CHM consists of the following steps:
1. Preparing and precleaning the workpiece surface. This provides goodadhesion of the masking material and assures the absence of con-taminants that might interfere with the machining process.
2. Masking using readily strippable mask, which is chemically impreg-nable and adherent enough to stand chemical abrasion during etching.
3. Scribing of the mask, which is guided by templates to expose theareas that receive CHM. The type of mask selected depends on thesize of the workpiece, the number of parts to be made, and the desiredresolution of details. Silk-screen masks are preferred for shallowcuts requiring close dimensional tolerances.
4. The workpiece is then etched and rinsed, and the mask is removedbefore the part is finished.During CHM (Fig. 3.1), the depth of the etch is controlled by the timeof immersion. In order to avoid uneven machining, the chemicals thatimpinge on the surface being machined should be fresh. The chemicalsused are very corrosive and, therefore, must be handled with adequatesafety precautions. Both the vapors and the effluents must be suitably