Manufacturing Processes Chap 21 Fundamentals of Machining Processes

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Manufacturing Processes Chap. 21 - Fundamentals of Machining Processes

Manufacturing Processes Chap. 21 - Fundamentals of Machining Processes

Fundamentals of Cutting • Definition: – Machining: the removal of unwanted material from a

Fundamentals of Cutting • Definition: – Machining: the removal of unwanted material from a workpiece in the form or chips. – High precision machining can involve tolerances of. 0001” or less. – There are 7 basic chip formation processes: – Shaping – Drilling – Turning – Sawing – Milling – Broaching – Grinding

Many factors influence cutting process • Independent – Tool material, coatings, condition. – Tool

Many factors influence cutting process • Independent – Tool material, coatings, condition. – Tool shape, finish, sharpness. – Workpiece material, condition, temperature. – Cutting parameters: feed, speed, depth of cut. – Cutting fluids. – Machine tool properties: stiffness & damping. – Workholding & fixturing.

Many factors influence cutting process • Dependent – Type of chip produced. – Force

Many factors influence cutting process • Dependent – Type of chip produced. – Force / Energy dissipated in cutting process. – Temperature rise in workpiece, chip, tool. – Tool Wear / Failure. – Final surface finish produced.

Metal Cutting Processes • Complex process due to many variables involved: – different materials

Metal Cutting Processes • Complex process due to many variables involved: – different materials behave differently – very large strains occur – sensitivity to tool geometry, material, environment, other. • One of the most common is turning: • workpiece is rotated and cutting tool removes material as it moves to the left after setting a depth of cut. • A chip is then produced which moves up the face of the tool.

Fundamentals of Cutting • Basic cutting operating parameters: Speed Feed Depth of Cut

Fundamentals of Cutting • Basic cutting operating parameters: Speed Feed Depth of Cut

Fundamentals of Cutting • Speed (V) – Velocity of the rotating workpiece to stationary

Fundamentals of Cutting • Speed (V) – Velocity of the rotating workpiece to stationary tool. (velocity of outermost point of workpiece) – Units: sfpm or ipm • Feed (fr) – Amount of material removed per rev. – Units: in/rev*, ipm (* in turning) • Depth of Cut (d) – Distance plunged into work. Units: in.

Basic Formulas • d = (D 1 - D 2)/2 = d. o. c.

Basic Formulas • d = (D 1 - D 2)/2 = d. o. c. (depth of cut) • V=( D N / 12) = surface speed; 1 s D 1 in inches, Ns in rpm, V in surface ft/min. • MRR ~ 12 Vfrd = Material Removal rate (MRR in 3/min); fr = feed rate (in/rev) • MRR is also viewed as Volume removed per Unit time = V / CT Cutting Time CT (in minutes) = (L + Allowance) / fr Ns – L is the length of the cut. A length allowance is added for tool entry and exit. – The allowance formulas vary by process.

How to select parameters? A. Select cutting tool: most critical component. • Select its

How to select parameters? A. Select cutting tool: most critical component. • Select its material & geometry: depends on (material to be cut) B. Determine input parameters: • Tables have been developed to associate the parameters. – They are conservative starting points for parameter selection. • RPM depends on V; • V, f and d. o. c. depend on many parameters: – (process, material, hardness, cutting tool material)

How to select parameters? B. Determine input parameters: (cont. ) – Table will show

How to select parameters? B. Determine input parameters: (cont. ) – Table will show initial values for V, f, d. o. c. (depth of cut) • fr, d. o. c. : usually larger for roughing, smaller for finishing. • V: usually smaller for roughing, larger for finishing. • They are conservative starting points for parameter selection. – Once V is selected from table, solve for Ns via V = ( D 1 Ns / 12) – Now, estimate MRR via MRR ~ 12 Vfrd

Understanding Chip Formation • Most cutting processes are three-dimensional. • A 2 D model

Understanding Chip Formation • Most cutting processes are three-dimensional. • A 2 D model is a simplified version of this process (orthogonal model).

