Mechanical Technology
Grade 11 Learner’s Book
FutureManagers
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ISBN 978-1-920133917
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First published 2007 Revised 2012
Published by
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How to use this book ... iv
Chapter 1: Safety... 1
Chapter 2: Tools ... 19
Chapter 3: Materials ... 57
Chapter 4: Terminology ... 79
Chapter 5: Joining methods ... 119
Chapter 6: Forces ... 157
Chapter 7: Maintenance ... 185
Chapter 8: Systems and control ... 209
Outline of the curriculum
Mechanical Technology Grade 11 Learner’s Book has been based on the new FET Curiculum for Mechanical Technology which entails 9 topics.
Spider diagrams
Each chapter is introduced by a spider diagram which is a diagrammatical summary of the content covered in a particular chapter. The following spider diagram is an example from Chapter 6:
Performing basic testing on basic mechanical principles Basic calculations on stress Moments found in engineering components Forces found in engineering components Forces
important concepts or activities.
Icon Description
Topics
Assessment
Did you know?
Key word
Pause for thought
Caution!
Note!
Besides the various icons, explanatory notes, Pause for thought and Did you know? boxes have been placed in the margin to give further insights.
huge skills shortages and desperately need skilled engineers, technologists, technicians and artisans. An introduction to Mechanical Technology aims to produce learners who have been exposed to knowledge, skills, attitudes and values (SKAVs) which will equip them for further study in Mechanical Engineering and related sectors.
The subject Mechanical Technology focuses on technological processes from conceptual design through to the process of practical problem solving to produce or improve on products which can enhance our quality of life.
Explanation of the key words
You may encounter many unfamiliar words in this course. For this reason, key words have been included in the margins, to explain the meanings of words that appear in bold print in the text. The key words also cover acronyms (words made up of the first letters of the name of something) and abbreviations that are used in the book.
Assessment activities
The assessment activities comprise individual, pair and group tasks. Some are pen-and-paper activities and some are practical tasks. The solutions to some tasks can be found in the text but others will require you to do further research. It is very important that you read the instructions carefully before attempting any of the tasks.
Message from the authors
You have the good fortune to be one of the first learners to choose Mechanical Technology as one of your FET subjects. It will definitely stand you in good stead for your future studies. To help you succeed in this subject, it is essential to apply the following principles:
• Go through your notes and make sure that you understand the work. • Learn the important concepts and definitions.
• Do as many problems as you can.
• You will find that regular revision will help you to understand and remember the work better.
Do not hesitate to refer to other relevant reading material to broaden your understanding of the subject.
Above all, think and work hard.
We wish you well for your studies this year. THE AUTHORS
Chapter 1
Safety
Topic 1
Joining equipment Handling and storage of gas cylinders Machine tools Grinding machines Cutting machines Safety Shearing machines Press machinesSafety
Introduction
People mistakenly believe that accidents will never happen to them. As a result, work accidents often occur because people are careless and take chances. In fact, every year in South Africa, nearly 220 000 serious industrial accidents occur.
Existing legislation aims to make workplaces as safe as possible. All South African safety regulations are based on the Occupational Health and Safety
(OHS) Act No. 85 of 1993. The handbook which deals with this Act is available from any local office of the Department of Labour.
Definition of an accident
The Act defines an accident as an unplanned, uncontrolled event caused by unsafe activities and conditions.
Figure 1.1: Know safety! No accidents!
Accidents do not simply happen by chance; usually they result from carelessness. DID YOU KNOW? Safety engineers work to prevent accidents. These experts design structures and equipment to make homes, schools, jobs, roads and communities safer.
Causes of accidents
The causes of accidents include: • poor housekeeping
• loose clothing
• improper use of tools
• inaccurate setting-up of machines.
Assessment
1. Name four causes of accidents.
2. What is the purpose of the Occupational Health and Safety Act?
General safety rules
Responsibility for safety
Although everyone in a workshop must practice safety, the Act makes the person in charge of machinery, ultimately responsible for safety. This person must:
• install and properly maintain machinery • repair machinery
• ensure that safety appliances, devices and guards are in good condition and properly used
• stop anyone from using a dangerous machine.
Obedience to instructions
Machines are to be used according to the manufacturers’ instructions. Do not use a machine without your teacher’s supervision.
Illumination and lighting
Machinery and workspaces should be adequately illuminated. Artificial light, if it is used, must not shine in a machine operator’s eyes.
Assessment
1. Name four responsibilities that a person has when in charge of machines.
Power-driven machines
Bench grinder
Figure 1.2: A bench grinder
When using a bench grinder, observe the following safety rules: • Only use a machine once the guards have been correctly fitted. • Ensure that there is no oil or grease on the floor around the machine. • Check that the tool rest is not more than 3 mm from the grinding
wheel surface.
• When starting the machine, do not stand in front of the wheel. Before you start the grinding, let the machine idle for a few seconds.
• If the wheel is running unevenly, dress it with an emery-wheel dresser.
• Grind only on the face of a straight grinding wheel and never on the side of the wheel.
• Use the various wheels only for their intended purpose.
• Approach the wheel carefully and gradually, and do not ‘jab’ materials onto it.
• Never ‘force grind’ so that you cause the motor to slow or stop. • Adjust the tool rest only when the wheel is stationary.
• Clamp workpieces and holding devices safely and firmly.
• Never allow the wheel to stand in cutting fluid as this may cause it to run ‘off balance’ when you switch it on again.
stationary
at rest and not rotating Head Wheel guard Maximum gap 3 mm Grinding wheel Stand Perspex shield Tool rest On/off switch
Mounting of a grinding wheel
The following steps are advised:
• Select the correct type of wheel for the job.
• Inspect the wheel for cracks and tap it to apply the ‘ringing test’. Never use a grinding wheel that is damaged or not properly dressed.
• Make sure that the wheel’s speed does not exceed the manufacturer’s recommendation. Below is an example of a manufacturer’s
recommendation.
• Never force the wheel onto the spindle.
• Use only one smooth paper spacer on each side of the wheel.
• Use true and correctly recessed flanges of the same size and at least one third the diameter of the wheel.
• Gently tighten the grinding wheel with a spanner only enough to hold it firmly.
• Replace the guards correctly.
• Stand aside and set the machine in motion. Let the machine idle before you dress the wheel, using an emery-wheel dresser.
• Finally stop the machine and reset the tool rest to within 2 mm of the wheel surface. Ensure that the tool rest is parallel to the wheel surface.
