Metal Forming Processes (Junior Cert Engineering): Revision Notes
Hot Forming Processes
Hot forming processes involve heating metal until it becomes soft and workable, then shaping it into the required form. These manufacturing techniques are essential in engineering for creating strong, durable metal components.
What is forgework?
Forgework is a manufacturing process where metal is heated until soft and then shaped using tools like hammers and anvils. The key advantage of forgework is that the metal grain structure follows the shape of the forged piece, making it much stronger than if it were simply cut from a block of metal.
In schools, forgework helps students develop important skills including judging material sizes and shapes, while also improving hand-eye coordination through practical metalworking experience.
Essential equipment for hot forming
The forge
The forge is used to heat metal to the correct working temperature. There are two main types you'll encounter:
- Coal or coke-fired forges: Traditional forges that burn solid fuel
- Gas forges: Modern forges that use gas burners - these are particularly suitable for school workshops because they're easier to control, produce less smoke and dirt, and don't create the same fumes as coal-fired versions
Some forges combine both gas and solid fuel systems, giving you flexibility in how you heat your metal. Gas forges are particularly recommended for educational environments due to their cleaner operation and better temperature control.
The anvil
The anvil is the main work surface for shaping hot metal. It's made from high carbon steel and has several important features:
- Main working face: The flat top surface where most shaping happens - this is hardened for durability
- Cutting table: A softer area used for chiselling operations (never chisel on the hardened face as this can damage both the anvil and your tools)
- Rounded edge: Used for bending operations
- Hardie hole: Square hole for holding special tools
- Punching hole: Round hole that allows punches to pass through when making holes
- Bick: The pointed end used for forming curves and eyes
Never chisel on the hardened face of the anvil as this can damage both the anvil and your tools. Always use the softer cutting table for chiselling operations.
The anvil sits on a solid stand to bring it to the right working height and provide stability during heavy hammering work.
The leg vice
The leg vice is specially designed to withstand the heavy hammering involved in forge work. Unlike regular bench vices, it's bolted to both the bench and floor for extra support. The moveable jaw pivots, which means there's only one position where the jaws are perfectly parallel - this affects how well it grips different sized pieces.
Tongs for hot metal handling
Tongs are essential for safely gripping and manipulating hot metal during forging. Different types serve different purposes:
- Close mouth tongs: Best for gripping thin materials
- Open mouth tongs: Designed for thick materials
- Hollow bit tongs: Shaped to grip round, square, hexagonal, and octagonal metal sections
- Bolt tongs: Made for holding articles with large ends
Always choose tongs that fit your work properly and provide a secure grip. If possible, hold the cold end of the work in your hand rather than relying entirely on tongs.
Drawing down operations
Drawing down is a forging technique that reduces the cross-sectional area of a metal bar while increasing its length. This is commonly used to create tapered sections, points, and necked areas.
Creating points
When forming a point, mark the required length on the anvil face with chalk. Start with a blunt point and work backwards until you achieve the desired length. This technique prevents "piping" - the formation of a hollow at the end of the bar.
Worked Example: Creating a Centred Point
Step 1: Mark the required length on the anvil face with chalk Step 2: Start with a blunt point and work backwards Step 3: Hold the bar at an angle equal to half the desired point angle Step 4: For square points, turn the bar 90° regularly to maintain the square cross-section Step 5: Continue until you achieve the desired length and shape
Safety in the forge
Critical Safety Rules:
- Only one student should use the anvil at a time
- Never leave hot tools or metal lying around the workshop
- Cool all tools completely before storing them
- Wear protective equipment including leather apron and safety goggles
- Hold hammers by the end of the handle, not near the head
- Keep the hammer head at the correct angle to avoid leaving marks
Forming operations
Creating an eye
Forming an eye in metal requires careful planning and technique:
Worked Example: Forming an Eye
Step 1: Calculate the material length needed using the mean diameter formula Step 2: Mark this length on the bar and heat the material Step 3: Make a right-angle bend at your mark Step 4: Reheat and form the eye around the bick of the anvil or a former Step 5: Work from the end back to the right angle Step 6: Complete the eye and flatten it on the anvil face
Forming a U-bend
U-bends are formed using similar techniques to eyes but create a different shape. Mark the position where the bend begins, start bending with this mark in contact with the former, and continue until both legs are parallel.
