Sheet metal projects often require edge treatment:
Common edge treatments used in basic sheet metalwork are:
These edge treatments are shown in the pictorial sketches and pattern developments on the right. The illustrations show necessary allowances. The allowance (A) for a folded edge is the width of the edge. The allowance for a wired edge using 2mm (14g) wire is 5mm.
Depending on the basic design, sheet metal projects usually require some form of overlap to hold joins together. The most common seams used in basic sheet metalwork are:
These seams are illustrated in the pictorial sketches and simple developments below. Folded seams will hold themselves together but lap seams require fixing with rivets, solder or adhesive.
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The grooved seam is similar to the folded seam in that the development requires a single allowance and a double allowance. The allowances are folded over, hooked together and the seam is formed using a special tool called a groover, as shown in the illustration on the right. |
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When a sheet metal project has seams and edges the development must allow for both to fit neatly together without overlapping. The illustrations which follow show typical notching (the part of the seam cut away to allow for the edge.)
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ Which of these is not a reason for edging a sheet metal project ][ * For decorative purposes ][ * To make edges stronger ][ * To make edges safe to handle ][ To make it easier to paint ][ Painting a project doesn't depend on how it is edged. ]]
[[ mc /f ][ Which of these is not a method of fixing a lap seam? ][ * Riveting ][ * Soldering ][ * Gluing ][ Grooving ][ A groover is used on a folded seam, not a lap seam. ]]
[[ sh ][ JOINING WITH RIVETS AND SCREWS ]]The three types of rivets commonly used to join sheet metals and light metal sections in basic metalwork projects are the tinman’s rivet, the solid rivet and the blind rivet which is commonly called a pop rivet.
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The tinman’s rivet is a short galvanised rivet with a flat head used mainly for joining thin sheet metals such as tinplate and galvabond. The hole for the rivet may be punched with a solid punch into the end grain of a block of wood or punched by the rivet itself by drawing the rivet through the metal with a rivet set. Waste material punched out of the metal is forced into the hole in the face of the rivet set and is eventually ejected from the hole in the side of the rivet set. The shallow hole in the face of the rivet set is used for doming the tail of the tinman’s rivet as shown in the illustration on the right. |
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Solid rivets are available with a variety of shaped heads. The tails of these rivets would be domed in a similar fashion to the tinman’s rivet. Most metal projects undertaken in the school workshop require flush rivets where this method is used to join light metal sections. The illustrations on the right show the four stages of the solid rivetting process using round steel rivet material.
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Blind rivets or pop rivets are used when:
The pop rivet is placed in a drilled hole and the wire mandrel through the centre of the rivet is gripped by special pliers and pulled through. The head of the wire mandrel spreads the tail of the rivet clamping the materials together firmly. The mandrel snaps when sufficient pressure is applied leaving the parts rivetted together. The illustrations on the right show how the tail of the blind rivet spreads when pressure is applied. |
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A basic blind rivet plier or pop riveter is illustrated below. Interchangeable tips allow the plier to be used for a range of rivet sizes.
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Self tappers are hardened screws often used in the assembly of sheet metal components. These screws cut their own thread in a hole drilled to the core size of the screw. Their main advantage over rivets as a fixing system is that self tapping screws allow the parts to be separated when necessary. A typical pan head self tapping screw is shown on the right with the main slot types that are available. |
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Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ How do you form the rounded head on a tinman's rivet? ][ * With a hammer ][ With a rivet set ][ * By heating it ][ * With a block of wood ][ The rivet set tool has an inverted dome underneath it. ]]
[[ mc /f ][ Why are blind rivets called "pop" rivets? ][ Because they go "pop" when the wire mandrel snaps ][ * It's a brand name ][ * Because the rivets aren't actually blind ][ * They're not very strong ][ They make a characteristic "pop" sound when the mandrel snaps. ]]
[[ mc /f ][ Why are self-tapping screws so called? ][ They cut their own thread into the sheet ][ * They can be fixed with a screwdriver ][ * They can be screwed down with different drivers ][ * They don't need to be drilled first ][ Self-tapping screws cut their own thread following a pre-drilled hole. ]]
[[ sh ][ SOFT SOLDERING ]]Soft solder is usually an alloy of the metals tin and lead but varies in composition depending on its use. It is made in stick form and is usually measured by weight.
For fine work solder is also made in wire form. Sometimes this wire has a core of flux (usually non-corrosive resin) and is most suited to electrical work.
