Material Removal Process (Metal Machining Process)

Material Removal Process (Metal Machining Process)
1.1 Theory of Metal Cutting overview, theory of chip formation, force & merchant equation, power & energy, cutting temperature 1.2 Machining Operations and Machine Tools turning, drilling, milling, machine centers, cutting tool technology BDA 3052 Manufacturing Technology

Material Removal Processes Introduction
It is shaping operations, it remove material from a starting workpart so the remaining part has the desired geometry Divided into three main groups: 1) Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling 2) Abrasive processes – material removal by hard, abrasive particles, e.g., grinding 3) Nontraditional processes - various energy forms other than sharp cutting tool to remove material BDA 3052 Manufacturing Technology

Why Machining is Important
Variety of work materials can be machined Most frequently used to cut metals Variety of part shapes and special geometric features possible, such as: Screw threads Accurate round holes Very straight edges and surfaces Good dimensional accuracy and surface finish BDA 3052 Manufacturing Technology

Disadvantages with Machining
Wasteful of material Chips generated in machining are wasted material, at least in the unit operation Time consuming A machining operation generally takes more time to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming BDA 3052 Manufacturing Technology

Machining in Manufacturing Sequence
Generally performed after other manufacturing processes, such as casting, forging, and bar drawing Other processes create the general shape of the starting workpart Machining provides the final shape, dimensions, finish, and special geometric details that other processes cannot create BDA 3052 Manufacturing Technology

Machining Nomenclature of single point tool
BDA 3052 Manufacturing Technology

Seven elements of single‑point tool geometry; and (b) the tool signature convention that defines the seven elements. BDA 3052 Manufacturing Technology

Orthogonal Cutting Model
Simplified 2-D model of machining that describes the mechanics of machining fairly accurately Figure Orthogonal cutting: (a) as a three‑dimensional process. BDA 3052 Manufacturing Technology

Orthogonal and Oblique cutting

Assumptions in orthogonal cutting (Merchant theory)
01.The tool is perfectly sharp and no contact along clearance face. 02. The shear surface is a plane extending upward from the cutting edge. 03.The cutting edge is a straight line, extending perpendicular to the direction of motion and generates a plane surface as the work moves past it. 04.The chip does not flow to either side. 05.The depth of cut is constant. 06.Width of the tool is greater than the work piece. 07.The work moves relative to the tool with uniform velocity. 08.A continuous chip is produced with no built up edge. BDA 3052 Manufacturing Technology

Assumptions in orthogonal cutting (Merchant theory)
09. Chip is assume to shear continuously across plane AB on which the shear stress reaches the value of shear flow stress. 10.Width of chip is remains equal to the width of the work piece. i.e. Plane strain conditions exist. BDA 3052 Manufacturing Technology

Difference between orthogonal and oblique cutting
Orthogonal cutting Oblique cutting 01.The cutting edge of the tool is perpendicular to the direction of the tool travel. The cutting edge is inclined at angle with the normal direction of the tool travel. 02. The cutting edge clears the width of the work piece on either ends. The cutting edge may or may not clear the width of the workpiece. 03. The chip flows over the tool. The chip coils in tight. The chip flows on the tool face making an angle with the normal cutting edge. The chip flows side ways in a long curl. 04. Only two components of the cutting force acting on the tool. Three components of the forces acting on the tool. 05.Maximum chip thickness occurs at the middle. Maximum chip thickness may not occur at middle. 06. For the given feed rate and DOC, the force which act or shears the metal acts on a smaller area and therefore, the heat developed per unit area due to friction along the tool work interface is less and the tool life is less. It acts on larger area and thus tool life is more.

