Lect5 Machining

transcript

Machining Processes and Machine

• Machining is the removal of material and modification

of the surfaces of a workpiece

• Machining involves secondary and finishing operations

• Parts can be manufactured by casting, forming and

shaping processes

• They often require further operations before the

product is ready for use

• Major types of material removal processes:

1. Cutting

2. Abrasive processes

3. Advanced machining processe

• Machining operations is a system consisting of the

1. Workpiece

2. Cutting tool

3. Machine tool

4. Production personnel

Common Machining Processes

Some examples of common machining processes.

Characteristics of Machining

General characteristics of machining processes.

Introduction

• Machining processes has the capability of producing

parts that are round in shape

• Such as miniature screws for the hinges of eyeglass

frames and turbine shafts for hydroelectric power plants

• Most basic machining processes is turning where part

is rotated while it is being machined

• Turning processes are carried out on a lathe or by

similar machine tools

• Highly versatile and produce a wide variety of shapes

Introduction

• In the turning process, the cutting tool is set at a certain

depth of cut (mm) and travels to the left as the

workpiece rotates

• Feed, or feed rate, is the distance the tool travels

horizontally per unit revolution of the workpiece

(mm/rev)

Lathe Operations

Variety of machining operations that can be

performed on a lathe.

Introduction

• Turning is performed at various:

1. Rotational speeds, N, of the workpiece clamped in a

spindle

2. Depths of cut, d

3. Feeds, f, depending on the workpiece materials,

cutting-tool materials, surface finish, dimensional

accuracy and characteristics of the machine tool

The Turning Process

Cutting Speeds for Turning

The range of applicable cutting speeds and

feeds for a variety of cutting-tool materials.

Approximate Ranges of Recommended Cutting

Speeds for Turning Operations

Tool Angles

Designations and symbols for a

right-hand cutting tool. The

designation “right hand” means that

the tool travels from right to left

General recommendations for

tool angles in turning.

Types of Chips

Basic types of chips produced in metal cutting and their micrographs: (a)

continuous chip with narrow, straight primary shear zone; (b) secondary

shear zone at the tool-chip interface; (c) continuous chip with built-up edge;

(d) segmented or nonhomogeneous chip; and (e) discontinuous chip.

FIGURE 8.5 Shiny (burnished)

surface on the tool side of a

continuous chip produced in turning.

Chip Breakers

FIGURE 8.7 (a) Schematic illustration of the action of a chip

breaker. Note that the chip breaker decreases the radius of

curvature of the chip. (b) Chip breaker clamped on the rake

face of a cutting tool. (c) Grooves on the rake face of cutting

tools, acting as chip breakers. Most cutting tools now are

inserts with built-in chip-breaker features.

FIGURE 8.8 Various chips produced in

turning: (a) tightly curled chip; (b) chip hits

workpiece and breaks; (c) continuous chip

moving radially outward from workpiece;

and (d) chip hits tool shank and breaks off.

Source: After G. Boothroyd.

FIGURE 8.44 General view of a typical lathe, showing various major components. Source: Courtesy

of Heidenreich & Harbeck.

CNC Lathe

(a) A computer-numerical-control lathe, with two turrets; these machines have higher power and spindle

speed than other lathes in order to take advantage of advanced cutting tools with enhanced properties;

(b) a typical turret equipped with ten cutting tools, some of which are powered.

Typical CNC Parts

Typical parts made on computer-numerical-control machine tools.

Typical Production Rates

Typical production rates for various cutting operations.

Boring Mill

Schematic illustration of the components of a vertical boring mill.

Drilling, Drills, and Drilling Machines

• Holes are used for assembly with fasteners, for design

purposes or for appearance

• Hole making is the most important operations in

manufacturing

• Drilling is a major and common hole-making process

Drills Two common types of drills: (a)

Chisel-point drill. The function of the

pair of margins is to provide a

bearing surface for the drill against

walls of the hole as it penetrates into

the workpiece. Drills with four

margins (double-margin) are

available for improved drill guidance

and accuracy. Drills with chip-

breaker features are also available.

