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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 23 — Nov. 18, 2013
  • pp: 28469–28482
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Development of pseudo-random diamond turning method for fabricating freeform optics with scattering homogenization

Zhiwei Zhu, Xiaoqin Zhou, Dan Luo, and Qiang Liu  »View Author Affiliations


Optics Express, Vol. 21, Issue 23, pp. 28469-28482 (2013)
http://dx.doi.org/10.1364/OE.21.028469


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Abstract

In this paper, a novel pseudo-random diamond turning (PRDT) method is proposed for the fabrication of freeform optics with scattering homogenization by means of actively eliminating the inherent periodicity of the residual tool marks. The strategy for accurately determining the spiral toolpath with pseudo-random vibration modulation is deliberately explained. Spatial geometric calculation method is employed to determine the toolpath in consideration of cutting tool geometries, and an iteration algorithm is further introduced to enhance the computation efficiency. Moreover, a novel two degree of freedom fast tool servo (2-DOF FTS) system with decoupled motions is developed to implement the PRDT method. Taking advantage of a novel surface topography generation algorithm, theoretical surfaces generated by using the calculated toolpaths are obtained, the accuracy of the toolpath generation and the efficiency of the PRDT method for breaking up the inherent periodicity of tool marks are examined. A series of preliminary cutting experiments are carried out to verify the efficiency of the proposed PRDT method, the experimental results obtained are in good agreement with the results obtained by numerical simulation. In addition, the results of scattering experiments indicate that the proposed PRDT method will be a very promising technique to achieve the scattering homogenization of machined surfaces with complicated shapes.

© 2013 Optical Society of America

1. Introduction

As for the FTS assisted diamond turning process, the cutting tool will follow a spatial spiral trajectory with constant radial spacing, resulting in the residual tool marks left on machined surfaces with strong periodicity in high spatial frequency domain. Unfortunately, the repetitive feature with high spatial frequency will give rise to undesired scattering effects on machined surfaces, leading to degradation performance of machined optics such as imagining distortions, breakup of high power laser systems and so on [12

12. A. Kotha and J. E. Harvey, “Scattering effects of machined optical surfaces,” Proc. SPIE 2541, 54–65 (1995). [CrossRef]

14

14. J. Xu, F. Wang, Q. Shi, and Y. Deng, “Statistical measurement of mid-spatial frequency defects of large optics,” Meas. Sci. Technol. 23(6), 065201 (2012). [CrossRef]

]. During the last two decades, the scattering behavior of diamond turned surfaces have been mentioned and characterized, and the relationship between the surface texture and the scattering effects has also been investigated. By means of experiments, Prof. Yi and his coauthors have studied the influences of turning parameters on scattering effects of obtained surfaces. According to their results, the first order diffraction intensity caused by the periodic tool marks can significantly decrease when the tool mark spacing is small enough [15

15. L. Li, S. A. Collins Jr, and A. Y. Yi, “Optical effects of surface finish by ultraprecision single point diamond machining,” J. Manuf. Sci. Eng. 132(2), 021002 (2010). [CrossRef]

, 16

16. L. Li, “Investigation of the optical effects of single point diamond machined surfaces and the applications of micro machining,” Ph.D thesis, The Ohio State University (2009).

]. However, the machining efficiency should be disastrously decreased by adopting this method. For practical applications of these optics, certain laborious post-processing approaches, such as abrasive jet polishing, magneto-rheological finishing, bonnet polishing and so on, should be further employed to remove these periodic tool marks [17

17. P. Dumas, D. Golini, and M. Tricard, “Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF),” Proc. SPIE 5786, 296–304 (2005). [CrossRef]

20

20. A. Beaucamp and Y. Namba, “Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing,” CIRP Ann. 62(1), 315–318 (2013). [CrossRef]

]. However, these post-processing approaches will greatly decrease the efficiency and increase the cost of optics. Moreover, it is generally difficult to perfectly implement these approaches on surfaces with intricate structures. Facing this dilemma, a much better solution should be to develop certain novel machining methods which can actively disturb the inherent periodicity of tool marks during the cutting process, accordingly achieving the scattering homogenization of machined surfaces.

Motivated by this, a novel pseudo-random diamond turning (PRDT) method is proposed in this paper to eliminate the periodicity of the residual tool marks during the turning process without loss of efficiency and accuracy, accordingly achieving scattering homogenization. The toolpath generation strategy (TGS) is introduced, and the corresponding theoretical surfaces are obtained and characterized by a novel surface topography generation algorithm (STGA). To implement the PRDT method, a novel two degree of freedom FTS (2-DOF FTS) is introduced, which possesses totally decoupled motions along the radial and axial directions of the workpiece. Machining experiments on flat and typical micro-structured surfaces are carried out to validate the effectiveness of the proposed PRDT method for fabricating surfaces with periodicity elimination and scattering homogenization.