Machining Cutting action involves shear deformation of work material to form a chip As chip is removed, new surface is exposed (a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a). BDA 3052 Manufacturing Technology

Cutting Process in Turning

Relationship between chip thickness, rake angle and shear plane angle

Chip Thickness Ratio where r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separation Chip thickness after cut always greater than before, so chip ratio always less than 1.0 BDA 3052 Manufacturing Technology

Determining Shear Plane Angle
Based on the geometric parameters of the orthogonal model, the shear plane angle  can be determined as: where r = chip ratio, and  = rake angle BDA 3052 Manufacturing Technology

Example of Problem In a machining operation that approximates orthogonal cutting, the cutting tool has a rake angle = 10. The chip Thickness before the cut to = 0.50 mm and the chip thickness after the cut tc = mm. Calculate the shear plane angle and the shear strain in the operation. Answer :  = 25.4  = 2.386 BDA 3052 Manufacturing Technology

Example of Problem 2 In an orthogonal cutting operation, the tool has a rake angle = 15. The chip thickness before the cut = 0.30 mm and the cut yields a deformed chip thickness = 0.65 mm. Calculate (a) the shear plane angle and (b) the shear strain for the operation. BDA 3052 Manufacturing Technology

Shear Strain in Chip Formation
Shear strain during chip formation: (a) chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation. BDA 3052 Manufacturing Technology

Shear Strain Shear strain in machining can be computed
from the following equation, based on the preceding parallel plate model:  = tan( - ) + cot  where  = shear strain,  = shear plane angle, and  = rake angle of cutting tool BDA 3052 Manufacturing Technology

Example of Problem 2 In an orthogonal cutting operation, the tool has a rake angle = 15. The chip thickness before the cut = 0.30 mm and the cut yields a deformed chip thickness = 0.65 mm. Calculate (a) the shear plane angle and (b) the shear strain for the operation. BDA 3052 Manufacturing Technology

Chip Formation More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction. BDA 3052 Manufacturing Technology

Four Basic Types of Chip in Machining
Discontinuous chip Continuous chip Continuous chip with Built-up Edge (BUE) Serrated chip BDA 3052 Manufacturing Technology

1. Discontinuous Chip Low cutting speeds Large feed and depth of cut
Brittle work materials Low cutting speeds Large feed and depth of cut High tool‑chip friction BDA 3052 Manufacturing Technology

2. Continuous Chip Ductile work materials High cutting speeds
Small feeds and depths Sharp cutting edge Low tool‑chip friction BDA 3052 Manufacturing Technology

Continuous with BUE Ductile materials Low‑to‑medium cutting speeds
Tool-chip friction causes portions of chip to adhere to rake face BUE forms, then breaks off, cyclically BDA 3052 Manufacturing Technology

Serrated Chip Semicontinuous - saw-tooth appearance
Cyclical chip forms with alternating high shear strain then low shear strain Associated with difficult-to-machine metals at high cutting speeds Figure 21.9 (d) serrated. BDA 3052 Manufacturing Technology

Effect of Higher Shear Plane Angle
Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature Effect of shear plane angle  : (a) higher  with a resulting lower shear plane area; (b) smaller  with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation BDA 3052 Manufacturing Technology

Forces Acting on Chip Friction force F and Normal force to friction N
Shear force Fs and Normal force to shear Fn Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting BDA 3052 Manufacturing Technology

Resultant Forces Vector addition of F and N = resultant R
Vector addition of Fs and Fn = resultant R' Forces acting on the chip must be in balance: R‘’ must be equal in magnitude to R R’ must be opposite in direction to R R’ must be collinear with R BDA 3052 Manufacturing Technology

Cutting Forces BDA 3052 Manufacturing Technology

Coefficient of Friction
Coefficient of friction between tool and chip: -(1) Friction angle related to coefficient of friction as follows: -(2) BDA 3052 Manufacturing Technology

Shear Stress Shear stress acting along the shear plane: -(3) -(4)
where As = area of the shear plane -(4) Shear stress = shear strength of work material during cutting BDA 3052 Manufacturing Technology

Cutting Force and Thrust Force
F, N, Fs, and Fn cannot be directly measured Forces acting on the tool that can be measured: Cutting force Fc and Thrust force Ft Forces in metal cutting: (b) forces acting on the tool that can be measured BDA 3052 Manufacturing Technology

