Welcome to Manufacturing Processes
Welcome to your study notes for Unit 2: Production. In engineering, having a brilliant design on paper is only half the battle. You also need to know how to build it accurately, safely, and efficiently in the workshop. In this chapter, you will learn the core manufacturing processes used to turn raw stock materials into finished engineering components.
Don't worry if there seem to be a lot of tools and names to remember at first. We will break everything down step-by-step into clear families of processes!
---1. Wasting Processes (Material Removal)
Wasting means taking a piece of material and removing the unwanted parts as chips, swarf, or dust until you are left with the shape you want. Think of it like a sculptor carving away stone.
A. Marking Out
Before cutting or drilling, engineers must mark precise lines on the raw material. Precision here prevents costly mistakes later.
• Steel Rule: Used for direct, accurate linear measurements.
• Engineer's Square: Used to mark lines at right angles (\(90^\circ\)) to an edge and to check surfaces for squareness.
• Scriber: A hardened steel pointer used like a pencil to scratch sharp, fine lines onto metal surfaces.
• Centre Punch: Used with a ball-peen hammer to make a small indentation at the exact centre point of a hole. This stops a drill bit from wandering off target.
• Oddleg Calipers (Jenny Calipers): Have one bent leg and one straight point; used to scribe lines parallel to an edge.
• Dividers: Used to scribe circles, arcs, and step off equal distances.
• Surface Gauge & Marking Table (Surface Plate): A heavy, perfectly flat reference surface used with a gauge to scribe precise parallel horizontal lines onto workpieces.
B. Sawing and Shearing
• Hacksaw: Used for cutting metal. Blade teeth point forward. The blade pitch (number of teeth per inch) is chosen based on material thickness: fine teeth for thin walls and tubing, coarser teeth for thick solid bar.
• Junior Hacksaw: Smaller hacksaw for lighter work and tight spaces.
• Piercing Saw: Uses very fine, thin blades to make intricate cuts in thin sheet metal.
• Guillotine / Bench Shears: Fast, waste-free cutting of sheet metal using a scissor-like shearing action.
C. Filing and Abrading
Files are used to shape, smooth, and deburr edges after cutting.
• File Shapes: Flat and Hand files (for flat surfaces), Half-round files (for curved and flat surfaces), Round files (for circular holes/slots), and Square files (for corners and rectangular slots).
• Cross Filing (Roughing): Filing across the work to remove material quickly.
• Draw Filing: Holding the file with both hands and drawing it along the edge to produce a smooth, satin finish.
• Deburring: Removing sharp, ragged edges (burrs) left after sawing or drilling to make the part safe to handle.
D. Drilling
Drilling produces round holes using rotating cutting tools.
• Machines: Pillar drill (fixed bench or floor machine), cordless hand drill, or lathe tailstock.
• Centre Drilling: Using a short, stiff centre drill bit to create a rigid starter dimple.
• Pilot Drilling: Drilling a small hole first to reduce cutting resistance before using a large drill bit.
• Hole Saws: Cylindrical cutters used to cut large-diameter circular holes in sheet material.
• Countersinking: Creating a chamfered (conical) enlargement at the top of a hole so countersunk screws sit flush with the surface.
E. Centre Lathe Operations (Turning)
On a centre lathe, the workpiece rotates in a chuck while a single-point cutting tool moves against it.
• Facing Off: Cutting across the end face of a workpiece to make it flat, smooth, and square to the centreline.
• Parallel Turning: Moving the tool along the length of the rotating bar to reduce its diameter evenly.
• Step Turning: Turning different sections of a bar to different diameters along its length.
• Chamfering: Cutting an angled bevel on an edge to remove sharp corners.
• Knurling: Pressing a hardened textured roller into the metal surface to create a diamond or straight textured grip.
• Parting Off: Using a narrow blade tool to slice through and separate the finished component from the raw stock.
• Centreline Drilling & Boring: Holding a drill chuck in the tailstock to drill or enlarge holes along the central axis.
F. Milling
Unlike turning, on a milling machine the workpiece is clamped rigidly while a multi-toothed cutter rotates at high speed.
• Face Milling: Creating large, flat horizontal surfaces using the face of the cutter.
• End Milling: Using the end and sides of a cutter to create slots, grooves, shoulders, and flat steps.
• Slotting & Profiling: Machining precise slots or following outer contours on manual or CNC (Computer Numerical Control) milling machines.
Key Takeaway for Wasting: Wasting always produces chips or swarf. Always mark out accurately first, choose the right blade or cutter, and clamp the workpiece securely before cutting.
