Terminology
Topic 4
Using instructions and drawings and
Terminology
Cutting procedures
Terminology
Introduction
Communicating ideas and thoughts through drawings is not new and its importance has increased in the modern world. Pictorially representing ideas is an important method of communication between product designers and manufacturers. Technological design would be impossible were it not for the several different ways in which a drawing can represent an idea. The drawing also provides a testing phase for the idea. Many drawings are rejected at the early drawing board stage, thus saving the investment of money in a manufacturing plant or risking the production of an inferior item.
Almost anything can be represented by drawing, even those designs that are quite impossible to fabricate. It is important that the designer is conscious of the possible problems that the machinist will encounter. A machinist must fully understand the symbols and terminology on the designer’s drawing. A good interpretation of these symbols and terms will enable the machinist to convert the designer’s ideas into useful products.
History of the Systême Internationale of Units
The creation of the decimal metric system during the French Revolution and the subsequent using of two platinum rods representing the metre (and now stored in vaults in Paris and New York), is really the first step in the development of the present SI system.
In 1832, Gauss promoted this metric system as a system of units for physical science. He was the first person to make absolute measurements of Earth’s magnetic force in terms of a decimal system based on the millimetre (length), gram (mass) and second (time).
Systems of measurement
Throughout history there have been many systems of measurement. Prior to industrial operations, individuals were often responsible for completely manufacturing products. Since the same person made the necessary parts and did the assembly alone, that person needed only to his own particular system of measurement.
DID YOU KNOW?
SI is the abbreviation for the Systême Internationale of Units.
However, as machines replaced people and mass production was established, the need for standardisation of measurement grew. Measurement throughout the world is still not fully standardised. Most measurement in the modern world does, however, conform to either the English (British Imperial) system or the metric (International System of Units). The metric system is now used chiefly by most industrialised nations of the world.
The British system of measurement
The British Imperial system of measurement uses the units of inches, pounds and seconds to measure length, mass and time.
The metric system of measurement
The basic unit for length in the metric system is the metre. Originally the length of a metre was defined by a natural standard, specifically, a portion of Earth’s circumference. Later, more convenient metal standards were constructed.
Although the metric system had been used for many years in different countries, it still lacked complete standardisation; therefore, an attempt was made to modernise and standardise the metric system. This led to the International System of Units (SI). The SI metre is defined by a physical standard that can be reproduced anywhere with accuracy.
Probably the biggest advantage of the metric system is its convenience in calculations. All subdivisions and multiples use 10 as a divisor or multiplier. Notice this in the following table.
Decimal fraction of 1 m Description of the
decimal fraction Name of unit
,000001 one millionth of a metre micrometre ,001 one thousanth of a metre millimetre ,01 one hundredth of a metre centimetre
,1 one tenth of a metre decimetre
1,00 unit metre metre
10 10 metres one decametre
100 100 metres one hectometre
1000 1 000 metres one kilometre
DID YOU KNOW?
The British
Imperial system is also referred to as the inch system.
NOTE!
1 metre = 1 650 763,73 wavelengths in a vacuum of orange- red light spectrum of the Krypton- 086 atom.
SI base units
Base quantity Name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature Kelvin K amount of substance mole mol luminous intensity candela cd
SI derived units
Other quantities, called derived quantities, are defined in relation to the seven base units.
Derived quantity Name Symbol
area metre square m2
volume cubic metre m3
speed, velocity metre per second m/s
acceleration metre per second squared m/s2
wave number reciprocal metre m-1
density kilogram per cubic metre kg/m3
specific volume cubic metre per kilogram m3/kg
current density ampere per square metre A/m2
magnetic field strength ampere per metre A/m concentration of a substance mole per cubic metre mol/m3
luminance candela per square metre cd/m2
mass fraction kilogram per kilogram, which may be represented by the number 1
kg/kg = 1
To make the system easy to understand, some units have been given special names and symbols. This was done especially where the quantities end with several units. As you can see from the following list, these quantities are named after influential scientists.
Derived quantity Name Symbol
Frequency Hertz Hz
Force Newton N
Pressure, stress Pascal Pa Energy, work, quantity of heat Joule J Power, radiant flux Watt W
Assessment
Write an essay of 500 words or more on the contribution of any one of the above-mentioned scientists to physical sciences.