Merchant’s circle diagram

Forces in Metal Cutting
Equations can be derived to relate the forces that cannot be measured to the forces that can be measured: F = Fc sin  + Ft cos  (5) N = Fc cos  ‑ Ft sin  (6) Fs = Fc cos  ‑ Ft sin  (7) Fn = Fc sin  + Ft cos  (8) Based on th ese calculated force, shear stress and coefficient of friction can be determined BDA 3052 Manufacturing Technology

Example of Problem 3 Cutting force = 1559 N Thrust force = 1271 N
Width of cutting = 3 mm Rake angle = 10 Shear plane angle = 25.4 Original Thickness = 0.5 mm Determine the shear strength of the work material. shear stress, S / shear strength,  = 247 N/mm2 BDA 3052 Manufacturing Technology

Forces in Metal Cutting
From equation 3, the force diagram (Merchant ‘s Circle Diagram), can be used to derived the following equations: BDA 3052 Manufacturing Technology

The Merchant Equation From equation 3, 4 and 7, Merchant Equation for shear stress can be expressed as, BDA 3052 Manufacturing Technology

The Merchant Equation Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle  that minimizes energy, given by Derived by Eugene Merchant Based on orthogonal cutting, but validity extends to 3-D machining BDA 3052 Manufacturing Technology

What the Merchant Equation Tells Us
To increase shear plane angle Increase the rake angle Reduce the friction angle (or coefficient of friction) BDA 3052 Manufacturing Technology

Power and Energy Relationships
A machining operation requires power The power to perform machining can be computed from: Pc = Fc  where Pc = cutting power (Nm/s); Fc = cutting force (N); and  = cutting speed (m/min) BDA 3052 Manufacturing Technology

In U.S. customary units, power is traditional expressed as horsepower HPc = Fc/33,000 where HPc = cutting horsepower, hp BDA 3052 Manufacturing Technology

Gross power to operate the machine tool Pg or HPg is given by or where E = mechanical efficiency of machine tool Typical E for machine tools  90% BDA 3052 Manufacturing Technology

Unit Power in Machining
Useful to convert power into power per unit volume rate of metal cut Called unit power, Pu or unit horsepower, HPu or where MRR = material removal rate (mm3/s) BDA 3052 Manufacturing Technology

Specific Energy in Machining
Unit power is also known as the specific energy U Units for specific energy are typically N‑m/mm3 or J/mm3 BDA 3052 Manufacturing Technology

Cutting Temperature Approximately 98% of the energy in machining is converted into heat This can cause temperatures to be very high at the tool‑chip The remaining energy (about 2%) is retained as elastic energy in the chip BDA 3052 Manufacturing Technology

Cutting Temperature is Important
High cutting temperatures Reduce tool life Produce hot chips that pose safety hazards to the machine operator Can cause inaccuracies in part dimensions due to thermal expansion of work material BDA 3052 Manufacturing Technology

Cutting Temperature Analytical method derived by Nathan Cook from dimensional analysis using experimental data for various work materials where T = temperature rise at tool‑chip interface; U = specific energy; v = cutting speed; to = chip thickness before cut; C = volumetric specific heat of work material; K = thermal diffusivity of work material BDA 3052 Manufacturing Technology

Example Problem 6 Cutting speed = 100 m/min
Chip original thickness = 0.5 mm Thermal diffusivity = 50 mm2/s Specific Energy = 1.038 Volumetric specific heat work material = 3 x10-3 J/mm3 Find the mean temperature rise at the tool-chip Interface. BDA 3052 Manufacturing Technology

Cutting Temperature Experimental methods can be used to measure temperatures in machining Most frequently used technique is the tool‑chip thermocouple Using this method, Ken Trigger determined the speed‑temperature relationship to be of the form: T = K vm where T = measured tool‑chip interface temperature, and v = cutting speed K and m depend on the cutting conditions and work material BDA 3052 Manufacturing Technology

Machining A material removal process in which a sharp cutting tool is used to mechanically cut away material so that the desired part geometry remains Most common application: to shape metal parts Most versatile of all manufacturing processes in its capability to produce a diversity of part geometries and geometric features with high precision and accuracy Casting can also produce a variety of shapes, but it lacks the precision and accuracy of machining

Classification of Machined Parts
Rotational - cylindrical or disk‑like shape Nonrotational (also called prismatic) - block‑like or plate‑like Machined parts are classified as: (a) rotational, or (b) nonrotational, shown here by block and flat parts.