Grinding wheels
All power-operated grinding machines should be clearly marked with the recommended speed of the spindle in revolutions per minute. This speed should not allow the peripheral speed of the wheel to exceed the manufacturer’s recommendation.
Other safety measures are:
• Every grinding wheel should have a guard that can withstand the force of a rupturing wheel.
• Bench grinders should have a transparent shield to protect an operator’s eyes.
• Each machine must carry a notice prohibiting persons from
performing, inspecting or observing grinding work without suitable
spindle
the spindle of the bench grinder is the rotating shaft onto which the grinding wheel is attached
recessed flanges
the recessed flanges fit onto the spindle of the bench grinder on either side of the grinding wheel
peripheral speed
the peripheral speed of the grinding wheel is the speed along the circumference of the grinding wheel
Angle grinder
Figure 1.3: An angle grinder
Observe the following safety precautions:
• The safety precautions that are applicable to other types of grinders are also applicable to an angle grinder.
• The safety guard must be in place before you can start the grinding process.
• Protective shields must be placed around the object being grinded to protect people passing by.
• Use the correct grinding wheel for the job. • Do not force the grinding wheel on the work.
• Make certain that there are no cracks on the stone before you start a job.
• Protective clothing and eye protection are essential when working with an angle grinder.
Surface grinder
Figure 1.4: A surface grinder
essential necessary Body Disk Safety guard Handle Vertical adjuster Horizontal adjuster Horizontal table On/off switch Grinding stone
Observe the following safety precautions when using a surface grinder: • The same safety precautions that are applicable to other types of
grinders are applicable when using a surface grinder.
• Protective clothes and eye protection are essential when working with a surface grinder.
• Before operating the surface grinder, be sure you have been properly taught how to control it and that you understand the potential dangers associated with it.
• Do not operate the surface grinder unless all guards and safety devices are in place and working correctly.
• Understand the operating instructions applicable to your machine. • Never clean or adjust the machine whilst it is in motion.
• Immediately report any dangerous aspect of the machine and stop using it until it has been repaired by a qualified person.
• You may have to stop your machine in an emergency. Learn how to do this without having to stop and think about it.
Assessment
1. Name five safety precautions to observe when working with a bench grinder.
2. Name five steps to follow when installing a grinding wheel.
3. Name five safety precautions to observe when working with a surface grinder.
Cutting machines
A drill press
Motor and gearbox
Feed lever
Table
Column Base Depth gauge
Observe the following safety precautions when using a drill press:
• Choose the correctly sharpened drill for the type of work you need to do and the material you are going to drill.
• Do not leave the chuck key in the chuck when you are not at the machine.
• Never leave the machine running if unattended.
• Clamp the workpiece securely to the table and do not hold it by hand. • Never try to stop the workpiece by hand if it slips from the clamp. • A drill should run at the correct speed for the job.
• Do not force a drill into the workpiece – this may cause broken or splintered drills and possible injury.
• Use a brush or wooden rod to remove chips from the drill – and not your fingers, waste or rags.
• When reaching around a revolving drill, be careful that your clothes do not get caught in the drill or drill chuck.
Power saws
Figure 1.6: A power saw
Observe the following precautions when using a power saw: • See that all guards are in place.
• Ensure that there is no oil, grease or obstacles around the machine. • Select the correct blade for the material to be cut.
• When changing blades, ensure that the machine is switched off at the main switch.
• When removing or replacing the blade, do it gently. Quick
movements, such as pulling off the blade, may result in a severe cut to your hand.
Power saw switch Blade
• Do not adjust guides whilst the machine is running.
• All material must be clamped properly before cutting commences. • Long pieces of material must be supported at the ends.
• Always stop the machine if you leave it unattended.
Lathes and milling machines
Figure 1.7: A centre lathe
Figure 1.8: A milling machine
Toolpost Top slide or compound slide Cross slide Cross-feed handle Saddle or carriage Thread dial Power-feed control Leadscrew engager Feed rod Carriage handwheel Rack Leadscrew Adjustable overarm Arbor Power-feed unit Base Arbor support Machine table Knee and saddle Handwheel
Observe the following safety precautions when working with a metal lathe or milling machine:
• Make sure that all guards are in place.
• Do not use a machine or come close to its moving parts whilst wearing loose clothing. Keep any cleaning materials, such as waste and rags, away from rotating parts.
• Check that there is no oil or grease on the floor around the machine. • Do not leave spanners or keys on rotary parts. Always disconnect,
remove or stand clear of hand wheels, levers or chuck keys before setting your machine or feeds in motion.
• Never apply a wrench to revolving work.
• Always clamp workpieces and holding devices safely and firmly. A loose fit, especially of spanners and keys, may cause slipping and result in injury.
• Do not use your hands to remove cuttings while a machine is in motion. Use a wire hook or a brush once the machine has stopped. • Never adjust the cutting tool while a machine is running.
• Resist the habit of leaning on machinery. This dangerous, ‘automatic’ practice often results in serious injury.
• Do not attempt to stop a machine by placing your hand on the chuck while the machine is slowing down.
• Pay attention to cutting-fluid control before switching on a machine.
Assessment
1. Name five safety precautions to follow when working with a drill press. 2. A power saw is a power tool that is very dangerous. Name five safety precautions that must be observed when working with the saw. 3. Name four safety precautions to follow when working with a lathe or a milling machine.
Guillotines
Figure 1.9: An electrical guillotine
Switch
Machine platform
Blade safety mechanism
Figure 1.10: A manual guillotine
Manual and electrical guillotines
Where the opening of a pair of shears or a guillotine at the point of operation is greater than 10 mm, the machine should have either a fixed guard that prevents hands or fingers from reaching through, over, under or around the guard or a self-adjusting guard which automatically adjusts to the thickness of the material being worked. Some machines have manual or automatic moving guards that completely enclose the point of operation so that the working stroke cannot be opened unless the ram or blade is stationary. Another safety device is the automatic sweep-away or push-away that pushes
any part of the operator’s body out of the danger zone when the working stroke starts. Today there are electronic, presence-sensing devices which stop the working stroke if the device senses any foreign object in the danger zone.
A hydraulic press
DID YOU KNOW? A guillotine is an instrument for inflicting capital punishment by decapitation, introduced into France in 1792 during the French Revolution. Blade Safety mechanism Operating pedal Pressure meter PlungerHydraulic press cylinder Return springs
Platform
Observe the following safety precautions when using a hydraulic press: • The predetermined, maximum pressure must never be exceeded. This
operating pressure is always less than the maximum safe pressure and is indicated by a pressure gauge on the apparatus.
