fabrication of arbitrary precision micro profiles by nano-grinding

Int. J. Nanomanufacturing, Vol. 8, No. 3, 2012 231

Copyright © 2012 Inderscience Enterprises Ltd.

Fabrication of arbitrary precision micro profiles by nano-grinding

Gareth E. Milton and Jay Katupitiya* School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney NSW 2052, Australia E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: A multi-axis machine tool and a control architecture that can be used to machine flat or concave axisymmetric or non-axisymmetric micro profiles on micro-scale components is presented. Using this system, the micro profiles can be produced to exact specifications within tolerances. The motivation is to exploit the precision of mechanical motion in conventional machining set ups to our advantage in producing high precision three-dimensional micro profiles. The machine is built by combining a suite of micro and nano stages together with a number of other high precision positioning devices. This paper describes both the machine architecture and the control architecture. The specific micro component used in this work is the tip of a 125 μm diameter optical fibre made out of silica glass. Grinding parameters published in the literature have been used to achieve nano-grinding of silica glass in ductile mode. The emphasis in this work is on achieving the precision of the micro profile. Results include the images of the profiles created and the comparisons of measured and specified profiles.

Keywords: precision micro-profiles; micro-lenses; nano-grinding.

Reference to this paper should be made as follows: Milton, G.E. and Katupitiya, J. (2012) ‘Fabrication of arbitrary precision micro profiles by nano-grinding’, Int. J. Nanomanufacturing, Vol. 8, No. 3, pp.231–246.

Biographical notes: Gareth E. Milton received his PhD in Mechanical Engineering from the University of New South Wales in Sydney, Australia. He now works as a Project Engineer for Ricardo UK on advanced control applications for the automotive sector.

Jay Katupitiya received his PhD in Robotics from the Katholieke University in Leuven, Belgium. He is currently an Associate Professor at the School of Mechanical and Manufacturing Engineering of The University of New South Wales, in Sydney, Australia. His research interests are in control of complex ultra-precision machinery, force control and guidance of off-road vehicles.

232 G.E. Milton and J. Katupitiya

1 Introduction

A nano-grinding process has been developed that is capable of producing micro-scale, high-precision, concave or flat three-dimensional profiles on the tips of optical fibres. Both, axisymmetric and non-axisymmetric profiles may be produced. The process uses in-feed rates that ensure the ductile mode machining of the brittle silica glass fibre. This process is primarily intended for use in the production of micro lenses directly on the tips of optical fibres, using the fibre itself as the lens material. However, the process may be used to produce larger scale lenses for uses in surgery, in the development of optic fibre sensors, or in the production of other micro-scale components from brittle glasses and ceramics for use in other precision industries.

Several methods for producing micro lensed optical fibres have been proposed in the literature. These production methods include using surface tension and cohesion of curable materials (Park et al., 1999), laser machining (Edwards et al., 1993), etching (Kumazaki et al., 2000; Ghafouri-Shiraz and Aruga, 1996), tapering (Hillerich and Guttmann, 1989) and electric arc-discharge (Thual et al., 2003). It is shown in the literature that different lens profiles perform well in differing situations. Profiles such as conical (Ghafouri-Shiraz and Aruga, 1996), hemispherical (Park et al., 1999) and hyperbolic (Thual et al., 2003) have been studied. Many of these studies have shown significant gains in the efficiency of laser diode to fibre coupling (Edwards et al., 1993; Ghafouri-Shiraz and Aruga, 1996). However, many of these methods have weaknesses, such as an inability to accurately centre the lens onto the fibre, a lack of repeatability and an inability to produce different lens profiles or adjust the lens radius. These factors are important, as studies have shown that a lens offset of a few μm can significantly reduce coupling efficiency (Hillerich and Guttmann, 1989) and an ability to repeatedly manufacture arbitrary profiles without significant retooling is desirable. With appropriately chosen feed rates mechanical grinding is capable of producing optical quality surface finishes, free from surface and sub-surface damage (Gharbia and Katupitiya, 2004).

A mechanical grinding method that is able to precisely control feed motions at nanometre scale has the advantage of being directly controlled and therefore able to produce any desired profile by simply altering tool paths. Studies have shown that mechanical grinding is a viable method of micro-scale material removal from optical fibres (Gharbia and Katupitiya, 2004).