2. Working principle of the PRDT method

Based on Harvey-Shack’s scattering theory, the scattering intensity of a rough surface will be proportional to the power spectral density (PSD) function of the micro-textures on the surfaces [12

12. A. Kotha and J. E. Harvey, “Scattering effects of machined optical surfaces,” Proc. SPIE 2541, 54–65 (1995). [CrossRef]

, 21

21. A. Krywonos, J. E. Harvey, and N. Choi, “Linear systems formulation of scattering theory for rough surfaces with arbitrary incident and scattering angles,” J. Opt. Soc. Am. A 28(6), 1121–1138 (2011). [CrossRef] [PubMed]

]. It suggests that the scattering effects can be well homogenized by actively eliminating the periodicity of residual tool marks. Thus, one direct way is to actively adjust motions of the cutting tool with pseudo-random vibration modulation (PRVM) along the radial direction during the cutting process. As for conventional FTS, it has but one degree of freedom (DOF) of motion which is parallel to the C-axis of the lathe, without the capability to actively change motions of the tool in the XZ plane. Although a novel two DOF slow slide servo (S3) technique has recently been introduced [22

22. E. Brinksmeier and W. Preuss, “How to diamond turn an elliptic half-shell?” Precis. Eng. 37(4), 944–947 (2013). [CrossRef]

], it will not be a good choice for modifying surface micro-textures in real time due to its relatively low operating bandwidth.

Motivated by this, a novel 2-DOF FTS based diamond turning technique is proposed and the configuration is illustrated in Fig. 1.
Fig. 1 Working principle of the PRDT method (a) Axis configuration of turning machine; (b) Schematic of the motions of the cutting tool.
As shown in Fig. 1, the machining system mainly consists of three key components, namely: a spindle to hold and rotate the workpiece, a X-axis and a Z-axis linear servo system to move the slide carriage of the lathe, and a 2-DOF FTS system mounted on the carriage serving as two auxiliary fast translation axes along the X- and Z-directions of the lathe. As for the fast motion of the 2-DOF FTS along the Z-axis, it can be regarded as the same one of conventional FTS, while the fast motion along the X-axis is especially adopted to actuate the cutting tool following pseudo-random motions along radial direction to break up the periodicity of the tool marks as shown in Fig. 1(b). Generally, the actual tool location position (TLP) is highly depended on tool contact position (TCP) for creating complex surfaces [23

23. F. Z. Fang, X. D. Zhang, and X. T. Hu, “Cylindrical coordinate machining of optical freeform surfaces,” Opt. Express 16(10), 7323–7329 (2008). [CrossRef] [PubMed]

, 24

24. D. Yu, Y. Wong, and G. Hong, “Optimal selection of machining parameters for fast tool servo diamond turning,” Int. J. Adv. Manuf. Technol. 57(1–4), 85–99 (2011). [CrossRef]

]. So, as is illustrated in Fig. 1(b), the TCP variations induced by PRVM motions along the X-axis should be compensated along the Z-axis to avoid machining errors. Characterized by PRVM motions of the cutting tool, the proposed PRDT method can be constructed.

3. Actual TGS for the PRDT

3.1 Description of the cutting tool

The coordinate systems of the workpiece and cutting tool during turning process are illustrated in Fig. 2, where oWxWyWzW and oTxTyTzT denote the local Cartesian coordinate system of the workpiece and the cutting tool, respectively.
Fig. 2 Schematic of toolpath generation.
In the local coordinate system of the workpiece, the desired surface can be expressed as zW=f(x,Wy)WW. However, the turning process can be naturally expressed in cylindrical coordinates (ρ,ϕ,z) [23

23. F. Z. Fang, X. D. Zhang, and X. T. Hu, “Cylindrical coordinate machining of optical freeform surfaces,” Opt. Express 16(10), 7323–7329 (2008). [CrossRef] [PubMed]

25

25. H. Gong, F. Fang, and X. Hu, “Accurate spiral tool path generation of ultraprecision three-axis turning for non-zero rake angle using symbolic computation,” Int. J. Adv. Manuf. Technol. 58(9–12), 841–847 (2012). [CrossRef]

]. Taking the following conversions: xW=ρcWosφW, yW=ρsWinφW, the desired surface can also be expressed as zW=g(ρ,Wφ)WW. In Cartesian coordinate system, the normal vector at any given point (xs,ys,zs) can be expressed as

VW=(fWx,fWy,1)|x=xs,y=ys
(1)

Assume that an arc-edged cutting tool with nose radius RT and rake angle γ0 is used, as shown in Fig. 2, it can be better modeled in its local spherical coordinates (Rt,γ,θ), where it can be expressed as
{xT=RTcosθyT=RTsinθsinγzT=RTsinθcosγ,θ[θmin,θmax]
(2)
where θmin and θmax denote the lower and upper angle boundary of the cutting edge, respectively.

To conduct numerical calculation, the cutting edge is uniformly discretized into N0 + 1 points. The i-th point in its local coordinate is represented as (xiT,yiT,ziT), the corresponding tangent vector can be obtained by

{Ti=(xTθ,yTθ,zTθ)|θ=θi,γ=γ0θi=θmin+(i1)(θmaxθmin)N0
(3)

3.2 Determination of the TCP

Similar to the discretization of the cutting edge, the rotational angle of the spindle is also uniformly discretized into Ns points. As for the l-th point in the k-th revolution of spindle, if ρk,l0, then it can be expressed in the cylindrical coordinate by
{φk,l=2π(k+l/Ns)ρk,l=ρmaxφk,lf0/2π+δk,l
(4)
where ρmax denotes the aperture of the desired surface, f0 denotes the constant feedrate per revolution of the 2-DOF FTS system driven by the lathe; δk,l represents the transient value of the PRVM. From Eq. (4), it can be observed that ρk,l is comprised of two terms, namely the smooth term (ST) and the PRVM term. Generally, the ST represents the conventional feeding motion along the radial direction, which will be executed by the lathe. The PRVM term will then be executed by the specified 2-DOF FTS system, constructing the PRDT method.

