Mechanical Engineers Data Handbook Episode 9 ppsx

Best combination of toughness and hardness High hardness, keen edge, low shock resistance Large taps and reamers Taps, screw dies, twist drills, mills for soft metals, files Water harden

1.6.6 Characteristics of steel tools

Carboo steels (for softer metals and wood; poor perfomace above 250°C)

General purpose Best combination

of toughness and hardness High hardness, keen edge, low shock resistance

Large taps and reamers

Taps, screw dies, twist drills, mills for soft metals, files

Water hardening, takes keen edge, more shock resistant than plain carbon steel

Screw taps and dies, twist drills, reamers, broaches

Water hardening, good abrasion Drawing dies, wood planes, chisels resistance, takes high compression

Oil hardening, tougher but less Bending form dies, hammers, tool hard, high shock resistance shanks

High-speed steels

Composition (%)

C W Cr Va Co Mo Characteristics Super

0.8 18/22 4.5 1.5 10112 - Highest temperature of HSS Very hard

but not so tough Most expensive For materials with tensile strength

=- 1225 MPa General purpose 0.75 18 4.15 1.2 - - Tougher than super and cheaper, for

materials over 1225 MPa tensile strength

than general purpose HSS High wear resistance

General purpose 1.25 7 4.3 2.8 6 5.5 Better impact resistance and cheaper tungsten/molybdenum

High vanadium 1.55 12.5 4.75 5.0 5 - Best abrasion resistance Used for

hinhlv abrasive materials

HSS, high-speed steels

5.6.7 Carbide and ceramic tools

Carbides are graded according to series (see table) and

by a number from 01 (hardest) to 50 (toughest), e.g

P Steel, steel castings

K Heat resistant steels, stainless steels W with Co binder

W, Ta, Tt, Ni with Co binder

SI&ered carbide tools - eollrdiriolllil ud poeitive rake

Cutting speed (mmin-') Top rake (")

120-210 90-180 75-120

90-120 6-4 240-360 240-360 240-300 180-225 240-300 15-21

0 3.5

Ceramic tools ( s i n t e d ahminiurn oxide witb grain

5.7 General information on metal cutting

5.7 I Cutting speeds and feed rates

R, rough; F, fine; R & T, reaming and threading; D, drilling

2.7.2 Power used and volume removed

f, = milling machine table feed (mm min- l )

V = volume of metal removed (cm3 min- ')

Different processes produce different degrees of finish

on machined surfaces These are graded from N1 with

an average height of roughness of 0.025 pm, up to N12 roughness 50pm The manner in which a machined surface is indicated is shown

a + b + c +

L

Average height of roughness, h, = -where a, b, c, etc =area on graph, and L =length of surface

Grind Hone, etc

5.7.4 Merchants circle for tool forces

‘Merchant’s circle’ is a well-known construction for

the analysis of cutting forces for a single-point tool If

the cutting and feed forces, the initial and final chip

thickness and the tool rake angle are known, then the

other forces, friction and shear angles can be found

Known:

F , =cutting force

F,=feed force

t , =initial chip thickness

t , =final chip thickness

a = tool rake angle

The diagram can be drawn to give:

F, =shear force

F , = resultant force F=friction force on tool face

F,, = force normal to shear force

F, =force normal to F

p =coefficient of friction = F / F ,

6 =friction angle = tan - p

4 = shear angle

5.7.5 Machining properties of

thermoplastics

Material ("1 ("1 (ms-') (mmrev.-') (ms-') (mmrev.-') (ms-') (mms-')

Polystyrene Of - 5 20130 1.5-5.0 0.05-0.25 0.5-10 0.1-0.38 5 9 4

Rigid PVC 01-10 20130 1.5-5.0 0.25-0.75 2.5-30 0.05-0.13 5 <4

5.7.6 Negative rake cutting

5.7.7 Calculation of machining cost

The ‘total-time cost per workpiece’ is made up of

‘machine-time cost’, ‘non-productive-time cost’ and

‘tool cost’ ‘Machining-time cost’ is for actual machin-

ing and includes overheads and wages ‘Non-produc-

tive-time cost’ covers ‘setting-up’ and ‘loading- and

unloading-time cost’ ‘Tool cost’ combines ‘tool-

change-time cost’ and actual ‘tool cost’ The former is

the cost of changing the cutting edge, the latter is the

cost of the cutting plus resharpening When ‘total cost’

is plotted against ‘cutting speed’ an optimum speed for

minimum cost is found

Let:

C, = machining-time cost per workpiece

C, = non-productive-time cost per workpiece

C, = tool-change-time cost per workpiece

C, = tool cost per workpiece

Total tool cost per workpiece C,, = C, + C, l + n , a t ,

Let :

t , = machining time per workpiece (min) 5.7.8 Cutting fluids

t, =loading and unloading time per workpiece (min)

t,=setting time per batch (min)

t , = tool life (min)

t, = tool change time (min)

t,, = tool sharpening time (min)