The work presented in this paper describes a mechanical grinding system that is capable of producing micro-scale profiles at the tips of optical fibres with very high precision. In the case of axisymmetric profiles, the system has the ability to produce components with excellent concentricity. This is a key step toward the flexible production of arbitrary axisymmetric micro profiles using a mechanical grinding technique.

2 System configuration

This section describes the different subsystems of the nano-grinding machine. The complete nano-grinding machine is shown in Figure 1(a). There are three subsystems that are combined to form a coherent ultra-precision machining system. Namely, the spindle system for positioning and manipulating the workpiece, the tooling system for

Fabrication of arbitrary precision micro profiles by nano-grinding 233

manipulating the nano-grinding tool and the visual feedback system that is used for localisation of the tool and the workpiece.

Figure 1 (a) The nano-grinding machine (b) the fibre centring assembly (see online version for colours)

(a) (b)

2.1 Spindle system

The spindle that carries the workpiece can operate in one of two modes: in speed control or in position control. Speed control is actuated during grinding and position control is actuated for workpiece centring and alignments. The spindle motor is a brush type DC motor with a direct flexible coupling to a high stiffness air bearing. The air bearing has an axial stiffness of 300 N/mm and a maximum run out of 10 nm. The motor has its own tacho metre to provide velocity feedback and an encoder to provide position feedback.

Mounted on this spindle is a device called the fibre centring assembly (FCA) shown in Figure 1(b). The workpiece in this system is an optical fibre of 125 μm diameter. In order to produce axisymmetric profiles, the grinding process requires that the workpiece be rotated precisely around its centreline during production. A simple way to rotate a given workpiece around its own centreline is to mount the workpiece on a spindle in such a way that its centreline is coaxial with the spindle’s axis of rotation. In a macro-scale machine, this is easy enough to achieve. On a micro-scale however, simply mounting the workpiece on a spindle in such a manner is far more difficult. The FCA is capable of automatically centring a workpiece with sub-micron accuracy using four linear stick-slip type actuators shown in Figure 1(b). They appear prominently as the four identical components in the figure and are called ‘pico’ motors. The feedback signal that determines the proper centring and alignment is via visual imaging (Milton et al., 2004b).


2.2 Tooling system

The tool is comprised of a flat grinding surface with embedded 0.25 μm synthetic diamond grits and is intended to carry out face grinding. The grinding surface is driven by a constant speed motor. However, various constant linear grinding speeds can be achieved by positioning the workpiece against different radii on the grinding disc.

The in-feed rates and tangential feed rates of the grinding surface with respect to the workpiece must be maintained in such a manner that the material removal takes place in ductile mode. As grinding parameters for machining silica glass, data from the literature has been used (Gharbia et al., 2004). A high precision linear stage is necessary to provide feed rates to ensure ductile mode grinding. In this work, a nano stage that has a minimum incremental motion of 2 nm is used as the high precision linear stage. For the experiments described in this paper, however, the minimum step size used is 25 nm. The tooling arrangement is shown in Figure 2. Here, the coarse stages have minimum incremental motions of 0.1 μm and the rotational stage has a minimum incremental motion of 5.0 μradians. As mentioned, ωt remains constant. The grinding face is illustrated as the hashed area in the figure.

In order to accurately compute the tool paths used in the nano-grinding operation, it is necessary to localise the tool face with respect to the workpiece mounted. Additionally, a one off task of localising the grinding face with respect to the centre of rotation of the rotary stage must also be carried out. Once these parameters are determined, a tool path can be generated for the tooling stages to follow so that the desired profile can be machined.

Figure 2 Schematic of tooling stages

2.3 Feedback system

As it is essential to determine the positions of various components relative to each other, some form of non-contact measuring method is needed. Note that providing position feedback is essential for effective control. The obvious choice of visual images has been


made as the non-contact sensing method. Due to the micro size components used in the experiments, a suitable magnification has to be chosen. A magnification of 24 times has been chosen as this magnification makes an optical fibre to cover about 70% of the image area. A zooming capability provides a range of magnification selections. Most of the image acquisition is done through back-lit images which allow the camera to capture the silhouettes of the components in the visual field. For all localisation purposes, what matters is the detection of edges of the micro components. The entire edge detection method, the associated image processing and its use to automatically centre a rotating fibre within 500 nm of eccentricity is described in Milton et al. (2005).

2.4 Control system

The control system schematic used in this work is shown in Figure 3. It shows control system components applicable to all subsystems mentioned above. At the heart of all these control systems is a computer that performs all control and image processing tasks. This computer runs the Linux kernel patched with Real Time Linux extensions on an Intel Pentium III CPU running at 733 MHz.