Terminology used in Orthogonal Cutting • Shear Plane • Shear angle • Shear Strain

Terminology used in Orthogonal Cutting • Shear Plane • Shear angle • Shear Strain • Rake side of Tool • Rake angle • Plate thickness w • Relief side of Tool • Relief Angle • Uncut Chip thickness t • Chip thickness tc • Depth of Cut to

Understanding Chip Formation • Refer to figure 20. 3 and 20. 4 – Rake

Understanding Chip Formation • Refer to figure 20. 3 and 20. 4 – Rake angle and Relief angle are the most critical elements of the tool geometry. – Workpiece moves at velocity V. – Uncut chip thickness is to, cut chips have thickness tc – Chip has velocity Vc. – Shearing takes place along a very narrow region called shear zone or shear plane at a shear angle .

2 D Model: Orthogonal Cutting • Below shear plane, work is undeformed. • Above

2 D Model: Orthogonal Cutting • Below shear plane, work is undeformed. • Above shear plane, chip is formed and moving up the face of the tool over a length lc • Computing the shear angle: – 1. Find Chip Thickness or Cutting Ratio: • r = to / tc = sin / (cos - ) where (0 < r < 1); – to = depth of cut; tc = chip thickness – tc is always greater that to so r < 1 – (to is usually measured with a micrometer)

2 D Model: Orthogonal Cutting 2. Replace rc in: • Tan = (rc cos

2 D Model: Orthogonal Cutting 2. Replace rc in: • Tan = (rc cos ) / (1 – rc sin ) and solve for shear angle . However, notice that rc = to/tc = sin / cos ( – ) = Vc / V and Vs / V = cos / cos ( – ) The shear strain = 2 cos / ( 1 + sin ) depends exclusively on the rake angle.

Orthogonal Cutting - Shear Angle • Shear angle influences force and power requirements, chip

Orthogonal Cutting - Shear Angle • Shear angle influences force and power requirements, chip thickness and temperature. • Therefore, relationship between shear angle, material properties and cutting process variables are important. • A relationship between shear angle and workpiece props. / cutting process variables was developed. = 45 o + /2 - /2 where = friction angle

Orthogonal Cutting - Friction Angle • Coefficient of friction at the interface => =

Orthogonal Cutting - Friction Angle • Coefficient of friction at the interface => = tan • Equation = 45 o + /2 - /2 shows: – As rake angle decreases / friction at tool-chip interface increases – The shear angle decreases and chip becomes thicker. – More energy has to be dissipated by the chip because the shear strain required is higher. – Temperature also increases.

Orthogonal Cutting - Cutting Speed V • Recall that: • From continuity: r =

Orthogonal Cutting - Cutting Speed V • Recall that: • From continuity: r = to / tc V c tc = V t o Since chip thickness (tc) > depth of cut (to), chip velocity Vc has to be lower than the cutting speed V. So Vc = Vr = V sin / (cos - )

Orthogonal Cutting - Cutting Speed V • To calculate Vs (velocity at which shearing

Orthogonal Cutting - Cutting Speed V • To calculate Vs (velocity at which shearing takes place): V / cos ( - ) = Vs / (cos ) = Vc / sin and r = to / tc = Vc / V

Cutting Forces and Power

Cutting Forces and Power

Cutting Forces and Power • The figure shows force diagram in orthogonal cutting. •

Cutting Forces and Power • The figure shows force diagram in orthogonal cutting. • Cutting Force Fc acts in direction of cutting speed (supplies energy for cutting). • Thrust Force Ft acts normal to cutting velocity (normal to workpiece). • Computing the resultant force of Fc and Ft we obtain R. • R is balanced by an equal and opposite force along the shear plane.

