Investment Casting Design Parameters - A.W.Bell

1 - 1-The following guidelines and technicalinformation outline what an investmentcasting is capable of offering. It will coverdimensional and structural topics. Theintegrity of a part, both dimensional andstructural is proportional to the cost of thefinished part. This is true of investmentcastings, as well as fabricated andmachined parts. Investment castings aredesigned to minimize the cost of producingclose tolerance close dimensionaltolerance in an Investment Casting isaffected by many factors. Most of thesefactors can be controlled by the foundry,although minor lot to lot variations willoccur. The tolerance bands provided forinvestment castings are determined bythese variations in the can achieve closertolerances than available in an investmentcasting. A review of the Design will oftenpermit expansion of some tolerances,undercuts, blind holes, etc. to allow thehigher production yields and lower piececosts possible with Investment castings. Ifcloser than cast tolerances are necessary,the machining required on an investmentcasting will still be substantially less thanon conventional cast or fabricated added benefit of an Investment castingis the ability to add radii and other detailthat would be more expensive withtraditional manufacturing size and shape determines the tolerancerequired to allow for process variables.

2 Manyprocess factors affect Investment Casting tolerances,such as:1. wax temperature2. wax die temperature3. the amount of pressure applied to inject wax into a die4. firing temperature of the ceramic shell5. ceramic mold refractory composition6. cooling rate of the metal7. lot to lot variation of raw materialsSTANDARD LINEAR TOLERANCESAs a general rule normal linear tolerance on aninvestment Casting can be +/- .005 of an inch per inchor +/- .125 of a mm per 25mm. The following table gives a partial listing ofdimensions and TOLERANCED imension InchesDimension MM(English)(Metric)(Metric)0" up to 1"+/- .0050 up to +/- .131" + up to 2" +/- . + up to 50+/- .252" + up to 3" +/- . + up to 75+/- .38 3" + up to 4" +/- . + up to 100+/- .504"+ up to 5" +/- . + up to 125 +/- .635" + up to 6" +/- . + up to 150 +/- .756" + up to 7" +/- . + up to 175 +/- .887" + up to 8" +/- . + up to 200 +/- " + up to 9" +/- . + up to 225 +/- inch +/.

3 005each 25 MM +/- .125afterafterInvestment Casting Design Parameters - 2-FLATNESSF latness and straightness are so closely related thatconfusion may arise unless the foundry andpurchaser clarify the definitions before tolerance is a tolerance zone defined bytwo parallel planes within which the surface mustlie. In measuring (see Fig. 1),the parallel planesmust be the minimum distance 1 Degree of flatness in an Investment Casting isalmost always determined by the volumetricshrinkage of the wax pattern and then by the metalduring cooling. Shrinkage usually occurs in thecenter of a mass, this shrinkage is called sink .Sink can be controlled by specialized techniques,but it will always occur to some extent. Generalflatness tolerances cannot be quoted because theyvary with the configuration of the part and alloyused. The following is a rough guide derived fromdata collected from standard test Volume of PossibleThickness Section Sink perFace of.

4 5 not not sink is in addition to the basic example (see figure 2), on a block 1" +/- .005thick ( +/- .13mm) the allowable sink mayaffect the tolerance of the part an additional .008":Figure 2 The method of measuring flatness should bespecified by the purchaser. It may vary from asimple surface plate and feeler gage to full layoutwith equalization and dial tolerance is a tolerance zone withinwhich an axis or the considered element must , to correctly measure axial straightness of ashaft, bar, or plate, (see & 4)the tolerancezone within which the axis or axial plane lies mustalso be 3 Figure 4 Straightness may be a real problem with certaintypes of castings. A relatively thin, short part maybend while a long, heavy part may not. Experiencetells us that a given Design may bend, but it cannotindicate to what extent. Ribs and gussets willinhibit warpage, but will also hinder mechanicalstraightening of whatever warpage did 3-PARALLELISM Parallelism is the condition of a surface equidistantat all points from a datum plane or an axisequidistant along its length to a datum axis.

5 Parallelism is difficult to control in the as castcondition and may require a straighteningoperation. Parallelism is a feature that is a functionof the complexity of the part. As the part is beingdesigned, consult your foundry for 5 For example, Casting parts with parallel prongssupported at only one end (see Fig. 5)and yokecastings (see Fig. 6a)are very specializedproblems which should be discussed fully with thefoundry before 6aIn Figure 6a, point X is the thickest section, it isthe ideal gate point. It is also where the greatestvolumetric shrinkage will occur. Dimension Y,however, will be restrained by the rigid mass ofrefractory used to produce the mold. Parallelism istherefore difficult to maintain. It can be improvedby control techniques and sizing. This condition will also affect any through holesusually found in yokes (see ).Whenspecified, such holes should carry considerablefinish stock for machining if they are to befinished truly 6bROUNDNESSR oundness tolerance specifies a tolerance zonebounded by two concentric circles within whicheach circular element of the surface must lie.