However, limited by dynamics of 2-DOF FTS system, the cutting tool cannot follow real pseudo-random motions. So, the generated pseudo-random sequence will be preprocessed by passing through a low-pass filter where the cut-off frequency is specially selected with consideration of the working bandwidth of the 2-DOF FTS system. In addition, the preprocessed sequence should be further scaled into the desired moving range
δk,l=Kf0(0.5Rk,l'),Rk,l'[0,1]
(5)
where Rk,l' represents the (kNs + l-Ns)-th number in the preprocessed pseudo-random sequence, and K is the scale gain to adjust the range of the sequence.

By means of the rotation transformation of vector, the point PiT(xiT,yiT,ziT) on the cutting edge in the local coordinate of the workpiece yields

[xi(k,l)yi(k,l)zi(k,l)]=[cosφk,lsinφk,l0sinφk,lcosφk,l0001][xiT+Δρk,lyiTziT]
(6)
Δρk,l=ρmaxρk,l
(7)

The corresponding projection of this point on the desired surface PiS(xi(k,l),yi(k,l),zS,i(k,l)) is

zS,i(k,l)=f(xi(k,l),yi(k,l))W
(8)

Substituting (xi(k,l),yi(k,l),zS,i(k,l)) into Eq. (1), the normal vector can be obtained by

Vi(k,l)=(fWx,fWy,1)|x=xi(k,l),y=yi(k,l)
(9)

How to determine the TCP can be reduced to solving a position, at which the cutting edge is tangential to the desired freeform surface. Mathematically, it is to find a proper point on the cutting edge where its tangent vector is perpendicular to the normal vector of its projection point on the desired surface [25

25. H. Gong, F. Fang, and X. Hu, “Accurate spiral tool path generation of ultraprecision three-axis turning for non-zero rake angle using symbolic computation,” Int. J. Adv. Manuf. Technol. 58(9–12), 841–847 (2012). [CrossRef]

]. As for discrete calculation, the TCP can be determined by the following approximation:

PC(k,l):=argPiTmin{|Vi(k,l)Ti|,i}
(10)

3.3 An iteration strategy for tool path generation

As is evident from Eq. (10), the calculation accuracy will highly depend on the sample size of the cutting edge, namely Ns. Larger Ns will lead to more accurate TCPs, just at expense of much lower computation efficiency. As for the material removal of continuous surface, the arbitrary neighborhood TCPs will also possess continuous features. So, an effective zone containing the l-th TCP can be defined according to the (l-1)-th TCP. With the assumption that the proper index i of the (l-1)-th TCP is m(k,l-1), an iteration strategy for the determination of the l-th TCP can be expressed by
{PC(k,l):=argPiTmin{|Vi(k,l)Ti|}i[m(k,l1)n0,m(k,l1)+n0]
(11)
where n0 is a user defined integer determining the size of the effective zone. Taking advantage of the iteration strategy, computational consumptions for the toolpath generation will be decreased significantly.

As shown in Fig. 2, the TLP PL(k,l) can further be obtained:

{x(k,l)=ρck,losk,ly(k,l)=ρsk,link,lz(k,l)=|zm(k,l)T|+zS,m(k,l)(k,l)
(12)

Following through all the k and l by repeating the aforementioned steps, the toolpath for the whole surface can be well generated.

4. Numerical investigation into the machined surface

4.1 Topography generation of the machined surface

Generally, there are two crucial folds determining performance of the developed TGS for the PRDT method, namely the forming accuracy and the periodicity elimination capacity. To theoretically examine the efficiency of the TGS, modeling the machined surface topography will be a crucial issue. In this paper, a novel STGA is proposed here, and the main procedure can be summarized as follows.

Step 2. Let Pm,nG be one of the grid nodes in this zone. The corresponding TLP PLIn can be obtained according to Pm,nG based on the linear interpolation method. Then the Euler distance d=Pm,nGPLIn between the two points can be obtained.

If dRT, Pm,nG will be the point to be removed; otherwise, it should be reserved. If Pm,nG is the point to be removed, the corresponding z-coordinate should be adjusted to the z-coordinate on the tool nose.

Step 3. Following through all the TCPs by repeating the steps (1) and (2), the machined surface topography will be generated.

