Specialist Techniques & Processes

Wastage and addition

Marking out and cutting

Marking out is a process where the required shape is marked onto stock material using tools such as:

• Try square, tape measure, steel rule

Materials must be held or clamped (using a vice or clamp) before cutting.

Cutting tools include:

• Hand tools — coping saw, tenon saw, hacksaw
• Machinery — bandsaw, laser cutter

Marking out metals

• Scriber — scratches a fine, accurate line on metal.
• Engineer’s square — marks or checks lines at 90° to a straight datum edge.
• Odd-leg callipers — mark lines parallel to an edge.
• Dividers — mark circles and arcs, and step out equal distances.
• Centre punch — makes a small dent to locate a drill bit at the centre of a hole.
• Marking blue (engineer’s blue) — a dye painted on bright metal so scribed lines show clearly.

Always mark out from a datum edge or face (a straight, true reference edge) to keep measurements accurate and reduce waste.

Drilling

Drilling is used when a hole is required in a material. Can be done using a hand drill or pillar drill. Jigs and formers ensure holes are drilled in correct positions.

Screws

Screws come with different head types:

• Flat or round headed
• Slots, crosses (Phillips) or square holes for different screwdriver fittings
• Woodscrew threads are wider than self-tapping screws
• Machine threads: M3, M4, M5 etc (fine standard threads)

Joining and reforming metals

Permanent joining:

• Welding — melting the edges of the metals so they fuse together, usually with a filler rod
• Brazing — joining with a brass filler (spelter) at bright red heat, without melting the parent metal
• Soldering — joining with a lower-temperature filler metal (soft solder or silver solder)
• Riveting — a rivet is deformed to clamp the parts together; it must be drilled out to remove it
• Epoxy resin adhesives

Temporary joining:

• Nuts, bolts and washers, machine screws, self-tapping screws, hinges and catches

Main stages: pop riveting

1. Clamp the parts together and drill a clearance hole through both.
2. Push the pop rivet into the hole.
3. Place the rivet gun over the mandrel and squeeze repeatedly. This pulls the mandrel and expands the rivet body on the far side.
4. The mandrel snaps off, leaving the parts permanently joined. Pop rivets are useful when you can only reach one side of the joint.

Main stages: brazing

1. Clean the joint area (emery cloth) and make sure the parts fit closely.
2. Apply flux to stop oxides forming and help the filler flow.
3. Support the work on a brazing hearth with fire bricks and heat it with a gas torch to bright red heat.
4. Touch the brass spelter (brazing rod) onto the joint. It melts and flows into the joint by capillary action.
5. Let it cool, then clean off the flux residue.

Machines

• Lathe — turns a piece of metal to create differently shaped round pieces
• Milling machine — cuts slots in blocks of metal and faces off edges

Wood joining

Temporary methods: screws (countersunk and round head), nuts and bolts, and knock-down (KD) fittings such as cam locks, corner blocks and scan fittings — used in flat-pack furniture so it can be assembled and taken apart.

Permanent methods: wood joints and adhesives (PVA, epoxy resin, contact adhesives)

Box/carcass joints

Frame joints

Producing wood products: veneering, laminating and steam bending

Veneering — a very thin slice of attractive (often expensive) timber is glued onto a cheaper, stable base such as MDF, chipboard or plywood. This gives a high-quality appearance at lower cost and makes better use of rare hardwoods.

Laminating — thin strips or veneers of wood are coated with adhesive (e.g. PVA or urea formaldehyde), stacked with the grain running the same way, and clamped around a former until the glue sets. When released, the timber keeps the curved shape. Laminating makes strong curved parts with little waste, e.g. chair frames and rocker legs.

Steam bending — solid timber (often beech or ash) is heated in a steam box until the fibres soften, then quickly bent around a former and clamped until dry. It keeps its new shape. Used for bentwood chairs and boat ribs.

Joining polymers

Plastics can be joined using:

• Mechanical fixings — nuts and bolts, screws, rivets, hinges
• Adhesives — bond polymers together
• Solvent cement (e.g. Tensol for acrylic) — dissolves the surfaces so they fuse as the solvent evaporates.
• Plastic welding — heat (e.g. a hot-air welding gun or ultrasonic vibration) softens the surfaces of two thermoforming polymer parts so they fuse together.

Bending and forming polymers

Line bending — a strip heater (line bender) heats a narrow line across a thermoforming sheet such as acrylic. When soft, the sheet is bent to the required angle and held in a bending jig until it cools.

Vacuum forming, press moulding and injection moulding — see Scales of Production.

3D printing

3D printing (fused deposition modelling) is an additive process. A CAD model is exported (e.g. as an STL file) and “sliced” into layers. A filament such as PLA or ABS is melted through a heated nozzle and laid down layer by layer.

• Advantages: complex shapes in one piece, very little waste, ideal for prototypes and one-off parts, no tooling costs.
• Disadvantages: slow, visible layer lines may need finishing, and limited strength compared with moulded parts.

Computer aided manufacture (CAM)

• All cuts are accurate and exact
• Laser cutters cut plastic sheets and certain types of MDF (correct settings essential to avoid burning)
• CAM machines can engrave materials and mark out where holes, joins and fixings are to go
• Can produce features that allow parts to be joined to another part

Shaping papers, cards and boards

• Scoring and folding — scoring (lightly cutting or creasing along a line) lets card fold cleanly without cracking.
• Embossing / debossing — a shape is pressed into the card so it is raised (embossed) or sunken (debossed).
• Cropping — trimming printed sheets to their final size along the crop marks, removing the excess (bleed).
• Die cutting — a shaped steel-rule die cuts and creases the net of a package in one action.
• Binding — see Surface Treatments & Finishes (stitching, comb binding, perfect binding, stapling).