Machining Operations and Part Geometry
Each machining operation produces a characteristic part geometry due to two factors: Relative motions between tool and workpart Generating – part geometry determined by feed trajectory of cutting tool Shape of the cutting tool Forming – part geometry is created by the shape of the cutting tool

Generating Shape Figure Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling.

Forming to Create Shape
Forming to create shape: (a) form turning, (b) drilling, and (c) broaching.

Forming and Generating
Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.

Turning Single point cutting tool removes material from a rotating workpiece to generate a cylinder Performed on a machine tool called a lathe Variations of turning performed on a lathe: Facing Contour turning Chamfering Cutoff Threading

Turning Turning operation.

Turning Operation Close-up view of a turning operation on steel using a titanium nitride coated carbide cutting insert (photo courtesy of Kennametal Inc.)

Facing Tool is fed radially inward

Contour Turning Instead of feeding tool parallel to axis of rotation, tool follows a contour that is other than straight, thus creating a contoured shape

Chamfering Cutting edge cuts an angle on the corner of the cylinder, forming a "chamfer"

Cutoff Tool is fed radially into rotating work at some location to cut off end of part

Threading Pointed form tool is fed linearly across surface of rotating workpart parallel to axis of rotation at a large feed rate, thus creating threads

Cutting Conditions in Turning - 1
The rotational speed in turning related to the desired cutting speed at the surface of the cylindrical workpiece by the equation: N = rotational speed, rev/min;  = cutting speed, m/min, And Do = original diameter of the part, m. 72

The change in diameter is determined by the depth of cut, d: Do – Df = 2d Do = original diameter, mm; Df = final diameter, mm d = depth of cut 73

The feed in turning is generally expressed in mm/rev. This feed can be converted to linear travel rate in mm/min by the formula: fr = Nf fr = feed rate, mm/min; f = feed mm/rev 74

The time to machine from one end of a cylindrical workpart to the other is given by: Tm = L/fr Tm = time of actual machining, minutes; and L = length of the cylindrical workpart, mm 75

The volumetric rate of material removal rate can be most conveniently determined by the following equation: MRR = vfd MRR = material removal rate, mm3/min, f = feed, mm 76

Cutting Conditions in Turning
Problem 1 A cylindrical workpart 200 mm in diameter and 700 mm long is to be turned in an engine lathe. Cutting conditions are as follows: cutting speed is 2.30 m/s, feed is 0.32 mm/rev, and depth of cut is 1.80 mm. Determine (a) cutting time, and (b) metal removal rate. 77

Problem 1 A cylindrical workpart 200 mm in diameter and 700 mm long is to be turned in an engine lathe. Cutting conditions are as follows: cutting speed is 2.30 m/s, feed is 0.32 mm/rev, and depth of cut is 1.80 mm. Determine (a) cutting time, and (b) metal removal rate. 78

Problem 2 A work materials are to be turned to final size of 175 mm length having diameter of 60 mm. Total length of the work material is 300 mm. A single point tool having a certain degree of rake angle is used. The work material rotates at 1400 RPM. The feed is 0.35 mm / revolution. Final size of the work material is 51 mm. Calculate: 1. Cutting velocity, 2. Time taken to machine to the length of 55 mm, 3. Total Material removal rate to get 51 mm diameter. 79