The most common types of soft solder are listed in the table on the right.
Unless metal is perfectly clean the solder will not adhere to it. All metals will oxidise when in contact with the air and these oxides are the main problem when soldering.
A flux is used to:
Common fluxes and some of the metals that can be soldered with them are listed in the table below:
| Flux | Metals |
| Zinc Chloride (Killed Spirits) | Tin, tinplate, mild steel, copper, brass |
| Hydrochloric Acid (Spirits Of Salts) | Zinc, galvanised iron, wire |
| Commercial Soldering Fluids | Most metals listed above |
The soldering bit (commonly called soldering ‘iron’) has a copper head, steel shaft and a wooden handle.
Copper is used in the head because of its good thermal conductivity. It will heat quickly and then distribute the heat quickly and evenly.
Before a soldering bit can be used the point must be ‘tinned’. This means the point has to be coated with a thin layer of solder which enables the bit to pick up and then distribute the solder to the job.
Procedure for tinning a soldering bit is as follows:
The key factors in successful soft soldering are:
The bit and the metal to be soldered must be perfectly cleaned by using dip solution (dilute flux) to clean the bit and flux to clean the metal. The heat which is applied by the soldering bit is the harder factor to control. The area of metal where the solder is required must be heated to a temperature in excess of the melting point of the solder so that the solder can flow.
The soldering bit should be moved slowly along the seam allowing the metal to heat sufficiently. All flux must be washed from the job to prevent corrosive action from chemicals in the flux solution.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mr /r /f ][ What metals are usually used to make solder? ][ Tin ][ Lead ][ * Zinc ][ * Copper ][ Most solders are made of Tin and Lead. ]]
[[ mc /f ][ Which of these is not one of the purposes of flux? ][ Improves the look of the finished joint ][ * Improves solder adhesion ][ * Removes oxides ][ * Helps the solder flow ][ Good-looking solder joints are the result of skill and practice. ]]
[[ so /f ][ Put these tinning steps in correct order ][ Heat the iron ][ File to shape required ][Reheat the bit ][ Clean by dipping in dilute flux ][ Roll in solder (with flux) ][ Heat the iron => File to shape required => Reheat the bit => Clean by dipping in dilute flux => Roll in solder (with flux) ]]
[[ mc /f ][ Why do you need to clean the flux off finished solder joints? ][ Flux is corrosive ][ * Flux smells terrible ][ * Flux doesn't conduct electricity ][ * Flux hardens when it cools ][ Flux will corrode metal if left on the surface of the joint. ]]
[[ sh ][ ART METALWORK ]]Art metalwork can take many forms. Some of the common ones practised in school workshops are briefly described below.
Copper tooling is the process of forming raised patterns and pictures in sheets of thin copper foil. The copper is usually annealed to a very soft state so that it can be easily worked.
A design is traced or drawn on the copper foil (usually with a ballpoint pen) and then shaped out with tooling sticks. These simple tools can be made from wooden dowel or similar material.
The thin copper foil can be easily stretched and formed to provide the three dimensional effect required. The hollows on the back of the finished work are usually filled with plaster of Paris or plastic putty.
Chasing is the process of decorating the surface of art metal by forming patterns and designs with special shaped punches and chisels. The chisels are used to chase around outlines, corners, etc. The punches can provide a variety of textured surfaces to decorate the work.
Matting is similar to chasing but is done with special tools with patterned faces to provide the texture required.
Planishing can produce quite smooth surfaces using a special planishing hammer and a specially shaped stake. Planishing stakes can vary in shape and size and planishing hammers are usually slightly round faced and very smooth. Planishing will work harden the metal which may be necessary to stiffen the job in the last shaping operation.
Beating the surface of art metal with a ball pein hammer can produce a very attractive finish but care must be taken to avoid thinning the metal too much. Beating will also work harden the metal quickly.
Dishing or hollowing involves the formation of bowl shapes in soft art metals such as brass, aluminium and copper.
