Talysurf probe. As well as allowing the Talysurf system (motorized column, horizontal traverse, etc.) to be used for aligning the probe. This configuration allows rapid switching between two similar instrument systems of different sensitively, as required.

The new scheme uses 0.2 mm ruby probe (Renishaw) and a nominal 50 mm probe arm. The magnetic-coil actuator ideal scale with volume so nominal design was taken by comparison with known characteristics of the small system on this original instrument. The dimensions of iron-neodymium magnet are 5 mm by 6 mm diameter and the coil dimensions are 10 ×14.7 mm, 5.9 mm internal diameter and 14.7 mm external diameter, 3776 with number of turns of 0.12 mm gauge copper wire.

The tests are essentially static and they were judged quite adequate and simplest to use mechanical control of null measurement. The force would be changed step-wise and restoring adjustment each time. Consequently, a small, stiff xyz adjuster table with fine thread manual screw was employed. The x- and y-axes are not used in the measurement but provide further aid when aligning specimen and probe. The z hight of the table was measured by a 1 µm resolution optical grating probe (Heidenhain HH60).

The inductive gauge was conducted to the Talysurf electronic unit, an output voltage proportional to displacement. The unit has variable gain, but was usually operated at 125 µm/V for tests reported here. The output was read from a simple digital voltmeter. Force is directly proportional to the coil current. The latter was set manually at each step using standard current source (Knick model). With the coil being used, this could be deliver up to±140mAwith 1 µA resolution.

and the current was reduced to increase the force on the triskelion samples i.e.

Fspring = Wprobe−Fcoil (6.1)