• Pressure gauges must be tested regularly and adjusted or replaced if any malfunction occurs.
• The platform on which the workpiece rests must be rigid and square with the cylinder of the press.
• The platform must rest on the supports provided and should not be supported by the cable by which it is raised or lowered.
• Place objects to be pressed in or out in suitable jigs. Ensure that the direction of pressure is always at 90° to the platform.
• Special tools and holding devices must be used to prevent damage to soft material.
• Relieve the cylinder of all pressure after use by opening the return valve. Also remember:
• The level of the hydraulic fluid in the reservoir should be checked regularly. If fluid has to be added frequently, it is an indication that an internal leakage is present.
• Regularly inspect the apparatus for rigidity and tighten all bolts and nuts. • Pins and/or other equipment that keep the platform at a desired
height on the frame must be inspected for damage.
• When the apparatus is equipped with cables to alter the working height of the platform, the cable and pulleys must be inspected for damage and lubricated with grease.
Assessment
1. Name five precautions that one must observe when operating a hydraulic press?
2. Which safety devices are used in conjunction with guillotines.
General machinery protection
Where possible, the moving parts of machinery (including parts not
mentioned specifically in this chapter) within reach of any person should be fenced or guarded. Fences and guard railings need to be at least 1 m high and of double rail construction.
Revolving machinery
Make sure that shafts, pulleys, wheels, gears, couplings, collars, clutches and friction drums are fenced. Similarly, set screws, keys and bolts on revolving
clutch
a clutch is a device that enables two shafts (or rotating members) to be connected or disconnected, either while at rest or when in motion
shafts, couplings, collars, friction drums, clutches, wheels, pulleys and gears should be countersunk, enclosed or otherwise guarded. Never use a damaged pulley.
Projecting shaft ends
All shaft or spindle ends must be enclosed by a cap or shroud if they protrude more than a quarter of their diameter.
Transmission belts
All driving belts, ropes, chains or sprockets within normal reach must be guarded. This includes the underside of overhead driving belts, ropes or chains above passages and workplaces. Driving belts must never be adjusted while the machine is in motion.
Conditions of safety appliances and machinery
All safety appliances, devices or guards must be maintained in good working condition. Turn off machinery if safety is compromised.
Starting and stopping machinery
• All machinery must be fitted with an efficient stopping and starting device. This device must be accessible for easy engaging.
• Never start a machine while another person is repairing, cleaning, oiling or adjusting, or even dangerously close to it.
• Machines with foot-operated pedals should have either an automatic locking device to stop the pedal being accidentally pressed or a stirrup guard over the pedal with only enough space for the operator’s foot.
Repairing and oiling machinery
Never clean, repair, adjust or lubricate a machine while it is in motion. All repairs should be done by a competent person.
Machine tools
Rotating stock bars which extend beyond the end of a machine should be
shroud
a shroud is a protective cap that fits over a projecting shaft
Lighting
• There must be adequate illumination (lighting) in the workplace. • The glare in any workplace must be reduced to a level that does not
impair vision.
• The lighting on rotating machinery must not cause a stroboscopic (flashing) effect.
• Lights and lamps must be kept clean and maintained.
Ventilation
Every workplace must be ventilated either by natural or mechanical means in such a way that:
• the air breathed by employees is safe.
• the air concentration of any explosive or flammable gas, vapour or dust does not exceed safety levels.
Assessment
1. State two safety precautions for each of the following: • transmission belts
• projecting shaft ends • revolving shafts
• general machine protection.
2. Name two safety precautions that must be observed when considering each of the following:
• ventilation • lighting • machine tools.
Joining equipment
Arc, spot and gas welding
The following general safety precautions are applicable to the welding processes listed above. Specific safety instructions for each apparatus are detailed in Chapter 5. When in doubt, the manufacturer’s instructions are always the final authority on safety precautions and procedures. African Oxygen (Afrox) freely supplies safety booklets on all aspects of welding safety at their outlets and depots.
Welding or flame-cutting operations may not be undertaken, unless: • an operator has been instructed on how to use the apparatus safely; • a workplace is effectively partitioned off;
• an operator uses protective equipment;
• effective ventilation is provided or masks or hoods maintaining a supply of safe air for breathing are available and used by the persons performing such operations;
• the insulation of electrical leads is in sound condition;
• the electrode holder is completely insulated to prevent accidental contact with current-carrying parts;
• the welder is completely insulated by boots, gloves or rubber mats; and
• any vessel that contains any substance which, under the action of heat, may ignite or explode (or react to form dangerous or poisonous substances) must not be welded or heated until it has been properly cleaned. Where hot work involving welding, cutting, brazing or soldering operations is carried out at places other than workplaces, which have been specifically designated and equipped for such work,
steps to ensure that proper and adequate fire precautions must be taken.
First aid, emergency equipment and procedures
Injuries and emergencies should be dealt with swiftly and carefully. Where more than five employees are employed at a workplace, the employer must provide an accessible first-aid box. The Act stipulates what the box should contain and states that the position of the first-aid box be clearly signposted.
Handling of gas cylinders
Figure 1.12: Oxy-acetylene welding apparatus
Gas cylinders must contain the following permanently marked, minimum particulars:
• name of the manufacturer • country of origin
• year of manufacturing • manufacture serial number
• name, number and date of the design • design gauge pressure in Pascal
• maximum permissible operating pressure in Pascal • operating temperature
• mark of an approved inspection authority.
Gas cylinders must be tested before being placed in service. People must use a portable gas container and fill, place in service, handle, modify, repair, or inspect and test any portable gas container in compliance with South African Bureau of Standards (SABS) standards.
Oxygen regulator Oxygen flashback arrestor Acetylene regulator Acetylene flashback arrestor
Acetylene hose Welding nozzle Welding torch Acetylene cylinder Oxygen cylinder Oxygen hose
The following safety precautions must be observed when handling gas cylinders:
• Store full cylinders apart from empty ones.
• Keep cylinders in a cool place and protect them from sunlight and other sources of heat.
• Always store and use cylinders in an upright position. • Store oxygen cylinders away from fuel cylinders. • Never stack cylinders on top of each other. • Do not bang or work on cylinders.
• Never allow cylinders to fall from any height. Cylinders must be chained to prevent this.
• Do not allow oil or grease to come into contact with oxygen fittings as oxygen and oil form an inflammable mixture.
• Keep the caps on the cylinders for protection.
• The thread on an oxygen cylinder is a right-hand thread. • The thread on an acetylene cylinder is a left-hand thread.