4.2 Characteristics of the machined surface

To examine the form accuracy and the periodicity of surface textures, a typical sinusoidal grid surface with z(x,y) = 0.005sin(2πx) + 0.005cos(2πy) mm is employed as the desired surface. The radius of the workpiece is 3 mm. The nose radius of the cutting tool is 0.5 mm, and the nominal rake angle is 0°. The feedrate of the carriage along the x-axis direction is set as 50 μm/rev. During the toolpath calculation, the discretization sampling numbers Ns is set as 720. As for the PRDT process, the pseudo-random vibrations along the x-axis are within ± 40 μm, and the cutoff frequency of the filter for preprocessing is set as 100 Hz. The obtained toolpaths for the conventional turning (CT) method and the PRDT method are illustrated in Figs. 4(a) and 4(b), respectively.
Fig. 4 Toolpaths of sinusoidal grid surfaces for (a) the CT method, and (b) the PRDT method.
By conducting the STGA, the machined surfaces with respect to the two generated toolpaths are obtained and illustrated in Figs. 5(a) and 5(b), and the corresponding error maps are shown in Figs. 5(c) and 5(d).
Fig. 5 Characteristics of machined surfaces (a) Surface topography generated by the CT method; (b) Surface topography generated by the PRDT method; (c) Error map generated by the CT method; (d) Error map generated by the PRDT method; (e) Features of the extracted 2-D profile of the surface generated by the CT method; (f) Features of the extracted 2-D profile of the surface generated by the PRDT method.
To clarify the periodicity of machined surfaces, two-dimensional (2-D) profiles along the x-axis and through the origin are extracted and characterized by the PSD function. The extracted profiles and the corresponding PSDs of the two generated surfaces are illustrated in Figs. 5(e) and 5(f).

As shown in Fig. 4(a), the toolpath of the CT method is the standard spatial spiral with constant spacing along the radial direction, while that for the PRDT can be regarded as the PRVM of the conventional toolpath, resulting in irregular spacing along the radial direction at any TLP as shown in Fig. 4(b). To generate the surface, the required TCPs of the CT method and the PRDT method are 51912 and 51516, respectively. The approximately identical numbers of TCPs indicate that there are no significant differences between the cutting efficiencies of the two methods.

From the generated surfaces shown in Figs. 5(a) and 5(b), the sinusoidal grid profile generated by the CT method is much smoother than that generated by the PRDT method, attributing to the constant feedrate of the CT process. The errors shown in Figs. 5(c) and 5(d) are just the residual tool marks, and no tendency form errors can be observed, well demonstrating the accuracy of the generated toolpaths. However, by adopting smaller feedrate and more discretization numbers Ns, the heights of the residuals can be further cut down. In addition, non-uniform distributions of residual heights can also be observed in Fig. 5(c), which may be caused by the non-uniform curvatures of the desired surfaces along both cutting and feeding directions. All these observations demonstrate the efficiency of the proposed TGS and the STGA.

From the resulted error topographies of machined surface, as shown in Figs. 5(c) and 5(d), regular tool marks are observed on the surface generated by the CT method, while irregular features with randomly distributed height and spacing can be observed on the surface generated by the PRDT method. More specified features of the two surfaces are characterized in Figs. 5(e) and 5(f). As shown in Fig. 5(e), the waviness of the residual tool marks along the x-axis is of strong periodicity with nearly constant amplitude, the location of the peak (f = 20 mm−1) in the PSD diagram indicates that the periodicity is caused by the constant feedrate of the cutting tool along the radial direction. As for the profile shown in Fig. 5(f), the variable spacing between any two adjacent peaks and the variable values of the peaks depict a disordered profile. The corresponding PSD diagram shows that the peak in the location f = 20 mm−1 is well eliminated, replaced by several chaotic peaks located in a wider domain with much lower frequencies. The results demonstrate that the PRDT method can effectively eliminate the inherent periodicity of the residual tool marks.

5. Development of the novel 2-DOF FTS system

A piezoelectrically actuated 2-DOF flexural mechanism with parallel configuration is developed for activating the cutting tool, schematic of the developed mechanism is illustrated in Fig. 6.
Fig. 6 Schematic of the designed 2-DOF FTS mechanism. 1. The base; 2. The right circle flexure hinge; 3. Piezoelectric actuator; 4. The cutting tool; 5. The Z-shaped flexure hinge.
As shown in Fig. 6, the mechanism consists of three key components, namely the two symmetric driving units (DUs) A and B, and the Z-shaped flexure guidance unit (ZFGU). The DU is a typical structure with a group of parallel and symmetric right circle flexure hinges. The ZFGU is constructed by a group of parallel and symmetric Z-shaped flexure hinges (ZFHs) and a platform holding the cutting tool, details of the structure and the moving principle of the ZFGU is illustrated in Fig. 7.
Fig. 7 Schematic of the ZFGU mechanism.
As shown in Fig. 7, when the two DUs push the two ends of the ZFGU, long beams of the ZFHs cannot shrink straightly, rather, they will bend to accommodate the space decrease, resulting in motions of the platform along the z-axis. Simultaneously, the difference between the driving forces of the two DUs will induce motions of the platform along the x-axis direction, obeying the well-known differential moving principle (DMP). Thus, two-DOF decoupled motions of the platform can be achieved, accompanying with a superior advantage that each actuator may contribute to motions along both directions.