Several real-time threads were then written to allow implementation of the controllers described. User-space command programs could then communicate with the real-time controller thread via a system of FIFO buffers.

Figure 3 The machine control block diagram

actuatorsPicomotor

u’rus u’f

ωs

θs

v

v

vsf

r

Encoder

Tacho

DC

actuator

Spindle

Rotary switch

Velocity

Host computer

Quadraturecounter

controller

outputAnalog

driver

Linear

servoHigh speed DC

motor

PWM controller

PWM controller

Tool and spindle

stageRotary motion

Nanostage

Linear micro−stage

IEEE1394

NanostageDriver

RS232

Workpiecemanipulationsystem

Camera

The ‘velocity controller’ block maintains the rotational speed of the spindle that carries the workpiece. In general, this spindle runs at low speed. During self-centring of the workpiece, the ‘linear actuator driver’ controls the pico motors mentioned earlier through the slip-ring type contacts. This control action is shown by the dotted lines. Through the slip ring contacts only two of the pico motors get connected at any one time. These are the diagonally opposite pico motors that lie on the horizontal plane containing the axis of rotation of the spindle. While monitoring the visual imaging an automatic control system


adjust the pico motors to reduce the workpiece eccentricity and the misalignment. The spindle axis must be indexed by 90° to get the other two pico motors connected. The workpiece centring procedure is completely automated and the workpiece spindle indexing and pico motor control continues until the visual feedback system reports satisfactory centring. This system is explained in detail in Milton et al. (2004b).

The tooling system is shown as a stack of stages on the right hand side of Figure 3. The top most stage is the 200 μm × 200 μm nano stage. The ‘nano stage driver’ is used to control the nano stage and its primary purpose is to provide controlled in-feed to ensure ductile mode grinding.

The ‘PWM controllers’ drive the linear stage and the rotary stage mentioned earlier. Note that, although the tooling system in Figure 2 shows two linear stages, only one linear stage in the y-direction is used to machine concave profiles.

3 Profile machining

The developed machine is able to produce many types of profiles on the tips of optical fibres. The complexity of path generation and machine control increases with the rd increasing complexity of the profiles.

3.1 Flat surfaces

Producing flat surfaces at an angle to the fibre axis, also called oblique faces, on a fibre with a flat tool requires the tool to be set to an appropriate angle and then to feed the tool on to the workpiece. This was achieved by setting the rotational motion stage to set the desired tooling angle and then positioning the linear stage such that the tool face was set some distance from the corner of the optical fibre. As shown in Figure 4, in-feed of up to 200 μm could then be performed using either the nano stage in a direction perpendicular to the tool face, or the linear micro motion stage in a direction parallel to the fibres axis. A single face could be machined in this way with no fibre rotation. Multiple facets could be machined by producing a single face, withdrawing the tool, indexing the workpiece spindle to the next position and then proceeding to machine the next facet. A curved profile can be machined by producing a sequence of facets. The smoothness of the curved profile can be controlled by appropriately choosing the number of facets. Tool paths used for grinding a single facet and a sequence of facets are shown in Figure 5.

The minimum in-feed the linear motion stage could produce for was v = 100 nm/step. However, as this in-feed did not occur perpendicular to the face being ground, the actual minimum in-feed rate, measured perpendicular to the face, is given by v cos α, where α is the acute angle between the fibre axis and the tool face. Using the nano stage, the in-feed rate was invariant to tool angle and average in-feed rates of practically zero could be achieved.

The only difference between the conical profiles and the flat surfaces is the rotation of the workpiece spindle. All parameters used for the flat surface grinding can remain the same (Milton et al., 2004a).


Figure 4 Oblique grinding method, (a) using the nano motion stage (b) using the micro motion stage

(a) (b)

Figure 5 (a) Grinding an oblique surface and (b) using oblique surfaces to approximate a curve (see online version for colours)

(a) (b)

Note: Dash-dot line shows the fibre axis.

3.2 Curved face and axisymmetric curved face grinding

3.2.1 Procedure

To produce curved profiles using flat tools on this machine, a number of geometric parameters must obtain. These are the location of the centre of rotation of the rotary stage, the perpendicular distance between the grinding surface and the centre of rotation of the rotary stage, the orientation of the grinding surface, the location of the origin of the optical fibre’s coordinate frame and the pixel resolution of the acquired image.


