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This page primarily discusses metals and plastics and the joining of materials using adhesives. It does not discuss wood joints. To learn specifically about wood joints which are often decorative in nature, please visit the 'woods' area of the website.
Any kind of product or structure, whether that is as large as a building, as small as a sandwich box, fast as a rocket, or as slow as a canal boat requires assembling. That is that it requires all the separate parts that make it up to be joined or 'jointed' together. The choice of method of joining it together will be determined by the following factors:
What is/are the materials being joined? Are they the same or are they different? Will they react together?
How long do I want the joint to last for?
Is it a permanent or a temporary joint? Do I want to get it off again?
How strong does it need to be? Will it be used a lot? Will there be any vibration? Is it a safety critical joint?
What environment does it need to be used in? Is that environment hazardous to the object?
How much money have I got to spend on it?
Temporary fasteners are any fastener that is considered to create a temporary joint. The most common example is a hex cap screw and nut. Bolts and nuts are combined to create a temporary joint between two materials. Most machine screws, nuts, bolts, are considered temporary fasteners.
Nuts and Bolts
Welded or drop-in studs
Captive Nuts
Screws
Each threaded fastener must be driven using a specific type of screw attachment according to the drive type of their 'head'.
Here is a comprehensive illustration of screw drive types by pihixxx.com.
Non-threaded fasteners are used as simple, low-strength fixings in many applications. They are often quick to put together and easy to remove. They can also often be found used in combination with threaded fasteners. For example a split pin is often used to help prevent vibration causing a threaded nut from un-tightening by itself (by preventing rotation of the nut).
Hairpin Cotter (Hitch Pin)
Example of the use of a hitch pin to prevent accidental disconnection of a tow hitch.
Split Pin
Example of a split pin in position (on a castellated nut).
Roll Pin / Spring Pin
A roll pin being inserted.
Circlip
A circlip being inserted
Taper Pin
Linch Pin
Linch pin being used to keep a bike axle from becoming detached due to the nut vibrating off.
When designing and manufacturing plastic components such as cases or enclosures for electronic products, it is not possible to use the fittings above as they would damage the plastic because they are much harder / stronger than the 'parent' object. For this reason a different approach is used. Below are some useful links and resources explaining 'snap' fittings and screw bosses (sometimes called 'risers, or 'posts'). There is also a useful guide to applying good design principles when designing 3D-printed cases.
Plastic_Snap_fit_design.pdfMetal Inserts.pdfAdditional information - Design guide for joining plastics
Plastic Joining Techniques.pdfAs the title implies, in order to bond (join) two surfaces of a metal or plastic together, heat is required. There are three accepted methods of joining metals using heat:
Welding
Brazing
Soldering
Welding is the general term given to the processes where heat is used to fuse two mating surfaces of an assembly together. To do this, the surfaces are heated to melting point allowing them to merge together at the molecular level.
Welding can provide a joint at least as strong as the original substrate (surface material).
It can be made stronger, however it is important to consider how this affects the whole assembly as the mass and mechanical properties of the assembly could change, altering how stresses are experienced across the part. This can mean that even though the joint is strong, stress concentrations are transferred to a weaker location elsewhere in the component, causing it to fail at a different point.
Welding can only be used when joining two surfaces of the same, identical material. If the two surfaces are different materials, a different process must be used, such as brazing.
There are multiple methods of welding, but here are the most common:
MIG (Metal Inert Gas)
TIG (Tungsten Inert Gas)
Electric Arc
Plasma
Brazing can be used when joining two different metallic materials together or two of the same material but when a fused joint is not needed.
Brazing is analogous (similar) to gluing, with the 'glue' in this case being a filler material, usually applied as a consumable 'rod'.
In brazing, the surfaces to be bonded are heated up to a very high temperature (but not melting point). In the case of steel, it is heated until it is 'cherry red'. A brazing 'torch' is used to heat up the mating surfaces. The torch never comes into contact with the brazing rod (as it would melt it, spattering it uncontrollably over the surface). Instead the residual heat of the surfaces to be mated is enough to melt the filler material. When the joint cools, the filler goes hard and provides a permanent, strong joint.
