ChM is a process of controlled dissolution w/p material by contact with strong reagent and protecting the part by maskants from which material is not to be removed. Used for material having high strength to weight ratioChM consists of following steps:Preparing and precleaning the w/p surfaceMasking using readily strippable mask, which is chemically impregnable and adherent enough to stand chemical abrasion during etchingMasking material depends onSize of w/p; number of parts to be made; desired resolution of details Scribing of mask to expose areas that receive ChM w/p is then etched and rinsed in etchant or reagent with temperature ranging from 37-85oC

Depth of cut is controlled by time of immersion

Chemical machining set-up

Machining process proceeds both inwardly from mask opening and laterally beneath mask, thus creating etch factor. Etch factor is ratio of undercut d to depth of etch T.

Radius of fillet is approx. equal to depth of cut.

Maskants should posses following properties Be tough enough to withstand handlingAdhere well to w/p Scribe easily Be inert to chemical reagent

Be able to withstand heat generated by etchingRemoved easily and inexpensively after etching

Maskants are of three types Cut and peelPhotographic Screen resists

Etchant Basic function of etchant is to convert a material in to a metallic salt that can be dissolved in etchant, thus removed from work surface. Etchant selection is based on Material to be etched Type of maskant or resist used Depth of etch Surface finish require dPotential damage to metallurgical properties of w/pRate of material removal Economics

Process parametersChM process parameters including reagent solution type, concentration, properties, mixing, operating temperature, and circulation will directly impact on w/p regarding following Etch factor Etching and machining rate Production tolerance Surface finish

The MRR depends on the chemical and metallurgical uniformity of w/p and uniformity of solution temperature with casting having largest grain size, show roughest SF together with lowest machining rate. Rolled metal sheets have highest machining rate accompanied by best surface quality Etching rates are high for hard metals and low for softer ones Generally, high etch rates is accompanied by low surface roughness, hence narrow machining tolerances

Advantages Several parts can be machined simultaneously Machined or extruded parts may be reduced on selected areas or all over by ChM to provide web sections thinner than those obtainable by conventional machining Casting are designed uniformly oversized, heat treated with little or no warpage, and then chemically machined to achieve final desired dimensions Extrusion, forging, casting, formed sections and deep drawn parts can be lightened considerably by ChM

DisadvantagesHandling and disposal of chemicals can be trouble some Surface imperfection are producedMetallurgical homogeneous surfaces are required Deep narrow cuts difficult to produce Steep tapers are not practical Welded areas frequently etch at rates that differ from base metal.