Assessment
1. To join material by means of arc or gas welding can be a dangerous activity. Name six safety precautions that a worker must observe to secure a safe environment and do a safe job.
2. The gas in the cylinders of the welding plant is highly inflammable. Name five safety precautions that must be observed when working with gas cylinders to prevent them from exploding.
Chapter 2
Tools
Topic 2
Tools Gas cylinders Dial indicators Inside micrometers Telescopic gauges Stocks Cutting machines Shearing machines Press machines Joining equipmentTools
This chapter mainly deals with precision-measuring tools. These tools are usually used to test for trueness or when setting up machines for precision work.
A dial gauge
Figure 2.1: A dial indicator
Dial indicators are used as precision-measuring tools in the setting up of work on machinery such as centre lathes or milling machines. (The use of dial indicators is referred to later in the text.)
A dial gauge magnifies the actual linear motion of the plunger. This highly accurate and versatile measuring instrument has a plunger attached to a rack and pinion for magnification. A pointer on the graduated dial shows the amount of movement. The outer rim is rotated so as to set the pointer to zero and then clamped into position. The dial indicator is marked off in hundredths of a millimetre.
Uses of a dial indicator
A dial indicator is used to determine: • the ‘runout’ of a flywheel • if a crankshaft is bent • if a workpiece in a lathe is running true • if two pieces of equipment are the same size • the bearing nip on a bearing shell used on a crankshaft • the end float on a crankshaft. DID YOU KNOW? The production of artificial abrasives in the late 19th century led to the introduction of grinding machines. Norton of Massachusetts, USA, illustrated the potential of grinding machines. He made a machine that could grind an automobile crankshaft in 15 minutes, a process that previously had required five hours. linear motion motion along a straight line graduated divided into degrees Lock screw Indicator pointer Rotation indicator Plunger Body
Care of a dial indicator • Over travel of the pointer on the scale must not result in damage to the dial indicator. • Care must be taken that the dial indicator is not knocked over since the sensitive dial will be damaged. • Store the dial indicator in a proper storage place after use. • Clean the dial indicator, including its magnetic base, after use.
Assessment
1. Make a drawing of a dial indicator and label all the parts. 2. Name four instances in which you would use a dial indicator.
An inside micrometer
Figure 2.2: An inside micrometer
The function of an inside micrometer
You can use an inside micrometer to measure an inside diameter or the inside of two parallel surfaces accurately.
Handle Interchangeable rod
Thimble
How to use an inside micrometer
You can use an outside micrometer to test an inside micrometer for accuracy. The size of the hole that can be measured depends on the size of the body of
the micrometer. Most common inside micrometers are able to measure holes of up to 50 mm in diameter.
To increase the range of the micrometer, the provided set of lengthening rods is used. The rods cover these sizes: 50 – 75, 75 – 100, 100 – 125, 125 – 150, 150 – 175 and 175 – 200 millimetres. To allow the inside micrometer to measure a 50 mm bore diameter, a special arrangement must be made. This is because the rods in the micrometer only cover 25 mm ranges. To overcome this, a 12 mm spacer is fitted to the micrometer as an extension piece. To measure a bore of 50 mm, the main scale is made to move only 13 mm, as 12 mm is already accounted for by the extension piece.
Adjustment of a inside micrometer
To test if a micrometer is accurate, do the following:
• Place the 50 – 75 mm extension shaft in the micrometer. • Adjust the micrometer on 0 to get dimensions of 50 mm. • Measure the inside micrometer with an outside micrometer of which the accuracy is known. • If a small adjustment is necessary you can adjust it in the same way that you would adjust the outside micrometer. For bigger adjustments the necessary adjustment is made on the front of the extension shaft. • Loosen the locknut on the extension shaft with a key which will be supplied in the set.
• Adjust the anvil in or out to get to the zero reading. • Check for correctness. If it is right, lock the lock nut on the anvil. • Test again for correctness. • Put the micrometer inside the hole. • Turn the micrometer till both ends touch the sides of the hole. • Move the micrometer handle to one side, and remove the micrometer. How to read the reading on the micrometer.
• Determine the length of the extension shaft. • Ex 100–125
• Check if the 12 mm spacer has been used. If so, increase the reading by 12 mm
• 100 + 12 = 112 mm
• Determine the reading on the inside micrometer in the same way as on the outside micrometer.
• Ex reading on micrometer = 10,68 mm • Add up.
• 100 + 12 + 10,68 = 122,68 mm
Assessment
1. What is the function of an inside micrometer?
Care of the micrometer
• Micrometers must be handled with care if the accuracy is to be maintained. Keep the face of the anvil and spindle clean to ensure accuracy. If the zero line on the thimble does not coincide with the zero line on the index line, loosen the cap on the end of the thimble. Reset the thimble to zero, adjust to the correct position and tighten the cap.
• When taking a reading, use very light pressure to obtain a correct reading. Never touch the anvil or spindle with bare fingers since it will cause the anvil and spindle to tarnish.
• Store micrometers in wooden boxes after use. Clean them after use.
A telescopic gauge
Figure 2.3: A telescopic gauge
Spring-loaded plungers
Function of a telescopic gauge
A telescopic gauge provides a quick and accurate means of checking inside measurements. Small gauges have two plungers which are internally spring-loaded. A telescopic gauge is inserted into the item to be measured, locked and then removed to check the size, using an outside micrometer.
Care of the telescopic gauge
• Do not over-tighten the locking screw. • Do not force the telescopic plungers into the bore. • Take care when removing the telescopic gauge after measurement was taken. • Store gauges safely away after use.
Assessment
1. State the function of a telescopic gauge.
A torque wrench
Figure 2.4: A torque wrench
The most convenient torque wrench is the adjustable, double audible type. Just before the final torque is reached, a distinct but soft audible click is heard, and the wrench makes a louder click when the final torque is reached. Rotation adjuster Nm. indicator Handle Nm. adjuster
Torque
Torque is indicated or calculated in Newton metres (Nm).
Reasons for using a torque wrench on an engine
• It prevents bolts or studs from breaking. • It prevents bolts and nuts from loosening. • It prevents castings from warping.
Some important applications
A torque wrench is used to tighten: • cylinder head bolts or nuts • main or big-end bearings, bolts or nuts • front wheel-bearing nuts • rear axle assemblies • bolts and nuts on automatic gearboxes. Note: When a bolt or nut has been tightened beyond the specified torque, it should first be loosened (but not with a torque wrench) and thereafter be torqued again. This practice ensures that readings on the torque wrench are accurate. Screw threads on bolts, nuts and tap holes must be clean otherwise the torque at which they are tightened is less than that required. Uneven torques may also result when a number of bolts or nuts are tightened.
