Mechanical Technology Grade 12 NSC November 2020 Memo (Eng)

Downloaded from hlayiso.com SENIOR CERTIFICATE/ NATIONAL SENIOR CERTIFICATE GRADE 12 MECHANICAL TECHNOLOGY: WELDING AND METALWORK NOVEMBER 2020 MARKING GUIDELINES MARKS: 200 These marking guidelines consist of 18 pages. Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 2 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 1: MULTIPLE-CHOICE QUESTIONS (GENERIC) 1.1 A (1) 1.2 D (1) 1.3 A (1) 1.4 C (1) 1.5 B (1) 1.6 B (1) [6] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 3 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 2: SAFETY (GENERIC) 2.1 Work procedures on machine: Switch off machine.  (1) 2.2 The horizontal band saw:  No adjustments to machine or work piece.   Ensure sufficient coolant.   Do not leave machine unattended while in operation.   Do not lean on machine.   Keep hands clear from blade.  (Any 2 x 1) (2) 2.3 Surgical gloves:  Prevent contamination of wound   To prevent transmission of HIV/AIDS or any blood related diseases to the first aid helper.  (2) 2.4 Personal protective equipment (PPE) during arc welding:  Welding helmet / Helmet   Safety goggles / Face shield   Leather apron / Apron   Leather gloves / Gloves   Leather spat / Spats   Safety boots / Safety shoes   Over-all   Skull cap   Neck protection   Ear plugs / Ear muffs.   Respirator  (Any 2 x 1) (2) 2.5 Responsibility of the employer regarding the health and safety:  Sufficient lighting   Sufficient ventilation   Provide first-aid equipment   Provide a safe / clean working environment   Provide personal protective equipment (PPE)   Provide safety training to employees  (Any 1 x 2) (2) 2.6 Responsible for administering first aid: A qualified / trained first aid person  (1) [10] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 4 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 3: MATERIALS (GENERIC) 3.1 Tests to identify various metals: 3.1.1 Sound test:  Tapping the metal with a hammer (any metal object)  and identify the sound.   Dropping the metal on the floor  and identify the sound.  (Any 1 x 2) (2) 3.1.2 File test: File the metal and pay attention to the bite of the file into the metal.  The bigger the bite the softer the metal. OR The smaller the bite the harder the metal.  (2) 3.2 Purpose of heat treatment of steel:  To change  the properties  of steel.  To change  the grain structure  of steel. (Any 1 x 2) (2) 3.3 Purpose of case hardening on steel: To create a hard / wear resistance surface / case  with a tough core.  (2) 3.4 The tempering process for steel:  Heat the steel to a temperature (temper colour) below the critical temperature.   Soak it at that temperature for a period.   Quench / cool in an appropriate quenching agent.  (water, brine, or oil) (3) 3.5 THREE factors for heat treatment of steel:  Heating temperature / Carbon content   Soaking (Time period at temperature) / Work piece size   Cooling rate / Quenching rate (Quenching medium)  (3) [14] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 5 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 4: MULTIPLE-CHOICE QUESTIONS (SPECIFIC) 4.1 B/D (1) 4.2 A (1) 4.3 C (1) 4.4 D (1) 4.5 B (1) 4.6 D (1) 4.7 A (1) 4.8 C (1) 4.9 D (1) 4.10 C (1) 4.11 B (1) 4.12 C (1) 4.13 A (1) 4.14 D (1) [14] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 6 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 5: TERMINOLOGY (TEMPLATES) (SPECIFIC) 5.1 Advantages of templates:  Quicker to use to improve mass production   Accurate production   Cheap to manufacture   Unskilled labour will be able to use it   Avoid unnecessary wastages / cost effective   Uniformity in production   Can be reused.  (Any 2 x 1) (2) 5.2 Use of templates: 5.2.1 Thin metal is used for profile cutting machines  (1) 5.2.2  Hardboard templates is used for templates for gussets.   Hardboard templates are used for checking sizes.   Hardboard templates is used for marking of holes.  (Any 1 x 1) (1) 5.3 Components of a roof truss: A. Rafter  B. Purlin  C. Internal bracing members / strut  D. Gusset plate  E. Main tie / Tie beam / Beam  (5) 5.4 A mild steel ring material: Calculate the dimensions of the required material: Mean   Outside   plate thickness  280  12   268 mm   Mean circumference  π  mean   π  268   841,95 mm   842 mm  (5) 5.5 Abbreviation 'SANS':  South African  National Standards  (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 7 DBE/November 2020 SC/NSC – Marking Guidelines 5.6 Resistance weld: 5.6.1 Foil seam  (1) 5.6.2 Flash or resistance butt  (1) 5.7 Weld dimensions:  5 – size (width) of weld   25 – length of weld   50 – pitch of welds  (3) 5.8 Position of the weld: 5.8.1 Weld on the arrow side  (1) 5.8.2 Weld on both sides  (1) [23] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 8 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 6: TOOLS AND EQUIPMENT (SPECIFIC) 6.1 Pedestal drill machine: A = Table / Machine table / Working table / Platform  B = Chuck / Drill bit holder  C = Motor  D = Pillar / Column  (4) 6.2 Operating principles: 6.2.1 Horizontal band saw:  The blade is tensioned around two pulleys.   The machine is driven by an electric motor.   The blade is fitted so that it cuts in a continuously forward motion.   The blade assembly is raised and lowered by hand or by hydraulic controls.   The metal being cut is held firmly in the stock clamp during the cutting process.  (Any 4 x 1) (4) 6.2.2 Punch and cropper (shear) machine:  It is an electrically driven machine.   It makes use of a flywheel and clutches  to engage various shearing blades or punches.  (3) 6.3 Primary function of flashback arrestors: It prevents  back-feeding/backfiring.  (3) 6.4 Use of taps and dies:  Taps are used to cut / clean  internal / nut screw threads   Dies are used to cut / clean  external / bolt screw threads  (4) [18] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 9 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 7: FORCES (SPECIFIC) 7.1 Frameworks: 7.1.1 Force diagram: Scale 1 : 100  Correct indication of ties and struts The space diagram can be a freehand sketch I Scale 1 mm: 10 kN 30   22,5  EF   EI    27,5  20 Markers need to draw the diagram to scale. (10) Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 10 DBE/November 2020 SC/NSC – Marking Guidelines 7.1.2 Magnitude and nature of the members: MEMBER MAGNITUDE (kN) NATURE AF 1050  (1020 – 1080) STRUT  BG 760  (730 – 790) STRUT  CH 760  (730 – 790) STRUT  DI 960  (930 – 990) STRUT  FG 300  (270 – 330) STRUT  HI 200  (170 – 230) STRUT  FE 930  (900 – 960) TIE  GH 250  (220 – 280) TIE  IE 830  (800 – 860) TIE  Minus 2 marks for incorrect conversion (mm to kN) (18) 7.2 BEAMS: 7.2.1 Bending moments: BMB  3,4  3    10,2 kN.m     BMC  (3,4 7)  (4 4)  23,8 16   7,8kN.m    BMD  0 kN.m  (6)    Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 11 DBE/November 2020 SC/NSC – Marking Guidelines 7.2.2 Shear force diagram: 3,4 kN   17 mm   0 kN 13 mm 0 kN  -0,6 kN 3 mm    -2,6 kN    (3)  7.2.3 Bending moment diagram: 10,2 kN.m 7,8 kN.m 51 mm  39 mm           0 kN.m 0 kN.m  (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 12 DBE/November 2020 SC/NSC – Marking Guidelines 7.3 Stress and Strain: 7.3.1 Maximum stress: Maximum Load Maximum Stress  Area 8  10 3   0,08  10 -3   100  10 6 Pa   100 MPa (3) 7.3.2 Safe working stress: Maximum Stress Safe Stress  Safety Factor 100  10 6   4  25  10 6 Pa   25 MPa (2) [45] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 13 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 8: JOINING METHODS (INSPECTION OF WELDS) (SPECIFIC) 8.1 Welding spatter:  Too high current / Amperage too high   Too long arc / Arc blow   Not using anti-spatter spray   Electrode angle too small   Welding speed too fast   Wet electrodes   Gas-flow too high (gas welding)   Incorrect polarity for electrode type  (Any 4 x 1) (4) 8.2 Gas cutting: 8.2.1 Nozzle too high from surface:  Excessive melting of the top edge.   Undercut at the top of the cut face with lower part square and sharp bottom corner.  (2) 8.2.2 Nozzle too close to the surface:  Top edge slightly rounded and heavily beaded.   Cut face usually square with fairly sharp bottom corner.  (2) 8.3 Causes of weld defects: 8.3.1 Porosity:  Dirty weld surface   Wet welding electrodes   Rust in the MIG wire electrode   Interruption of shielding gas supply   Welding in windy conditions where effectiveness of shielding gas is compromised   Wrong gas used on the specific metal   Weld ability of parent metal not good   Wrong electrode used on the specific metal   Too high temperature  (Any 2 x 1) (2) 8.3.2 Poor penetration:  Welding current is too low   Travel speed is too fast   Incorrect electrode angle   Poor edge preparation   Insufficient root gap   Gas flow too low (gas welding)  (Any 2 x 1) (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 14 DBE/November 2020 SC/NSC – Marking Guidelines 8.4 Heat Affected Zone (HAZ) Cracks:  Excessive hydrogen   High residual stress levels (work piece cooled too fast)   High carbon content on the base metal  (Any 2 x 1) (2) 8.5 Visual inspection:  Shape of the profile   Uniformity of the surface   Overlap   Undercutting   Penetration bead   Root groove   Surface cracks  (Any 3 x 1) (3) 8.6 Ultrasonic test:  Gel should be put on the surface of the work piece.   