Milling Machining operation in which work is fed past a rotating tool with multiple cutting edges Axis of tool rotation is perpendicular to feed Creates a planar surface Other geometries possible either by cutter path or shape Other factors and terms: Interrupted cutting operation Cutting tool called a milling cutter, cutting edges called "teeth" Machine tool called a milling machine 80

Peripheral Milling vs. Face Milling
Cutter axis parallel to surface being machined Cutting edges on outside periphery of cutter Face milling Cutter axis perpendicular to surface being milled Cutting edges on both the end and outside periphery of the cutter 81

METHODS OF MILLING-1 1) Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the workpiece is fed to the right in up milling. 82

METHODS OF MILLING-2 2) Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling 83

Slab Milling Basic form of peripheral milling in which the cutter width extends beyond the workpiece on both sides 84

Slotting Width of cutter is less than workpiece width, creating a slot in the work 85

Conventional Face Milling
Cutter overhangs work on both sides 86

Profile Milling Form of end milling in which the outside periphery of a flat part is cut 87

Pocket Milling Another form of end milling used to mill shallow pockets into flat parts 88

Surface Contouring Ball‑nose cutter fed back and forth across work along a curvilinear path at close intervals to create a three dimensional surface form 89

End Milling Cutter diameter is less than work width, so a slot is cut into part 90

Machining Centers Highly automated machine tool can perform multiple machining operations under CNC control in one setup with minimal human attention Typical operations are milling and drilling Three, four, or five axes Other features: Automatic tool‑changing Pallet shuttles Automatic workpart positioning 91

Milling Operation High speed face milling using indexable inserts (photo courtesy of Kennametal Inc.). 92

Cutting Conditions in Milling - 1
The cutting speed is determined at the outside diameter of a milling cutter. N = rotational speed, rev/min;  = cutting speed, m/min, And Do = outside diameter of a milling cutter,mm. 93

The feed, f in milling is usually given as a feed per cutter tooth; called the chip load, it represents the size of the chip formed by each cutting edge. fr = Nnt f fr = feed rate, mm/min; N = spindle speed, rev/min; nt = number of teeth on the cutter; f = chip load in mm/tooth 94

The feed, f in milling is usually given as a feed per cutter tooth; called the chip load, it represents the size of the chip formed by each cutting edge. fr = Nnt f fr = feed rate, mm/min; N = spindle speed, rev/min; nt = number of teeth on the cutter; f = chip load in mm/tooth 95

The material removal rate, MRR = wdfr w = width; d = depth of cut; fr = feed rate, mm/min; 96

Peripheral Milling 97

Approach distance A, to reach full cutter depth given by: d = depth of cut, mm, and D = diameter of the milling cutter, mm 98

The time to mill the workiece Tm is therefore; 99

Face Milling – cutter is centered
100

Where A and O are each to half the cutter diameter; A = O = D/2 D= cutter diameter, mm 101

Face Milling – cutter is offset
102

Where A and O are each to half the cutter diameter; w= width of the cut, mm 103

The time to mill the workiece in face milling,Tm is therefore; 104

Cutting Conditions in Milling
Problem 1 A peripheral milling operation is performed on the top surface of a rectangular workpart which is 400 mm long by 60 mm wide. The milling cutter, which is 80 mm in diameter and has five teeth, overhangs the width of the part on both sides. The cutting speed is 70 m/min, the chip load is 0.25 mm/tooth, and the depth of cut is 5.0 mm. Determine (a) the time to make one pass across the surface, and (b) the maximum material removal rate during the cut. 105

Cutting Conditions in Milling
Problem 2 A face milling operation is performed to finish the top surface of a steel rectangular work piece 350 mm long by 55 mm wide. The milling cutter has four teeth (cemented carbide inserts) and a 85 mm diameter. Cutting conditions are: v = 600 m/min, f = 0.35 mm / tooth, and d = 3.5 mm. Determine: a) the time to make one pass across the surface. b) the metal removal rate during the cut. 106

Drilling Creates a round hole in a workpart
Compare to boring which can only enlarge an existing hole Cutting tool called a drill or drill bit Machine tool: drill press

Through Holes vs. Blind Holes
Through‑holes - drill exits opposite side of work Blind‑holes – does not exit work opposite side Two hole types: (a) through‑hole, and (b) blind hole.