SI prefixes
Prefixes, combined with the unit name, form smaller or larger units by factors of powers of 10. For example, exponent (base 10) of decimal number: En = 10n (where n refers to the number of powers the base number 10 should be
raised by.)
Prefix Symbol Factor Multiple SI unit with
Yotta Y 1024 E24 1 000 000 000 000 000 000 000 000 Zeta Z 1021 E21 1 000 000 000 000 000 000 000 Exa E 1018 E18 1 000 000 000 000 000 000 Peta P 1015 E15 1 000 000 000 000 000 Tera T 1012 E12 1 000 000 000 000 Giga G 109 E9 1 000 000 000 Mega M 106 E6 1 000 000 Kilo k 103 E3 1 000 Hector h 102 E2 1 00 Deca da 101 E0 1 0 Deci d 101 E-1 0,1 Centi c 102 E-2 0,01 Milli m 103 E-3 0,001 Micro μ 106 E-6 0,000 001 Nano n 109 E-9 0,000 000 001 Pico p 1012 E-12 0,000 000 000 001 Femto f 1015 E-15 0,000 000 000 000 001 Atto DID YOU KNOW? A prefix is a group of letters placed in front of a word to change its meaning.
The bold prefixes in the table above are the most common prefixes for metric units.
Assessment
Complete the following questions:
1. (a) one metre (m) = … millimetres (mm). (b) 50 mm = … centimetres (cm)
(c) five kilometres = … metres (d) 682 mm = … centimetres (cm) 2. What is meant by the abbreviation SI? 3. What is mass production?
4. What are the three elevations most commonly used in a drawing?
Symbols and abbreviations
Besides the elevations and dimensions, more information is needed to manufacture a product. This includes things like the kind of material to be used, its treatment, the number of parts required, size and shape of each part, type of finish, and the tools and dies needed.
Using symbols saves a lot of time and gives the worker much needed
information. The two most common symbols used on machine drawings are those that show screw threads and finish marks. Screw thread symbols may be either regular or simplified. Finished or machined surfaces are indicated by a ‘V’-like mark. The point of the ‘V’ should rest on the line of the metal in a manner similar to that of a tool bit.
When it is necessary to control surface roughness of finished surfaces, the ‘vee’ is used as a base for more elaborate surface quality symbols.
Many abbreviations are placed on drawings. A few of the most common are listed in the following table.
Abbreviation Meaning Abbreviation Meaning
ASSY assembly M/C machine
CBORE counter bore I/D inside diameter
CHAM chamfered CYL cylinder or cylindrical
CI cast iron O/D outside diameter
CL centre line A area
CP circular pitch M/CD machined
CSK countersink Galv galvanised
DIA / Ø diameter PCD pitch circle diameter
DR drill SPEC specification
FAO finish all over HEX hexagon
GA gauge U’CUT undercut
HDN harden MATL material
LH left hand A/F across flats
MOD module VOL volume
NC national coarse M7 × 1 metric tap size and pitch
NF national fine RPM revolutions per minute
P pitch M metric thread nominal
diameter
R radius B.D.C. diameter bolt circle
RH right hand SAE Society of Automotive Engineers
SQ square SPH spherical
THD thread ISO International
Organisation of Standardisation
UNC United NC BSP British standard pipe thread
UNF United NF BSW British standard
Whitworth thread
Colour-coding of metals
There are numerous different metals. It is often necessary to identify and distinguished between these metals. For this reason, steel manufacturers supply the various metals with colour codes. These colour codes are standardised by the SABS (South African Bureau of Standards) and are regularly used in industry. The following table provides the most frequently used metals and their colour codes.
metal colour code
carbon and alloy spring steel black
case hardened steel orange
cast steel blue
high-carbon steel brown
low-alloy steel light purple
low-carbon steel orange
pipeline steel grey
silicon chrome steel black
stainless steel black
steel for lifting machines green steel for pressure containers white
structural steel red
Suppliers of steel use different conventions on different steel sections or sizes. The colours are usually painted on the ends of the stock.
Machining symbols
Finishing marks indicate the particular surface of a rough casting or forging that is to be machined or ‘finished’. They are placed in all views, touching the visible or invisible lines that are the edge views of surfaces to be machined. When a surface is to be finished by the removal of material and it is not
necessary to indicate surface quality, a bar is added to the check-mark portion of the texture symbol at the top of the short leg. When material removal is prohibited, a small circle is added to the ‘V’ in place of the horizontal bar.
