relative to the circuit card network but allowed to fl oat more generously with respect to the mechanical superstruc- ture of the housing. Clever processing and tooling within the metalcasting operations ensured the required toler- ances were attained on each casting. A series of cost tradeoff s were
performed to determine which features to cast net versus which features would be machined. Optical alignment fea- tures, precision mounts and gasketed cover seal surfaces required machining, sometimes with dimensional precision on the order of +/- 0.001 in. and better. It is not enough for these features
to comply under constraint—dimen- sional requirements for precision features must be validated in the “free state,” unconstrained and after temperature cycling simulations of the in-service environment. Close coordination between the designer, the metalcaster and the machine shop ensure end item requirements are achieved in the most cost-eff ective
Figure 4. The pattern was produced with stereolithography.
manner. Special processes are added to the foundry operations and also to the machine shop operations to enhance dimensional stability and ensure piece by piece product con- sistency. Features and requirements that move upstream into the net-cast confi guration add to the casting unit price and tooling requirements, but reduce the total cost of ownership for the end item.
Ensuring Quality First article process qualifi cation
included dimensional compliance of the casting, validation of mechani- cal properties, fl uorescent penetrant inspection, radiographic inspection and all other characteristics applicable to the casting. First article approval formally notifi es the foundry of process acceptance, and introduces an obligation to continue production using the same processes and controls. Proposals for change must be commu- nicated through the buyer for review by Raytheon engineers, who evaluate the potential impact and establish a test plan appropriate for reapproval. Simple changes might not justify any extra testing, but other changes might require revalidation of dimensions, strength or internal soundness. Com- plex changes might require repetition of the entire fi rst article process ap- proval test program. Studies of complex thin-walled aluminum investment castings have shown the greatest potential for vari- ability lies with dimensional integrity. T is is because wax pattern sag, solidi- fi cation stresses and quenching distor- tion are facts of life. Fortunately, these can be addressed by foundry processes, straightening tooling, and dimensional check fi xtures. Consistent quality from the casting
supplier results from capable equip- ment and processes, skilled work- manship, and a well documented set of work instructions. To ensure a consistent and well-focused appraisal process by all parties, Raytheon Preci- sion Manufacturing shares a Supplier Quality Plan unique to each casting part number. T is SQP documents the parameters for casting inspection
Figure 5. High stress areas were determined. 24 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2013
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