TECHNOLOGY
A contour panel. Chem-milling can b e applied to complex contoured or formed parts in which metal removal may b e performed simultaneously in one operation
Etch Process Beats Milling Etch process, C h e m - M i l l , has a d v a n t a g e s o v e r m a c h i n e milling . . . w e i g h t savings, p a r t s p r o d u c e d in batches, a n d cost of e q u i p m e n t a b o u t 7°?o of skin mills \ NOVEL process for milling of metal •**• parts by chemical means has been developed by North American Aviation. T h e process, d u b b e d Chem-Mill, is faster, considerably cheaper, and offers design possibilities not available b y machine milling, says Manuel C. Sanz, reporting for North American. The new process is carried out b y partially masking a metal sheet with a chemically resistant coating and then submerging the sheet in a modified h o t alkaline solution which removes t h e metal in the unmasked areas. Indications were that alkaline etchants possessed certain advantages over acid etchants, says North American, so t h e former were tried first. However, company is also investigating acid etchants as well as electrochemical means. Chem-milling by acid, alkaline, and electrochemical means has been a p plied to aluminum, steels, and titanium. However, North American reports t h a t its test results and work at the moment concerns etching of aluminum. Company says idea of applying process in an exacting degree and under precise control is distinctly an innovation, particularly to large areas to produce high complex shapes, nonsymmetrical patterns, integrally stiffened structures, and tapered skins. Chem-milling affords numerous advantages over machine milling, reports North American. Two types predominate: (1) engineering design advantages, which result in lighter-weight construction, and (2) operational and V O L U M E
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economic advantages, which result in lower cost per part. T h e process permits design not permissible b y other means to obtain lighter-weight construction of integrally stiffened structures simplified b y elimination of riveting, seam and spot welding, and stiffeners and doublers, according to company. Moreover, chemmilling can be performed after forming operations; there are no machining restrictions, and applications can be made to complex contours. Other engineering advantages are closer tolerances, and permission of: tapering of sheets, production of integral stiffeners and various depths of cut on one sheet. T h e new process lends itself to automation b y electronic instrumentation, says North American. Consequently, no highly skilled operators are required. Multiplicity of production is also facilitated since the n u m b e r of parts t h a t can b e etched simultaneously is limited only b y size of t h e tank. Other economic and also operational advantages are that process allows use of conventional equipment (cleaning and pickling type already available in aircraft plants for metal cleaning) and no sanding or polishing is required on the finished chem-milled parts. North American says there are various methods of obtaining selective etching b y masking. Current methods, in order of greatest economy are: mechanical masking b y use of chemical resistant templates, use of special chemical resistant adhesive tapes, spray
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or b r u s h application of chemical resistant organic coatings, use of silkscreen process, photosensitive organic coatings and plating. Chem-milling of aluminum results in h y d r o g e n and alumina as by-products. At present, North American is disposing of hydrogen gas evolved by conventional electroplating tank venting e q u i p m e n t , without a n y efforts toward recovery. Solids may b e removed by centrifuging, which permits continuous a n d uniform operation of the process. At present, solids are disposed of as w a s t e . However, w h e n chem-milling goes into full production at Downey, it is p l a n n e d to market t h e m commercially. Cost comparisons m a d e between chemical and machine milHng show considerable advantage for chem-milling. However, these cost comparisons are limited only to machine milled parts that can b e duplicated by chem-milling. C o m p a n y says that t h e chemical composition of the solution (based on strong alkalinity) it is using to submerge t h e metal sheets m u s t be enh a n c e d b y additional agents to p r o d u c e smoother surface characteristics, more uniform etching w i t h the best fatigue characteristics, b e t t e r wetting, and to make easier the disposal of byproducts. T h e solution n o w used, subject t o further improvement, is supplied by T u r c o Products of Los Angeles.
On left is a ch em-milled surface. On right is a machine milled, unpolished surface. Both surfaces are magnified five times N o r t h American says initial equipm e n t cost for chem-milling is considerably less than for machine milling. For machine milling, the cost has b e e n considered at over $200,000 per milling machine. For chem-milling, the cost of a complete facility (independent of other processing equipment t h a t would b e available) would not exceed $30,000. Moreover, the space requirements for chem-milling machinery, including conventional cleaning equipment, is approximately 5000 square feet. On the basis of releasing parts on a time schedule for one manufacturing requirement, it was estimated that t h r e e milling machines at a total cost of $600,000 would b e necessary, as against one chem-milling installation costing approximately $30,000. A t present, North American has one pilot plant i n operation and a production facility is being planned for installation in the near future. 3051