It is not necessary to show finish marks on holes. They are omitted, and a title note, ‘finish all over’ (FAO), is substituted if the piece is to be completely machined. Finish marks are not required when limited dimension are used.
Surface texture symbols
Surface texture, finish, roughness, and other surface characteristics are generally used interchangeably in the workshop. However, application of these terms to drawings must follow precise Code of Practice standards. Surface texture symbols and values provide specific standards according to which finished parts may be accurately produced, uniformly inspected and measured. Common surface characteristics are surface texture, surface finish, micrometer value, lay, waviness, and flaws.
The following table is an example of these surface texture symbols.
Symbol Details
The surface may be produced by any manufacturing process.
The basic machining symbol, a 60º with uneven legs.
Add a bar to the basic machining symbol. Material removal from the surface by machining is required.
Add a circle to the basic machining symbol. The surface is to be produced by no machining process such as casting, forging, welding, rolling, powder metallurgy, etc. There is no subsequent machining of the surface.
The addition of a decimal or metric value to the basic machining symbol indicates the amount of stock to be removed by any machining process. This symbol indicates (the
permissible range of roughness) that the high and low limits are set for the roughness value.
This symbol indicates that the required finish texture is to be obtained without the removal of material.
This symbol indicates the suitable fabrication process, treatment or coating or preservation method to obtain a specific surface texture. Place it above the horizontal bar of the machining symbol.
This symbol indicates the sampling length in millimetres (length cut off). Place it below the horizontal bar of the machining symbol
This symbol indicates the
required direction of lay (right of lay symbol). Place it to the right of and next to the machining symbol.
This symbol indicates the required machining allowance to be made in a process, in millimetres. Place it to the left of the machining symbol.
To avoid repetition of a symbol only one appropriate symbol may be used when the same surface texture is required on all the surfaces of an object. Place the suitable symbol, followed by the phrase “all over” near the drawing of the object.
Centre lathe
Introduction
• The centre lathe is one of the most multipurpose machines in the Me- chanical Technology centre. Even though the centre lathe is mainly a ma- chine for the producing cylindrical work, it is by nature much more than that. Being so versatile, it is a readily adaptable piece of machinery which can be used to carry out various other machining operations in addition to its fundamental operations.
• The operational principles of the centre lathe were discussed at length in Gr. 10. These uses included facing, parallel turning, drilling, boring, taper- turning, screw-thread cutting, parting and knurling.
• You should remember that a centre lathe is a machine tool that cuts by rotating the workpiece against the cutting edge of the cutting tool. The cutting tool can move across the bed (facing) and also along the lathe bed (sliding).
• A centre lathe enables the operator to execute a variety of processes on a workpiece. These processes may involve different actions carried out either by a cutting tool or by another kind of tool (e.g. a knurling tool, reamer, drill bit) acting on the turning workpiece.
• In grade 11 you are required to do taper turning as centre lathe work. There are a number of ways taper turning can be done on a centre lathe: – The tail stock can be offset for longer external tapers.
– The taper turning attachment can be used for external tapers and for short internal boring
– The compound slide rest can be rotated round for turning short internal and external tapers.
• Tapers can be expressed in three ways: – It can be expressed as an included angle.
– As a given amount on the diameter per unit length, e.g. 12 mm per 300 mm.
– As an incline, e.g. 1 in 25 on the diameter.
• For grade 11 purposes you are only required to concentrate extensively on taper turning using the compound slide, set over method.
Taper turning
Tapers are very useful machine elements that are used on machines because of their capacity to align and hold machine parts and to realign them when they are frequently disassembled and assembled. A well-known example of a taper is the Morse tapered shank of a drill bit.
Taper turning is the process of producing a conical (pointed) profile which equally increases or decreases in diameter (externally or internally) as the cutting tool is fed longitudinally along the rotating workpiece on the lathe. There are, of course, many types of taper work that have to be carried out
repeatedly on the centre lathe. These could be roughly arranged in categories as follows: • long, slow tapers under approximately 14° included angle. • long, steep tapers over approximately 14° included angle. • short, slow tapers • short, steep tapers • combinations of two or a number of the previous categories. There are a number of methods in which tapers can be turned: 1. The tailstock can be set over (off set) for longer external tapers. 2. The taper turning attachment can be used for external tapers and for
boring short internal tapers.
