Lecture No 111 Fundamentals of Metal removal processes Dr. Ramon E. Goforth Adjunct Professor of Mechanical Engineering Southern Methodist University

Lecture No 112 Outline of Lecture Basic information on material removal Factors involved in material removal Independent variables Dependent variables Machining Processes Machining Economics Machines Lecture 10 Lecture 11 Lecture 12

Lecture No 113 Independent variablesIndependent variables in cutting Workpiece material - "machinability" Cutting tools Cutting parameters Presence or absence of fluid

Lecture No 114 Functions of Cutting Fluids Reduce friction and wear Reduce forces and energy consumption –30% of total energy can go into friction and heat generated Cooling the cutting zone Wash away chips Protect new surfaces from corrosion

Lecture No 115 Cutting fluids Basically four types –Oils –Emulsions –Semisynthetic –Synthetics

Lecture No 116 Dry Machining Scenario Turn off fluid –Friction increases –Shear angle decreases –Shear strain increases –Chip is thicker –Can form built up edge Friction increases further –Total energy increases Temperature increases –Dimensions change »machining inaccurate –Surface finish likely to deteriorate –Tool wear increases

Lecture No 117 Dry Machining Scenario HOWEVER –Recent studies are taking a hard look at dry machining to minimize environmental impact of waste fluids

Lecture No 118 Good cooling practice

Lecture No 119 Independent variablesIndependent variables in cutting Workpiece material - "machinability" Cutting tools Cutting parameters Presence or absence of fluid Characteristics of the machine tool

Lecture No 1110 Characteristics of the Machine The machine provides the power and ensures that the tool is maintained in the chosen location relative to the workpiece –Stiffness Deflection under load - inaccurate cuts –Dynamic response Vibrations - chatter - rough surfaces –Horse power available Determines the maximum material removal rate –Gearing Determines the speeds and feeds available

Lecture No 1111 Independent variablesIndependent variables in cutting Workpiece material - "machinability" Cutting tools Cutting parameters Presence or absence of fluid Characteristics of the machine tool Fixture design

Lecture No 1112 Fixture Design Fixtures hold the workpiece fixed while the cutting tool acts on it –Movement under the cutting force not desirable –Deflection of the workpiece under cutting force not desirable Not truly independent –Vibration of the workpiece undesirable

Lecture No 1113 Summary of Independent Variables in Metal Removal Workpiece material - "machinability" Cutting tools Cutting parameters Presence or absence of fluid Characteristics of the machine tool

Lecture No 1114 Dependent Variables Forces and energy dissipated Temperature rise Tolerances of workpiece after machining Surface finish of workpiece after machining Wear and failure of tool Type of chip produced

Lecture No 1115 Relationships among the variables Forces Power Temp Rise Tolerances Surface Finish Chip Type Tool Degradation Tool Choice Workpiece properties Cutting fluid Machine Tool Characteristics Fixture design Process Variables

Lecture No 1116 Force and Energy Consumption Important to know force to –Avoid excessive distortion in workpiece, tools Distortion gives rise to inaccuracies - tolerances –Allow adequate fixturing to be designed –Determine the work done by force which ends up as heat Important to know Power to –Choose a machine with adequate power capabilities –Estimate how long it will take to machine a part –Estimate the rate at which heat is generated

Lecture No 1117 The Common Cutting Diagram Kalpakjian p 595/546 Chip Tool Workpiece R FnFn V   Shear Zone Friction Zone

Lecture No 1118 Forces F = R sin  N= R cos  F s = F cos  F t cos  F s = F sin  F t sin   F/N = (F t + F c tan  )/(F t - F c tan   is rake angle:  is shear angle:  is friction angle Kalpakjian p 608/546

Lecture No 1119 Forces R is the resultant force consisting of –thrust force, F t and –cutting force, F c –OR –F, Frictional force, –N normal force perpendicular to –friction force F t can be + or - depending on rake angle and friction R is balanced by an equal and opposite force which resolves into –Shear force, F s and –Shear normal force F n Kalpakjian p 608

Lecture No 1120 Force Diagram FtFt N R F FcFc   FnFn R FsFs Workpiece tool Shear Zone V chip Friction Zone  But, forces usually computed from measured machine power or measured with sensors