6 It isthe total indicator reading (TIR) when the part isrotated 360 , or calculated by taking half thedifference between the maximum and minimumcondition. The latter is usually preferred becauseit is quicker to determine. The actual inspectionmethod, however, should be specified by is a condition in which two or morefeatures ( cylinders, cones, spheres, hexagon, etc.)in any combination have a common axis. Forexample, any two cylindrical surfaces sharing acommon point or axis as their center areconcentric. Any dimensional difference in thelocation of one center with another is the extent 7 Figure 7 shows that out of roundness in eitherdiameter does not affect concentricity becauseconcentricity is determined by the centers or axis.(Continued)- 4-CONCENTRICITY(Continued)Out of roundness is variance from a true does influence concentricity if thecasting has a shaft or tube 8In Figure 8, diameters A and B may be true circles,but the out of straightness affects the length of a bar, a Casting , or tube is notmore than 2 times its component diameters, thediameters will be concentric within.

7 005" per 1" ofseparation (see Fig. 9 below).Figure 95"OD x 2"ID x 2" longThe 5"OD x 2"ID will be concentric within .015" TIR5"OD - 2"ID = 3" the length is more than two times thediameters, out of straightness should be added toeccentricity (see Fig. 10 below).Figure 10 3"OD x 2"ID x 4" long3"OD x 2"ID will be concentric within .005" TIR 3"OD-2"ID = 1" is the condition of a surface, axis, orcenter plane which is at a specified angle (otherthan 90 degrees) from a datum plane or axis (seeFig. 11 below). As a designer you should be awarethe angularity of a component part to bemaintained may require the part to be mechanicallystraightened or the die reworked. In heavy sections,the die may need to be reworked, and in thinsections, the Casting may be straightened to achieveangularity tolerances. Angular tolerance dependson the configuration forming the angle. Sketch A cannot be sized, but in certain cases aftersufficient data review, the die can be reworked tobring the part closer to optimum.

8 Sketches B and C represent castings that can be reworked to +/- 1degree, depending on the alloy and its condition. Figure 11 POSITIONINGP osition tolerance is dependent on the parentcasting configuration. A Casting that is lesssymmetrical in confirguration yields greaterdeviation in position tolerancing. A part that issymmetrical, with consistent cross section, willyield the best positioning tolerances. Features suchas holes, pockets or slots that can restrict thecasting s ability to shrink properly will effect theposition features around the restricting feature. Alinear dimension may be affected approximately5% because of a restricting feature. Features thathave heavy sections near, can be effected by thevolumetric shrinkage of the heavy section. Thiswill pull or distort the feature in the direction of theheavy feature. It is difficult to predict the exactamount of the pull or distortion, and the foundrymay wish to rework tooling to minimize its effectand to improve the position general, a consistent cross section with gradualchanges from heavy to light sections that leaves theheavy section open for gating purposes will yieldthe best over all Casting and position " TIR- 5-HOLE TOLERANCES tandard linear tolerances will apply(see page 1).

9 But a cast hole s roundness is affected by thesurrounding metal. If an uneven mass is adjacent,the hole will be pulled out of round. If thesurrounding metal is symmetrical, holes up to "( ) diameter can be held to + "( ).Figure 12 The longer the hole or the more mass in the sectionaround it, the more pronounced the pull effect. A (see Fig. 12)shows the effect of holeshrinkage concavity which will be present to someextent in all castings. The top and bottomopenings will be approximate dimensions, whilethe center will be a larger diameter. Thru holesthat require clearance can be held to fairly closetolerances if the large diameter (bow) in the centeris ignored. If the sidewalls of the hole are used asbearing surfaces, simple reaming should besufficient. B shows how a heavier section closeto the hole distorts the shrinkage pattern. The holediameter is distorted by the additional shrinkage ofthe heavier section. The figure graphically showsthe pattern of distortion present to a greater orlesser degree in every Casting with heavier massaffecting HOLESS ince curved holes are formed by either solublewax or preformed ceramic cores, normal tolerancerequirements tend to double (see ).

10 Multiply the tolerance on all dimensionscontrolling such a feature by two. Since such holescannot be sized, a diameter tolerance of anadditional +/- " per inch may be RADII, FILLETSI nternal radii and fillets improve the strength andintegrity of the Casting . Shrinkage and crackingare reduced. Internal radii and fillets require thewidest tolerance possible. They are difficult tocontrol and can receive approximate checks byradius 13 CONTOURS, RADII AND CAMSWhen dimensioning a Casting radii, it must beremembered that volumetric shrinkage occursduring cooling which affects external radii andcontours. Most of the volumetric shrinkage willoccur in the center of the regards to a flat Casting , the designconsiderations for concavity are discussed in theFlatness section, page 2. The same concavity alsoaffects castings that have contour, but with moredramatic results. In this case, it is advised thatwhen dimensioning your drawing that a basicdimension along with a profile tolerance be used.