Lecture 1 EBB440 Applied Metallurgy Material Removal Process

INTRODUCTION There are two distinct classes of solid-state manufacturing processes involving plastic deformation. Bulk deformation processes which cause significant shape change by deformation in parts but volume is conserved. Machining process is in which material is removed by plastically straining a local region of a workpiece by relative motion of tool and workpiece.

MACHINING IN THE MANUFACTURING SEQUENCE Machining is a secondary processing operation Generally performed after other manufacturing processes, such as casting, forging and bar drawing Other processes create the general shape of the starting work piece. Machining provides the final shape, dimensions, finish, and special geometric details that other processes cannot generate.

Variety of work materials : machining can be applied to a wide variety of work materials. Variety of part shapes and geometric features : may create any regular geometries such as flat planes, round holes, sharp corners, cylinders and etc. e.g : Screw threads, accurate round holes, very straight edges and surfaces Dimensional accuracy : Very close dimensional tolerances can be achieved. Good surface finishes : machining capable of creating smooth surface finishes. ADVANTAGES OF MACHINING

Wasteful of material : Material wastage. More capital, energy and labour are required. Time consuming : More time consuming for shaping parts. Improper machining can affect surface characteristics and other properties. DISADVANTAGES OF MACHINING

MATERIAL REMOVAL PROCESS Categories: Cutting–material removal by a sharp cutting tool, e.g., turning, milling, drilling. Abrasive processes –material removal by hard, abrasive particles, e.g., grinding. Nontraditional processes -various energy forms other than sharp cutting tool to remove material

Conventional machining Turning and related operations Drilling and related operations Milling Other machining operations (shaping, planing, broaching & sawing) Abrasive processes Nontraditional machining Grinding operations Other abrasive processes (honing, lapping & superfinishing) Mechanical energy process Electrochemical machining Thermal energy machining CLASSIFICATION OF MATERIAL REMOVAL PROCESS Material Removal Processes Chemical 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 MACHINING  Cutting tool’s 2 surfaces rake face – directs flow of newly formed chip,oriented at rake angle,  flank – clearance between tool & new work surface oriented at relief angle.  Cutting tool’s 2 surfaces rake face – directs flow of newly formed chip,oriented at rake angle,  flank – clearance between tool & new work surface oriented at relief angle.

Major independent (Inputs) variables affecting the machining process are: The machine tool selected to perform the process. The cutting tool selected (geometry, material, condition, surface finish and sharpness). The properties and parameter of workpiece material. The cutting parameter – speed, feed and depth of cut. Use of cutting fluids. The workpiece holding devices or fixtures or jigs MECHANICS OF MACHINING

Dependant (outputs) variables: Type of chip formed (depends on the workpiece and tool). Force and energy dissipated in the cutting process. Temperature in the workpiece, chip and the tool. Wear and failure of the tool. Surface finish produced on the workpiece after machining MECHANICS OF MACHINING

FUNDAMENTAL INPUTS & OUTPUTS TO MACHINING PROCESS

Most important machining operations: –Turning –Drilling –Milling Other machining operations: –Shaping and planing –Broaching –Sawing MACHINING OPERATION

TURNING using lathe machine single cutting edge remove material from rotating workpiece to form cylindrical shape. speed motion = rotating workpart feed motion = cutting tool moving parallel to axis of rotation of workpiece

using milling machine rotating tool with multiple edges is moved slowly relative to part to generate plane or straight surface. feed motion = work moved perpendicular to tool’s axis of rotation. speed motion = rotating milling cutter Two forms : peripheral milling and face milling (c) peripheral milling, and (d) face milling. MILLING

using drill press machining operation used to create round hole. use rotating cylindrical tool with 2 cutting edges tool is fed in a direction parallel to its axis of rotational into workpiece (b) drilling DRILLING

SHAPING & PLANING (a)Shaping ; speed motion accomplished by moving cutting tool (b) Planing ; speed motion by moving part. Using machine tool called shaper straight, flat surface is created. Interrupted operation also  impact loading to tool upon entry into work. Also limited to low speed

using broaching machine use multiple tooth cutting tool by moving tool linearly relative to work in direction of tool axis Highly productive method, but limited to certain jobs. BROACHING Broaching ; multiple tooth cutting moves linearly relative to work in direction of tool axis.

Boring similar to turning uses single-point tool against a rotating workpiece the difference from turning is that boring performed on inside diameter of an existing hole Sawing Tool consists of a series of narrowly spaced teeth. 3 basic types ; (a) hacksawing, (b) bandsawing & (c) circular sawing. BORING & SAWING

1.Single-Point Tools One dominant cutting edge Point is usually rounded to form a nose radius Turning uses single point tools 2.Multiple Cutting Edge Tools More than one cutting edge Motion relative to work achieved by rotating Drilling and milling use rotating multiple cutting edge tools CUTTING TOOL CLASIFICATION

(a)A single ‑ point tool showing rake face, flank, and tool point (b)a helical milling cutter, representative of tools with multiple cutting edges. CUTTING TOOL Two basic types : Single point tool – turning, boring, shaping, planing Multiple cutting edge tool – drilling, milling, broaching, sawing

Three dimensions of a machining process: –Cutting speed v – primary motion –Feed f – secondary motion –Depth of cut d – penetration of tool below original work surface For certain operations, material removal rate can be computed as MRR = v f d where v = cutting speed; f = feed; d = depth of cut CUTTING CONDITION IN MACHINING MRR (mm 3 /s) = v(mm/s) x f(mm) x d(mm).

