6.2.3.2 machining accuracy influenced by forces
（1）influencedbythepositionvariationoftheforces
Oneofthefeaturesofthemachiningsystemstiffnessisthatthestiffness
varieswiththepositionvariationoftheforces,whichmakesthevariationofthediameterindifferentsection,andcausestheshapeerror.
（2）Influenced by the variation of the force size
Thenon-uniformofthematerialhardnessormetalremovalwillcausethevariationofthecuttingforce,whichwillmakethenon-uniformdeformationandleadtomachiningerror.Fig.6.20showsasituationwhenashortcylindricalstockwithellipseerrorwasmachinedonalathe.Ineachrotation,thebackengagementwillvaryfromthemaximumvalueap1totheminimumvalueap2，whilst,thecuttingforcewillalsovaryfromthemaximumFY1totheminimumFY2，fromtheformulaY=Fy/K,theyielddeformationofthecuttercausedbythevariationofthecuttingforceisY1andY2.
Fig.6.20 the copy of the stock shape error movie
According to the cutting theory,the cutting force is proportional to the back engagement under certain condition
FY1?CaP1;FY2?CaP2where：C?CFYfYFYKFY——constatnt N／mm；
——back engagement，mm ——normal cutting forces N
aP1FY1ap2FY2