Cutting Forces and Power • R can be decomposed along the shear plane: F

Cutting Forces and Power • R can be decomposed along the shear plane: F = R sin and N = R cos • Each is balanced by a normal force Fn and a shear force Fs respectively, where Fs = Fc cos - Ft sin Fn = Fc sin - Ft cos • F / N Ratio is m = F/N = (Ft + Fc tan ) / (Fc - Ft tan ) where typically 0. 5 < < 2

Cutting Forces and Power • Fs is used to compute the shear stress along

Cutting Forces and Power • Fs is used to compute the shear stress along the shear stress, defined as: ts = Fs / As where As = to w / sin and t = the uncut chip thickness, w = workpiece width Substituting, ts = ( Fc sin cos - Ft sin 2 / t w ) (in psi)

Cutting Forces and Power • Thrust force is important: – tool holder, fixturing &

Cutting Forces and Power • Thrust force is important: – tool holder, fixturing & machine tool must be stiff enough to withstand thrust force. Ft = R sin ( - ) or Ft = Fc tan ( - ) where = tan-1 ( F / N ) Note: when rake angle is 0 o, F = Ft and N = Fc • Ft can be positive or negative: – If > , Ft is positive (downward). – If < , Ft is negative (upward). • Conclusion: it is possible to have an upward thrust force if rake angles are high and/or with low friction between tool and chip.

Cutting Forces and Power • Power requirements are important for proper machine tool selection.

Cutting Forces and Power • Power requirements are important for proper machine tool selection. • Cutting force data is used to: – properly design machine tools to maintain desired tolerances. – determine if the workpiece can withstand cutting forces without distortion.

Cutting Forces and Power In Handout 21 -11: • Fc = primary cutting force

Cutting Forces and Power In Handout 21 -11: • Fc = primary cutting force in the direction of velocity vector • (largest force; accounts for ~99% of power requirements) • Ff = feed force acting in direction of tool feed. • (usually 50% if Fc ; accounts for a small % of power requirements) • Fr = radial/thrust force acting perp. to machined surface. • (usually 50% if Ff ; accounts for very little % of power requirements)

How do cutting forces vary with cutting parameters? • As V is increased, all

How do cutting forces vary with cutting parameters? • As V is increased, all forces drop some and then stay constant. • As d. o. c. is increased, Fc and Ff increase, Fr stays constant*. • As feed rate is increased, Fc, Ff and Fr increase*. (*) due to change in As • Conclusions: – In general, increasing speed, feed or d. o. c. will increase power reqs. – Doubling speed doubles required HP. – Doubling feed or d. o. c. doubles Fc. – In general, increasing speed does not increase Fc, but increases chip length.

Cutting Forces and Power • Power = Force x Velocity P = Fc. V

Cutting Forces and Power • Power = Force x Velocity P = Fc. V (ft-lb/min) • Power is mainly dissipated in the shear zone. Shearing Power = Fs. Vs • If w is the width of cut, specific energy for shearing is Us = Fs. Vs / w d V = Fs cos / w d cos ( - ) (psi)

Cutting Forces and Power • Power dissipated by friction: Pfriction = F. Vc (units:

Cutting Forces and Power • Power dissipated by friction: Pfriction = F. Vc (units: ft-lb/min) • Specific energy for friction Uf = F V c / w d V = F r c / w d (psi) • Total specific energy Ut = Fc V / w d V = Fc / w d and Ut = Us + Uf (psi) Note: when rake angle is 0, F = Ft and N = Fc

Cutting Forces and Power • Horsepower at the spindle: – HP = Fc. V

Cutting Forces and Power • Horsepower at the spindle: – HP = Fc. V / 33, 000 • Specific Horsepower: – HPs = HP/MRR (hp/in 3/min) = Fc / 396, 000 frd • Motor Horsepower: – HPm = (HPs x MRR x CF) / E • (CF is a correction factor ~1. 25, E is machine efficiency ~80%) • Max. d. o. c. = HPm E / 12 HPs V fr CF • Primary cutting force can be estimated by – Fc ~ (HPs x MRR x 33, 000) / V

Cutting Forces and Power • Horsepower conversions 1 Horsepower = 42. 43 British Thermal

Cutting Forces and Power • Horsepower conversions 1 Horsepower = 42. 43 British Thermal Units per minute 33000 foot-pounds-force per minute 550 foot-pounds-force per second 10. 69 kilocalories per minute 0. 7457 kilowatts 745. 7 watts 1. 0139 horsepower (metric)

Chips produced in Metal Cutting • Different materials will generate different types of chips

Chips produced in Metal Cutting • Different materials will generate different types of chips depending on their properties (ductility, hardness, structure, composition). • The type of chip produced significantly influences the surface finish of the workpiece and the overall cutting operation. • There are two sides to a chip: – a shiny burnished side, which is in contact with the cutting tool. – the other side does not come in contact with the tool, it is jagged and rough.

