机械制造技术基础课件-新第2章课件(4).ppt

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1、2.1.4 Tool wear and tool life,2.1.4.1 Types of tool wear,2.1.4.2 Tool wear process and criteria,2.1.4.3 Causes of tool wear,2.1.4.4 Tool life,2.1.4.5 Influence factors of tool life,Edge chopping,Wear on rake face,Wear on rake face,2.1.4 Tool wear and tool life,Metallurgical Structure before wear,Met

2、allurgical Structure after wear,2.1.4 Tool wear and tool life,2.1.4.1 Types of tool wear,Tool wear can be divided into normal wear and non-normal wear.,The tool wear increases with time is called normal wear. It can be further divided into the following kinds according to the worn region,(1) normal

3、wear,Flank wear,Wear on the rake face,Simultaneous wear on rake face and flank,The non-normal wear is not gradually formed as time goes by, but formed when the cutting edge is suddenly broken, or the cutting edge is curled, or the tool bit is crushed etc.,(2) non-normal wear,Wear on rake face,Wear o

4、n flank,2.1.4.1 Types of tool wear,2.1.4.1 Types of tool wear,Tool Wear Criteria：when the tool becomes so dull that it can not work again, the wear caused in this limitation is called the Wear Criteria .Usually it is judged by observation,As flank wear is easy to measure, it is often used as tool we

5、ar criteria. ISO regulates that VB measured on flank surface, half way of the back engagement ，is taken as the tool wear criteria,2.1.4.1 Types of tool wear,Homogeneous wear within the region B VB=0.3mm; Inhomogeneous wear within the region B VB=0.6mm; Depth of crater on rake face KT =0.06+0.3f (f f

6、eed, mm/r).,Table 2.9 Wear criteria VB(mm),In ISO standards, the tool wear criteria (high speed steel and carbide insert) from metal cutting research are:,The wear criteria for carbide turning tools obtained in practice are listed in Table 2.9.,Tool Wear Criteria：,2.1.4.1 Types of tool wear,2.1.4.3

7、Causes of tool wear,Cutting tool wear takes place under conditions of high temperature and pressure, therefore, the causes of tool wear are very complicated, involving mechanical, physical, chemical effects and phase variation.,1 Abrasive wear,2 Adhesive wear,3 Diffusion wear,4 Phase change wear,5 O

8、xidizing wear,2.1.4.4 Tool life,(1) Definition of tool life,Tool life is defined as the time that the wear land develops from zero to the wear criterion at which the tool is regarded as “dull”, and is denoted by T.,Tool life can also be expressed as the cutting distance lm or the number N of machine

9、d parts before the tool reaches the wear criterion.,Tool life is an important index to judge the tools cutting property. It is convenient to judge where the tool is dull or not by using tool life rather than inspecting the width of the wear land.,vc,(a) Cutting speed,Where Acoefficient related to th

10、e experiment condition, minfluence index of vc on T, for HSS tool: m=0.11, for Carbide alloy tool, m=0.20.3, for ceramics m=0.4,(b) Feed and back engagement fT and apT have the similar relationship, f , ap=,(2) Factors affecting tool life,2.1.4.4 Tool life,(3) Selection of rational tool life,there a

11、re two methods to correctly select the tool life:,Tool life Tp of the maximum productive rate, or the minimum time required to machine a part.,The total time required to perform an operation is: tprtmtltc tm/T,where tmcutting time; tl support time(non-cutting time); tctime per tool changing (min/cha

12、nge ); Ttool life.,2.1.4.4 Tool life,Tool Life Tp for Maximum productive rate,2.1.4.4 Tool life,The average cost Cpr for each workpiece in an operation is:., Tool life Tc of the minimum productive cost,Mthe unit expenses, including labor, machine and etc Cttool cost, including resharpening and labor

13、 etc,2.1.4.4 Tool life,Differentiate the equation and let it to be zero , the tool life Tc for minimum productive cost is:,Notice: TcTp Could you tell why?,The detailed values of tool life can be selected from technical information and handbooks.,2.1.4.4 Tool life,2.1.4.5 Influence factors of the to

14、ol life,(1) Influence of cutting parameters;,(2) Influence of tools geometric parameters ;,(3) Influence of workpiece materials ;,(4) Influence of tool materials .,(1) Cutting parameters,An experiential formula of tool life,Allowable Cutting speed,Question: in what sequence to select the cutting par

15、ameters in order to raise the machine efficiency ?,2.1.4.5 Influence factors of the tool life,(2) Cutting tool geometry,Rake angle,Relief angle,Main cutting edge angle , minor cutting edge angle and nose radius,(3) Material of workpiece,Strength, hardness, elongation, heat conductivity,(4) Material of Cutting tool,Hot hardness, impact toughness,2.1.4.5 Influence factors of the tool life,