The tool surface location and the centre of rotation of the rotary stage can be obtained easily by imaging the back lit tool plane at three different orientations with incremental orientations known. The pixel resolution of the camera can be obtained by imaging an object of known size a number of times. The optical fibres are ideally placed to be imaged for this purpose. Imaging the optical fibre’s tip will also localise the local coordinate frame of the optical fibre. Having obtained all these parameters the tool path can be calculated. It is very important to notice that the tool path is not a simple off-set path from the desired micro profile as would have been the case with conventional CNC machining. Generally, there are multiple solutions for the tool path. However, as the tool path calculation is an offline process, the tool path that is best suited for the current machine configuration can be easily chosen. An example coarse tool path that is calculated in this manner is shown in Figure 6.

Figure 6 Continuous flat-face grinding (see online version for colours)

Note: Dash-dot line shows the fibre axis.

The curved-profile machining operation runs as follows:

1 Operation commences with the tool set to an initial position, its face perpendicular to the fibre’s axis.

2 The tool is then fed into the fibre some short distance. This creates a flat face at the end of the fibre, removing any burrs that might remain from fibre cleaving and ensuring high quality surface conditions before actual curved grinding begins. This is important, as burrs will sometimes concentrate stresses during grinding and cause fractures at the fibres edges.


3 Using the current position of the tool and the known parameters of the machine, a tool path is calculated based on a desired workpiece profile. This path is written to a script file.

4 The script file is executed, command by command, causing the tool to be fed through the calculated path using the linear and rotational motion stages. If axisymmetric grinding is being performed, the workpiece spindle will also be rotated accordingly.

The machining path is broken up into a number of ‘way-points’ that the tool passes through as it machines a curve. These way-points are shown as crosses in Figure 6. Tool motion is linear between each of these way-points, so a number are required to ensure that the resulting profile does not have flat segments. Determining the in-feed rates that occur when performing this operation is more complex than for oblique faces, as the machining is not taking place at all points evenly. Further, tool velocities are calculated only for the current way-point that is being machined. Points further along the profile may experience higher in-feed rates than the current way-point, as illustrated in Figure 7. As for oblique face grinding, the in-feed rate at the point of grinding is calculated by v cos α. Other points further along the profile will also be machined but with an increased in-feed rate of v cos α + Lω, where L is the distance from the current way-point to this other point and ω the rotational velocity of the tool between way-points. This was taken into account in tool path calculation and it was ensured that the in-feed velocity remained low enough for all points during a grinding operation to ensure ductile mode grinding and to prevent fracture.

Figure 7 Increase in in-feed rate due to tool rotation

4 Results

A variety of different profiles were machined on the nano-grinding machine. A number of flat surfaces are shown in Figure 8. These images were obtained while the workpieces are in-situ on the nano-grinding machine. SEM images of flat surfaces are shown in Figure 9. The stability of the grinding operation is clearly evident in the quality of the machined surfaces.


Figure 8 In-machine optical microscope images of the results of several oblique grinding experiments, (a) a single oblique face in profile and (b) observing the same face directly (c) a symmetric oblique faces forming a ‘wedge’ with a large included angle (150°) and (d) a wedge with a smaller included angle (80°) (e) and (f) two views of a three sided tetrahedral structure

(a) (b) (c)

(d) (e) (f)

Figure 9 SEM images of oblique profiles, (a) a 45° oblique surface machined half way into the fibre, and (b) a 50° oblique profile machined right through the fibre

(a) (b)


4.1 Conical profiles

The SEM images of conical grinding is shown in Figure 10. Once again the high surface quality of the conical profiles shown in the figure demonstrates the stability of the machining process.

Figure 10 Two cones produced using a flat grinding surface

4.2 Curves and axisymmetric profiles

Many curved and axisymmetric curved profiles were produced using the micro-grinding machine. Several examples of curved faces are shown in Figure 11, whilst axisymmetric curved faces can be seen in Figure 12. Figure 13 then show some SEM images of axisymmetric curved profiles.

Note that for the cases shown in Figure 11, the workpiece spindle was not rotated. The workpiece shown in (a) and (b) was ground with a desired radius of curvature of 40 μm, centred on the fibre’s axis. Image (a) shows a profile view of the face, clearly illustrating the curve, whilst (b) shows the face from an angle, illustrating that the surface quality is maintained across the entire curve. The images (c) and (d) show a face with a larger radius of curvature of 120 μm, still centred on the fibre’s axis. Finally images (e) and (f) show a workpiece ground with radius of curvature of 180 μm with the radius centred at the edge of the fibre rather than its axis.