Types of Chips produced in Metal Cutting • Types include: – Continuous Chips –

Types of Chips produced in Metal Cutting • Types include: – Continuous Chips – Discontinuous – Built-up edge (BUE) – Serrated / segmented

Types of Chips produced in Metal Cutting • Continuous Chips – Ductile materials produce

Types of Chips produced in Metal Cutting • Continuous Chips – Ductile materials produce a Continuous chip (i. e. aluminum) – Chip rubs against tool longer and generates added heat. – Chip is long and curly. – usually associated with good surface finish – not always desirable: can become tangled in machine tool holder or fixture if too long.

Types of Chips produced in Metal Cutting • Discontinuous Chips – Brittle materials produce

Types of Chips produced in Metal Cutting • Discontinuous Chips – Brittle materials produce a Discontinuous chip (i. e. gray cast iron) – Chip fails not in shear but in brittle fracture. – Segments are attached to each other loosely or firmly. – Usually form with: • materials contains inclusions/impurities • very low/high cutting speeds. • large docs • low rake angles • poor cutting fluid

Types of Chips produced in Metal Cutting • Build-Up Edge Chips – form when

Types of Chips produced in Metal Cutting • Build-Up Edge Chips – form when there is bonding affinity between tool and work material. – affinity is higher in cold work than in annealed metals. – layers of workpiece material are deposited on tool. – build-up becomes larger and breaks off. – part is carried by tool side of chip, other part is deposited on work surface: bad for surface finish.

Types of Chips produced in Metal Cutting • Build-Up Edge Chips – usually undesirable,

Types of Chips produced in Metal Cutting • Build-Up Edge Chips – usually undesirable, a thin stable BUE protects the rake face of the tool. – Can reduce by: • decreasing d. o. c. • increasing rake angle • using sharp tools • using effective cutting fluid

Types of Chips produced in Metal Cutting • Serrated Chips – Semi-continuous chips with

Types of Chips produced in Metal Cutting • Serrated Chips – Semi-continuous chips with zones of high / low shear strain. – Usually found in materials with low thermal conductivity. – Have sawtooth like appearance.

Surface Finish Integrity • Surface finish affects dimensional accuracy & geometric tolerances. • Surface

Surface Finish Integrity • Surface finish affects dimensional accuracy & geometric tolerances. • Surface integrity affects properties: fatigue life, corrosion resistance. • Build-up edge (BUE) has the greatest influence on surface finish. • Ceramic tools have less tendency to form BUE.

Chip Curl • Chips develop a natural curl as they leave the work surface.

Chip Curl • Chips develop a natural curl as they leave the work surface. • increases with smaller d. o. c. • larger rake angle • decreases tool-chip friction

Chip Breakers • Used to avoid long continuous chips to become entangled in machine

Chip Breakers • Used to avoid long continuous chips to become entangled in machine tool/fixturing. • Traditional chip breaker consists of piece of metal attached to rake face of tool which bends chip & breaks it. • Cutting tools / inserts now have built-in chip breaker features. • They increase effective rake angle and therefore shear angle.

Tool Life, Wear and Failure • Tool wear is caused by: – high localized

Tool Life, Wear and Failure • Tool wear is caused by: – high localized stresses – high temperatures – sliding of chip along rake face – sliding of tool along cut surface

Tool Life, Wear and Failure • Tool wear: – Affects quality of machined surface

Tool Life, Wear and Failure • Tool wear: – Affects quality of machined surface & dimensional accuracy. – Is a gradual process. – Wear rate depends on material being cut, tool shape, cutting fluids, cutting parameters, among others. • Two types: – flank wear and crater wear.