Practically, the actuators are embedded into the linkage platforms of the ZFGU to achieve impact structure sizes. Capacity transducers (Micro-sense II 5300) are chosen for dynamic position measurements of the cutting tool. Two piezoelectric stack actuators (Polytec PI, Inc., Karlsruhe, Germany) are employed for the mechanism. The amplifier module PI E-617 with a nominal amplification factor 10 ± 0.1 is chosen to amplify the driving signal of the actuators. The measured displacement signals are gathered through the Power PMAC control card sampled at 0.45 ms, which is also used for closed-loop control of the system. Taking advantage of the decoupled motions in the two moving directions, the system is regarded as two single input and single output (SISO) sub-systems. A simple proportional, integral and derivative (PID) controller is developed for the SISO system. To reject vibrations and external noises with high frequencies, a simple low pass filter is also embedded in the feedback control loop. Additionally, a velocity feedforward compensation loop is also utilized to enhance tracking accuracies.

By constructing performance testing experiments, the stroke of the mechanism along the z-axis can reach up to 27.03 μm with a resolution of 14 nm, and bi-directional motions are achieved along the x-axis ranging from −8.379 μm to 7.544 μm with a resolution of 8 nm. The working bandwidth along the two directions can both reach up to 200 Hz by implementing sweep excitation tests. The obtained performance demonstrates that the developed 2-DOF FTS system is suitable for micro/nano machining of complicated surfaces.

6. Preliminary machining results and discussions

The newly developed 2-DOF FTS system is integrated into a Spinner SB/C-TMC precision lathe as shown in Fig. 8(a), front view and back view of the developed 2-DOF FTS mechanism are illustrated in Figs. 8(b) and 8(c), respectively.
Fig. 8 Configuration of the hardware of the PRDT system (a) The machining system; (b) Front view of the 2-DOF FTS; (c) Back view of the 2-DOF FTS. (1. The 2-DOF FTS mechanism; 2. The workpiece; 3. Spindle of the lathe; 4. Machine bed; 5. Slide carriage of the lathe; 6. The cover; 7. The PEA; 8. Height adjustment screws; 9. Flexure mechanism for height adjustment; 10. Preloading screws of the PEAs; 11. The diamond cutting tool; 12. Probes of capacity transducers; 13. Connection part for displacement measurement; 14. The base; 15. Fastening screws).
The cutting tool is a synthetic polycrystalline diamond (PCD) with the nose radius of 0.4 mm, nominal rake angle of 0°, clearance angle of 15°. The Zr-based bulk metallic glasses (BMG) is employed as the workpiece. During the cutting process, half of the region of the work surface is generated by conventional turning method and the other half is generated by the PRDT method to guarantee the identity of the cutting process. A flat surface and a typical micro-structured surface with sinusoidal wave along the radial direction (SWR) [11

11. D. Yu, Y. Wong, and G. Hong, “Ultraprecision machining of micro-structured functional surfaces on brittle materials,” J. Micromech. Microeng. 21(9), 095011 (2011). [CrossRef]

] are machined, and scattering features are examined by a simple testing method as shown in Fig. 9.
Fig. 9 Principle of scattering tests.
During the scattering testing process, the workpiece is fixed on a holder and will be irradiated by a laser beam with proper incidence angles. The reflect lights will irradiate on a screen with certain scattering fringes. If energies of the scattering fringes are weaken or the fringes are gone, then it suggests that surfaces with scattering homogenizations are obtained.

6.1 Flat surface machining

As for the flat surface machining, the spindle speed and the feedrate of the slide carriage are chosen to be r = 60 rev/min and f0 = 5 μm/rev, respectively. The pseudo-random vibrations of the cutting tool along the X-axis are within ± 4.5 μm. The nominal cutting depth is 10 μm. Trajectories of the cutting tool along the two directions and the tracking performance of the 2-DOF FTS system are shown in Fig. 10.
Fig. 10 Motions of the cutting tool. The upper and lower red lines denote the desired trajectories along the z and the x-axis, respectively. The black line and the blue line denote the practical motions of the cutting tool along the z and the x-axis, respectively.
Overall, the cutting tool can well follow the desired pseudo-random trajectories. Simultaneously, small vibrations can also be observed along the Z-axis which may be excited by the cutting forces. To avoid the undesired vibrations, more sophisticated control strategies with high robustness and disturbance rejection capacities should be further developed in the future.

As for the scattering testing process, the wavelength of the laser beam is 632.8 nm and the diameter of the beam is about 700 μm. From the scattering results shown in Fig. 11, it can be obviously observed that the reflected light from zone A which is machined by the CT method will induce strong scattering fringes on the screen.
Fig. 11 Observed scattering effects of the flat surface.
While the reflected light from zone B which is machined by the developed PRDT method will well focus on a spot, verifying that the PRDT method can well homogenize the scattering effects of the turned surfaces.

6.2 Machining of typical micro-structured surfaces

A typical SWR surface with 500 μm wavelength and 4 μm amplitude is fabricated on the BMG material to investigate the generation and scattering homogenization capacity of the PRDT method for complex surfaces. The spindle speed and the feedrate of the slide carriage are chosen to be r = 30 rev/min and f0 = 5 μm/rev, respectively. The pseudo-random vibrations are also within ± 4.5 μm, and the nominal cutting depth is 15 μm.

