AUTOMATED POLISHING OF MOULDS TO REPLACE MANUAL FINISHING

With increasing demand for low cost, high accuracy optical systems, manufacture of lenses and mirrors is moving away from direct machining of glass blanks towards glass press moulding. Aware of this trend, British CNC polishing machine and metrology equipment manufacturer, Zeeko, has developed an automated process for improving substantially the surface quality of such moulds.

Used for replicating optics, ranging from lenses in consumer digital cameras to high-accuracy thin mirrors for X-ray telescopes and microscopes, the moulds have an internal shape that is produced either by micro-grinding of tungsten carbide (WC) with a polycrystalline diamond tool, or by turning of electroless nickel (ENi) plating with a diamond tip.

However, both machining methods generally fail to meet the ultra-precision criteria required by an increasing number of applications due to the creation of cyclic micro-structures that can cause diffraction or stray light effects in the finished glass product. Consequently, such moulds are traditionally polished by hand, which is time-consuming, inaccurate and expensive.

As an alternative, a Zeeko 7-axis CNC intelligent robot polisher (IRP), using either precessed-bonnet or fluid-jet polishing, has been shown to be able to correctively polish moulds automatically and consistently to very high accuracy in terms of form error peak-to-valley (P-V) and surface texture (Ra). The result is a significant improvement in the quality of the optics, which can be of any shape: concave, flat, convex, aspheric or free-form.

During trials, flat WC discs of 25 mm diameter and 0.2 micron grain size were ground using a wheel moving at an angle across the spinning surface to replicate aspheric mode conditions. Surface roughness of 8.86 nm Ra and form error of 905 nm P-V were achieved.

The samples were then polished on a Zeeko IRP using the manufacturer's proprietary 'precessions' corrective polishing software. A 4.5 mm radius, polyurethane bonnet of 90 Shore A hardness held at a 20-degree precess angle was rotated at 2,000 rpm and programmed to follow a raster tool path at 500 mm/min in the presence of monocrystalline diamond grain abrasive.

Surface texture measurements with a white-light interferometer showed that all of the grinding marks in the surface were removed after polishing away a 200 nm layer of material. Surface roughness down to 1.6 nm Ra was achieved with 0.25 μm diamond grit size, while a further improvement to 1.03 nm Ra – eight times smoother than the unpolished surface – was obtained by applying pitch on the tip of the bonnet tool. Form error was reduced to 29 nm P-V, 31 times better than the original condition of 905 nm P-V.

Concurrent tests carried out on flat, diamond turned ENi samples of 50 mm diameter were similarly impressive, with form error reduced from 400 to 30 nm P-V in two iterations. Residual turning grooves were also efficiently removed from the surface. Subsequent smoothing by bonnet polishing with 30 nm silicon dioxide particles reduced the surface roughness from 2.37 to 0.52 nm Ra.

These findings also have relevance to finishing moulds for intra-ocular and ophthalmic lenses; and indeed any moulds of any material designed to manufacture super-high-precision parts, with form error in the region of one twentieth of the wavelength of visible light.

The test results were presented by Zeeko at the Euspen international conference and exhibition, held in Como, Italy, 23-26 May 2011. The process and results have been peer-reviewed and accepted for publication at the forthcoming annual meeting of the international academy for production engineering (CIRP).