This chapter describes the newly-formulated set of design considerations for stylus systems for micro-CMMs and micro scale measurement. Five major factors that influence the measurements using micro-CMMs, especially at micro- and nanometre scales, have been identified. These influence factors then lead to the development of the design considerations for a new stylus with tip diameter in the sub-10 ยตm region. Thus, the outcome of this chapter is a set of the design rules which are interlinked to each other, satisfying Thesis Objective 1. Moreover, the explanation of the influence factors and the development of design rules are essential in answering Research Question 1.1. This set of design rules is also expected to be a general recommendation in manufacturing and properties characterization of styli. In summary, the set a design rules can be listed as follows:
(i) The diameter of Stylus tip sphere must be smaller compared to the dimensions of the features of the surface to be measured.
(ii) The diameter of the stylus tip should be bigger than the diameter of the stylus shaft.
(iii) The aspect ratio of the stylus should be higher than the aspect ratio of the features that intends to be measured
(iv) Selection for the diameter of the stylus tip is depend on the geometrical condition of the stylus shaft, including its diameter and effective length. The mathematical relationship between the diameter of the stylus shaft, length of the stylus shaft and the diameter of the stylus tip is:
๐
๐๐๐=
๐
๐๐๐๐๐๐ยฑ โ๐
๐๐๐๐๐๐โ ๐ (๐๐๐ ๐
๐๐
๐๐ ๐ฌ ๐ฌ
โ๐) ๐
๐๐๐๐๐๐๐ (๐๐๐ ๐
๐๐
๐๐ ๐ฌ ๐ฌ
โ๐)
(v) In the micro-scale range, the probing force for single point probing is a combination of impact force, overtravel force and surface interaction forces. The actual probing force must always be less than allowable probing force. The allowable force depends on the material properties and the radius of stylus tip. Using the previously defined notation, the allowable probing force should obey the relationship:
๐ญ
๐= ๐๐ ๐
๐
๐
๐106 (vi) The impact force in the single point probing will be influenced by the probing speed,
material properties, probing mass and stylus tip diameter with this relationship:
๐ญ
๐๐๐= โ๐๐๐
๐๐
๐
๐๐
โ๐
๐๐ฌ
๐๐๐๐
๐๐
(vii) The overtravel force depends on the stylus stiffness with the following relationship
๐ญ
๐๐๐= ๐ช
๐๐
๐๐๐(viii) The surface interaction force has potential contributions from the van der Waals force, electrostatic attraction force, Casimir force, and hydrostatic or capillary force. The surface interaction force has an important influence in micro-scale range compared to the gravitational force and cannot be neglected.
(ix) To reduce the van der Waals force, the contact area between the surface should be as small as possible and it is preferable to select a hard material for the stylus tip.
(x) To reduce the electrostatic force, a stylus tip material with a small contact potential difference between the tip and the measured surface is preferred.
(xi) In order to reduce the surface force related to the hydrostatic or capillary force, it is recommended that the relative humidity should be below 60%, with a preference that it should be between 30 to 40% RH.
(xii) The scanning mode is influenced by the surface force, friction force and the stylus stiffness with the following relationship:
โ๐ = ๐ (โ๐ +
๐ญ๐๐ช๐)
(xiii) To avoid the collision between stylus shaft and measured workpiece, elastic deflection of the stylus shaft , Ws, should be smaller than allowable stylus deflection, Wa
(xiv) The maximum safe tip force should be determined and should be bigger than allowable probing force
(xv) Relationship between (maximum) stylus speed and effective mass for a given
107
โ๐
๐๐๐= โ(๐. ๐๐๐ )
๐๐
๐๐
๐.๐๐๐๐ ๐ฌ
๐๐๐ ๐๐
๐ ๐(xvi) In general, higher stylus speeds are preferred in order to reduce overall measurement times, reduce risk of thermal drift, and so on. Selection of the stylus speed depends on the effective mass, stiffness at the stylus tip, diameter of stylus tip sphere, measurement range travels by stylus, and the sensitivity of the measurement.
(xvii) A good surface quality is necessary at the stylus tip sphere, evaluated in comparison to the desired measurement precision and uncertainty. It should have a low surface roughness, a uniform sphere diameter, a small value of roundness deviation and a small centric offset of the sphere from the stylus shaft.
(xviii) The surface roughness and roundness of the stylus tip sphere must be smaller than the surface roughness and the (local) roundness of features on the measured surface.
(xix) The contamination at the surface of stylus tip sphere should be minimized (ideally, eliminated) by using appropriate cleaning methods..
(xx) The effective stiffness at the stylus contact point depends on the stiffness of the stylus shaft, which is influenced by the length and diameter of the stylus shaft and the Youngโs modulus of its material, and the type of suspension or probing that the stylus shaft is attached to.
(xxi) Stiffness of the stylus shaft should be higher than the stiffness of the suspension for probe sensor mechanism or that the overall movement at the stylus tip arising from the suspension is bigger than the estimated deflection of the stylus shaft.
(xxii) Stiffness should be isotropic (or in practical terms, nearly equal in all directions).
(xxiii) As mentioned in design rule (VII), (XII), and (XIV), the stiffness at stylus tip sphere will give significant effect to the probing force, stylus speed, surface force and stick slip effect during scanning
(xxiv) With reference to design rules number (V) and number (XIX), the material for stylus shaft with higher value of Youngโs modulus will decrease the allowable probing force and will influence the stiffness of the stylus shaft.
(xxv) The material of the measured workpiece should be considered in designing a measurement task and selecting the appropriate styli in order to avoid plastic
108 deformation in the measurement. Generally, a stylus tip material with high Youngโs modulus and higher strength will be preferable.
(xxvi) The selection material for stylus tip sphere should take into account the material of the surface intended to be measured to avoid or minimize the abrasive and adhesive wear
(xxvii) Manufacturing techniques applied to styli must be able to deliver consistent control over sizes, tolerances and so on sufficient to achieve the performance required in the stylus design rules, especially those for geometrical considerations and physical condition.
(xxviii) As there are variant manufacturing techniques and processes for producing stylus systems for micro-CMMs, the important control parameters associated with these manufacturing processes depend on their specific manufacturing technique themselves. The relevant factors must be properly investigated and understood before any process is introduced.
Finally, the next step is to bring these design rules together and consult with the manufacturer for the manufacturing a stylus system with the dimension of stylus tip less than 10 ยตm or in sub-10 ยตm range. In addition, prior to characterising the new stylus design, analytical modelling based on the design rules will be explored in chapter 4
109