Care of the torque wrench
• Clean torque wrench after use. • Make sure that sockets fit properly. • Store properly after use.
Assessment
1. Give three reasons for using a torque wrench on a motor engine.
Taps and dies
Figure 2.5: Taps and dies
The stock is the tool that holds the tap or die. Taps and dies cut internal and external threads. It is advisable to cut internal threads first, as the taps are non-adjustable for size, whereas dies are adjustable. Taps used for cutting internal threads are extremely brittle and easily broken. They are made from hardened and tempered cast steel and are generally available in sets of three, namely taper taps, second or intermediate taps and bottoming or plug taps.
Taper taps
Figure 2.6: A taper tap
Tap
Die
Full thread Taper Pilot
Taper taps are used to start cutting the thread. The taper enables the tap to take the cutting force gradually. The remaining portion of the tap cuts the threads to its full depth.
Second taps or intermediate taps
Figure 2.7: An intermediate tap
Intermediate or second taps are tapered for one-third of their length. After the first tap has cut the initial thread, the second is used, and finally the third tap. Each tap cuts a little more thread than the previous one.
Bottoming taps or plug taps
Figure 2.8: A bottoming or plug tap
Bottoming taps are of uniform thickness throughout their length. This kind of tap is the last of the set to be used and is necessary when a blind hole has to be tapped.
Tapping size
Before tapping, a suitable hole must be drilled a little larger than the core diameter of the tap. The tapping size can be obtained by referring to tables. Tables are published in engineering handbooks and brochures printed by tap and die manufacturers. The tables indicate which tapping size to use with which tap.Clearance size
Clearance size is the size of the hole that must be drilled so that it (the hole) will clear the outside diameter of a screw or bolt.Tap wrenches
Figure 2.9: Taps and tap wrenches
Function/Uses
Tap wrenches are used to hold the taps while cutting. They are usually adjustable. Two types of tap wrenches are commonly used. Tap wrenches are used for general work and T-handle type tap wrenches are used for light work. As taps are brittle, delicate and easily broken, it is essential that you take the following precautions: • Taps must be used in the correct order (taper tap first). • A tap must be entered squarely in the tap wrench. • The correct size tapping drill must be used. • A tap is at a right angle to the stock once cutting has started. Take care not to bear too heavily on only one handle of the tap wrench or to force the tap, as it is likely to break off in the hole. • The tap is turned forwards a part-turn and then turned backward about half a turn to break off the chippings. • A suitable lubricant must be used. For steel, bronze, copper and wrought iron, use cutting fluid or cutting paste. For aluminium, use paraffin or soluble oil. No lubricant is required for brass and cast iron. • When deep, blind holes are being tapped, withdraw the tap occasionally to clear the hole of chippings.
T-handle type tap wrench
Tap wrench
External threads
External threads are cut by dies that fit into die stocks or die holders. The face of the die has a chamfer lead to enable the cutting to start more
easily. Ensure that the die is square with the work. Note the following:
• After squaring the die, use a generous amount of cutting oil, otherwise the threads are likely to tear.
• Turn the die backwards frequently (quarter or half a turn) in order to break the chippings.
• After cutting a few threads, back the die off the work and test the thread for size.
• Turning the spreading screw in or out, opens or closes the die. Always start with the die fairly open and then gradually release the spreading screw and tighten the set screw.
Tapping and clearance sizes ISO metric
Diameter Pitch Tapping drill Clearance drill
2 0,4 1,6 2,1 2,5 0,45 2,05 2,6 3 0,5 2,5 3,1 4 0,7 3,3 4,1 5 0,8 4,2 5,1 6 1,0 5,0 6,1 8 1,25 6,8 8,2 10 1,5 8,5 10,2 11 1,5 9,5 11,2 12 1,75 10,2 12,2 The tapping sizes given are for 75% depth of thread. For threads 3 mm to 12 mm diameter clearance = nominal dia. +0,1 mm For threads above 12 mm diameter clearance = nominal dia. +0,4 or 0,5 mm The tapping size can be found by subtracting the pitch from the nominal diameter. chamfer lead tapered opening
Care of taps
• Taps should be lubricated when used on all materials except brass and cast iron. • Regulary remove cutting otherwise taps will clog up and can break. • Turn the tap about half a turn forward and reverse slightly to clear the thread. • When a blind hole is being tapped, the tap should be removed occassionally to clear the metal cuttings from the bottom of the hole.Assessment
1. What do you use taps and dies for? 2. What material are taps made from?
3. Name the three types of taps you will use to cut an internal screw thread and explain what each tap is used for.
4. What do you understand by ‘clearance size’?
5. Name the safety precaution that you must observe when working with taps. 6. Explain how you will cut an external screw thread.
Thread pitch gauge
Selection of the thread pitch gauge
• Ensure that you choose the correct thread pitch gauge, e.g. is it a metric or imperial thread? Is a V-screw thread or Acme thread to be measured?
Figure 2.10: A pitch gauge
Function of a pitch gauge
A thread pitch gauge is used to compare the threads on a bolt to the teeth cut on the gauge in order to assess the pitch of the bolt. The pitch is normally displayed on each blade of the thread gauge.
Care of the thread pitch gauge
• Clean after use. • Avoid finger contact with the blade as it may tarnish and destroy the numbering on the blade. • Oil after use. • Store in wax paper after use.Assessment
1. State the function of a pitch gauge.
Grinding machines
Figure 2.11: A bench grinder
Function of a grinder
A grinder is used to take wrought edges off objects, to make objects smaller and to shape objects into certain forms.
Care of grinding machines
• Never force-grind the workpiece in the grinding wheel. • Ensure the grinding wheel is properly dressed.
• Ensure that the transparent shields are clean and in place.
• Always fit the correct wheel to the grinding machine according to the rated speed of wheel and machine.
Head Wheel guard Maximum gap 3 mm Grinding wheel Stand Perspex shield Tool rest On/off switch
Angle grinder
Figure 2.12: An angle grinder
Function
• To grind off sharp edges • To cut material.
Care of angle grinder
• Never force-grind.
• Ensure that the correct grinding disc is fitted, e.g. cutting vs. grinding; metal disc vs. masonry disc.
• Do a physical inspection of the grinder before use. • Only use in dry conditions. • Replace broken or frayed wires immediately. • Clean and store after use.
Surface grinder
FunctionSurface grinding is the precision grinding of a planed surface. While all grinding, strictly, is surface grinding, the term is commonly used to describe the grinding of flat surfaces.
