Chapter 13: Forming Processes. Forming Processes Forming Processes: the choice of manufacturing...

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Transcript of Chapter 13: Forming Processes. Forming Processes Forming Processes: the choice of manufacturing...

Chapter 13: Forming Processes

Chapter 13: Forming Processes1Forming ProcessesForming Processes: the choice of manufacturing process depends on the size, shape and quality of the componentDesigners must be familiar with different manufacturing processes (advantages, disadvantages, cost, machines necessary) to wisely recommend a cost effective method

2Casting ProcessesCasting is the process whereby parts are produced by pouring molten metal into a mold. Casting processes classified byType of mold or patternPressure or force to fill mold

Conventional sand, shall and plaster molds use a permanent pattern, but the mold is used only once.Permanent molds and die-casting dies are machined in metal or graphite sections and are employed for a large number of castingsInvestment castings and the relatively new full mold process involve both an expendable mold and an expendable pattern3Sand mold castingMost widely used for metalsPermanent pattern of metal or wood that shapes the mold cavity when loose molding material (fine sand & a binder)is compacted around the patternSections:Bottom (drag)Top (cope)Intermediate (cheeks) when requiredMolten metal is poured into the sprue, connecting runners channel metal to the moldRiser cavities located over heavier sections of the castingVents release gasesCore for hollow castings4Sand Mold CastingsShrinkage allowance-Metal or wooden pattern slightly larger than part to be castDrafts or slight tapers allow for easy withdrawl from the sand moldAfter a sand mold is used the sand is broken and casting removed. Excess metal, gates, risers are removed and remelted5MANUFACTURING MATERIALSEQUENCE IN PREPARING A SAND CASTING

6MANUFACTURING MATERIALMOLD CASTING TECHNIQUES

7Other CastingsShell mold refractory sand is bonded by a resin forming a thin shell mold and a reusable, heated metal pattern platePlaster mold plaster of paris and fillers mixed with water form a slurry which is poured around a reusable metal or rubber pattern and set to form a gypsum moldPermanent mold uses a permanent metal mold to produce many castingsInvestment mold investment refers to the refractory material used to encase the wax patterns. An expendable pattern and mold usedFull mold a consumable pattern is made of foamed plastic (individual castings)8Other Casting ContinuedCentrifugal a permanent mold is rotated rapidly about the axis of the casting. Castings are smooth, sound, and clean on the outsideContinuous continuously pouring molten metal into a water-jacketed mold. The metal solidifies in the mold & the solid billet exits continuously into a water spray (uniform section rounds, ovals, squares, rectangles, plates)Die one of the least expensive, fastest & most efficient methods to produce metal parts. Molten metal is formed into a die or mold. Used for nonferrous alloys. Submerged-plunger & cold-chamber types

9MANUFACTURING MATERIALPERMANENT MOLD CASTING

10MANUFACTURING MATERIALINVESTMENT MOLD CASTING

11MANUFACTURING MATERIALCASTING EQUIPMENT AND PROCESSES

12Selection of ProcessType of metalNumber of castings requiredShape & size of castingsDimensional accuracy requiredCasting finish requiredEconomicsThe number of finishing operations-the processes that provide the closest dimensions, best surface finish, & the most intricate detail usually require the smallest number of finishing operations13Solidification of metal in a moldHeat dissipates from the surface through the moldSolidification commences from the outside & progresses inward in a series of layersSolidifying metal contracts in volume & a shrinkage cavity may form in the centerFinal castings are smaller than the mold cavityFillets and radii at corners prevent rapid cooling/shrinkage at corners hot spots

14MANUFACTURING MATERIALCOOLING EFFECTS ON MOLD CAVITIESFILLED WITH MOLTEN METAL

15General design rulesCasting soundness-feeder heads can be placed to offset liquid shrinkageFillet or round all sharp anglesBring the minimum number of adjoining sections togetherDesign all sections as nearly uniform in thickness as possibleAvoid abrupt section changes-eliminate sharp corners at adjoining sections: not exceed a 2:1 ratioDesign ribs for maximum effectiveness-increase stiffness and reduce massAvoid bosses & pads unless absolutely necessary

16General design rules continuedUse curved spokes-less likely to crackUse an odd number of spokes-more resilient to casting stresses Consider wall thicknessesGray-iron & aluminum: .16 in minimumMalleable iron & steel: .18 in minimumBronze,brass,magnesium: .10 minimumParting lines: a line along which the pattern is divided for molding or along which sections of the mold separate (consider shape of casting, elimination of machining on draft surfaces, methods of supporting cores, location of gates & feeders)Drill holes in castings: small holes are drilled and not cored

17MANUFACTURING MATERIALDESIGN MEMBERS SO THAT ALL PARTS INCREASEPROGRESSIVELY TO FEEDER RISERS

18MANUFACTURING MATERIALFILLET ALL SHARP ANGLES

19Drafting practices-information for cast partsMaterial-physical characteristics of the metalMachining allowances- sufficient excess metal should be provided for all machined surfaces (Table 13-2 guidelines)Surface textureDraft anglesLimits of cast surfaces that must be controlledLocating pointsParting linesFillets & radii-generous should be specified on the drawingCasting tolerances-Table 13-2 general guidelinesDraft-a draft or taper on all surfaces perpendicular to the parting line to facilitate removal of the pattern & ejection of the casting1 for external surfaces and 2 for internal surfaces20MANUFACTURING MATERIALCAST PART WORKING DRAWING

21MANUFACTURING MATERIALPATTERN WORKING DRAWING

22MANUFACTURING MATERIALDRAFT ANGLES

23Casting DatumsIn many cases a drawing is made of the fully machined end product, & casting dimensions, draft, machining allowances are left entirely to the pattern maker or foundry worker. For mass-production it is advisable to make a separate casting drawing with carefully selected datums to ensure that parts will fit into machining jigs and fixtures and will meet final requirements after machining.Two sets of datum surfaces to provide reference pointsCastingMachiningDatum surface or base surface for casting-Datum ASecondary & tertiary surfaces at right angles to eachother & primary datum surface- Datum B & Datum C24Machining Datum

Primary datum surface for machining-Datum D is the first surface on the casting to be machinedSecondary & Tertiary datum surfaces for dimensioning purposesDatum-locating dimension-the dimension between eaching casting datum surface and the corresponding machining datum surfaceDimensions-directly from the datums to all main surfacesRegular point-to-point used to maintain a particular relationship between 2 or more surfaces or features (thickness of ribs, height of bosses, projections, depth of grooves, diameters, radii, center distances between holes or similar featuresDimension to surfaces or surface intersections, not radii centers or nonexistent center lines & dimensions should not be repeated

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