Reaming Used to slightly enlarge a hole, provide better tolerance on diameter, and improve surface finish

Tapping Used to provide internal screw threads on an existing hole
Tool called a tap

Counterboring Provides a stepped hole, in which a larger diameter follows smaller diameter partially into the hole

Cutting Conditions in Drilling - 1
Letting N represent the spindle rev/min,  = cutting speed, m/min; D = the drill diameter,mm. 112

Feed can be converted to feed rate using the the same equation as for turning: fr = Nf fr = feed rate, mm/min; N = spindle speed, rev/min; f = feed in drilling, mm/rev 113

The time to drill through holes; Tm= machining time, min; t = work thickness, mm; fr = feed rate, mm/min 114

The allowance is given by; A = approach allowance, mm;  = drill point angle 115

The time to drill blind holes; Tm= machining time, min; d = hole depth, mm; fr = feed rate, mm/min 116

Cutting Conditions in Drilling
Problem 1 A drilling operation is to be performed with a 12.7 mm diameter twist drill in a steel workpart. The hole is a blind hole at a depth of 60 mm and the point angle is 118. The cutting speed is 25 m/min and the feed is 0.30 mm/rev. Determine (a) the cutting time to complete the drilling operation, and (b) metal removal rate during the operation, after the drill bit reaches full diameter. 117

CUTTING TOOL TECHNOLOGY
Tool Life Tool Materials Tool Geometry Cutting Fluids

Cutting Tool Technology
Two principal aspects: Tool material Tool geometry

Three Modes of Tool Failure
Fracture failure Cutting force becomes excessive and/or dynamic, leading to brittle fracture Temperature failure Cutting temperature is too high for the tool material Gradual wear Gradual wearing of the cutting tool

Preferred Mode: Gradual Wear
Fracture and temperature failures are premature failures Gradual wear is preferred because it leads to the longest possible use of the tool Gradual wear occurs at two locations on a tool: Crater wear – occurs on top rake face Flank wear – occurs on flank (side of tool)

Tool Wear Figure Diagram of worn cutting tool, showing the principal locations and types of wear that occur.

Figure Crater wear, (above), and flank wear (right) on a cemented carbide tool, as seen through a toolmaker's microscope (photos by K. C. Keefe, Manufacturing Technology Lab, Lehigh University).

FLANK WEAR and BUE

CRATER WEAR

BUILT UP EDE

Taylor Tool Life Equation
Relationship is credited to F. W. Taylor where v = cutting speed; T = tool life; and n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion used n is the slope of the plot C is the intercept on the speed axis at one minute tool life

Tool Life Criteria in Production
Complete failure of cutting edge Visual inspection of flank wear (or crater wear) by the machine operator Fingernail test across cutting edge Changes in sound emitted from operation Chips become ribbon-like, stringy, and difficult to dispose of Degradation of surface finish Increased power Workpiece count Cumulative cutting time

Tool Materials Tool failure modes identify the important properties that a tool material should possess: Toughness ‑ to avoid fracture failure Hot hardness ‑ ability to retain hardness at high temperatures Wear resistance ‑ hardness is the most important property to resist abrasive wear

Quiz In a machining operation that approximates orthogonal cutting, the cutting tool has a rake angle = 20. The chip thickness before the cut to = 0.80 mm and the chip thickness after the cut tc = 1.35 mm. Calculate the shear plane angle and the shear strain in the operation.  = tan( - ) + cot  BDA 3052 Manufacturing Technology

Kuiz 3) Determine time processing for face milling
1) Determine time processing to drill through hole 2) Determine approach distance A in peripheral milling process 3) Determine time processing for face milling BDA 3052 Manufacturing Technology

Material Removal Process (Metal Machining Process)

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