1. Diamond turning of contact lens polymer
3rd UNESCO AFRICA ENGINEERING WEEK – POSTGRADUATE RESEARCH POSTER
Diamond turning of contact lens polymer
M.M. Liman*, K. Abou-El-Hossein and A.I. Jumare
Precision Engineering Laboratory, Nelson Mandela Metropolitan University, Port Elizabeth, 6031 – South Africa.
2.0 Materials and methods
1.0 Introduction
Precitech Nanoform 250 Ultra-grind lathe (Fig. 1) was used to machine the ONSI-56 contact lens button (Fig. 2) with single-crystal single-point diamond cutting tool (Fig. 3). The workpiece is tested after each pass with Taylor Hobson optical profiler and Ra values recorded (Fig. 4).
Figure1: Precitech Nanoform 250 Ultra-grind lathe Fig. 2: ONSI-56 lens button (workpiece)
In the years since polymers’ introduction, contact lens technology has been increasing at a rapid rate. There have been improvements in manufacturing techniques as well as an increase in the type of polymers used in making lenses. Single point diamond turning (SPDT) is an ultra-precision machining (UPM) process for the production of high quality optical surfaces on metals, polymers and crystals. [1].
An SPDT process plays an important role in advanced manufacturing industries, especially in the field of optics, aerospace technology, information technology and clean energy [2]. The technique is regarded as an effective process for the generation of high-quality functional surfaces. It produces surfaces with minimal defects at nanometric surface roughness in the superficial surface layer from various materials especially from the thermoplastic amorphous polymers and their composition for optical, photonic and bioengineering applications [2, 3]. The nanometric surface roughness heavily reduces the requirement for time-consuming post-polishing operation. Today, contact lenses for vision correction can be manufactured via injection molding or lathe techniques (using SPDT) due to high precision and surface finish requirements.
However, despite advances in ultra-precision turning, it is not always easy to achieve a high quality surface finish. Lots of factors such as machine tool performance, cutting tool geometry, tool and workpiece material and machining process affect surface quality during turning operation [4]. Hence, the need for a method to determine the most appropriate combination of these factors that will produce the best surface quality with minimum tool wear [5, 6].
Surface quality is considered as one of the most important requirements for the customer and performance of the machined part. One of the critical measures for surface quality is the surface roughness and contact lens manufacture requires high accuracy and surface integrity. Surface roughness is widely used to measure the index quality of a turning process. It has been an important response due to its direct influence on the part’s performance, manufacturing cost and profit for the manufacturer [7]. Thus, the choice of optimum cutting parameters can improve not only the quality measure but also productivity of the entire process, yielding maximum material removal rates.
This research is therefore aimed at investigating the optimal cutting conditions of contact lens polymer machined with a single-crystal diamond cutting tool and developing a surface roughness prediction model to avoid the trial- and-error experimental approach.
3.0 Results
The statistical analysis shows that the model is quadratic. During the experiment, the best and worse average surface roughness were found to be 6 nm and nm respectively (Figures 5 & 6).
From the RSM, the quadratic polynomial model equation was developed to correlate the independent variables (machining parameters) with the surface roughness (dependent variable) as shown below:
In (Ra) = – F E-07S2 – E–05SF
Or Ra = exp( – F E–07S2 – E–05SF)
Where: F = Feedrate and S = Cutting speed. (i.e. depth of cut is of less significance on the surface quality of polymeric material).
The best Ra value of 6 nm was achieved, which greatly satisfies the requirement for optical quality and this surface quality level is much below the acceptable average surface roughness of 25 nm, as determined by SPI A-1 specification of the Society for the Plastic Industry [1].
The worse Ra = nm was achieved at low speed of 200 rpm and high feed rate of 12 µm/rev.
apter
Figure 2.1: Workpiece pressed on adapter
Figure 3: Diamond cutting tool centering
Figure 4: Surface roughness measurement
5.0 Conclusion
4.0 Plots
In this work, a novel surface roughness prediction model, in which the cutting parameters (feedrate, cutting speed and depth of cut) were successfully modelled and optimized using a BBD with RSM. The effects of feedrate, cutting speed and depth of cut were investigated. The ANOVA indicated that the proposed quadratic model successfully interpreted the experimental data with coefficients of determination, R2 = 0.89 and adjusted R2 = Through this model, the surface roughness can be predicted and controlled under different conditions. Cutting speed was found to be the most dominant factor during finish turning of ONSI-56 contact lens. Furthermore, the optimal conditions for the best surface roughness were found to be at the feedrate of 7 µm/rev, cutting speed of 2100 rpm and depth of cut of 25 µm. Whereas the worse Ra achieved at low speed and high feedrate is because ONSI-56 is a soft material, which results in broken chips formation that adhered to the surface of the workpiece during the machining process (fig. 7). Hence, for a soft polymer material (such as ONSI-56), cutting parameters combination of low cutting speed and high feed rate should be avoided for better surface finish.
Figure 5: Best surface roughness
Figure 6: Worse surface roughness
3D Optimization surface plot
Fig. 7: Chips stick to the workpiece
6.0 References
[1] Bolat, M Machining of Polycarbonate for Optical Applications, in the M.Sc. Mechanical Engineering Thesis, Graduate School of Natural and Applied Sciences, Middle East Technical University. pp
[2] Shore, P. & Morantz, P Ultra-precision: enabling our future. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 370,
[3] Mamalis, A. and Lavrynenko, S On the precision single-point diamond machining of polymeric materials. Journal of materials processing technology, 181,
[4] He, C., Zong, W. & Sun, T Origins for the size effect of surface roughness in diamond turning. International Journal of Machine Tools and Manufacture, 106,
[5] Zong, W.J., Huang, Y.H., Zhang, Y.L. and Sun, T Conservation law of surface roughness in single point diamond turning. International Journal of Machine Tools and Manufacture, 84:58-63.
[6] Yan, J., Asami, T. and Kuriyagawa, T Response of machining-damaged single-crystalline silicon wafers to nanosecond pulsed laser irradiation. Semiconductor Science and Technology, 22(4):
[7] Nalbant, M., Gökkaya, H. and Sur, G Application of Taguchi method in the optimization of cutting parameters for surface roughness in turning. Materials & design, 28(4):