(b) Crankshaft drills. These drills

have good centering ability, and

because chips tend to break up

easily, they are suitable for producing

deep holes.

Various types of drills and drilling operations.

Drilling, Drills, and Drilling Machines:

Material-removal Rate in Drilling

• The material-removal rate (MRR) in drilling is the

volume of material removed per unit time

Speeds and Feeds in Drilling

General recommendations for speeds and feeds in drilling.

Reamers and Taps

Terminology for a helical reamer.

(a) Terminology for a tap; (b)

illustration of tapping of steel nuts in

high production.

CASE STUDY

Bone Screw Retainer

• A cervical spine implant

Introduction: Milling

Typical parts and shapes produced by the machining processes

Milling and Milling Machines

• Milling is machining operation for a variety of

configurations with the use of a milling cutter

Conventional and Climb Milling

(a) Illustration showing the difference between conventional milling and climb milling.

(b) Slab-milling operation, showing depth of cut, d; feed per tooth, f; chip depth of cut,

tc and workpiece speed, v. (c) Schematic illustration of cutter travel distance, lc, to

reach full depth of cut.

Milling and Milling Machines:

Peripheral Milling

Milling Parameters

• The cutting speed in peripheral milling is the surface

speed of the cutter is

Milling and Milling Machines:

Peripheral Milling

Milling Parameters

Milling Operations

Cutters for (a) straddle milling; (b) form

milling; (c) slotting; and (d) slitting

operations.

Approximate range of recommended cutting speeds for

milling operations.

Milling Machines

(a) Schematic illustration of a horizontal-spindle column-and-knee-type milling machine. (b)

Schematic illustration of a vertical-spindle column-and-knee-type milling machine.

Broaching

(a) Typical parts finished by internal broaching. (b) Parts finished by surface broaching.

The heavy lines indicate broached surfaces; (c) a vertical broaching machine. Source:

(a) and (b) Courtesy of General Broach and Engineering Company, (c) Courtesy of Ty

Miles, Inc.

Broaches

FIGURE 8.61 (a) Cutting action of a

broach, showing various features. (b)

Terminology for a broach.

Terminology for a pull-type internal

broach, typically used for enlarging long

holes.

Saws and Saw Teeth

FIGURE 8.63 (a) Terminology

for saw teeth. (b) Types of saw

teeth, staggered to provide

clearance for the saw blade to

prevent binding during sawing.

(a) High-speed-steel teeth welded on a steel

blade. (b) Carbide inserts brazed to blade

teeth.

Manufacture

(a) Schematic illustration of gear

generating with a pinion-shaped gear

cutter. (b) Schematic illustration of

gear generating in a gear shaper,

using a pinion-shaped cutter; note that

the cutter reciprocates vertically. (c)

Gear generating with a rack-shaped

cutter. (d) Three views of gear cutting

with a hob.

Machining Centers

Schematic illustration of a computer

numerical-controlled turning center. Note

that the machine has two spindle heads and

three turret heads, making the machine tool

very flexible in its capabilities.

A horizontal-spindle machining center, equipped with

an automatic tool changer. Tool magazines in such

machines can store as many as 200 cutting tools,

each with its own holder.

Reconfigurable Machines

Schematic illustration of a reconfigurable modular machining center, capable of

accommodating workpieces of different shapes and sizes, and requiring different machining

operations on their various surfaces. Source: After Y. Koren.

Reconfigurable Machining Center

Schematic illustration of assembly of different components of a reconfigurable

machining center.

Machining of Bearing Races

Sequences involved in machining outer bearing races on a turning center.

Hexapod

(a) A hexapod machine tool, showing its major components. (b) Closeup view of the cutting tool

and its head in a hexapod machining center. Source: National Institute of Standards and

Technology.

Machining Mechanics: Orthogonal

Cutting

Schematic illustration of a two-dimensional cutting

process, or orthogonal cutting. (a) Orthogonal cutting with

a well-defined shear plane, also known as the Merchant

model; (b) Orthogonal cutting without a well-defined shear

plane.