Just to confuse you, brazing is sometimes referred to as a method of soldering (see further down the page).
Copper, brass, bronze.
Copper-phosphorous-silver brazing rods can be used.
Cast iron, galvanised steel, nickel, mild steel, malleable iron.
Bronze-silver brazing rods are used.
Stainless steel, nickel alloys, cast iron, copper alloys, tungsten carbide.
Nickel-silver brazing rods are used.
Brazing torches can come in two types:
Single-gas (e.g. propane or butane). These are the one most commonly used by plumbers and mobile professionals brazing soft copper/brass/bronze joints as it means there is no need to carry big separate gas canisters around. Small canisters found in local trade counters or DIY stores can be used.
Dual-gas (e.g. oxygen-acetylene, or oxyacetylene). This requires a more expensive permanent set-up with a 'hearth' (a dedicated heat-proof work area) and gas storage. These are found in larger, industrial facilities.
Oxygen-boosted torches generate much greater burning temperatures than propane or butane torches and are used for welding higher-temperature alloys.
In the image below, you can see a brazing torch being used to heat up the surfaces of the two components being joined (in this case two sections of tubular steel).
The brazing rod is being pushed into the seam between the two components.
As the material is hot enough to melt the brazing rod, it flows in between the joint acting as a bonding surface.
Flux (a chemical paste) is used to encourage the filler alloy to flow freely, reducing friction and preventing it from sticking in 'blobs'.
GH-Brazing-Guide.pdf
There are three types of soldering:
Hard (filler material melts at temperatures greater than 450°c)
Silver-solder is often used to join jewellery together.
Soft (filler material melts at temperatures lower than 450°c)
Soft-soldering is a specific method of joining metals together using a tin alloy called solder. Solder used to consist of lead and tin, but due to the toxicity of lead it has been replaced by lead-free solder. Lead-free solders can consist of alloys of copper, tin, bismuth, indium, antimony or zinc. They vary in their specific constituents.
Soft-solder is often used with flux to improve the flow of the solder, however they can also be used flux-free.
Soft-solder is most commonly used to join electronic components together in an circuit. This is because solder is a good conductor of electricity and provides a strong, permanent joint.
Because polymers are homogeneous (composed of the same material) and isotropic (have the same mechanical properties in all-directions), they can be welded, just like metals. This can be done in two main ways:
Using chemical solvents.
Using heat.
Obviously because plastics melt at much lower temperatures than metals, we cannot use the same techniques as metals and so different methods need to be used. These are listed below.
Friction welding
Extruded bead welding
High frequency (radio) welding
Hot gas welding
Hot plate welding
Laser welding
Solvent welding PVC
'Floplast' PVC solvent cement
Solvent welding acrylic
'Tensol 12' acrylic solvent cement
Riveting is the process of compressing metal pins with 'heads' on either side to hold two or more plate sections together. Rivets provide very high strength joints which are able to resist high shear forces. There are two main types of rivets:
Blind or 'Pop' rivets
Solid rivets
Blind riveting is the process of using a rivet to join two or more plates of material when only one side of the joint is accessible (hence 'blind').
Blind rivets are not generally as strong as solid rivets but are more commonly used for thinner sheet materials, e.g. aluminium sheet.
A blind rivet being pulled and compressed.
Cross-section showing the process of compressing the rivet and the 'break-stem' mandrel shearing off.
Blind rivet hole size chart
Blind rivet nomenclature (terminology)
Additional supporting blind rivet data
A 'lazy tong' riveter / rivet 'gun'.
A manual blind / pop rivet 'gun'.
How to use a pop rivet gun
Rivet types and how to identify them
POP Rivet Types and Materials _ Fasteners 101.mp4How to remove a blind rivet (if it has failed or is spinning).
This video from Albany County Fasteners show the rivet removal process.
How to Remove POP Rivets Without Surface Damage _ Fasteners 101.mp4This one shows how to remove a spinning blind rivet.
Remove a Spinning or Loose POP Rivet With Duck Tape _ Fasteners 101.mp4