DEFINATION TERM Speed (V) -Relates velocity of the cutting tool to the work piece Feed (f) -Amount of material removed per revolution or per pass of the tool over the work piece. linear translation of tool with respect to the work piece. Depth of Cut (d) -Distance the tool has plunged into the surface. Material Removal Rate (MRR) -Mass of material removed per unit machining time

Speed, feed, and depth of cut in turning. CUTTING CONDITION FOR TURNING

In production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts. Roughing - removes large amounts of material from starting workpart –Creates shape close to desired geometry, but leaves some material for finish cutting –High feeds and depths, low speeds Finishing - completes part geometry –Final dimensions, tolerances, and finish –Low feeds and depths, high cutting speeds ROUGHING vs. FINISHING

Machine tool - a power ‑ driven machine that perform machining operation, including grinding Functions in machining: Holds workpart Positions tool relative to work Provides power at speed, feed, and depth that have been set The term is also applied to machines that perform metal forming operations MACHINE TOOLS

CUTTING MODEL Cutting edge is perpendicular to the direction of cutting speed Cutting edge is oblique to the direction of cutting speed Orthogonal MachiningOblique Machining

Simplified 2-D model of machining that describes the mechanics of machining fairly accurately. uses wedge-shaped tool, cutting edge normal to direction of cutting speed. Chip is formed by shear deformation along shear plane, oriented at angle  with surface of work. Orthogonal cutting: (a) as a three ‑ dimensional process (b) two dimensions side view ORTHOGONAL CUTTING MODEL

where r = chip thickness ratio; t o = thickness of the chip prior to chip formation; and t c = chip thickness after separation Chip thickness after cut always greater than before, so chip ratio always less than 1.0. Orthogonal cut has width dimension, w. Chip Thickness Ratio

Geometry cutting model allows to establish relationship between the chip thickness, rake angle, and shear plane angle. l s = length of the shear plane, therefore t o = l s sin  and t c = l s cos (  – α). where r = chip ratio, and  = rake angle Determining Shear Plane Angle This can be arranged to determine the shear plane angle  as follow:

Figure show 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. Shear Strain in Chip Formation

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 Shear Strain

Examples In machining operation that approximates orthogonal cutting, the cutting tool has a rake angle = 10°. The chip thickness before the cut t o = 0.5 mm and the chip thickness after the cut t c = mm. Calculate the shear plane angle and the shear strain in the operation. The chip thickness ratio, r = = The shear plane angle tan  = (cos 10) 1 – (sin 10)  = 25.4 ° Shear strain  = tan(  -  ) + cot  = tan ( ) + cot 25.4 = = 2.386

Chip Formation Figure show view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool ‑ chip friction.

Four Basic Types of Chip in Machining : Discontinuous chip Continuous chip Continuous chip with Built-up Edge (BUE) Serrated chip Chip Formation

Discontinuous Chip Brittle work materials Low cutting speeds Large feed and depth of cut High tool ‑ chip friction

Ductile work materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool ‑ chip friction Continuous Chip

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. Continuous with BUE Build up edge Tool

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

Friction force F and Normal force to friction N Shear force F s and Normal force to shear F n Figure shows forces in metal cutting: (a) forces acting on the chip in orthogonal cutting Forces Acting on Chip

F, N, F s, and F n cannot be directly measured Forces acting on the tool that can be measured: –Cutting force F c and Thrust force F t Figure shows forces in metal cutting: forces acting on the tool that can be measured Cutting Force and Thrust Force

A machining operation requires power. The power to perform machining can be computed from: P c = F c v where P c = cutting power; F c = cutting force (N); and v = cutting speed (m/s) Power and Energy Relationships

In U.S. customary units, power is traditional expressed as horsepower (dividing ft ‑ lb/min by 33,000) where HP c = cutting horsepower, hp Power and Energy Relationships

Gross power to operate the machine tool P g or HP g is given by or where E = mechanical efficiency of machine tool Typical E for machine tools  90% Power and Energy Relationships

Useful to convert power into power per unit volume rate of metal cut Called unit power, P u or unit horsepower, HP u or where MRR = material removal rate Unit Power in Machining

Unit power is also known as the specific energy U Units for specific energy are typically N ‑ m/mm 3 or J/mm 3 (in ‑ lb/in 3 ) Specific Energy in Machining

Examples Continuing from the previous examples, determine cutting power and specific energy to perform the machining process if the cutting speed = 100 m/min. Summarizing the data and results from previous examples, to = 0.5 mm, w = 3.0 mm, Fc = 1557 N. Power in the operation, Specific energy, P c = F c v = 1557 N x 100 m/min = 155,700 N-m/min = 155,700 J/min = 2595 J/s = 2595 W = 155, (10 3 )(3)(0.5) = N-m/mm 3