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A dishing block and bossing mallet are used as shown in the illustration on the right. The best technique is to work gradually around the circumference of the piece of art metal using overlapping strokes of the bossing mallet. Uneven areas should be smoothed out by working lightly and the surface could be finished by beating or planishing. The metal will work harden very quickly and may have to be annealed several times before the job is complete. |
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Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ How do you protect the delicate patterns that are pressed into copper sheet? ][ * Spray paint the surface ][ Fill the back with plastic putty ][ * Glue timber into the back of the sheet ][ * Colour the surface with felt-tip pen ][ Plastic putty pressed into the back preserves the delicate patterns. ]]
[[ mc /f ][ What is meant by "work-hardening"? ][ Making thin metal stiffer by hammering it ][ * Bending metal until it snaps ][ * Heating and cooling the metal ][ * Soldering the face ][ Work-hardening is stiffening of metal due to work like hammering. ]]
[[ sh ][ DECORATIVE SURFACE FINISHING ]]Metal surfaces sometimes require further work after procedures such as cutting, grinding or filing, particularly where a decorative finish is required on the project.
Sheet abrasives are generally used to further smooth the metal surface after fine filing. Sheet abrasives consist of an abrasive grit bonded to a cloth or paper backing.
Emery cloth can be wrapped around a file to enable even pressure to be applied when rubbing the metal surface. A light lubricating oil used on the surface can often improve the finish.
The most common grits used in the manufacture of sheet abrasive materials are silicon carbide and alminium oxide.
Silicon carbide is manufactured from coke and sand fused together in a furnace and then ground into small particles. It is a shiny black synthetic abrasive which is brittle and fractures into sharp wedge-shaped slivers. It is very hard and is generally used on waterproof (wet-and-dry) abrasive papers.
Aluminium oxide is manufactured from bauxite, iron filings and coke fused together in a furnace and then ground. It is usually a grey-brown colour, extremely tough and resistant to normal wear.
After crushing and grinding, abrasive materials are sieved through accurately woven silk screens. The size of the mesh is determined by the number of openings per given area and this in turn determines the grade of the abrasive. The more openings per given area the finer the grit and the larger the number used to describe the grade.
When metal such as copper has been exposed to the elements or heated, a film of oxide forms on the surface. This film can be removed by placing the metal into a bath of some chemical agent that will react with and remove the oxide, exposing the true natural colour of the metal.
Dilute nitric or sulphuric acids are used in industry but these are quite dangerous to handle. A safe and convenient pickling solution for copper can be made by dissolving salt in vinegar until the vinegar is saturated, which means it cannot absorb any more salt.
After pickling, filing or rubbing with fine abrasives, metal surfaces can generally be highly polished using polishes such as ‘Brasso’, ‘Silvo’ or other liquid metal polishes that are available. Fine abrasive powder polishes are also available. Care must be taken to clean all traces of polish from the surface before coating with lacquer or paint.
Lacquer is usually sprayed on art metalwork projects after polishing to prevent tarnishing and to maintain the natural colour of the metal and polished finish. A coat of lacquer seals the metal surface off from the air thus preventing the surface oxidising.
Electroplating is the process of coating the surface of one metal with another metal. This process is described in the sections of this text which relate to galvanising and to refining of copper.
Anodising is the process of thickening the oxide layer on the surface of aluminium using an electric current which is passed through a chemical solution. The thickened oxide layer provides a barrier to further corrosion.
This protective layer is often coloured with special dyes to provide an attractive finish such as on aluminium saucepan or canister lids.
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A metal surface can be decorated with an attractive pattern which resembles scales on a fish. The pattern is produced on the surface of the metal by using a short length of wooden dowel in the chuck of the drill press. An abrasive powder is sprinkled on the metal surface. When the drill is switched on and the dowel is brought into contact with the metal surface the end of the dowel picks up abrasive particles. The circular motion of the dowel causes the abrasive grit to scratch a pattern on the metal surface. Overlapping of the circular pattern can produce a fish scale effect. |
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Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ What could you use to safely pickle copper sheet? ][ Salt dissolved in vinegar ][ * Dilute hydrochloric acid ][ * Dilute sulphuric acid ][ * Dilute nitric acid ][ Salt and vinegar are safe, everyday substances. ]]
[[ mm /f ][ Match the term with its description: ][ Pickling ~ bathing in a chemical ][ Lacquering ~ applying a clear coat ][ Electroplating ~ coating with another metal ][ Anodising ~ thickening the oxide layer ][ Pickling - bathing in a chemical; Lacquering - applying a clear coat; Electroplating - coating with another metal; Anodising - thickening the oxide layer. ]]
[[ mc /f ][ Which of these statements is not true of the fish-scaling process? ][ Oil is essential for a good result ][ * Abrasive particles produce the pattern ][ * Overlapping the circular patterns looks like fish scales ][ * A dowel is fitted in the drill press ][ Oil might help, but it isn't essential. ]]