The investment casting process can produce

Transcription

1 T E C H N I C L U P T E esigning for Investment Castings Te investment casting process can prouce almost any sape from almost any alloy. s wit all processes, owever, esigning for te process can lower cost wile acieving te esire performance. Te guielines note ere for investment casting esign apply to part configuration as cast, witout macining. For complex parts wit a imension larger tan 3 inces (76mm), it is a goo iea to make a casting rawing base on geometric imensioning an tolerancing (SME Stanar Y14.5M), make a finise part rawing an establis inspection metos tat assure (1) te casting will fit into te macining fixtures an (2) a imensionally accurate finise part. lloy Selection Because service conitions corrosion, local ig-wear areas, weigt limitations, appearance, etc. may require caracteristics in aition to strengt, te alloy soul often be selecte after establising te part s strengt. (For a full treatment of alloys an specifications, see Hitciner Tecnical Upate 22 an talk wit your company s or Hitciner s metallurgists.) esign Information Overall size. Hitciner can provie most parts tat will fit into a box of about 18 inces (457mm) cube (an larger in aluminum). Wall tickness. Hitciner can provie castings as tin as inc (0.4mm) in any of te alloys it casts an usually in as large an area as suc a section woul be mecanically suitable. Walls as tick as 6 inces (152mm) can be cast in all alloys. Some areas of te part require ticker walls an, werever possible, it is better to ave te walls go from tin to tick in all irections, rater tan from tin to tick to tin to tick again. In oter wors, avoi ticker sections surroune by tinner ones. Figure 1 sows recommene ways of canging wall tickness. Joining walls. Figure 2 sows te best ways to join two or tree walls. Corners. Figure 3 sows te best corner esigns. Tooling outsie corners sarp reuces tooling cost. Normal processing in most alloys will create a rouning to a maximum of about inc (0.4mm). Special processing can keep te eges sarp if esire. Fillets on insie corners soul ave raii equal to te wall tickness. Isolate bosses. Stock must often be ae locally in a more or less uniform wall to provie macining stock for metal-to-metal contact or bolting. Keep te boss eigt to te maximums sown in Figure 4 to minimize cost. Casting oles troug te bosses is more economic for tall bosses. Internal features. Flui flow passages or internal contours of almost any sape an size can be cast. If te passage lengt to te smallestwit imension is more tan 3:1 on a close passage or 6:1 on a troug passage, more expensive metos must be use to cast tem. If possible, internal walls soul be tinner tan te external walls surrouning tem (Figure 5). Try to avoi tick sections on tin internal walls, owever, unless tey meet te criteria for isolate bosses. Linear tolerances for cast imensions. Te tolerances sown in Table 1 Linear Tolerances for Cast imensions can be applie to lengt, concentricity, fillet, an corner raii, oles (straigt an curve), flatness an straigtness. Te table sows tree levels of tolerances, from lowest to igest quality (also lowest to igest cost). Holing closer tolerances requires more expensive tooling, more extensive process control an often aitional operations (coining, for example), all of wic increase cost. Hitciner s capabilities are worl class an it can meet te tolerances of any oter founry. Before specifying te igest-cost tolerances, owever, we recommen you work wit our engineers to assure you get suc tolerances at te lowest possible cost. Wall tickness tolerances. ue to te nature of te process, investment castings nee more allowance for wall tickness variation, regarless of te wall tickness imension. For lowest to igest cost, te wall tolerances are ±0.015 inc (0.38mm), ±0.010 inc (0.25mm), an ±0.005 inc Tecnical Upate 21

