【正文】 3σis very important , and is widely used in the analysis of machining errors. 6σrepresents the machining accuracy reachable in a certain machining condition for a kind of process operation.Generally speaking, for the process chosen of a sequence, the tolerance of the sequence T and 8 / 9the standard difference σ obey 6σ≤T. For some reason, σ and μ is unknown. Fortunately, we can use the total mean value of the sample , and the standard difference SUsage of the distribution graph1) Assess the feature of the machining error2) Determine the sequential ability and the level number The sequential ability the undulation of the machining error under steady machining conditions. If the sizes obey the normal distribution, the range of the sizes should fall into 6σ. So the sequential ability is 6σ. The level number of the sequential ability represents the degree of satisfaction to the machining accuracy, in terms of Cp Cp=T/ (6σ)3) Estimate the waste rate or the qualification rate.CHAPTER FOUR INTEGRITY OF MACHINED SURFACES1 Concept of the machined surface integrity and it’s influence on the ponent functions2 Processing factors influencing the surface roughness and the improvement3 Processing factors influencing the physical and mechanical properties of the surface layer of the ponent and the improvement4 Brief introduction to the vibration in machiningSection 1 Concept of the machined surface integrity and it’s influence on the ponent functionsA Geometric form errors a) Surface roughness(粗糙度)：irregularities with very high frequencyb) Waviness（波度）： irregularities with high frequencyc) Lay/vein direction（紋理方向）：the direction of the marks left by the cutting tool during the forming operationsd) Scars（傷痕）：rarely presented defects on the surface, such as sand pits, air hole, and cracks etc. B The physical, chemical, and mechanical properties of the surface layera) Strain hardening, with the degree and depth of the hardeningb) Metallographic position Changes may occur of the metallographic position due to the cutting heat. c) Residual stress within the surface layerSection 2 Processing factors influencing the surface roughness and the improvementSection 3 Processing factors influencing the physical and mechanical properties of the surface layer of the ponent and the improvementA Strain hardeningThe strain hardening occurs whenever there is any form of plastic deformation, such as warp and aberrance of the crystal lattice , and slippage between the crystal grains, as well as the elongation 9 / 9of the crystal grains. In this procedure the surface layer gains energy and bees hard. So we call it a strengthening procedure. However it is in the state of instability, and it will relieve if there is a chance, say absorbing (taking in) heat (weakening).B Change of the metallographic positionC Residual stressWith factors of plastic deformation (residually pressive stress) and heat (residually tensile stress)residual stresses arising in I turning (speed, rake angle, feed ) 。 Composite fixtures。 Lathe fixtures。 Borer jigs。 c the main surfaces first。 finishing datum: machined locating datum.b) additional/auxiliary datum: the specially designed datum in the workpiece because of the requirement of the mechanical process operations.Gauging datum: during and after machining, datum used for measuring and checking the form, position, and error of size , of the workpiece.Assembling datum: in assembling datum adopted to determine the relative positions of the work or the ponent in the product.Section 3 Design of the routes of machining processes（工藝路線的制定）Consideration must be taken with the locating datum of the workpiece(定位基準(zhǔn)）, the process methods（加工方法）, the order of the processes（加工順序）, heat treatment（熱處理）, testing（檢驗） ， and the related operations（其他工序）. A selection of locating datums(定位基準(zhǔn)的選擇)B selection of economic accuracy and process methods of machining（機加的經(jīng)濟精度與方法）C machining routes for the typical surfaces（典型表面的加工路線——外圓、孔和平面）D order of sequences（工序順序安排）Selecting rules of rough machining datum:I guarantee the proper relative positions .II guarantee reasonable allocation of the allowance for the surface being machined .III facilitate the locating and clamping of the workpiece. For this reason, the selected surfaces as the datums must be to some extent smooth, enoughsized, flasherfree , without other defects such as reservoir and riser or the like. 4 / 9IV the rough locating datum used only for one time (without repetition)Selecting rules of finishing machining datum:I overlapping principle(基準(zhǔn)重合） datum used as the same as design datumII unifying principle（基準(zhǔn)統(tǒng)一）use