3. A straight-edged turning tool (formed tool) can be used for very short tapers (this method is not well-known or generally in used).
4. The compound slide rest can be swivelled around for turning external tapers or short internal tapers.
5. Use of a tracer or computer numeral controlled lathe.
These methods are basically mentioned to make you aware of the broad range of methods that can be employed for taper turning. It is expected that you familiarise yourself with the following method of taper turning:
The compound slide method of taper turning
The compound slide method can be used for both internal and external short, steep tapers on the lathe, by hand feeding the compound slide. The same method can also be used for boring work, lathe centres and bevel gears, etc. This method is best described by H. C. Bogaard.
Did you know?
A taper is a shaft or hole that grows gradually smaller towards the one end.
Did you know?
A slight taper, like a Morse taper, is called a self-holding taper since it is held in and driven by friction.
The compound slide is always set at an angle, equal to half the included angle of the desired angle, to the axis of the workpiece. The base of the compound slide is graduated in degrees to enable it to be easily set to cut the required taper.
The following formula is used to calculate the angle to which the compound slide must be set.
large diameter of taper – small diameter of taper Tan θ =
2 × length of taper D – d
= 2 × L Example 1
A taper 150 mm long, has to be turned on the end of a 75 mm diameter shaft. If the diameter of the small end of the taper is 60 mm, calculate the angle to which the compound slide must be set in order to cut this taper.
Solution: D – d = 2 × L 75 – 60 = 2 × 150 15 = 300 1 = 20 = 0,05 θ = 2° 52´
The compound (top) slide must be set at the required angle for turning. The workpiece can either be held in the chuck or between centres. The
circular graduated, compound slide base is slackened at the two holding down bolts. Sway the compound slide to the correct angle and clamp
Example 2
An internal taper 150 mm long, has to be bored in a bush. The large diameter of the bush hole is 60 mm. Calculate the small diameter of the taper hole if the included angle is 8°. Solution: NOTE: θ/2 = 8°/2 = 4° Tan θ = D – d 2 × L Tan 4° = 60 – d 2 × 150 0.0699 = 60 – d 300 300 × 0.0699 = 60 – d 20,97 = 60 – d 20,97 + d = 60 – 20,97 d = 39.03 mm
Example 3
A taper of 1 in 15 must be turned by means of the compound slide method at the end of a shaft. Calculate the angle at which the compound slide must be set to cut the taper, as well as the included angle of the taper. Ex- press the angle in degrees (°) and minutes (‘).
Solution:
NOTE: A taper of 1 in 15 means that the diameter increases 1 mm for
every 15 mm of taper length, or 10 mm for every 150 mm of taper length, etc. Tan θ = D – d 2 × L Tan θ = 1 2 × 15 0.Tan θ = 1 30 Tan θ = 0,03333 Tan θ = 1,909° Tan θ = 1° 55‘
The angle is 1 degree and 55 minutes
DID YOU KNOW? 1 degree (°) = 60 minutes (‘) 1 minute (‘) = 60 seconds (“)
REMEMBER: That compound
slide will only be set to half the included angle of the taper. 5
Tan θ = 150 Tan θ = 0,03333 Tan θ = 1,909° Tan θ = 1° 55‘
Advantages of the compound slide method
• The main advantage of this method is the fact that compound slide can be set at any required angle.
• Tapers with large angles can be turned.
• This method can also be used for external and internal taper cutting.
Disadvantages of the compound slide method
• The compound slide can only move a short distance.
• Only short tapers can be turned, because of the limited length of the compound slide.
• The taper can be done in stages, but the accuracy of the taper is impaired.
• This method can only be done by hand, which will make the surface tex- ture irregular.
• Taper turning using this method tends to be monotonous and can cause fatigue in the operator.
Cutting procedure for cutting a taper by using the compound slide method
• Release the lock nuts of the compound slide.
• Swing the compound slide to half the included angle.
• Tighten the lock nuts (take care not to over tighten).
• Mount the cutting tool in the tool holder in the toolpost; the cutting tool must be set centre height.
• Use the compound slide feed handle and feed the cutting tool slowly into the workpiece.
• Proceed to the end of cutting length, return to starting position, and feed the cutting tool in for the next cut.