2 (0.13mm) (cannot be el in all wall ticknesses). Geometric surfaces. Tolerances require for profiles, contours, perpeniculars, etc. epen on teir lengt, so te table for linear tolerances can generally be use. For example, for lowest cost, a contour coul be el to witin ±0.010 inc (0.25mm) if it is one inc (25mm) long (or one-byone inc (25-by-25mm), ±0.013 inc (0.33mm) if it is 2 inces long, etc. Many cast contours are functional treas, bearing surfaces an airfoils, for example an soul be cecke wit gauges to assure functionality. rtwork. lmost any texture, lettering, knurl, company logo, lines, rule an oter types of fine etail can be cast in a way tat will convey te intent effectively an permanently. For ease of casting, negative lettering wit soul be aroun inc (0.64mm) or larger, te ept soul be el to te same imension as te wit an a minimum raius of inc (0.13mm) soul be at te base of te letter cross section. Positive artwork can be cast to tese an larger imensions an can be isplaye on a sunken surface as well. Surfaces to macine. epening on te alloy, te cast surface finis ranges from 90 to 150 µinc R (2.2 to 3.8 µm). In most cases, terefore, te only surfaces tat nee macining (any metal removal) are tose tat require visual appeal suc as a golf club ea an tose wic are involve in metal-to-metal contact bearing, rubbing, bolting, fitting, treaing, etc. ny surfaces tat nee to be el more closely tan te tolerances outline above will ave to be macine. Straigt roun oles requiring tolerances better tan tose mentione are normally rille, unless te alloy is ifficult to macine, in wic case precision cores may be economical. Hitciner as extensive macining facilities for proviing macine finise castings to final print, regarless of tolerance requirements. If te alloy is ifficult to macine (e.g., a nickel or cobalt alloy), it may be economical to cast treas or cam or rubbing surfaces. Some alloys can be coine to acieve a surface tat functions quite well as a metal-to-metal operating interface. Macining stock. Surfaces polise for visual appeal nee at least inc (0.13mm) stock (epening on te alloy) to provie a great looking surface after polising. Te amount of stock neee to establis tigt imensions for surfaces working against oter metal parts increases as te part size increases to assure enoug stock for imensional variation an to assure surface irregularities (rougness) are remove uring macining. Since gauging oes not pick up suc irregularities, at least inc (0.25mm) more stock tan is neee to accommoate imensional variation soul be ae. If te alloy is ifficult to macine or te part volume is ig, work wit Hitciner to assure minimum stock aition an reuce macining cost. ll areas to be macine soul be marke clearly on te casting rawing an te finise part rawing. Geometric imensioning an tolerancing. Geometric imensioning an tolerancing soul be use for larger an more complex parts. It is a formal system approve internationally an ocumente by te merican National Stanars Institute (NSI), merican Society of Mecanical Engineers (SME) an International Organization for Stanarization (ISO). Hitciner strongly recommens tat its customers get tis etaile system of esign language from SME, asking for Stanar SME Y 14.5M. Tis system avois builup of tolerances, wic can appen wen coorinate tolerancing is use, an makes it easy to convey te maximum amount of information. Tis elps to avoi costly elays in te early stages of tool esign an buil as well as later on in prouction. Casting esigners soul use te system of atum target points, lines or areas tat can ten be te basis for inspection an macining fixtures. Figures 6 an 7 illustrate ow tey can be use. rawing notes. Most rawings ave stanar blocks containing information elpful to te user general tolerance, fillets, corner raii, general wall ticknesses (wit tolerance), surface finises, some rawing symbols, material, eat treat conition, special finises, an more. Suc notes are elpful to te founry. itional notes relating to te specific part are also esirable nonestructive testing metos, alloy arness, wel repair approval, special treatment areas, visual quality, microstructure, grain size, sampling plans, references to macining or gauging rawings or requirements anyting tat ais in communicating your nees to te founry. Tecnical Upate 21

3 Linear Tolerances for Cast imensions Lowest-to-Higest Quality & Cost Inces (mm) imension Lowest (±) Meium (±) Higest (±) <0.25 (6) (0.25) (0.13) (0.06) <0.50 (12) (0.25) (0.13) (0.06) < 1 (25) (0.25) (0.13) (0.13) <2 (50) (0.33) (0.25) (0.20) <3 (76) (0.41) (0.38) (0.25) <4 (101) (0.49) (0.44) (0.30) <5 (127) (0.56) (0.51) (0.34) >6 (152) 0.25 (0.63) (0.56) (0.38) <7 (178) (0.72) (0.63) (0.42) <8 (203) (0.079) (0.72) (0.46) <9 (229) (0.87) (0.77) (0.50) <10 (254) (0.95) (0.82) (0.52) >10 (254) (0.13)* (0.10)* (0.08)* * Per inc (25mm) of imension lengt. Table 1 Canges in Wall Tickness Poor esign Fair esign Goo esign Best esign Figure 1 Tecnical Upate 21

4 Joining Walls Goo Tin to Tick Fair Optimum For Same Wall Ticknesses Poor Goo Rigt is Goo for Equal Wall Ticknesses s Well Poor 3 Times Wall Tickness Better, Best If Contoure s Top Rigt Figure 2 Tecnical Upate 21

5 Corners Poor esign Goo esign Wen =t=r r t r Goo esign Wen r= Rib Tickness = or < 1.5 Tinning Flanges an Ribbing May Lower Cost Figure 3 Tecnical Upate 21

6 Low-Cost Bosses or Raise Pas 3 Min. Wen =, Keep < Min. Wen = 1.3, Keep < Min. Wen = 2, Keep < 0.25 Wen = 1.3, Keep < Min. Wen = 2, Keep < Min Min. If bosses must be taller tan above, coring can reuce cost. Figure 4 Tecnical Upate 21

7 Keep Internal Walls Tinner Tan External Walls Section Figure 5 atum Feature Symbol For atum Plane (Open or fille triangles) 6 1 Target area size Target number atum ientity 1 6 lternate Symbols for atum Target rea an Point 1 If a atum point Tis Means tis 1 Symbols for atum Target Line Figure 6 Tecnical Upate 21

8 C C B B1 400 B B1 B2 imensioning by atum Target Points for a Block. is te Primary atum Plane Line Contact Tis 3 6mm ia. Target reas, 1, 2 Means tis Primary atum Plane is Establise by Two Target reas an One Target Line Figure 7 PO Box 2001 Milfor, NH (603)