Many parts formerly milled, planed or hand scraped, are now precision ground.
With the introduction of large, high-powered surface grinders, it is obvious that steel or other metals are actually machined away.
Body
Handle
Safety guard Disk
Types of surface grinding machines
Among the numerous types of surface grinders that are manufactured, the following are the principal ones:
• Horizontal spindle reciprocating table • Horizontal spindle rotary table
• Vertical spindle reciprocating table • Vertical spindle rotary table.
Care of surface grinder
• Never force-grind a workpiece.
• Dress the wheel with a diamond wheel dresser regulary. • Do not take big cuts as the wheel will bite into the workpiece. • Inspect wheels for cracks regulary by doing the ring test.
Figure 2.13: A surface grinder
Assessment
1. Name five safety precautions to observe when working with a bench grinder.
2. Name five steps to follow when installing a grinding wheel.
3. Name five safety precautions to observe when working with a surface grinder. Vertical adjuster Horizontal adjuster Horizontal table On/off switch Grinding stone
Drilling machines
The selection of the drilling machine depends on the operation, the size of the workpiece and the size of hole to be drilled.
Function
• Drilling machines are used mainly for drilling holes.
• Their work includes reaming, countersinking, boring, spot facing, honing, lapping, and tapping.
• The ability to drill accurately is essential. It requires a high degree or skill.
Portable drills
Figure 2.14: A portable drill
Function Manually held, electric, portable drills are recommended only for operations that cannot be performed on drilling machines, especially in assembly work. Portable drills are powered by small electric motors. They contain chucks that will handle drill bits from 1 mm to 13 mm in diameter.
Drilling machines
It is not known who invented the drilling machine. However, Elias Howe, Eli Whitney and J.P. Brown are credited with contributing to its development. Drilling machines are used mainly for drilling holes. In addition, their functions include reaming, countersinking, boring, spot facing, honing, lapping and tapping.Machinists need to be able to use several types of complex and powerful drilling machines. Safety is the starting point for any machinist. If you are unsure about safety in the workshop, reread the safety precautions discussed in Chapter 1. Because of the great power exerted by these machines, holding devices for workpieces must be used to secure the work and to keep the operator safe. DID YOU KNOW? The American inventors Elias Howe, Eli Whitney, and J.P. Brown contributed to the development of the drilling machine. In 1798, Whitney took a contract with the United States government to manufacture several thousand muskets. He decided to make them on the principle of the interchangeable parts. He had to design and build his own machinery. One of these machines was a drilling machine. He thus invented a highly important manufacturing method.
Drilling machines cut by rotating a multi-edged cutting tool. The
combination of the straight-line and rotating motion of the tool produces a circular hole in the workpiece. Figure 2.19 shows some basic drilling operations.
Figure 2.15: Some basic drilling operations
Three types of drilling machines are used in machine shops. They are the sensitive drilling machine, used for light drilling on small parts; the upright drill press, used for heavy-duty drilling; and the radial drill press, used for drilling large, heavy workpieces.
Sensitive drill presses
Figure 2.16: A sensitive drill press
Drill multiple diameters Multiple drill, countersink and counterbore Drill, countersink and counterbore Drill and chamfer
Drill and countersink Drill and counterbore Motor Feed lever Up and down control arm Base Column Table Chuck Fanbelt and pulleys
As the name implies, this machine allows you to ‘feel’ the cutting action as you hand-feed it into the work. This drill press is usually belt driven. A sensitive drilling machine is either bench- or floor-mounted. Since these drill presses are designed for light-duty applications only, they are used to drill holes up to 12,5 mm in diameter.
The machine consists of a base, column (which holds the motor), vertical spindle and horizontal table. The spindle is provided with a morse-tapered hole to accommodate a taper-like drill sleeve. The size of a sensitive drilling machine is determined by the diameter of the largest workpiece that can be drilled from its centre.
Upright drill presses
An upright drill press is very similar to a sensitive drill press, but is used for heavier work. The machine has a column rising from the base. It carries a table for the workpiece and spindle head. The drive is very powerful. Some types are driven by belts and pulleys while others are gear driven. They can drill holes of 50 mm or more, in diameter and have the capacity to drill holes of 38 mm diameter in thick steel.
Motor and gearbox
Depth gauge
Feed lever
Table
Column
Base
Radial drilling machines
These are the most versatile drilling machines. Machine size is determined by the diameter of the column and the length of the arm, measured from the centre of the spindle to the outer edge of the column. The machine is used for operations on large castings too heavy to be repositioned for drilling each hole.
The work is clamped to the table or base, and the drill can then be positioned by swinging the arm and moving the head along the arm. This can also hold revolving fixtures and jigs for drilling. The arm and head can be raised or lowered on the column and then locked into place.
A radial drilling machine is used for drilling large holes and for boring, reaming, counter-boring and countersinking. The radial drilling machine has a power-feed mechanism, a wide range of spindle speeds and a hand-feed lever.
Care of drilling machines
• Clean after use, especially the T-slots of the table. • Never force-drill. • Oil machine regulary. • Check rack on side of pillar column. • Release the table lock before adjusting the table. Drilling head Arm Spindle Column Table Base
Assessment
1. Name the function of a drilling machine.
2. How do you determine the size of a sensitive drilling machine? 3. What is a radial drilling machine used for?
4. Name five safety precaution on a drill press
Power saw
FunctionReciprocating power saws are used mainly for cutting to length metal of various sizes, kinds, and shapes.
These sawing machines vary in design. Some are light-duty, crank-driven machines. Others are large, heavy-duty machines that are hydraulically driven. Actual cutting takes place in only one direction. The saw blade is lifted slightly on the return stroke. This saves wear on the saw teeth. On the lighter reciprocating machines, the saw is fed into the workpiece by gravity, through the weight of the saw frame and blade. The saw blade is usually lowered a fixed amount on each stroke. Some machines are designed with faster return strokes for the saw blade.
Horizontal band saws
This saw has a continuous blade that travels in a horizontal plane or a plane slightly inclined from the horizontal. The blades are made of high-carbon steel with a flexible back and hardened teeth. They may have from six to 24 teeth per 25 mm that are raker set. Some of these machines have a
hydraulically operated feed, an adjustable vice, an adjustable stock stop and a means of varying the cutting speed and downward pressure. They can cut stock square or at an angle.