R =cost rate per hour (E)

nb = number per batch

n, = number of resharpenings

It is necessary when machining to use some form of fluid which acts as a coolant and lubricant, resulting in

a better finish and longer tool life The fluid also acts as

a rust preventative and assists in swarf removal The following table lists various fluids and their advan- tages

Soluble oil

~ ~~~~~

Oil, emulsifier and 2-10% water Good coolant Poor lubricant

Clear soluble oil As above, with more emulsifier Good coolant Poor lubricant

Water based fluids Solution of sodium nitride and Good coolant Poor lubricant

triethanolamine

EP soluble oils Soluble oils with EP additives, e.g Fairly good lubricant

sulphur and/or chlorine

Cutting fluid applications (continued)

Straight oils Mineral or fatty oils (lard, sperm, Good lubricant Often unstable

olive, neat’s foot, rape, etc.) alone

or compounded Sulphurized EP oils Straight oils with sulphur, zinc oxide

of chip on tool Sulphochlorinated EP oils

on tool

Chlorinated materials Carbon tetrachloride and Very good EP fluid Highly

trichlorethylene alone or blended with oils

Casting is the forming of metal or plastic parts by

introducing the liquid material to a suitably shaped

cavity (mould), allowing it to solidify, and then

removing it from the mould Further processing is usually required

In sand casting the mould is made in a ‘moulding box’

produced by inserting previously made ‘cores’ of

baked sand Molten metal is poured into runners until

grinding and sandblasting Practically any metal can

be cast

using a special sand and a wooden ‘pattern’ Holes are .-

it appears in risers The casting is cleaned by chipping, Required casting

Runner Risers

INVESTMENT CASTING

Turbine biada

5.8.2 Shell moulding

This is a form of sand casting done using a very fine

sand mixed with synthetic resin The pattern is made of

machined and polished iron The sand mixture is

blown into a box containing the pattern which is

heated to produce a hard, thin (6-10mm) mould

which is split and removed from the pattern and then

glued together It is a high-speed process, producing

highly accurate castings

5.8.3 Investment a r t l n g (lost wax

casting)

Wax patterns are made from a permanent metal

mould The wax patterns are coated with ceramic

slurry which is hardened and baked so that the wax is

melted out The cavity is filled with molten metal to

give a precision casting Any metal can be cast using

this process

Wax panern

.ylil

Fan impeller

5.0.4 Die casting

The mould is of steel in several parts dowelled

together Molten metal is fed by gravity or pressure

and, when solid, is ejected by pins Aluminium, copper,

manganese and zinc alloy are suitable for casting by

5.9 Metal forming processes

5.9 I

‘Forging’ is the forming of metal parts by hammering,

pressing, or bending to the required shape, usually at

red heat ‘Hand forging’ involves the use of an anvil

and special hammers, chisels and swages A ‘drop

forging machine’ uses pneumatic or hydraulic pressure

to compress hot metal blanks between hard steel dies

Hand f o r l r y Md drop hwng

Fo@ngwithfiashnmwval

Vehicle axle

FORGINGS

5.9.2 Drawing process

This is the forming of flat metal blanks into box and

cup-like shapes by pressing them with a shaped punch

into a die The process is used for cartridge cases,

boxes, electrical fittings, etc

First stage Second stage Deep drawing

f-\

h

Deep-drawn components

A

5.9.3 Extrusion

Hot extrusion

A piece of red-hot bar or billet is placed in a cylinder

and forced through a specially shaped die by a piston

to produce long lengths of bar Hollow sections can be

made by placing a mandrel in the die orifice

Cold extrusion

Soft metals such as aluminium and copper can be

extruded cold Practically all metals may be extruded

cylinder The process is used for manufacturing tooth-

paste tubes, ignition coil cans, etc

Impact extrusion

5.9.5 Press work Rolling

A press is used for a wide range of processes such as

punching, piercing, blanking, notching, bending,

drawing, and folding It may be operated by means of a

crank connected to a heavy flywheel or by hydraulic

power Formulae are given for various processes

Bending plate Flanging a pipe

In a rolling mill, red-hot ingots of steel or other metals are passed through successive pairs of specially shaped rollers to produce flat bar, sheet, I, T, channel, angle or other section bar Final cold rolling may be camed out

to give a better finish

Universal Beams, Universal Columns, Joists, Angles, and Channels are made to British Standards

BS 4: Part 1 and BS 4848: Part 4

Rolls for I section

=-

Press work

5.9.6 Press tool theory

Sheet metal work

In sheet metal work allowance must be made for bends

depending on the thickness of the material, the radius

of the bend and bend angle

Punching process

Symbols used:

F,,, = maximum shear force

7u =ultimate shear stress

t =material thickness

x = penetration

p = perimeter of profile

Maximum shear force F,,, = 7 t ~

Work done W = Fmaxx

X

Penetration ratio c = -

t

Planishing force F , = a,Lb

where: a,=yield stress

Initial length of strip Li = h - t - 2r + b + - I( r + - :)

5.9.7 Sheet metal work

Allowance for right angle bend

Lengths a and b are reduced by an ‘allowance’ c, and

c = r + t -a (r +;)

When r=2t (as is often the case), c = 1.037t

Allowance for bend with outside angle 0

c=(r+t)tan - 2” :( r + - :) , (6 in degrees) When r=2t, c = 3tan 0.02188 ( : ) t

5.9.8 Rolled sections

Rolled sections are made to British Standards BS 4:

Part 1 and BS 4848: Part 4

t and T are in several sizes in each case

Beams columns and joists

Miscellaneous rolled sections

5.10 Soldering and brazing

In soldering and brazing, bonding takes place at a

temperature below the melting points of the metals

being joined The bond consists of a thin film of low-melting-point alloy known as 'solder' or 'filler'

5 IO I

For small parts, a 'soldering iron', which is heated by

gas or an internal electric element, is used For large

joints a gas flame is used

Solders and soldering

Soji solder

This is a mixture of lead, tin and sometimes antimony

Typical solders are 50% tin and 50% lead (melting

range 182-21SoC), 60% tin and 40% lead (melting

range 182-188°C) and 95% tin and 5% antimony

(melting range 238-243 "C) Solder is available in the

form of bar or wire with cores of resin flux Flux is used

to prevent oxidation by forming a gas which excludes

air from the joint A solution of zinc chloride (killed spirits) or resin are commonly used as fluxes

Silver solder

This is an alloy of silver, copper and zinc with a melting point of about 700°C used mainly for joining brass and copper It is in strip form and is used with a flux powder

Above about 800 "C the process is called 'brazing' (or

hard soldering) Brazing rod (50Y0 copper and 50%

zinc) is used for general work, with a flux consisting of

borax mixed to a paste with water A torch supplied

with mains gas and compressed air is used Taps

control the flow and mixture For large-scale produc-

tion work, induction and furnace heating are used

Gas-alr brazing torch

5.10.4 Brazed joints

In the figure, several types of brazed joint are shown;

the arrows indicate the direction of the load

-5.1 I Gas welding

In gas welding the heat to melt the metal parts being

welded is produced by the combination of oxygen and

an inflammable gas such as acetylene, propane, bu-

tane, etc Acetylene is the most commonly used gas; propane and butane are cheaper but less efficient

5 I I I Oxyacetylene welding

A flame temperature of about 3250 "C melts the metals

which fuse together to form a strong joint Extra metal

may be supplied from a filler rod and a flux may be

used to prevent oxidation The gas is supplied from

high pressure bottles fitted with special regulators

which reduce the pressure to 0.134.5 bar Gauges

indicate the pressures before and after the regulators

A torch mixes the gases which issue from a copper

nozzle designed to suit the weld size The process

produces harmful radiation and goggles must be worn

The process is suitable for steel plate up to 25mm

thick, but is mostly used for plate about 2 mm thick

2.5 3.0

4 .O

4.8

6.0

8.0

Gas welding - edge preparation, speed, and metal thickness (continued)

It is essential to have the correct type of flame which

depends on the proportions of the gases

Neutral flame

This is the type most used since it least affects the metal

being welded The almost transparent flame has a well

defined blue core with a rounded end Roughly equal

amounts of gas are used

Carburizing flame

This flame contains excess acetylene and hence car-

bon Carbides are formed which produce brittleness

The flame is used when 'hard facing' The blue core is

surrounded by a white plume

Oxidizing flame

This flame contains an excess of oxygen which pro-

duces brittle low-strength oxides Use of this flame should be avoided when welding brass and bronze

Rightward welding

This is used for plate thicker than 4.5 mm For larger

plate the edges are chamfered Bo give an included angle

of about 80"

Rihtward welding

5 I I 4 Fillers and fluxas

The table below gives recommended filler rod ma-

terials and fluxes for gas welding

Low carbon steels

Special steel rod for each type

High silicon cast iron rod 5 or 6mm square

Silicon bronze sometimes flux

coated 1 &6 mm diameter

Pure aluminium or alloy 1.6-5 mm diameter

Copper-silver low melting point rods 3.2 mm diameter

No flux required

Grey powder in paste with water (m.p 910 "C) Weld cleaned with 5% caustic soda solution, then with hot water

Grey powder in paste with water (m.p 850 "C) Excess removed by

chipping and wire brushing Pale blue powder (m.p 875 "C) in paste with alcohol Cleaning is with boiling water and by brushing

(m.p 570 "C) Cleaning by dipping in 5% nitric acid solution and hot water wash

White powder in paste with water

White powder in paste with water Cleaning is with boiling water and

by wire brushing