Hon-Yuen Tam

Investigation on removal features of multidistribution fixed abrasive diamond pellets used in the polishing of SiC mirrors

To obtain removal functions (RFs) with high removal rate and stability in the polishing of silicon carbide mirror, an optical fabrication technology based on fixed abrasive diamond pellets (FADPs) is adopted. In this paper, we focus on the removal characteristics of FADPs, including removal profile, removal rate, stability of RFs, and surface roughness. Diamond pellets polishing is analyzed theoretically with respect to removal rate and stability. A universal algorithm is proposed for computing theoretical removal profile of different distribution models. By evaluating the cutoff frequency of RFs, optimized parameters including speed ratio and eccentricity are confirmed. A series of experiments are conducted to verify the effectiveness of the algorithm; within 300xa0min (even more), the pellets could provide highly stable RFs with about 5 times removal rate than loose abrasives; the surface roughness 4.86xa0nm is obtained.

Study of a wheel-like electrorheological finishing tool and its applications to small parts

A wheel-like electrorheological finishing (ERF) tool for small parts polishing is proposed and thoroughly studied. First, the electrorheological polishing fluid is tested, and its properties suggest usability for electrorheological fluid-assisted finishing. Then, the mathematical removal model of the ERF tool is built employing the conformal mapping method and high-order multipolar moment theory. Finally, a micropattern of trough is fabricated on a slide glass (7 mm wide and 1 mm thick). The trough is 70 nm deep, and its flat bottom is 1.5 m wide (peak to valley of 3.16 nm and root mean square of 1.27 nm); the surface roughness finally achieves 0.86 nm. The results demonstrate the stable machining capability of the ERF tool for miniature parts.

Design of compact LED free-form optical system for aeronautical illumination

A type of runway centerline light is designed for the application of light-emitting diode (LED) aeronautical illumination. A total internal reflection collimating lens and an integrated prism are designed, respectively, to meet the intensity distribution of International Convention on Civil Aviation (ICAO) regulations. The principle of geometric optics is adopted to construct the free-form surfaces of a collimating lens, which is simple. Different variations are used in the process of free-form surface calculation. An integrated prism with a diffuser is used for uniformly diffusing rays and then decreasing the central maximum intensity to avoid glare. The structure of the optical system is compact. Computer simulation results show that an optical efficiency of 79.2% is achieved for a 1u2009u2009mm×1u2009u2009mm LED source. Tolerance analysis is carried out to determine tolerance limits of manufacture and installation errors. To verify the optical performance of the proposed runway centerline light, the practical illumination distribution is measured by using Cree XP-E2 LED, which can comply with ICAO regulations.

Compensating for velocity truncation during subaperture polishing by controllable and time-variant tool influence functions

The velocity-varying regime used in deterministic subaperture polishing employs a time-invariant tool influence function (TIF) to figure localized surface errors by varying the transverse velocities of polishing tools. Desired transverse velocities have to be truncated if they exceed the maximal velocity of computer numerical control (CNC) machines, which induces excessive material removal and reduces figuring efficiency (FE). A time-variant (TV) TIF regime is presented, in which a TIF serves as a variable to compensate for excessive material removal when the transverse velocities are truncated. Compared with other methods, the TV-TIF regime exhibits better performance in terms of convergence rate, FE, and versatility; its operability can also be strengthened by a TIF library. Comparative experiments were conducted on a magnetorheological finishing machine to validate the effectiveness of the TV-TIF regime. Without a TV-TIF, the tool made an unwished dent (depth of 76 nm) at the center because of the velocity truncation problem. Through compensation with a TV-TIF, the dent was completely removed by the second figuring process, and a TV-TIF improved the FE from 0.029 to 0.066u2009u2009mm(3)/h.

Robust linear equation dwell time model compatible with large scale discrete surface error matrix

The linear equation dwell time model can translate the 2D convolution process of material removal during subaperture polishing into a more intuitional expression, and may provide relatively fast and reliable results. However, the accurate solution of this ill-posed equation is not so easy, and its practicability for a large scale surface error matrix is still limited. This study first solves this ill-posed equation by Tikhonov regularization and the least square QR decomposition (LSQR) method, and automatically determines an optional interval and a typical value for the damped factor of regularization, which are dependent on the peak removal rate of tool influence functions. Then, a constrained LSQR method is presented to increase the robustness of the damped factor, which can provide more consistent dwell time maps than traditional LSQR. Finally, a matrix segmentation and stitching method is used to cope with large scale surface error matrices. Using these proposed methods, the linear equation model becomes more reliable and efficient in practical engineering.

Off-Axis Aspheric Surfacing Using Sub-Aperture Tools

The off-axis aspheric surface used in modern optical systems widely can obtain nearly perfect quality, realize both small packet-size and low-mass, and avoid the central obscuration. But it is difficult to fabricate because of asymmetry. There are some key technologies during the testing and fabrication of off-axis asphere. In this article, we proposed a method of the best fit sphere based on non-negative minimized removal criterion. And a measured data fitting algorithm was presented to estimate the geometry parameters of off-axis aspheric surface. Then an off-axis mirror was fabricated, and the interferometrically measured data was corrected to eliminate the distortion introduced by null compensator in every run. Finally, the surface error of off-axis mirror reduced to pv = 0.372λ, rms = 0.046λ, the surface roughness reached 0.72 nm. These methods mentioned in the article are suitable for off-axis aspheric optics.