Figure 2.19: A horizontal band saw machine
Blade tensioner Coolant control
Blade guard Machine vice
Coolant reservoir
Feed control
Vertical band saw machines
Vertical band sawing machines are available with either fixed or variable speeds. The proper cutting speed is vital in band machining. If the machine is operated too fast for the type of material being cut, the teeth are not allowed sufficient time to dig into the material. As a result, they merely rub over the work. This creates friction and dulls the cutting edge of the teeth. Blades for profile sawing are always raker set.
Figure 2.20: A vertical band saw
This type of tooth provides the necessary side clearance. Profile or contour sawing is a fast, accurate and efficient method of producing intricately curved or irregular cuts in almost any machinable metal. Where internal contours are to be cut, it is necessary to first drill a hole within the contour to admit the saw blade. The blade is cut at a convenient point, threaded through the pilot hole, and rewelded on the butt welder. Machines have a built-in butt welder for this purpose. Observe the following precautions when using a power saw: • See that all guards are in place. • See that there is no oil, grease or obstacles around the machine. • Select the correct blade for the material to be cut. • When changing blades, ensure that the machine is switched off at the main switch. Blade adjuster
Blade spot welder
On/off switch
Inspection light
Table Blade
• When removing or replacing the blade, do it gently. Quick movements, such as pulling off the blade, may result in a severe cut to your hand. • Do not adjust guides whilst the machine is running. • All material must be clamped properly before cutting commences. • Long pieces of material must be supported at the ends. • Always stop the machine if you leave it unattended.
Care of power saws
• Always select the proper blade. • Always clean cuttings out of slots and guides to prevent blades from becoming clogged. • Always adjust down pressure in order not to overload. • Check that the filler tank is in a serviceable state.
Assessment
1. Describe the function of a power saw.
2. What are the blades of a horizontal band saw made of?
3. Name five safety precautions you must observe when working with a power saw.
Lathes and milling machines
Centre lathes
FunctionTo cut a workpiece that is turning while the cutter is stationary.
With the earliest lathes, the workpiece was held at either end by a forked stick, an unsatisfactory method that seldom produced accurate work. To overcome this, pins were later used to rotate the workpiece between centres. Wooden pins were first used but were later replaced by metal pins. With early lathes, the workpiece was rotated by pulling a rope around it. However, the cutting action was sporadic, and this method was succeeded by continuous rotation; a rope or belt was run around the workpiece and then around a wheel or pulley (which was driven by a pedal device).
Rapid developments in the engine lathe followed James Watt’s improvement of the steam engine. However, the operator’s holding and guiding the tool by hand worked for turning wood, but was unsatisfactory for metal. The inability of early lathes to produce screw threads created serious problems and bottlenecks in the production of machine tools. The problem was overcome by an English mechanic, Henry Maudsley, who added the slide rest, lead screw and gears, to perfect the first practical, screw-cutting lathe.
Between 1800 and 1830 the first lathes were built in the United States of America, with a wooden bed covered with iron. In 1850, solid iron beds were
made, in New Haven, Connecticut.
The centre lathe produces a cutting action by rotating the workpiece
against the cutting edge of the tool. As the cutting tool is moved lengthwise and cross-wise to the axis of the workpiece, the shape of the workpiece is generated. The shape produced is basically cylindrical.
All parts are designed to hold and rotate the workpiece, and to hold and control the movement of the tool. If you understand these two fundamentals, you will find it easy to learn the parts and functions of the centre lathe.
Care of centre lathes
• Clean lathe after use. • Cover lathe with canvas or plastic covers during holiday breaks. • Check and fill up gear boxes. • Oil lathe beds and parts to prevent corrosion. • Always keep service records up to date. • Do not turn wood on a metal lathe.
Figure 2.21: The centre lathe
Milling machines
Vertical milling machines are relatively new. The first vertical milling machine appeared in the 1860s and was closely related to the drill press, the basic difference being that the spindle assembly and pulleys moved vertically. The next significant step came in the mid-1880s with the adaptation of the ‘knee and column’ part of the horizontal milling machine. This step allowed the milling machine table to be raised or lowered in relation to the spindle. In the early 1920s, vertical milling machines appeared with power feeds on the spindle and were housed in a heavy-duty quill. Machines with automatic table cycles followed and, by 1920, electrical servo-mechanisms were used on vertical milling machines for operations such as die sinking. In 1927 hydraulic tracing controls were developed and applied to vertical milling machines.Control systems, not limited to vertical milling machines, that activate machine control movements from information stored on punched or
magnetic tape called numerical control (NC), or from a computer numerical controlled machine (CNC), have been developed. Toolpost Top-slide or compound slide Cross slide Cross-feed handle Saddle or carriage Thread dial Power-feed control Leadscrew engager Feed rod Carriage handwheel Rack Leadscrew
A milling machine provides cutting action to a rotating cutting tool. In milling, a multi-toothed cutter rotating at a fixed position on the machine shapes the work as it is traversed across the cutter. The workpiece is firmly and safely secured in a machine vice or on the machine table, which can be adjusted to set the depth of the cut and can be traversed in at least two directions on the horizontal plane.
Figure 2.22:
A universal milling machine
Care of milling machines
• Clean after use. • Cover milling machine with canvas or plastic covers during holiday breaks. • Check and fill up gear boxes. • Oil moving parts. • Keep service record up to date.
Assessment
1. Name the function of a lath and a milling machine.
2. Name four safety precautions to follow when working with a lathe or a milling machine.
Adjustable overarm Arbor
Power feed unit
Base Arbor support Machine table Knee and saddle Handwheel
Switch
Machine platform
Blade safety mechanism
Activating mechanism
Guillotines
Figure 2.23: An electrical guillotine
Function
It is a machine that cuts material by means of a mechanical or electrical method.
Figure 2.24: A manual guillotine
Manual and electrical guillotines
Do not use the guillotine unless a teacher has instructed you in its safe use and operation and has given permission.
Only one person may operate this machine at any one time. DID YOU KNOW? ‘A guillotine is an instrument for inflicting capital punishment by decapitation, introduced into France in 1792 during the French Revolution.
Blade
Operating pedal Safety mechanism
Safety glasses must be worn at all times in work areas.
Sturdy footwear must be worn at all times in work areas. Rings and jewellery must not be worn. Long and loose hair must be contained. Close-fitting/protective clothing must be worn. Gloves must not be worn when using this machine.
Pre-operational safety checks
• Ensure fixed guards are in place to prevent hands or other parts of the body from entering the trapping space.
• Guards or safety devices must never be removed or adjusted, except by an authorized person for maintenance purposes.
• Working parts should be well lubricated and free of rust and dirt. • The area around the machine must be adequately lit and kept free of
materials, which might cause slips or trips.