The obtained micro-topographies generated by the CT method and the PRDT method are captured by OLYMPUS OLS3000 and illustrated in Figs. 12(a) and 12(b), respectively.
Fig. 12 Micro-topographies on machined surfaces generated by (a) The CT method, and (b) The PRDT method.
As shown in Fig. 12, surface generated by the CT method possesses regular structures with obvious periodicity, while the surface generated by the PRDT method presents non-regular behaviors along the radial direction. The roughness of the two machined surfaces are Sa = 28.2 nm and Sa = 49.1 nm, respectively. In view of the vibrations along the X-axis direction, the maximum feedrate between the l-th cutting point of the k-th and the (k + 1)-th revolution can reach up to 19 μm. If it happens, the height of local residual tool marks will be much higher than that of the CT method. Thus, it is reasonable that the roughness of the machined surface generated by the PRDT method is about double of that generated by the CT method. However, it should be noticed that the relative large roughness maybe partially caused by undesired vibrations along the z-axis. Thus, by using sophisticated tool positioning strategy with high robustness and disturbance rejection capacities, surfaces with optical qualities can be achieved.

To have more specified view of machined surface properties, the scanned 2D profiles along three arbitrary lines, which are all parallel to the x-axis of the measurement system shown in Fig. 12, are extracted and then characterized by PSD function, as shown in Figs. 13(a) and 13(b).
Fig. 13 The extracted 2D profiles and the corresponding PSD features of surfaces generated by (a) The CT method, and (b) The PRDT method.
During the PSD analysis process, the trend components of the profiles are removed by polynomial fitting. As shown in Fig. 13(a), the profiles generated by the CT method appear to be regular with an identical spatial frequency about 0.2 μm−1, which corresponds to the feeding of cutting tool. From the profiles shown in Fig. 13(b), irregular residuals can be observed. Comparing with the PSD results shown in Fig. 13(a), the frequency component caused by the tool feeding disappears, while the components with much lower frequencies are strengthened. The experiment results show a good agreement with the numerical simulation results, demonstrating the efficiency of the proposed PRDT method for actively eliminating the periodicity of surface textures.

The scattering effects of the obtained surface are further explored by the aforementioned testing method, the results obtained are shown in Fig. 14.
Fig. 14 Scattering effects of machined surface.
It can be obviously observed that the reflected light from zone A which is machined by the CT method will induce strong scattering fringes on the screen. While the reflected light from zone B which is machined by the proposed PRDT method will well focus on a narrow ribbons with no dissipation fringes. These qualitative observations well demonstrate the efficiency of the developed PRDT method for fabricating complicated surfaces with scattering homogenizations.

7. Conclusions

In this paper, a novel pseudo-random diamond turning (PRDT) method is proposed to fabricate freeform optics with scattering homogenization by means of actively eliminating the inherent periodicity of residual tool marks. The strategy for accurately determining the spiral toolpath with pseudo-random vibration modulation (PRVM) is deliberately explained. A novel two degree of freedom fast tool servo (2-DOF FTS) system with decoupled motions is introduced to implement the PRDT method. The main conclusions can be summarized as follows.

  • (1). It is more convenient to represent the the cutting tool geometries in spherical coordinate frame. The issue of accurately determining the tool contact point can be reduced to solving a proper point on the cutting edge where its tangent vector is perpendicular to the normal vector of the same point on the desired machined surface. With the assumption of the continuation of the cutting process, the toolpath computation efficiency can be significantly improved by adopting the proposed iteration strategy.
  • (2). A novel topography generation algorithm (TGA) is introduced to theoretically obtain the machined surface. Numerical simulation of generating a sinusoidal grid surface well demonstrates the accuracy of the toolpath generation strategy. Besides, the results also indicate that the periodicity of the residual tool marks induced by tool feeding can be well eliminated by adopting the PRVM strategy.
  • (3). A series of preliminary machining experiments are conducted. The results indicate that the PRDT method can effectively break up the inherent periodicity of the residual tool marks. The PSD features of the profiles of the machined surface are in good agreement with the numerical simulation results, demonstrating the efficiency of both the TGA and the PRDT method. In addition, the scattering testing results demonstrate that it is very promising for the PRDT method to suppress scattering fringes of machined surfaces with complicated shapes, accordingly achieving scattering homogenization.

Acknowledgments

The authors are grateful to the financial support from the National Natural Science Foundation of China (50775099, 51075041, 51175221), and the Department of Science and Technology of Jilin Province (20130522155JH).

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C. Brecher, S. Lange, M. Merz, F. Niehaus, C. Wenzel, M. Winterschladen, and M. Weck, “NURBS based ultra-precision free-form machining,” CIRP Ann. 55(1), 547–550 (2006). [CrossRef]

3.

E. Brinksmeier and W. Preuss, “Micro-machining,” Philos. Trans. A Math. Phys. Eng. Sci. 370(1973), 3973–3992 (2012). [CrossRef] [PubMed]

4.

S. Rakuff and J. F. Cuttino, “Design and testing of a long-range, precision fast tool servo system for diamond turning,” Precis. Eng. 33(1), 18–25 (2009). [CrossRef]

5.

S. J. Ludwick, D. A. Chargin, J. A. Calzaretta, and D. L. Trumper, “Design of a rotary fast tool servo for ophthalmic lens fabrication,” Precis. Eng. 23(4), 253–259 (1999). [CrossRef]

6.

Z. Zhu, X. Zhou, Q. Liu, J. Lin, and S. Zhao, “Fabrication of micro-structured surfaces on bulk metallic glasses based on fast tool servo assisted diamond turning,” Sci. Adv. Mater. 4(9), 906–911 (2012). [CrossRef]

7.

