The term 'chemical machining' can cover a wide range of processes, but all are basically characterised by slow rates of material removal which varies according to the geometry of the part, the material and the chemical agent used to remove the material from the surface of the part. Because they don't use physical 'hard' tooling, the processes, aren't restricted by difficult geometry such as undercuts, or very narrow or very deep features.
The essence of the process is to dissolve the substrate material using a chemical reagent, such as an acid or alkali. Of course, immersing a workpiece in a strong reactive liquid will simply tend to remove material from all surfaces, so masking the component allows selective material removal, and this is an important aspect of any chemical machining process.
Chemical machining is well suited to the manufacture of very complex geometry on thin sheet-metal parts, although we seldom have much call for such pieces in a race engine. One exception could be the production of bespoke flat filter elements for fluid filtration, which can easily be made by the process of photochemical machining, where precision masking techniques using light-sensitive masking materials give very accurate and fine geometry.
Anyone who studied chemistry at school might recall that the rate of reaction between a liquid and a solid is not only a function of the reagents but also the surface area. A finely divided solid reacts more quickly than a single block, and there is a direct analogy in an engine, where well dispersed fuel sprays burn more quickly. Where this affects chemical machining is in areas with a large or small surface area ratio. External sharp corners have a higher ratio of surface area to material volume compared to an internal corner and therefore material removal will tend to be higher on external corners. Perhaps the best example of a high ratio of surface area to volume is a burr; these will be rapidly removed by chemical machining, and the same is true of surface asperities. So, we can see that one ideal application of chemical material removal is deburring and improving surface finish.
Since the material removal rate can be accurately predicted, chemical machining can be used to reduce the thickness of metallic components. This is sometimes used where there is a limit to what can be fabricated in terms of thin metallic sheet metalwork; the finished fabrication can be chemically machined to produce a component that is thinner than can be manufactured practically.
In a similar way, castings can be reduced in thickness to below the practical limit for the usual production method. Titanium castings are very often chemically machined as a matter of course, owing to the propensity for titanium to react with the moulding material, normally causing the casting to have a brittle case. Such brittle material has poor mechanical properties, and the resulting parts are not suited to cyclic loading. Chemical machining removes this brittle case, leaving the casting with the desired amount of ductility.
Written by Wayne Ward