• Be aware of other personnel in the immediate vicinity and ensure the area is clear before using equipment. • Familiarise yourself with and check all machine operations and controls. • Ensure that the cutting table is clear of scrap and tools. • Faulty equipment must not be used. Immediately report suspect machinery.
Operational safety checks
• Do not attempt to cut material beyond the capacity of the machine. • Never attempt to cut rod, strap or wire with this machine. • Use correct lifting procedures when handling large sheets of material. • Take extreme care during the initial feeding of the workpiece into the machine. • The workpiece should always be held sufficiently far back from the edge being fed into the guillotine. • Ensure fingers and limbs are clear before actuating the guillotine. • Hold material firmly to prevent inaccurate cutting due to creep. •
Ensure that your feet are positioned to avoid contact with the foot-operated lever when cutting.
Where the opening of a pair of shears or a guillotine at the point of operation is greater than 10 mm, the machine should have either a fixed guard that prevents hands or fingers from reaching through, over, under or around the guard or a self-adjusting guard which automatically adjusts to the thickness of the material being worked. Some machines have manual or automatic moving guards that completely enclose the point of operation so that the working stroke cannot be opened unless the ram or blade is stationary. Another safety device is the automatic sweep-away or push-away that pushes
any part of the operator’s body out of the danger zone when the working stroke starts. Today there are electronic presence-sensing devices which stop the working stroke if the device senses any foreign object in the danger zone.
Care of manual and electric guillotines • Always keep blades sharp and in good condition. • Ensure guards are in place and operational.
A hydraulic press
Function • The hydraulic press is used when you want to press bearings on shafts or remove them from shafts. • You use it to push a shaft with a bearing into a housing like the water pump of a motor car.• You can also use it to press gears on shafts in a gearbox.
• A hydraulic system uses force that is applied at one point and is transmitted to another point using an incompressible fluid. A simple hydraulic system consists of two pistons and an oil-filled pipe connecting them.
The hydraulic press employs the principle of the multiplication of force within a closed system, which means that the force exerted on the smaller cylinder is transferred to the larger cylinder, in proportion to the diameter of the larger cylinder.
Figure 2.25: Hydraulic cylinders
Hydraulic motor car crusher
A large motor powers a pump that pushes hydraulic fluid to drive large cylinders. Using principles of force-multiplication, a hydraulic system can generate over 2,000 psi and impart more than 150 tons of crushing force onto a pile of scrap cars.
Small cylinder
Reservoir Piston/plunger
Big cylinder
Figure 2.26: Hydraulic press
Care of a hydraulic press
Observe the following safety precautions when using a hydraulic press: • The predetermined, maximum pressure must never be exceeded. This
operating pressure is always less than the maximum safe pressure and is indicated by a pressure gauge on the apparatus.
• Pressure gauges must be tested regularly and adjusted or be replaced if any malfunction occurs.
• The platform on which the workpiece rests must be rigid and square with the cylinder of the press.
• The platform must rest on the supports provided and should not be supported by the cable by which it is raised or lowered. • Place objects to be pressed in or out in suitable jigs. Ensure that the direction of pressure is always at 90° to the platform. • Special tools and holding devices must be used to prevent damage to soft material. • Relieve the cylinder of all pressure after use by opening the return valve. Also remember: • The level of the hydraulic fluid in the reservoir should be checked regularly. If fluid has to be added frequently, it is an indication that an internal leakage is present.
• Regularly inspect the apparatus for rigidity and tighten all bolts and nuts. • Pins and/or other equipment that keep the platform at a desired height on the frame must be inspected for damage. Pressure gauge Plunger Hydraulic press cylinder Return springs Platform Adjustment holes
• When the apparatus is equipped with cables to alter the working height of the platform, the cables and pulleys must be inspected for damage and lubricated with grease.
Assessment
1. Name five precautions that one must observe when operating a hydraulic press.
2. Which safety devices are used in conjunction with guillotines?
Joining equipment
Arc welding machines
Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is usually protected by shielding gas and/or slag. There are 4 main types of arc welding machines namely, AC machines, inverters, MIG and TIG welding machines.
Figure 2.27: Arc welding equipment
Selection The appropriate welding machine must be selected according to what type of welding is to be performed. The use of each machine is, therefore, determined by what it is designed to weld, as described in the following section: Current adjuster On/off switch Electrode terminal Electrode holder Electrode Arc gap Current scale Earth terminal Earth clamp
AC welding machines
AC (alternating current) welding machines are usually very basic and relatively inexpensive. They make use of a transformer to transform the voltage and current of an electrical outlet to that which can be used to strike an arc and weld with. They are either air or oil cooled.
Use
They are primarily used for manual metal arc welding (stick welding) mild steel, stainless, cast iron and some alloys.
DC welding machines (inverters)
Inverters use electronics to change the welding current from AC to DC. This allows them to weld a greater variety of material than an AC welding
machine. The steady arc provided by the inverter results in neater weld beads and less spatter.
Use
They are primarily used for manual metal arc welding (stick welding) mild steel, stainless, cast iron and a wide range of alloys.
MIG (Metal Inert Gas) welding machines
Gas metal arc welding (GMAW), commonly called MIG (metal inert gas), is a semi-automatic or automatic welding process with a continuously fed, consumable wire acting as both electrode and filler metal, along with an inert or semi-inert, shielding gas flow around the wire to prevent the weld site from contamination.
Use
MIG welding machines are commonly used in industries such as the
automobile industry for its quality, versatility and speed in conjunction with production line robotics.
TIG (Tungsten Inert Gas) welding machines
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten, an inert or semi-inert gas mixture, and a separate filler material.
Use TIG welding is especially useful for welding thin materials. This method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle frames and aerospace applications.
General care of arc welding equipment
Ensure that:• all connections are secure.
• insulation and electrical leads are in sound condition.
• electrode holders are properly insulated to prevent accidental contact with current carrying components.
• machines are regularly serviced and well maintained.
Resistance (spot) welding machines
Spot welding is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow. Workpieces are held together under pressure exerted by electrodes. Typically the sheets are 0,5 to 3 mm thick. The process uses two shaped, copper alloy electrodes to concentrate welding current into a small “spot” and to simultaneously clamp the sheets together. Forcing a large current through the spot will melt the metal and form the weld. The advantage of spot welding is that a lot of energy can be delivered to the spot in a very short time (approximately ten milliseconds). That permits the welding to occur without excessive heating to the rest of the sheet and, thereby, minimises distortion.