S. Scheiding, A. Y. Yi, A. Gebhardt, L. Li, S. Risse, R. Eberhardt, and A. Tünnermann, “Freeform manufacturing of a microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo,” Opt. Express 19(24), 23938–23951 (2011). [CrossRef] [PubMed]

8.

E. Brinksmeier, O. Riemer, R. Gläbe, B. Lünemann, C. Kopylow, C. Dankwart, and A. Meier, “Submicron functional surfaces generated by diamond machining,” CIRP Ann. 59(1), 535–538 (2010). [CrossRef]

9.

X.-D. Lu and D. L. Trumper, “Ultrafast tool servos for diamond turning,” CIRP Ann. 54(1), 383–388 (2005). [CrossRef]

10.

Y. Nie, F. Fang, and X. Zhang, “System design of Maxwell force driving fast tool servos based on model analysis,” Int. J. Adv. Manuf. Technol., doi: (2013). [CrossRef]

11.

D. Yu, Y. Wong, and G. Hong, “Ultraprecision machining of micro-structured functional surfaces on brittle materials,” J. Micromech. Microeng. 21(9), 095011 (2011). [CrossRef]

12.

A. Kotha and J. E. Harvey, “Scattering effects of machined optical surfaces,” Proc. SPIE 2541, 54–65 (1995). [CrossRef]

13.

J. M. Tamkin and T. D. Milster, “Effects of structured mid-spatial frequency surface errors on image performance,” Appl. Opt. 49(33), 6522–6536 (2010). [CrossRef] [PubMed]

14.

J. Xu, F. Wang, Q. Shi, and Y. Deng, “Statistical measurement of mid-spatial frequency defects of large optics,” Meas. Sci. Technol. 23(6), 065201 (2012). [CrossRef]

15.

L. Li, S. A. Collins Jr, and A. Y. Yi, “Optical effects of surface finish by ultraprecision single point diamond machining,” J. Manuf. Sci. Eng. 132(2), 021002 (2010). [CrossRef]

16.

L. Li, “Investigation of the optical effects of single point diamond machined surfaces and the applications of micro machining,” Ph.D thesis, The Ohio State University (2009).

17.

P. Dumas, D. Golini, and M. Tricard, “Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF),” Proc. SPIE 5786, 296–304 (2005). [CrossRef]

18.

Z. Z. Li, J. M. Wang, X. Q. Peng, L. T. Ho, Z. Q. Yin, S. Y. Li, and C. F. Cheung, “Removal of single point diamond-turning marks by abrasive jet polishing,” Appl. Opt. 50(16), 2458–2463 (2011). [CrossRef] [PubMed]

19.

A. Beaucamp, R. Freeman, R. Morton, K. Ponudurai, and D. Walker, “Removal of diamond-turning signatures on x-ray mandrels and metal optics by fluid-jet polishing,” Proc. SPIE 7018, 701835 (2008). [CrossRef]

20.

A. Beaucamp and Y. Namba, “Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing,” CIRP Ann. 62(1), 315–318 (2013). [CrossRef]

21.

A. Krywonos, J. E. Harvey, and N. Choi, “Linear systems formulation of scattering theory for rough surfaces with arbitrary incident and scattering angles,” J. Opt. Soc. Am. A 28(6), 1121–1138 (2011). [CrossRef] [PubMed]

22.

E. Brinksmeier and W. Preuss, “How to diamond turn an elliptic half-shell?” Precis. Eng. 37(4), 944–947 (2013). [CrossRef]

23.

F. Z. Fang, X. D. Zhang, and X. T. Hu, “Cylindrical coordinate machining of optical freeform surfaces,” Opt. Express 16(10), 7323–7329 (2008). [CrossRef] [PubMed]

24.

D. Yu, Y. Wong, and G. Hong, “Optimal selection of machining parameters for fast tool servo diamond turning,” Int. J. Adv. Manuf. Technol. 57(1–4), 85–99 (2011). [CrossRef]

25.

H. Gong, F. Fang, and X. Hu, “Accurate spiral tool path generation of ultraprecision three-axis turning for non-zero rake angle using symbolic computation,” Int. J. Adv. Manuf. Technol. 58(9–12), 841–847 (2012). [CrossRef]

OCIS Codes
(220.1920) Optical design and fabrication : Diamond machining
(240.6700) Optics at surfaces : Surfaces

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: August 22, 2013
Revised Manuscript: October 28, 2013
Manuscript Accepted: October 29, 2013
Published: November 12, 2013

Citation
Zhiwei Zhu, Xiaoqin Zhou, Dan Luo, and Qiang Liu, "Development of pseudo-random diamond turning method for fabricating freeform optics with scattering homogenization," Opt. Express 21, 28469-28482 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-23-28469