Lecture No 1121 Power Power = F c V where F c is the cutting force and V is the tool velocity Specific Energy = power/volume = shearing energy + friction energy = F s V/wt 0 V + F/wt o (V c /V) where t 0 is the depth of cut, w is the width of cut Example on page 611 illustrates that 30% of the energy can go into friction Energy can also go into rubbing friction if tool is dull

Lecture No 1122 Specific Energy Kalpakjian p 611/548

Lecture No 1123 Where the Energy Goes Overcoming friction Shearing the metal Temperature rise in the Workpiece Tool Chip Creating chips Higher temperatures Cause dimensional changes in the workpiece Induce thermal damage in the machine surface Affect strength, hardness and wear resistance of the cutting tool Eventually distort the machine tool itself Shearing normally gives good surfaces Poor tool/feed/speed selection can produce poor surfaces Dull tools also generate heat through rubbing of the workpiece surface

Lecture No 1124 Dependent Variables Forces and energy dissipated Temperature rise

Lecture No 1125 Temperature Distribution Kalpakjian p 613/550

Lecture No 1126 Temperature Distribution Kalpakjian p 613/550

Lecture No 1127 Dependent Variables Forces and energy dissipated Temperature rise Tolerances of workpiece after machining

Lecture No 1128 Tolerances Tolerances on a machine part depend on –Forces generated Distort the part and its fixturing –Software Tools Distort the tool and its holder –Depends on machine and tool design Distort the machine itself –Depends on the machine design –Temperature generated Thermal induced expansion of all components in the system results in machining errors

Lecture No 1129 Tolerances

Lecture No 1130 Dependent Variables Forces and energy dissipated Temperature rise Tolerances of workpiece after machining Surface finish of workpiece after machining

Lecture No 1131 Surface Finish and Integrity Surface finish describes the geometry Surface Integrity pertains to the mechanical properties –Fatigue life, corrosion resistance

Lecture No 1132 Surface Finish and Integrity Factors affecting surface integrity include –Workpiece temperature during processing –Residual stresses induced by the shearing –Metallurgical effects (phase transformations) –Plastic deformation –Tearing –Built up edge on chip Table on Page 685/616 provides surface roughness for various processes

Lecture No 1133 Machining Processes and Surface Finish

Lecture No 1134 Dependent Variables Forces and energy dissipated Temperature rise Tolerances of workpiece after machining Surface finish of workpiece after machining Wear and failure of tool

Lecture No 1135 Tool Life Very important economic factor –Cost of tools –Cost of damaged workpiece –Cost of rework due to inaccurate machining Machinability of part has direct influence

Lecture No 1136 Tool Life Abrasion and high temperature cause wear on –The face mostly craters –The flank High forces and shocks (interrupted cutting)cause chipping –Fracture of the tool –Produces holes and gouges in part Poorly machinable materials can give a built up edge –Material adheres to edge of tool and causes inaccuracies and extra friction

Lecture No 1137 Tool Life Formula for tool life due to wear and abrasion –VT n = C C, n are constants V is cutting speed, T is time in minutes –VT n d x f y = C d is depth of cut, f is feed rate, x and y are constants –Tool life, T given by T = C 1/n V -1/n d -x/n f -y/n –For n=0.15, x= 0.15, y=.06 T=C 7 V -7 d -1 f -4

Lecture No 1138 Tool Life for Different Materials Kalpakjian P 617/553

Lecture No 1139 Dependent Variables Forces and energy dissipated Temperature rise Tolerances of workpiece after machining Surface finish of workpiece after machining Wear and failure of tool Type of chip produced

Lecture No 1140 Type of Chip Produced Discontinous chips, continuous strands, continuous serrated strands, built up edge (on tool)

Lecture No 1141 Type of Chip Produced Depends on the: –machinability of the workpiece –the design of the cutting tool –the design of the tool holder

Lecture No 1142 Summary The results of choosing a specific set of the independent variables influences: –Forces and energy dissipated –Temperature rise –Tolerances of workpiece after machining –Surface finish of workpiece after machining –Wear and failure of tool –Type of chip produced Forces and power are important for choice of machine for a job Next week we discuss some of the individual processes