机械精度设计与检测技术 (14).pdf
FLATNESS MEASUREMENTSECTION 4.3Flatness MeasurementFlatness measurement is required for a lot of components including wafers,optical and mechanical parts.Flatness MeasurementFlatness comes in useful if a feature is to be definedon a drawing that needs to be uniformly flat withouttightening any other dimensions on the drawing.The flatness tolerance references two parallel planesthat define a zone where the entire reference surfacemust lie.Method(1):Maximum Straightness Method By measuring the straightness deviation in several differentdirections,maximum straightness method takes the maximumstraightness deviation as the flatness deviation.A5A4A3A2A1B1B3B5C5C4C3C2C1000008210+60+4+3+4+2+5+101A5A3A4A2A1B1C1C5Straight edge workpieceMeasuredIndicatorSupportUnit:11164101484Method(2):Three Point MethodThe flatness deviation is measured by supporting the workpiece on threewidely separated supports.By adjusting the supports,three corners areadjusted to same height.The maximum height difference is the flatnessdeviation.Flatness devition：=6-(-3.5)=9.5+3.5+5.5+1+63.50.5222maxminBut,If we chose another three corners00-8-4+8+4-0.51.5+1-20.53.5262Flatness devition：=6-(-2)=89.5This means,there is measurement error because of different corner choice.CDEFBAHGIPlateSupportDatum PlateWorkpiece MeasuredindicatorMethod(3):Diagonal MethodBy adjusting the supports,two diagonals are adjusted toparallel to the plate.The maximum height difference is theflatness deviation.-0.51.5+1-20.53.5262Flatness deviation：=3.5-(-2)=5.500+2+4-4-21.53.5+12-1.51.5222CDEFBAHGIPlateSupportDatum PlateWorkpiece MeasuredindicatorMethod(4):Minimum Zone MethodBy adjusting the supports,two maximum points and twominimum point are adjusted to parallel to the plate.Themaximum height difference is the flatness deviation.Flatness deviation：=3-(-2)=51.53.5+12-1.51.522200-1-0.5+1+0.513+11-12232CDEFBAHGIPlateSupportDatum PlateWorkpiece MeasuredindicatorSummaryAmong four measurement methods,the most accuratemethod is the minimum zone method.But this method isalso the most difficult method to operate.The relationship between the four methods is shown inthe chart bellow.Minimum zone methodDiagonal methodThree point methodMaximum straightness methodAccuracyDifficulty