Tool Life, Wear and Failure • Flank Wear – Occurs on relief face of

Tool Life, Wear and Failure • Flank Wear – Occurs on relief face of tool. (See Fig 20. 15 a) – Due to rubbing of tool on machined surface, causing abrasive wear. – Due to high temperatures that affect tool material.

Taylor Tool Life Formula VTn = C – V: – T: – n: –

Taylor Tool Life Formula VTn = C – V: – T: – n: – C: cutting speed time in minutes to develop a flank wear land depends on tool/work materials and cutting conditions constant Notes: • When T=1, C = cutting speed. • Each combo of tool material/cutting condition has its own n & C values. • These values are determined experimentally.

Predicting Tool Life & Wear • Cutting speed is the most significant process variable

Predicting Tool Life & Wear • Cutting speed is the most significant process variable in tool life, along with d. o. c. and feed rate. • Introducing into the Taylor formula. . . VTn dx fy = C – – d: depth of cut f: feed rate (in/rev in turning) x, y: determined experimentally for each cutting condition. Typical values are: n=0. 15, x=0. 15, y=0. 6.

Taylor Tool Life Formula • Rearranging, T = C 1/n V -1/n d -x/n

Taylor Tool Life Formula • Rearranging, T = C 1/n V -1/n d -x/n f -y/n • Plugging in the typical values, T = C 7 V-7 d-1 f -4 • Notes: – If feed rate or d. o. c. are increased, must decrease cutting speed V. – A reduction in speed can result in an increase in material removed due to increased d. o. c. & feed rate.

Tool Life Curves • Are plots of experimental data obtained by performing cutting tests

Tool Life Curves • Are plots of experimental data obtained by performing cutting tests at different conditions with different tool / work material combinations. • Cutting parameters are varied. • Generally tool life decreases with cutting speed. • Work material has a strong impact on tool life. • Harder materials will wear tool faster. • Tool life curves have a valid range and should be applied within it.

Tool Life Curves • Recommended cutting speed for HSS tools yields a tool life

Tool Life Curves • Recommended cutting speed for HSS tools yields a tool life of 60120 min. • For carbide tools, 30 -60 min. • There is an optimum cutting speed for each work / tool material combination. • Increasing cutting speed reduces tool life. Reducing speed increases tool life but decreases material removal rate. • Material removed is directly proportional to distance traveled by the tool: decreasing cutting speed helps remove more material between tool changes.

Tool Life • Example: • Let n = 0. 5, C = 400. Calculate

Tool Life • Example: • Let n = 0. 5, C = 400. Calculate % increase in tool life is cutting speed is reduced by 50%. • Solution: – Taylor Formula: VTn = C – V 1 = initial speed – V 2 = reduced speed – C = constant V 2 T 2 n = V 1 T 1 n

Tool Life • Let V 2= 0. 5 V 1 • Replacing, 0. 5

Tool Life • Let V 2= 0. 5 V 1 • Replacing, 0. 5 V 1 T 20. 5 = V 1 T 10. 5 (T 2/T 1)0. 5 = 1 / 0. 5 T 2 / T 1 = 4 • Tool life change = ((T 2 - T 1) / (T 1)) = T 2/T 1 - T 1/T 1 = 4 - 1 = 3 (> 1) • Tool life is increased by 300%.

Tool Life, Wear and Failure • Crater Wear – See Fig. 20. 15 and

Tool Life, Wear and Failure • Crater Wear – See Fig. 20. 15 and 20. 18. – Occurs of rake face of tool. – Changes the chip-tool interface geometry. – Caused by temperature at interface and chemical affinity between tool and work. – Due to diffusion of atoms of cutting tool to chip. – Location of maximum crater wear coincides with max. temp. at the tool-chip interface.

Tool Life, Wear and Failure • Chipping – Breaking away of a small piece

Tool Life, Wear and Failure • Chipping – Breaking away of a small piece of cutting tool edge. – Can be large or small pieces. – Sudden process. – Can really damage part surface finish, integrity, part accuracy. • Causes: – Mechanical shock (due to interrupted cutting) – Thermal shock (cycles in tool temperature) develops thermal cracks. – High positive rake angles.