Calculating material lengths
When forming circular shapes, you need to calculate how much straight material will be needed:
Mean Diameter =
Length of material required =
Worked Example: Material Length Calculation
For a complete circle:
- Internal diameter = 20mm, External diameter = 24mm
- Mean diameter = mm
- Material length = mm
For partial circles:
- Semicircle: divide by 2 = 34.6mm
- Quarter circle: divide by 4 = 17.3mm
Heat treatment processes
Heat treatment changes the properties of metals through controlled heating and cooling cycles. These processes are essential for making tools and components with the right characteristics.
Hardening
Hardening makes high carbon steel very hard but brittle. The process involves:
- Heating the steel to cherry red (approximately 750°C)
- Rapidly cooling or "quenching" in water, oil, or brine
The exact temperature depends on the carbon content of the steel. Different quenching media produce different results - oil gives slower cooling with less risk of cracking, while brine provides the fiercest quench but with higher cracking risk.
Tempering
Hardened steel is often too brittle for practical use, so tempering reduces some hardness to improve toughness. This involves reheating the hardened steel to a lower temperature and then cooling it. The tempering temperature depends on what the tool will be used for.
Temperature Colour Guide for Tempering:
- Pale straw (230°C): For scribers, dividers, scrapers
- Dark straw (240°C): For cold chisels
- Brown (250°C): For shear blades, centre punches
- Purple (270°C): For axes, press punches
- Blue (300°C): For springs, screwdrivers
You can judge the temperature by the colour that appears on the polished steel surface.
Annealing
Annealing softens metal and relieves internal stresses. Heat the metal to cherry red and allow it to cool very slowly - either by switching off the forge when it reaches temperature or by burying the hot metal in sand or lime to slow the cooling process.
Normalising
Normalising refines the grain structure of steel and removes stresses caused by cold working like hammering or bending. Heat the steel to cherry red and allow it to cool naturally in still air - this gives faster cooling than annealing, resulting in a harder structure.
The key difference between annealing and normalising is the cooling rate. Annealing uses very slow cooling for maximum softness, while normalising uses natural air cooling for a refined but harder structure.
Practical application: making a cold chisel
Creating a cold chisel demonstrates many hot forming and heat treatment techniques:
Worked Example: Making a Cold Chisel
- Cut suitable length material from high carbon steel bar
- Heat one end to medium red and draw down to required shape
- Reheat to cherry red and allow to cool slowly (anneal)
- Cut to exact length and file the forged sides straight
- File a chamfer on the other end and draw-file all sides
- Polish using emery cloth wrapped around a file
- Harden the cutting end by heating to cherry red and quenching in clean water
- Polish again to see tempering colours
- Temper by heating slowly from behind until dark straw colour appears at the point
- Cool in oil and grind the cutting edge to 60°
Metal casting
Metal casting involves pouring molten metal into moulds to create shaped components. While not traditional hot forming, it's an important related process.
Sand casting
Sand casting uses sand moulds created around patterns. The pattern (usually wooden) is removed to leave a cavity matching the desired component shape. Molten metal is poured through gates and risers into this cavity.
School workshop casting
Safe Casting for Educational Environments:
For safety in schools, low-melting alloys are used:
- Tin/bismuth alloys (melting point 138°C)
- Lead-free pewter (melting point approximately 245°C)
MDF board can be used for moulds due to these low temperatures, and special low-temperature melting units provide safe, controlled heating.
Key Points to Remember:
- Hot forming strengthens metal by aligning grain structure with the component shape
- Safety first - always wear protective equipment and follow proper procedures when working with hot metals
- Temperature control is crucial - use colour guides to judge heat treatment temperatures accurately
- Different tools for different jobs - select the right tongs, hammers, and forming techniques for each specific task
- Practice makes perfect - hot forming requires skill and experience to master the techniques effectively