---2. Forming, Moulding, and Casting Processes
Forming processes change the shape of a material through heat, pressure, or liquid casting without cutting material away.
A. Bending and Folding
• Sheet Metal Bending: Metal sheets are clamped between folding bars in a bench vice or placed into a box and pan folder (sheet metal brake) to create straight, crisp bends.
• Thermoplastic Line Bending: Acrylic sheet is heated locally along a narrow line using an electric strip heater (line bender). Once softened, the plastic is bent by hand against a former until it cools and sets.
B. Forging and Cold Working
• Forging: Shaping hot metal by compressive forces using hammers, presses, or dies.
• Cold Working & Work Hardening: Deforming ductile metals at room temperature. Bending or hammering cold metal alters its crystal structure, making it harder and stiffer (work hardened), but also more brittle.
C. Casting
Casting involves pouring liquid molten metal into a mould cavity shaped like the desired object.
• Sand Casting: A wood or resin pattern is packed into sand held in two mould boxes: the top half (cope) and bottom half (drag). Molten metal is poured down the runner, fills the cavity, and rises up the riser (which ensures complete filling and allows trapped air to escape). It is ideal for low-cost, one-off, or batch production of engine blocks and heavy components.
• Gravity / Die Casting: Molten metal is poured or injected under pressure into reusable metal moulds (dies). This produces high-precision components with smooth surface finishes from low-melting-point alloys (e.g., aluminium and zinc).
D. Polymer Processing
• Injection Moulding: Plastic pellets are melted in a heated barrel, then an archimedean screw forces the molten plastic under high pressure into a closed metal mould. Ideal for mass producing complex solid items like casing shells and bottle caps.
• Vacuum Forming: A sheet of thermoplastic is clamped, heated until rubbery, pulled over a mould, and atmospheric pressure forces the sheet down as a vacuum sucks out the air underneath. Ideal for trays and packaging.
• Blow Moulding: A tube of hot softened polymer (parison) is clamped in a split mould, and compressed air is blown inside, inflating it against the mould walls. Used for hollow bottles and containers.
• Extrusion: Molten plastic is pushed continuously through a shaped die nozzle to produce continuous profiles, pipes, and tubing.
Key Takeaway for Forming: Forming reshapes materials without waste. Thermoplastics soften when heated and can be reshaped repeatedly; metals can be shaped hot (forging/casting) or cold (work hardening).
---3. Joining and Assembly Processes
Components can be assembled using permanent methods (cannot be undone without damaging the parts) or temporary/demountable methods (can be dismantled for maintenance or repair).
A. Permanent Joining
1. Thermal Joining:
• Oxy-Acetylene Welding: Burns oxygen and acetylene gas to produce an intense flame that melts parent metal edges together with an optional filler rod.
• MIG / TIG / MMA Welding: Arc welding processes using electricity to create high heat that melts and fuses parent metals together.
• Brazing: Joins metals using a brass/spelter filler rod and flux at temperatures above \(450^\circ\text{C}\) (typically \(800^\circ\text{C}+\)). The parent metal is heated red hot but does not melt; the filler melts and flows into the joint by capillary action.
• Hard Soldering (Silver Soldering): Similar to brazing, using silver-alloy solder at high temperatures for strong, neat joints.
• Soft Soldering: Uses a low-melting-point tin-based solder (below \(450^\circ\text{C}\)) with an electric soldering iron or torch for electrical circuits and copper plumbing pipes.
2. Mechanical & Chemical Joining:
• Solid Rivets: Permanent metal fasteners inserted through pre-drilled holes and hammered over to form a second head.
• Pop / Blind Rivets: Rivets inserted from one side only using a pop-rivet gun (riveter); ideal when the back of a panel cannot be reached.
• Adhesives:
- Epoxy Resin: A two-part adhesive (resin + hardener) providing high strength for metals, ceramics, and dissimilar materials.
- Cyanoacrylate (Superglue): Fast-acting adhesive that cures instantly with surface moisture; ideal for non-porous parts.
- Tensile Acrylic Cements (e.g., Tensol): Chemical solvent cements that weld acrylic sheets by chemically dissolving and fusing the contact surfaces.
- PVA: Water-based adhesive for bonding wood and porous timber joints.
- Contact Adhesive: Applied to both mating surfaces, allowed to become tacky, and pressed together for instant bonding of large laminates and sheets.
B. Temporary / Demountable Joining
• Threaded Fasteners: Machine screws, bolts and nuts (standard hex nuts, nyloc locking nuts containing nylon inserts to resist vibration), self-tapping screws, Allen head cap screws, and threaded studs.