The sender/receiver is moved in a zigzag motion across the weld to broaden its detection range.   A high frequency sound wave is send into the metal.   When the wave is stopped, the sender serves then as a receiver.   The receiver monitors the waves as it is reflected through the metal.   Each wave is visually represented on an oscilloscope.   The calibrated oscilloscope will then indicate the deviations in the waves which represents the defects in the metal.  (Any 6 x 1) (6) [23] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 15 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 9: JOINING METHODS (STRESSES AND DISTORTION) (SPECIFIC) 9.1 Residual stress: Residual stresses are stresses that exist in a metal  after cooling.  (2) 9.2 Effect of hot working on steel:  In hot working, deformation and recrystallization occur simultaneously so that the rate of softening is greater than work hardening.   Hot-working should be finished at a temperature just above the recrystallization temperature.   To obtain a fine grain structure.   If the finishing temperature is too high, grain growth will occur while the metal is cooling above the recrystallization temperature and inferior properties will develop.  (4) 9.3 Iron-carbon diagram: 9.3.1 Iron-carbon equilibrium diagram  (1) 9.3.2 A. Temperature / Degrees Celsius  B. Austenite  C. Austenite and Cementite  D. Ferrite and Pearlite  E. Carbon content  (5) 9.4 Result when metal is cooled rapidly:  The metal sets up internal and external stresses.   Causes cracks on the surface of the metal.   Causes deformation.   Hardness increases.   Martensite forms.  (Any 2 x 1) (2) 9.5 Quenching media:  Water   Brine   Oil   Air   Metal / Molten salts   Lime   Sand   Ash  (Any 4 x 1) (4) [18] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 16 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 10: MAINTENANCE (SPECIFIC) 10.1 Types of maintenance:  Preventative   Reliable centred   Predictive   Routine   Corrective   Condition based   Reactive maintenance  (Any 2 x 1) (2) 10.2 Lockout on machines: To ensure that nobody can turn on  the machine while maintenance is being carried out.  (2) 10.3 Rules to be observed before machine start up:  Check if there is lock out tag   Confirm that the machine is safe and operational.   Check that all guards and safety devices are in position and operative.   Inform workers that the machine is being brought back into service.   Check that the area surrounding the machine is clear and that turning it on will not endanger anyone.   Check that all relevant fasteners have been properly tightened.   Check that all lockout devices have been removed.   Turn on the machine's power supply.   Know where the stop switch or emergency switch is located.  (Any 2 x 1) (2) 10.4 Factors to be observed when selecting the cutting speed of a drilling machine:  Type of material   Diameter of drill bit   Material of which the drill bit is made   Type of drill bit   Firmness with which the work is clamped   Condition of the machine   Use of cutting fluids   Rate of feed  (Any 3 x 1) (3) [9] Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology (Welding and Metalwork) 17 DBE/November 2020 SC/NSC – Marking Guidelines QUESTION 11: DEVELOPMENT (SPECIFIC) Square-to-round transition: 11.1 The true length FG: IK  300 (2 units) IH  150 (1unit) HK  1 3 (1unit 3 ) The true length FG : Plan length FG  FK  GK   400  300 100 mm  The true FG isequalto H'F CG2  C'F 2  FG 2   4002  1002  C' G  170000  CG  412,31 mm  True length C' G  412,312  8002   900mm  (7)    Copyright reserved Please turn over

Downloaded from hlayiso.com Mechanical Technology: Welding and Metalwork 18 DBE/November 2020 SC/NSC – Marking Guidelines 11.2 Length CI, the sides CE and EI of triangle CEI must be calculated: CE  CF  EF  400  150   250mm  But EI  FH HK  1 unit  3  150 3   259,81 mm  FH  FK  HK  400  259,81   140,19 mm  CI2  CE2  EI2  250 2  140,192   82153,24 CI  286,62 mm  True length CI2  CI2  Perpendicular Height 2  286,622  8002   722151,02  849,79 mm  (10) 11.3 JI is one-twelfth of the circumference: MD True Length of JI Circumference  π   12 1 1884,9 Circumference   12 12  157,08 mm  (3) [20]  GRAND TOTAL: 200  Copyright reserved