Terminology in Turning

FIGURE 8.19 Terminology used in a turning operation on a lathe, where f is the feed (in mm/rev or

in./rev) and d is the depth of cut. Note that feed in turning is equivalent to the depth of cut in orthogonal

cutting (see Fig. 8.2), and the depth of cut in turning is equivalent to the width of cut in orthogonal

cutting. See also Fig. 8.42.

Chip Formation

(a) Schematic illustration of the basic mechanism of chip formation in cutting. (b) Velocity diagram

in the cutting zone.

Oblique Cutting

FIGURE 8.9 (a) Schematic illustration of cutting with an oblique tool. (b) Top view, showing the

inclination angle, i. (c) Types of chips produced with different inclination angles.

Right-Hand Cutting Tool

(a) Schematic illustration of a right-hand cutting tool for turning. Although these tools have

traditionally been produced from solid tool-steel bars, they are now replaced by inserts of

carbide or other tool materials of various shapes and sizes, as shown in (b).

Chip Mechanics

Cutting Forces

FIGURE 8.11 (a) Forces acting on a

cutting tool in two-dimensional cutting.

Note that the resultant forces, R, must

be collinear to balance the forces. (b)

Force circle to determine various

forces acting in the cutting zone.

Source: After M.E. Merchant.

Cutting force Friction coefficient

Cutting Forces

Specific Energy

Approximate Specific-Energy Requirements in

Machining Operations

Tool Wear

Examples of wear in cutting tools. (a) Flank wear; (b)

crater wear; (c) chipped cutting edge; (d) thermal

cracking on rake face; (e) flank wear and built-up edge;

(f) catastrophic failure (fracture). Source: Courtesy of

Kennametal, Inc.

Taylor tool life equation:

Range of n values for various cutting tools.

Surface Finish

FIGURE 8.26 Range of surface roughnesses

obtained in various machining processes. Note

the wide range within each group, especially in

turning and boring. (See also Fig. 9.27).

Hardness of Cutting Tools

Hardness of various cutting-tool materials as a

function of temperature (hot hardness). The wide

range in each group of tool materials results from

the variety of compositions and treatments available

for that group.

Tool Materials Typical range of properties of various tool materials.

Properties of Tungsten-Carbide Tools

Effect of cobalt content in tungsten-carbide tools on mechanical properties. Note that

hardness is directly related to compressive strength (see Section 2.6.8) and hence,

inversely to wear [see Eq. (4.6)].

Inserts

Methods of mounting inserts on toolholders: (a) clamping, and (b) wing lockpins. (c) Examples

of inserts mounted using threadless lockpins, which are secured with side screws. Source:

Courtesy of Valenite.

Insert Strength

FIGURE 8.33 Relative edge strength and tendency for

chipping and breaking of inserts with various shapes.

Strength refers to that of the cutting edge shown by the

included angles. Source: Courtesy of Kennametal, Inc.

FIGURE 8.34 Edge preparations for inserts to improve

edge strength. Source: Courtesy of Kennametal, Inc.

Historical Tool Improvement

Relative time required to machine with various cutting-tool materials, with indication of the

year the tool materials were introduced. Note that, within one century, machining time

has been reduced by two orders of magnitude. Source: After Sandvik Coromant.

Properties of Cutting Tool Materials

Ranges of properties for various groups of cutting-tool materials. (

Construction of polycrystalline cubic-boron-nitride

or diamond layer on a tungsten-carbide insert.

Machining Economics

FIGURE 8.75 Qualitative plots showing (a) cost per

piece, and (b) time per piece in machining. Note that

there is an optimum cutting speed for both cost and time,

respectively. The range between the two optimum speeds

is known as the high-efficiency machining range.

Process Planning

Processes Planning

Case Study: Ping Golf Putters

FIGURE 8.76 (a) The Ping Anser® golf putter; (b) CAD model of rough machining of the putter outer surface; (c) rough

machining on a vertical machining center; (d) machining of the lettering in a vertical machining center; the operation

was paused to take the photo, as normally the cutting zone is flooded with a coolant; Source: Courtesy of Ping Golf, Inc.

Lect5 Machining

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