• Washers: Flat washers distribute clamping force over a larger area; spring/locking washers prevent nuts and bolts from vibrating loose.
• Taps and Tap Wrenches (Internal Threads): Used to cut female screw threads inside a pre-drilled blind or through hole. Used in a set of three: Taper tap (leads the thread), Plug/Second tap (deepens the thread), and Bottoming tap (cuts full thread right to the bottom of a blind hole).
• Split Dies and Die Stocks (External Threads): Used to cut male screw threads on the outside of a solid metal bar.
Key Takeaway for Joining: Choose thermal or adhesive joining for permanent, leak-proof, or sleek joints. Choose threaded fixings (bolts, machine screws) when products must be disassembled for servicing.
---4. Heat Treatment of Metals
Heat treatment alters the internal grain structure and mechanical properties of ferrous metals (especially medium and high-carbon steels) through controlled heating and cooling.
A. Hardening
• Process: Heating high-carbon steel to its critical temperature (cherry red, approximately \(750^\circ\text{C}\)–\(850^\circ\text{C}\)), followed by rapid cooling (quenching) in clean water, oil, or brine.
• Result: Traps the carbon to form a hard, needle-like crystal structure called martensite.
• Outcome: The steel becomes extremely hard and wear-resistant, but also very brittle (it will shatter like glass if dropped or struck).
B. Tempering
• Process: After hardening, the metal surface is cleaned with abrasive cloth. It is then gently reheated to a lower temperature (between \(230^\circ\text{C}\) pale straw colour and \(300^\circ\text{C}\) blue colour) and quenched again.
• Result: Relieves internal stresses.
• Outcome: Sacrifices a small amount of hardness to restore toughness and significantly reduce brittleness. Essential for cutting tools like chisels, punches, and drill bits.
C. Annealing
• Process: Heating metal to its recrystallisation temperature and allowing it to cool down very slowly (e.g., buried in hot ash, sand, or left inside a switching-off furnace).
• Result & Outcome: Softens the metal, relieves internal stress, and restores ductility so it can be cold worked or machined easily without cracking.
D. Normalising
• Process: Heating ferrous metals above their upper critical temperature and allowing them to cool in still air at room temperature.
• Result & Outcome: Refines the crystal grain structure and eliminates internal stresses caused by forging or rolling, leaving the metal with uniform, predictable mechanical properties.
Key Takeaway for Heat Treatment: Hardening makes steel hard and brittle. Tempering reduces that brittleness to make it tough. Annealing softens metal, while normalising creates a uniform grain structure.
---5. Surface Finishing Techniques
Engineered parts must be finished to prevent corrosion, improve wear resistance, or improve visual appearance.
• Painting and Spraying: Applying a primer followed by topcoats to seal the surface against oxygen and moisture.
• Powder Coating: Positively charged dry polymer powder is electrostatically sprayed onto an earthed metal component. The part is then baked in an oven, melting the powder into a tough, durable, uniform plastic coating.
• Electroplating: Immersing a metal component in an electrolyte bath and using an electric current to deposit a thin protective and decorative layer of another metal (such as zinc, nickel, or chromium).
• Anodising: An electrolytic process used on aluminium that builds up a durable, controlled aluminium oxide layer. This layer resists corrosion and wear, and can be dyed in bright, vibrant colours.
• Polishing and Buffing: Using rotating cloth mops loaded with fine abrasive compounds to buff away micro-scratches and produce a reflective, mirror-like finish.
• Galvanising: Dipping cleaned ferrous steel parts into a bath of molten zinc (at approximately \(450^\circ\text{C}\)). The zinc bonds to the steel, providing a weather-resistant barrier and sacrificial corrosion protection (the zinc corrodes before the steel).
Key Takeaway for Finishing: Surface finishes protect against rust (corrosion), resist wear, improve appearance, or provide electrical insulation.
---6. Scales of Production & Systems Conventions
The number of identical parts required determines the manufacturing scale and setup used in industry.
• One-Off / Jobbing Production: A single, bespoke item made to exact customer specifications (e.g., custom architectural gate, specialist prototype). High unit cost, highly skilled workforce, and general-purpose tools.
• Batch Production: A specific, limited quantity of identical items manufactured together in a production run (e.g., 500 bicycle frames). When finished, tooling is swapped to make a different batch. Uses jigs, fixtures, and templates for consistency.
• Mass / Flow Production: High-volume continuous line production of identical components (e.g., automotive parts, white goods). Highly automated, high initial setup cost, but very low unit cost.
