Page 23 - PCMI Journal December 2020

Manufacturing Bespoke Aesthetic and Functional Surfaces via Photochemical Machining, Electrolytic Photoetching and Electrolytic Photopolishing Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
Figure 8a (left). Etched 0.25 mm thick AISI 302 half hard stainless steel Christmas decoration showing half-etched Latin text, church architecture and horizontal and vertical fold-lines for converting the planar part into a 3-dimensional piece as shown in Figure 8b (right). Courtesy of NW Etch, Tacoma, WA, USA.
Surfaces for quantitative contrast analysis
As already discussed, visual contrast is enhanced when a matt etched surface is compared
with a shiny, reflective surface. The difference in reflectivity has been used to photochemically machine a surface etched stainless steel Ø32 mm spherical rotor for a gyroscopic missile guidance system [7]. The orientation of the rotor is determined by measuring light reflection from polished (non-etched ) areas in comparison to light scattered from etched areas of the rotor such that any line of latitude crosses a certain ratio of etched to unetched surface. This mark : space ratio, P, was to vary from 3:1 at +50° latitude to 1:3 at -50° and there was to be a linear relationship between roll angle, θ, and mark to space ratio, P such that:
• P = 1 + θ/25 above equator and
• P = 1 /(1 + θ/25) below equator (Figure 9a).
Thus, by measuring P from the light reflection intensity, the roll angle may be determined precisely and utilised for orientation and guidance purposes.
The rotor (Figure 9b) was fabricated by using a CNC-machined 3-D aluminium mask (Figure 10) and positive-working liquid photoresist to avoid 3-D imaging in a vacuum that would have been necessary if a negative-working photoresist had been used. The resist was applied by dip-coating and imaging was achieved by rotating the rotor and mask in front of a UV light. Immersion etching was carried out at 30°C in 39° Bé ferric chloride to achieve an acceptable light-scattering Ra ≈ 2 μm.
Surface finish range
From the above case histories, it appears that using ferric chloride and conventional
PCM limits the surface finish range achievable. An important question to be answered is: “Can a different range of surface finishes be acquired on stainless steel by a change in etchant formulation or technique?”. For instance, would electrolytic photoetching in 10% HCl or electrolytic photoetching in a polishing medium (known as electrolytic photopolishing) produce a wider range of surface finishes?
Issue 136 December 2020 PCMI Journal 23