Thiết kế flash - Tolerancing

TolerancingMotivationProducts are mass-producedParts manufactured in different parts of world, brought together for assemblyParts must be interchangeableVariability always exists in manufacturing processesDesigner must consider:Allowable range of variation of part featuresHow parts fit together in assemblyTolerancesTolerance specifies range for size and location of features on a part that will allow part to function properlyUsed to control variance that exists on all manufactured partsState tolerances as generously as possibleProvides for a wider variety of processes that can be used to manufacture partKeeps part cost lowA tolerance is the difference between the maximum and minimum size limits on a partDeclaring TolerancesMethods to indicate tolerances include:General tolerance notes E.G., ALL DIMENSIONS HELD TO ± .05Not good practiceLimit tolerancesPlus and minus tolerancesGeometric Dimensioning and Tolerancing (GDT)Limit TolerancesLimit tolerances state upper and lower limits for the dimension range The tolerance is the difference between the upper and lower limitsPlus/Minus TolerancesPlus/minus tolerances specify a range that is added or subtracted from a dimension value Tolerance StackingTolerance for one dimension added to next dimension in a chain Results in a large variation in location of last feature in chain Baseline DimensioningLocates a series of features from a common base featureTolerances do not stack up FitRefers to how tightly (loosely) mating parts must fit together when assembledTolerances specified based upon fit Fit TypesClearance – internal member (e.g., shaft) always smaller than external member (e.g., hole)Interference – internal member always larger than external memberTransition – either internal or external may be larger so that parts either slide together or can be forced togetherFit Types - 2AllowanceDifference between smallest hole size and largest shaft sizeAllowance is either minimum clearance (+) or maximum interference (-) between parts Basic Hole SystemUses external member (often a hole) as basic size from which tolerances (to achieve a given fit) are determinedSmallest hole used as basic size A hole can be made larger, but not smallerChoose standard drill size for hole, turn down shaft to fit holeEnglish units Basic size – theoretical size from which a tolerance is assignedBasic Shaft SystemUses internal member (often a shaft) as basic size from which tolerances are derivedLargest shaft size chosen as basic sizeA shaft can be made smaller, but not largerChoose standard shaft diameter, drill hole to suit fitLess common than basic hole systemEnglish unitsMetric Fit TablesHole-basis systemShaft-basis system Tolerancing