• Continuous Production: Non-stop 24/7 processing of bulk raw materials (e.g., oil refining, raw steel manufacturing, bulk polymer extrusion).
• Just-In-Time (JIT): A lean production strategy where parts and raw materials are ordered and delivered only as they are needed on the assembly line, minimising warehouse storage costs and eliminating work-in-progress waste.
Jigs vs. Fixtures — Don't Mix Them Up!
• Jig: Holds and locates the workpiece AND guides the cutting tool (e.g., a drill jig with hardened steel bushings that guide a drill bit straight into the right spot).
• Fixture: Securely locates and holds the workpiece in a fixed position relative to the machine table, WITHOUT guiding the cutting tool directly (e.g., a custom milling vice or clamp plate).
7. Engineering Calculations & Formulae
In Unit 2, you are expected to apply standard engineering formulae to calculate machine settings, costs, and tolerances.
A. Spindle / Cutting Speed Formula (Drilling and Lathe Turning)
To cut material efficiently without burning the tool, the spindle rotational speed (\(\text{RPM}\)) must match the material's recommended cutting speed (\(V\)) and diameter (\(D\)):
\(\text{RPM } (N) = \frac{1000 \times V}{\pi \times D}\)
Where:
• \(\text{RPM } (N)\) = Spindle rotational speed (revolutions per minute)
• \(V\) = Cutting speed of the material in metres per minute (\(\text{m/min}\))
• \(D\) = Diameter of the workpiece (on a lathe) or drill bit (on a drill) in millimetres (\(\text{mm}\))
• \(\pi \approx 3.142\)
Worked Example:
Calculate the required spindle speed (\(\text{RPM}\)) to turn a mild steel bar of diameter \(D = 50\text{ mm}\) with a recommended cutting speed \(V = 30\text{ m/min}\).
\(\text{RPM} = \frac{1000 \times 30}{3.142 \times 50} = \frac{30000}{157.1} \approx 190.96\text{ RPM}\)
Answer: Set the lathe to approximately \(191\text{ RPM}\).
Rule of Thumb: Larger diameter (\(D\)) \(\implies\) Lower \(\text{RPM}\). Smaller diameter (\(D\)) \(\implies\) Higher \(\text{RPM}\).
B. Production Costing Formula
To determine the total cost of making an engineered batch:
\(\text{Total Production Cost} = \text{Direct Costs} + \text{Indirect Costs}\)
\(\text{Total Production Cost} = (\text{Materials} + \text{Direct Labour}) + (\text{Overheads} + \text{Machinery Depreciation})\)
C. Engineering Tolerances & Limits
Engineering drawings specify dimensions with tolerances to ensure mating parts fit correctly without needing expensive, impossible perfection.
• \(\text{Upper Limit} = \text{Nominal Size} + \text{Upper Deviation}\)
• \(\text{Lower Limit} = \text{Nominal Size} - \text{Lower Deviation}\)
• \(\text{Tolerance Band} = \text{Upper Limit} - \text{Lower Limit}\)
Worked Example:
A turned pin has a drawing dimension of \(25.00\text{ mm} \pm 0.05\text{ mm}\).
• \(\text{Upper Limit} = 25.00 + 0.05 = 25.05\text{ mm}\)
• \(\text{Lower Limit} = 25.00 - 0.05 = 24.95\text{ mm}\)
• \(\text{Tolerance Band} = 25.05 - 24.95 = 0.10\text{ mm}\)
8. Common Exam Pitfalls & Workshop Safety
Mistakes to Avoid:
• Brazing vs. Soft Soldering: Brazing uses a copper-zinc (brass/spelter) filler rod and occurs at much higher temperatures (above \(450^\circ\text{C}\), typically \(800^\circ\text{C}+\)). Soft soldering uses tin-based filler at temperatures below \(450^\circ\text{C}\).
• Hardening vs. Tempering: Hardening does not make metal tough—it makes it hard and brittle. Tempering restores toughness by slightly reducing hardness.
• Jig vs. Fixture: Remember: A Jig Guides the tool (think "Jig Guides"), while a Fixture Fixes the work securely in place.
Workshop Safety Checklist (Essential for Unit 2 Assessment):
• Always remove the chuck key from the lathe chuck or drill chuck immediately after use before turning on the power.
• Clamp work securely in a machine vice or to the drill table using T-bolts; never hold metal workpieces by hand while drilling.
• Wear correct PPE (Personal Protective Equipment): eye protection/safety goggles, sturdy footwear, and tie back long hair / remove loose jewellery.
• Ensure local exhaust ventilation (dust/fume extraction) is operating when brazing, welding, or using adhesives.