Metal processes

Question 1

What is (MIG) welding?

Answer 1

An arc welding process in which a continuous solid wire electrode is fed through a welding gun. It is a fabrication process used to weld thin gauge metal, particularly tube and box section.

Question 2

What is (TIG) welding

Answer 2

A precise and high quality form of electric arc welding that uses a non-consumable tungsten electrode. It is an electric arc welding process and is different to (MIG) welding except that the electrode made from tungsten does not melt in the process.

Question 3

Spot Welding

Answer 3

This welding process is used primarily for welding two or more metal sheets together by applying pressure and heat.

Question 4

What is oxy-acetylene welding

Answer 4

Oxy-actelyne welding is used to weld low carbon steel sheet, tube or plate where arc welding process are not available. It uses a mixture of two high pressure gases - oxygen and actelyne.

Question 5

What is Soldering (soft and hard)

Answer 5

Soldering is a joining process used to join different types of metals together by melting solder. It is a similar process to brazing, but it is only used for lightweight applications and thin gauge materials.

Question 6

What is brazing?

Answer 6

A way of forming permanent joints in metal by melting a brazing rod at 880°C between two parts. The process uses a lower temperature than welding, so it is suitable for jointing thinner gauge low carbon steel tube or bar.

Question 7

What is riveting?

Answer 7

A rivet is a mechanical fastener composed of a head on one end a cylindrical stem on another (called the tail). In traditional, cold riveting, the two pieces to be joined are over slapped and drilled.

Question 8

Self tapping screws

Answer 8

These are used for joining thin sheets of metal. A pivot hole is drilled through the metal parts to be joined.

Question 9

Machine screws

Answer 9

They area type of bolt that is used to join thicker pieces of metal together - typically machine parts to be joined.

Question 10

Nut and bolt

Answer 10

Bolts are similar to machine screws, but instead into a threaded hole, they are put all the way through both pieces of metal, and then a hut is tightened on to the end of the bolt to keep the pieces together.

Question 11

Press forming process

Answer 11

1 Sheet metal is clamped over a die of the product that will determine the final shape of the pressing.

2 A hydraulic press pushes the die into the sheet metal. Cutting blades may be included to punch holes into the sheet and trim the excess from the edges.

3 The hydraulic die is lowered and the pressed sheet component is removed.

4 The sheet may be placed into further press forming machines for additional pressing, where the shape is complex.

Question 12

Spinning process

Answer 12

1 A former called a ‘mandrel’ is put into the chuck. The sheet metal blank is held in place between the mandrel and the tail stock.

2 The roller tool is moved into the blank and is rotated with the mandrel. This starts to stretch the metal over the mandrel.

3 The roller tool is moved along the mandrel as pressure is maintained against the rotating blank.

4 The roller tool is moved to the end of the mandrel, while still maintaining contact with the blank. This finishes the shape of the product.

5 The finished product is removed from the mandrel.

6 Excess material is trimmed off following the spirring process.

Question 13

Cupping and deep drawing process

Answer 13

1 The pressing blank is clamped over a deep drawing die using a pressure pad or clamping ring known as a retainer.

2 A hydraulic press moves the deep drawing punch to be in contact with the blank. It then pushes the blank into the die cavity to make a cup shape.

3 The ‘cup’ is then pressed further down through the deep drawing die to make the desired tube shape.

Question 14

Drop forging process

Answer 14

1 A die is made from cast tool steel (which resembles a mould) and this is secured to the top of an anvil.

2 A ram is also equipped with a die that resembles a mould.

3 The metal ‘billet’ to be forged is heated to above its recrystallisation temperature (the temperature below the melting point shape of the metal at which point it is possible to change the size and shape of the grains that make up the metal). This stops the product from work hardening as it cools, which would make it brittle.

4 Using tongs, the heated billet is placed by an operator into the anvil die, and the hydraulic ram is brought down with force. This makes the hot billet spread around the shape of the die.

5 The ram is lifted and the completed product removed for cooling and finishing

Question 15

Wrought iron forging process

Answer 15

Typically, the wrought iron is heated in a gas or coke-fired forge. It is then shaped by holding it with tongs, hammering it over an anvil or using other tools such as scroll formers or twisting bars.

Wrought iron forging is suitable for one-off or limited batch production because there is no requirement to make formers or dies to shape the product. The process in its simplest terms uses heat, an anvil and a hammer.

Question 16

Bending process

Answer 16

Bending sheet or plate metal in industry is done using a machine called a press brake. The desired bends are achieved by clamping the stock metal between a matching punch and die. A hydraulic, pneumatic or mechanical brake holds the metal sheet or plate, and lowers the punch to bend the material to shape.

Question 17

Rolling process

Answer 17

Rolling is a metal forming process in which the stock metal material is passed through sets of rollers to reduce the thickness of the material. This process is usually carried out with hot metal that has been heated to above its recrystallisation temperature (known as hot rolling), but can be rolled below its recrystallisation temperature (cold rolling).

Question 18

Sand casting process

Answer 18

1 A ‘pattern’ is made, usually from wood. This is a replica (sometimes divided into two halves) of the item that will be cast, and it is placed in the bottom of a steel box called a ‘drag’. The drag is then filled with sand which is packed or rammed in tight around the pattern and levelled.

2 The drag is turned over and a second box called the ‘cope’ is clamped into position over the top of the drag. The top half of the pattern is placed into this to mate with the bottom half of the pattern. Wooden stakes are positioned in the cope. These will form the sprue or runner, and riser later in the process.