In Figure 12, six different workpieces are shown. Note that in all the cases shown in this figure the fibre rotates around its axis. The curves on these workpieces are all centred on the fibre’s longitudinal axis and were produced with a range of values for the desired radius of curvature. It can be seen in these images that the axisymmetric curved profiles are highly symmetric and that the surfaces produced are of high quality.

Figure 13 shows SEM images of axisymmetric curved profiles. Figure 13(a) shows a 40 μm radius profile, while (b) shows detail of a curve and a surface from a fibre with an 80 μm radius profile.


Figure 11 Images of three workpieces with curved profiles taken in-machine using the optical microscope

(a) (b) (c)

(d) (e) (f)

Notes: Images (a) and (b) shows a centred 40 μm curve in profile and at an angle. Images (c) and (d) show a centred 120 μm curve and (e) and (f) show an 180 μm radius with the centre at the edge of the fibre.

Figure 12 Images of six workpieces with axisymmetric curved profiles taken in-machine using the optical microscope

(a) (b) (c)

(d) (e) (f)

Notes: The workpieces shown in (a) and (b) have a radius of curvature of 40 μm. Images (c), (d) and (e) show workpieces with radius of curvatures of 80 μm. Image (f) shows a workpiece with a radius of curvature of 180 μm.


Figure 13 SEM images of two axisymmetric curved profiles

(a) (b)

Note: Image (a) shows a workpiece with a 40 μm curve, whilst (b) shows a workpiece with a 90 μm profile.

4.3 Analysis of precision

4.3.1 Accuracy of machined profiles

To demonstrate the accuracy of profiles produced, ground fibres were analysed and their profiles were compared with desired profiles. This was performed using the edge detection process on the images being analysed. This is the same edge detection that has been used to centre the workpiece. It has a centring accuracy of 500 nm. Both the left and right edges of the fibre as well as the profile was measured. The desired profile was then fitted to the data using the left and right edges, and the peak value in the curve, to locate it.

Figure 14 shows how accurately the axisymmetric curved profiles pictured in Figure 12 compare with the desired profiles. In this figure, each graph (a) through (f) corresponds to image (a) through (f) in Figure 12. As can be seen, the results were consistently good. The maximum error was 3.5 μm in case (a) at an edge location away from the centre. For (a) and (b), the curve was within 2 μm of the desired profile near the centre region where profile accuracy is most important. In all other cases, the error was always less than 2 μm, with that value only being reached in a very limited edge region, and was less than 1 μm for the remainder of the profile.

Figure 15 shows the same analysis performed on the high resolution SEM images shown in Figures 13(a) and 13(b). Although the curve shown in Figure 13(a) appears very smooth, it is slightly off-centre on the fibre and consequently shows quite a high error as seen in Figure 15(a).


Figure 14 Profile measurements of the workpieces illustrated in Figure 12

(a) (b)

(c) (d)

(e) (f)

Note: Subfigures (a) through (f) correspond to the equivalent letters in that figure.


The accuracy requirements of the components machined depends on the application of the component. The errors in the axial direction are within the 0.5–1 μm range. The axisymmetric nature of the profiles can be seen by the axisymmetric nature of the error profiles. As far as the fibre lenses are concerned eccentricities in excess of 1–2 μm significantly downgrade the performance of the coupling efficiency of the optical fibres (Hillerich and Guttmann, 1989). As the axial and lateral symmetries are maintained well within these limits the machine and the described machining method is highly suitable for the fibre lens manufacture.

Figure 15 Profile measurements of two workpieces (a) from Figure 13(a) and (b) form Figure 13(b)

(a) (b)

5 Conclusions

This paper described a complex nano-grinding machine that allows the use of sophisticated tool path generation and its execution to fabricate micro profiles. The grinding parameters used are those that had been validated earlier. The results shown illustrate the potential of the micro-grinding system to fabricate not only the micro profiles, but also the millimetre scale components.

It has been shown that flat tool surfaces may be used to produce accurate micro-scale profiles consisting of oblique and curved surfaces with both repeatability and accuracy. Further, the images presented clearly illustrate that these profiles are produced with excellent surface quality. These results also serve to illustrate the versatility of the micro-grinding system with a wide range of profile types and profile sizes all being produced on the same machining system without the need for reconfiguration.


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fabrication of arbitrary precision micro profiles by nano-grinding

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