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References

  1. W. Gao, T. Araki, S. Kiyono, Y. Okazaki, and M. Yamanaka, “Precision nano-fabrication and evaluation of a large area sinusoidal grid surface for a surface encoder,” Precis. Eng.27(3), 289–298 (2003). [CrossRef]
  2. C. Brecher, S. Lange, M. Merz, F. Niehaus, C. Wenzel, M. Winterschladen, and M. Weck, “NURBS based ultra-precision free-form machining,” CIRP Ann.55(1), 547–550 (2006). [CrossRef]
  3. E. Brinksmeier and W. Preuss, “Micro-machining,” Philos. Trans. A Math. Phys. Eng. Sci.370(1973), 3973–3992 (2012). [CrossRef] [PubMed]
  4. S. Rakuff and J. F. Cuttino, “Design and testing of a long-range, precision fast tool servo system for diamond turning,” Precis. Eng.33(1), 18–25 (2009). [CrossRef]
  5. S. J. Ludwick, D. A. Chargin, J. A. Calzaretta, and D. L. Trumper, “Design of a rotary fast tool servo for ophthalmic lens fabrication,” Precis. Eng.23(4), 253–259 (1999). [CrossRef]
  6. Z. Zhu, X. Zhou, Q. Liu, J. Lin, and S. Zhao, “Fabrication of micro-structured surfaces on bulk metallic glasses based on fast tool servo assisted diamond turning,” Sci. Adv. Mater.4(9), 906–911 (2012). [CrossRef]
  7. S. Scheiding, A. Y. Yi, A. Gebhardt, L. Li, S. Risse, R. Eberhardt, and A. Tünnermann, “Freeform manufacturing of a microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo,” Opt. Express19(24), 23938–23951 (2011). [CrossRef] [PubMed]
  8. E. Brinksmeier, O. Riemer, R. Gläbe, B. Lünemann, C. Kopylow, C. Dankwart, and A. Meier, “Submicron functional surfaces generated by diamond machining,” CIRP Ann.59(1), 535–538 (2010). [CrossRef]
  9. X.-D. Lu and D. L. Trumper, “Ultrafast tool servos for diamond turning,” CIRP Ann.54(1), 383–388 (2005). [CrossRef]
  10. Y. Nie, F. Fang, and X. Zhang, “System design of Maxwell force driving fast tool servos based on model analysis,” Int. J. Adv. Manuf. Technol., doi: (2013). [CrossRef]
  11. D. Yu, Y. Wong, and G. Hong, “Ultraprecision machining of micro-structured functional surfaces on brittle materials,” J. Micromech. Microeng.21(9), 095011 (2011). [CrossRef]
  12. A. Kotha and J. E. Harvey, “Scattering effects of machined optical surfaces,” Proc. SPIE2541, 54–65 (1995). [CrossRef]
  13. J. M. Tamkin and T. D. Milster, “Effects of structured mid-spatial frequency surface errors on image performance,” Appl. Opt.49(33), 6522–6536 (2010). [CrossRef] [PubMed]
  14. J. Xu, F. Wang, Q. Shi, and Y. Deng, “Statistical measurement of mid-spatial frequency defects of large optics,” Meas. Sci. Technol.23(6), 065201 (2012). [CrossRef]
  15. L. Li, S. A. Collins, and A. Y. Yi, “Optical effects of surface finish by ultraprecision single point diamond machining,” J. Manuf. Sci. Eng.132(2), 021002 (2010). [CrossRef]
  16. L. Li, “Investigation of the optical effects of single point diamond machined surfaces and the applications of micro machining,” Ph.D thesis, The Ohio State University (2009).
  17. P. Dumas, D. Golini, and M. Tricard, “Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF),” Proc. SPIE5786, 296–304 (2005). [CrossRef]
  18. Z. Z. Li, J. M. Wang, X. Q. Peng, L. T. Ho, Z. Q. Yin, S. Y. Li, and C. F. Cheung, “Removal of single point diamond-turning marks by abrasive jet polishing,” Appl. Opt.50(16), 2458–2463 (2011). [CrossRef] [PubMed]
  19. A. Beaucamp, R. Freeman, R. Morton, K. Ponudurai, and D. Walker, “Removal of diamond-turning signatures on x-ray mandrels and metal optics by fluid-jet polishing,” Proc. SPIE7018, 701835 (2008). [CrossRef]
  20. A. Beaucamp and Y. Namba, “Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing,” CIRP Ann.62(1), 315–318 (2013). [CrossRef]
  21. A. Krywonos, J. E. Harvey, and N. Choi, “Linear systems formulation of scattering theory for rough surfaces with arbitrary incident and scattering angles,” J. Opt. Soc. Am. A28(6), 1121–1138 (2011). [CrossRef] [PubMed]
  22. E. Brinksmeier and W. Preuss, “How to diamond turn an elliptic half-shell?” Precis. Eng.37(4), 944–947 (2013). [CrossRef]
  23. F. Z. Fang, X. D. Zhang, and X. T. Hu, “Cylindrical coordinate machining of optical freeform surfaces,” Opt. Express16(10), 7323–7329 (2008). [CrossRef] [PubMed]
  24. D. Yu, Y. Wong, and G. Hong, “Optimal selection of machining parameters for fast tool servo diamond turning,” Int. J. Adv. Manuf. Technol.57(1–4), 85–99 (2011). [CrossRef]
  25. H. Gong, F. Fang, and X. Hu, “Accurate spiral tool path generation of ultraprecision three-axis turning for non-zero rake angle using symbolic computation,” Int. J. Adv. Manuf. Technol.58(9–12), 841–847 (2012). [CrossRef]

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