3 Sand is packed into the cope around the runner, riser and pattern. A small depression is made on the surface around the sprue to make a pouring basin.

4 The cope and drag are separated, and the stakes and patterns are carefully removed. Connecting channels are cut to join the sprue to the pattern cavity, and from this to the riser. The cope and drag are then re-assembled and the mould is ready for pouring to begin.

5 Small metal spikes may be inserted and removed to make vent holes. These will allow gases from the casting process to escape.

6 The molten metal is poured into the pouring basin. It flows down the runner, into the cavity. When the cavity is full, the molten metal flows up the riser, indicating to the worker that the cavity is full. Once cool, the sand is removed to reveal the casting. The runner, channels and riser are cut off with a hack saw and the casting is ready for machining.

Question 19

Gravity die casting process

Answer 19

Gravity die casting is the simplest form of die casting because it involves melting the metal and then pouring it into the mould. This process relies on gravity to help the metal to flow through the mould. As in sand casting, there is a runner and riser. The runner is used to pour the molten metal into the mould, while the riser will indicate when the mould is full. Once the metal is cool, the mould is opened and the casting can be removed.

Question 20

Hot chamber pressure die casting process

Answer 20

With hot chamber pressure die casting, the molten metal is stored in a chamber which is part of the high pressure die casting machine. A pneumatic or hydraulic plunger forces a ‘shot’ of molten metal through the ‘goose neck’ into the die.

Question 21

Cold chamber pressure die casting process

Answer 21

With cold chamber high pressure die casting, the molten metal is kept separately in a melting crucible. The molten metal is then ladled into the shot chamber, and a hydraulic ram forces the molten metal into the mould cavity. When the metal has hardened, the mould opens and ejector pins push the finished casting out.

Question 22

Investment casting process

Answer 22

1 An exact replica or pattern of the product to be cast is made using wax. (This might be made using a master mould machined in steel or aluminium if the product is to be batch produced.) Where several items are to be cast, further wax patterns might be joined together in a ‘tree’, including a replica of the runner that will be used to pour the molten metal in.

2 The wax pattern is dip coated with a refractory clay. It is then fired in a kiln to bake the clay hard. The wax is burned away, leaving a hollow clay mould.

3 Molten metal is poured into the clay mould.

4 Once the metal has filled the mould, it is allowed to cool.

5 The clay mould is then broken away, revealing the casting.

6 The runner and any connecting channels are machined off.

Question 23

Low temperature pewter casting process

Answer 23

1 A mould is made from MDF, plywood or high density modelling foam. If made from MDF or plywood, the mould might be laser cut or cut with a fret saw. The mould will include a sprue or runner which will be used to pour the pewter into.

2 The mould is sandwiched between two pieces of MDF and clamped together. The top of the mould will be level with the top of the side pieces.

3 The pewter is melted in a ladle and then ladled into the sprue.

4 Once the casting is cooled, it is removed from the mould.

5 The sprue or riser is removed with a junior hacksaw.

6 The casting is then filed, and cleaned up using abrasive wet and dry paper.

7 The casting would then be polished.

Question 24

Milling process

Answer 24

Milling uses a machine that looks similar to a drill. The work to be machined is clamped onto the table of the machine. The table can run in the x direction (left and right horizontally), y direction (forwards and backwards horizontally) and z direction (up and down vertically); therefore the workpiece can be machined in a range of directions.

Question 25

Turning process

Answer 25

Turning is a process carried out on a centre lathe. Work such as a bar can be held in a rotating chuck and machined to reduce the diameter, and to square or ‘face off’ the end, thread and drill. Where the piece being machined is long, it can be supported in the centre by a ‘tail stock’ on the lathe. A variety of shaped bars can be turned including round, hexagonal and square bars. Tubes can also be machined on lathes.

Question 26

Flame cutting process

Answer 26

Flame cutting is a wasting process that uses oxy-acetylene gas and a special flame-cutting torch to deliver a very intense and focused flame above 3,500 °C. It is used to cut low carbon and alloy steel plate.

The flame cutting process resembles welding – the metal is heated and a melt pool begins to form. At this point, an additional jet of oxygen is introduced. This intensifies the flame and pierces the metal, forcing a jet of melted metal and carbon (known as slag) with it. The flame is then moved along the cutting path to continue the cut.

Question 27

Plasma cutting process

Answer 27

lasma is a super-heated ionised gas that is electrically conductive. A plasma cutter will use this conductive gas to transfer the energy from the power supply to a conductive material such as steel plate. The resulting cut is faster and cleaner than using oxy-acetylene.

The plasma arc is directed out of a torch where a gas such as oxygen, nitrogen, argon or compressed air is forced through a tiny nozzle. An electric arc is generated from a transformer and, combined with the gas, forms a jet of plasma. The heat generated by the plasma can be as high as 28,000 °C, which quickly burns through the material and blows it way.

Question 28

Laser cutting process

Answer 28

Laser cutting uses the power from a high-powered laser that is directed through optics. The laser melts the material and a high-pressure gas or compressed air blows the melted material through the sheet.

The laser beam is emitted from a ‘laser tube’ where it is reflected through a series of mirrors in a similar way to a periscope into a ‘laser head’. The head contains a lens which focuses the laser into a fine beam for cutting and engraving. Laser beams usually have a very fine tolerance, and the amount of material removed in the cutting process can be less than 1 mm.

Question 29

Punching/stamping process

Answer 29

Punching is a wastage process that uses computer controlled machines which stamp out sections of sheet material using hardened punches following a CNC program. The program moves the table of the machine in the x and y direction (x moves left and right, y moves forwards and backwards) under the punch.