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Four castings (left) show variety of shapes and ma- terials made by Engineered Precision Casting Co, Mat- awan, NJ. Stainless steel (302) cam and holding plate (left) is used in mak- ing shaped medical cotton swabs. At top is type 440-C stainless stripper finger for removing trim on cardboard container die-cutting machine. Type 312 stainless burner mixer nozzle is at bottom and on right is type 356 alu- minum forming plate for packaging machine.


Tool bit investment cast from tool steel by Casting Engineers, Chicago60614. With it is type 8620 steel safety selector for the M-16 rifle.


p 42-Seven-step technique developed for model nosecones, quickly and eco- nomically makes metal and plastic parts with walls down to 0.0 I 0 in. thick. For tiny parts-minicast does it cheaper, Feb I '60, p 44-Minicast is a mass-production technique com- bining investment casting with preci- sion post forming for small parts with thin walls, complex shapes, and radi- cal changes from thick to thin sec- tions.


curing, and casting characteristics of all the specific materials to be han- dled. You will not be required to know their characteristics in detail, but you will have to understand why restric- tive critical tolerances specified for noncritical dimensions can depreciate the value of a casting while multiply- ing its cost. The case against overspecification


cannot be overstated. The advantages of the process can be enjoyed only by respecting its limitations, and its limitations are the dead opposite of those of machined precision. If there is a real necessity for simple round- ness or flatness, concentricity or paral- lelism, don't attempt to cast to size unless the alloy compels it. Leave some generous slop (design cast in fixture- saving registration points where you can), and turn or plane the part to size in secondary operations. Identify unambiguously the sur-


faces and dimensions that are critical to the successful function of the part, and give the foundryman the most generous envelope of tolerances the design allows, with upper and lower limits clearly specified. Establish in- spection standards based on casting precision, rather than on machine pre- cision. If a part is a likely candidate for the process, invite consultation at the earliest opportunity. And never forget that investment


casting is a recently industrialized jewelry process that can operate com- petitively only where late advances in pattern molding, mold production, and the casting process itself are auto- mated to the highest possible degree. Every restriction on your drawing or specification that demands artisanship instead of automatic control, or that raises inspection standards beyond rea- sonable functional requirements, or that commissions casting to do what machining can do better and cheaper -every such restriction pushes the process back, until you reach a point where you find that you are buying custom jewelry, instead of custom hardware.


EDITOR'S NOTE: This article up- dates our comprehensive reports on investment casting published in Nov 1951. The following articles deal with special problems or opportunities faced when designing with investment cast- ing. The last two articles are compre- hensive design guides: Investment-cast parts, Feb 5 '62, p 58 -Shows how multiple stacking gives produc·tion economies and lists best ferrous and non-ferrous alloys for this process. Four parts each costing 50¢ are pictured. Thermoforming and investment proc· ess-ultrathin-wall castings, Feb 27 '61 ,


Cheaper prototypes from one-shot in- vestment castings, Jan 4 '60, p 55- Here's how to make a casting from a wax pattern in 6 illustrated steps. With this method, one-of-a-kind in- vestment castings, look like produc- tion parts and are usually faster and cheaper to make than one-piece ma- chinings or welded assemblies. Investment castings . . . tight toler- ances can be even tighter, July 20 '59, p 49-Here's how to handle critical dimensions for lower costs that come from using the casting "as is". How to choose-investment casting or sintered powder, June 8 '59 p 86- Although investment casting and sin- tered powder seem competitive, know- ing shape, tolerances, and application makes the choice easy. Designing for investment casting, April '56, p 166-Advantages and limita- tions, tolerances available, types of pattern materials and casting alloys, preparation of alloy specification, and inspection procedures. Investment casting: alloy selection and design, Nov '51, p 184-How to se- lect alloys. Specific design suggestions cover changes in section, core design, bosses, and ribs. Three-page table gives castability rating, properties, and applications for 30 materials. For other articles in our current alternative process series, see: Design for instant parts, May 11


'63, p 75-Sixteen-page special report explains how to design for die casting, investment casting, and injection mold- ing. Eight tables compare these proc- esses as well as appropriate materials: zinc, aluminum, magnesium, acetal resin, nylon, polycarbonate, ABS, and acrylic resin. Powder metallurgy parts, April 1 3


'63, p 71-Gives advantages and limi- tations of this fast-growing process, plus a look at where it is staking out new territories. Shell molding and coring, Nov 11


'63, p 100-Shows how this process can reduce machining costs of castings and how shell cores are used with green and dry sand molds for low cost grey iron castings.


-Frank Hall 9


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