Joining Materials
PERMANENT FIXINGS
Pop riveting
Pop riveting is a technique that is used to join thin pieces of metal and it can also be used to join plastic sheet. The rivet has two parts; the pin and the rivet. The pop rivet pliers are used to pull the pin through the rivet and as this happens the rivet is deformed slightly so that it joins the metal or plastic pieces. This is explained in detail below.
This technique is used where the metal or plastic is thin and where the joint does not have to be very strong. It is ideal for joining aluminum or even thin sheet plastic
1. The two pieces of plastic or aluminum are drilled to a size slightly larger than the rivet
2. The pop rivet is passed through both holes in the sheet plastic / aluminum.
3. The rivet pliers are pushed on to the pin of the rivet and the handles are pulled together. As this happens the pin head is pulled into the rivet and the end of the rivet is expanded. Eventually the pin will break off leaving the rivet permanently fixed in position holding the two pieces of plastic / aluminum together.
Welding
Welding is used where high strength joints are required. Oxyacetylene welding uses an oxygen and acetylene gas to produce an extremely hot flame. Electric arc welding uses a high electrical current through a welding electrode to carry out the welding process. Arc welding is a quick and effective method of forming steel structures and large assemblies
Joining Plastics
Permanent jointing can be achieved by using adhesives or hot-air welding. When using adhesives appropriate types must be used. Tensol is a solvent based adhesive used for bonding acrylics. A hot air gun and filler rod can be used in plastics welding. The hot air has a high enough temperature to soften the material. The joints can be prepared in the same way as for metal welding.
Wood Joints
Wood joints are used in a wide range of applications from furniture to picture frames. Traditional joints such as housing joints, mortise and tenons and dovetails are used with natural timbers. Joints in natural timber products are used for strength purposes and to enhance the appearance of the wood.
TEMPORARY FIXINGS
Screws
COUNTERSUNK - SLOT HEAD: This can be used for general woodworking for example fitting hinges to doors. Because the screw is countersunk it can be tightened 'flush' to the surface of the material.
POZIDRIV HEAD: Used with special screw drivers which will not slip when pressure is applied. This is ideal when using screws in corners or confined spaces.
ROUND HEAD SCREW: These are used for fixing pieces of material together where countersunk holes are not being used. Round head screws can look quite decorative especially if they are made of brass.
Joining materials with screws
1. Mark the position of the screw on the top of the wood.
2. Drill a clearance hole through the top piece - the same diameter as the shank.
3. Countersink the hole if using a countersunk screw.
4. Put the two pieces of material together and drill a smaller diameter hole in the second piece of material
5. Place the screw in the top first hole and tighten carefully with a screwdriver.
Bolts
Bolts are generally made from mild steel. High tensile strength steels are used for higher strength applications. The thread of a bolt does not go all the way to the head. Set screws have threads that go all the way along its length. They are used where an accurate assembly of parts is required.
Knock-down fittings
Knock down fittings are used widely ion the construction of flat pack furniture. Knock-down fittings can be used for school based products, especially when manufactured boards are being used. The fittings allow furniture to be taken apart and re-assembled. The main types of knock-down furniture are scan fittings, block-joint fittings, modesty blocks and disk and pegs.