These sheets will be more useful to you in future construction projects rather than theory notes, as they cover methods of joining that are unlikely to be used in Household and Domestic Products.

Temporary Fixings

Screws

These include methods of joining two or more pieces such that they may be disassembled without causing lasting damage. They include Screws and Bolts.

Screws exist in a massive variety of shapes and forms and whilst they are used mainly in woodwork construction, they can be used in both metal work and plastics. (Your radios were originally held together with self tapping screws).

The defining characteristic of a screw is that it passes cleanly through the first piece of material and cuts it's own thread into the second piece of material. Therefore two different size holes need to be drilled, a clearance hole and a smaller pilot hole.
The diameters of these holes obviously varies according to the diameter of the screw and here matters are complicated by the coding system of wood screws which is a mixture of archaic inches and gauges or numbers. So a screw will be referred to as a 1.5 inch number 8. The 1.5 inches refers to the length of the screw and the number 8, it's gauge or diameter.
Fill in the chart below to indicate the clearance and pilot hole diameters in mm.

Countersink screw require the use of a third drill bit known as a countersink bit to create the tapered recess for the head of the screw.

Nuts and Bolts

The thread on bolts tends to be much finer than that on screws. Generally the bolt passes cleanly through both pieces to be joined and is secured by a nut on the other side. In more substantial sections, however, it is possible to cut a matching thread into the material, doing away with the need for a nut.

Fortunately the system for nuts and bolts has become metricated so a bolt described as a 25mm M6 is 25 mm long and is 6 mm in diameter. Simple. Like screw, however, there are a wide variety of bolt head shapes such as hex (hexagonal), cheese (cylindrical), countersunk etc. It is also possible to get different nuts such as wing-nuts and nyloc nuts (which have a nylon insert to prevent them shaking loose).

To cut your own thread on a piece of material you will need either a tap (for an internal thread) or a die (for an external thread). You must bear in mind that if you cutting an internal thread, say an M6 thread, if you drill a hole in preparation of 6mm, there will be no material left to cut the thread into. Therefore the preparatioon holes, when tapping, need to be smaller than the diameter of the external thread.

This category of fixings includes those which can be removed but not without a certain measure of force or the possibility of damaging the two materials. The two principle methods are nails and rivets.

Nails are usually made of steel and are used to fix timber products together quickly. Whilst they can provide sufficient strength on their own it is more common to use them in conjunction with wood glue. It is not necessary to drill a preparatory hole.

Rivets are fixtures which are passed through two layers of materials and then deformed, thus sandwiching them.

In industrial applications such as the construction industry and ship-building, the rivets are heated to soften the free end. In the workshop, however, you are more likely to encounter pop-rivets which are made from Aluminium and thus deform easily at room temperature

Permanent Joints

Permanent joints are not designed to be taken apart. That is not to say that they cannot be taken apart but in doing so, damage is likely to occur. They include the use of glues or adhesives which are used to permanently join wood, plastics and metals in addition to soldering and brazing, used to join metals, and welding which can be used to join both metals and plastics.

Adhesives

These have been used in various forms since the earliest civilisations. Animal and vegetable glues were used in ancient Egypt and even Copydex is an animal based adhesive (which is why it smells so vile).
Most modern adhesives are based upon either thermoplastics or thermosetting plastics.
Epoxy based adhesives such as Araldite consist of two chemicals (usually contained in two tubes) which react and set when mixed. These are thermosetting adhesives and can join virtually all materials.
PVA (wood glue) is a thermoplastic glue used to bond porous materials (such as wood, oddly enough).
Hot melt glue guns comprise thermoplastic sticks which melt and then solidify.
Solvent based adhesives such as Tensol are used to glue plastics together and work by chemically melting the two plastics together.

Soldering and Brazing

Each of these techniques relies upon heat. With soldering and brazing the two metals are joined by melting a second metal between them. With welding the two metals are melted and fused together.

Soldering occurs at a much lower temperature than brazing and lead/tin alloys are used as the filler metal. The work can be heated by a propane blow torch which gives the advantage of portability and minimal heat distortion. The joint is not particularly strong, however.

In brazing, brass is used as the filler metal. A brazing hearth or oxy-acetylene equipment is required to heat the metal up to the required higher temperatures and then the filler metal added.

In both soldering and brazing it is important that the metal is cleaned thoroughly and flux (a cleaning compound) applied.

Welding

There are numerous forms of welding each one using a different method to heat the metals up to the required temperatures.

Oxy-acetylene welding relies upon the mixture of acetylene and oxygen to create a flame hot enough to melt the metal whilst arc welding relies upon an electric arc to generate the temperatures.

Other techniques include spot welding in which two electrodes squeeze layers of metal together. A current is passed through the metal causing it to heat up, melt and fuse. This technique is very common in the motor industry where pressed steel panels are assembled in this way to form vehicle bodies.

In ultrasonic welding, the two materials are pressed together and then one of them vibrated at high frequency. The ensuing friction causes the materials to melt. This method can be used to weld thermoplastics together.

Conclusion

The selection of the best joining technique is dependent upon a number of factors.

the degree of permanency required,
the necessary strength of the joint,
the stiffness needed,
the type of loading ~ static or fatigue,
the effect of heat on the materials involved,
the appearance of the joint.

Stiffness is more important than strength in certain joints such as those used in chairs. This is because the loads are unlikely to be that high but a certain amount of give is required. Some joints are subject to continuous or static loads whereas others involve changing loads which can cause fatigue. Adhesive joints and welds are better equipped to cope with the latter whilst mechanical fixings are poor in this respect. Adhesives, however, are less able to cope with extremes of heat.


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