81894
Descripción
Fichas por Alec Stevens, actualizado hace más de 1 año
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Creado por Alec Stevens
hace más de 11 años
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| Pregunta | Respuesta |
| Centre-Drilling and Boring | Where a lathe is used to drill a hole in the end of a bar or rod along it's axis. Work rotates and drill is held stationary in the tailstock. Smaller hole must first be drilled with a centre drill. Then a twist drill is used to finish hole. If a larger hole is needed, a boring tool may be used - similar to a cutting tool but used internally to enlarge holes. Careful of damaging tip of drill bit through the huge amounts of heat that build up from friction. Soluble oil applied as a coolant constantly - in industry this is pumped on but applied with brush in school workshop. Drill regularly removed from hole being drilled to keep tip free from swarf. |
| Sand Casting | Molten metal is poured into a sand mould with a cavity of the desired shape of a component. Inexpensive and allows large or complex components to be produced. Can only be used once and very labour intensive/slow production rate with not necessarily a good surface finish. Once the metal has solidified, the sand is broken open and the component removed. The mould has to be made using a 'pattern'. This pattern has to be identical to the finished product apart from being slightly oversized to allow for the contraction of the metal when it cools. The majority of patterns are made in two parts, cut down the middle and held together using dowels. The pattern must be designed with sloping sides to allow it to be easily removed from the mould without damaging the walls of sand. No undercuts or sharp edges - all corners must be finished of with 'fillet' radii. Often finished in gloss paint. The 'drag' is the bottom half of the sand mould, the 'cope' the top. Parting powder is sprinkled over so that the pattern doesn't stick. The sand can't be wet when the molten metal is poured in! Sieved facing sand is placed around the pattern, then the rest of the mould is filled. Sprue pins make the cavity in the sand where the metal is poured in. Gates or channels are cut between the pattern and the holes created by the sprue pins before the pattern is removed. The hole where the metal is poured in is referred to as the 'runner' and the hole where the air is released is known as the 'riser'. Once done, the component will need to be 'fettled' - where runners, risers, flashings or other waste is removed using a hacksaw. |
| Die Casting | Mostly used for non-ferrous metals. Molten metal is forced under pressure into a die (pressure die casting) or allowed to flow into the mould by gravity (gravity die casting). Die are reusable moulds made from steel. A high volume, mass production process. The mould is sprayed with lubricant. Pressure is maintained till metal cools. Very high quality castings. Ejector pins push out cast component. It can produce a number of castings in one cycle, unlike sand casting. Things like screw threads can be included in the casting. High set-up costs, limited sizes, not suitable for all alloys. Need large volume production to make it economical. |
| Milling and Routing | Milling is where metal is cut away by feeding a piece of work past a rotating cutter. Routing is similar but associated with wood, plastics and composites. Often used to cut slots into material. Two types - vertical machines and horizontal machines. Speed of cutter and speed work is moved past cutter are variables. Fence used on handheld routers for wood. Material must be clamped very firmly and moved in opposite direction to cutter movement when 'upcut' milling on a horizontal table. |
| Drilling | Most common type the twist drill, manufactured from high speed steel and can be used to produce holes in most materials. Parallel shank twists are held in the machine by a chuck, whilst taper shanks are held by friction. The fluting/grooves on the drill bits allow swarf to be carried away. When drilling metals, lubricants should be used to keep the tip of the drill cool. Flat bit drills are used to make deep holes in wood. Forstner bits are used to drill flat-bottomed holes in wood. Auger bits are used to drill deep holes using a carpenters brace. Countersink bits allow countersink head screws to sit flush with surface. Hole saws are circular shaped cutting rings that can cut large holes. Tank cutters are circular cutters which are adjustable and can cut holes in sheet material. Centre-punch metal before drilling. Don't burn wood - sharp bit with correct speed. |
| Turning | Metalworking or woodturning lathes. Facing off (smoothing the end of a piece of metal), turning down (reducing diameter of a piece of material). On a metal lathe, the tools are fixed to the lathe on a tool post, movement is controlled by wheels and work is held in a chuck. On a wood lathe the tools are hand-held and rested on a toolrest and the work is secured to a faceplate or turned between centres. Make sure wood is clear from knots before lathing. Wood lathes: Gouges, chisels, scrapers. Plane now corners/edges before starting. Metal lathe: Knurling, parting off, facing off, making a screw thread, turning down, centre drilling. |
| Blow Moulding | Used to produce hollow products such as bottles from thermoplastics. Hot floppy plastic tube (parison) created by extruding heated plastic. Tube clamped in a metal mould and then air pumped in. Parison inflates to shape of the mould and the product is ejected when the plastic has cooled and the mould has opened. Intricate shapes can be formed, produces hollow shapes with thin walls, ideal for mass production with low unit cost and fast cycle time. High set up costs. Used for plastic bottles/containers of all shapes and sizes. |
| Injection Moulding | Thermoplastics, thermosets and even carbon fibre now. Molten plastic injected into mould under pressure from archemidian screw or hydraulics. Two part mould made from steel using spark erosion. Component rapidly cooled (usually with water) and ejected. Ideal for mass production, low unit cost for each moulding and precision finish with texture added to mould if necessary. High set up costs as mould expensive to produce and test. Most commonly used: ABS, HDPE, Nylon, PP and PS. Mould must be specially controlled in terms of temperature to injection mould thermosets. |
| Vacuum Forming | Thermoplastic sheets are heated till they are soft a pliable. A vacuum is then produced by a pump and the plastic is pulled to the shape of the mould. Can do everything from packaging right up to baths. Ideal for batch production. Inexpensive but mould must be accurate. A large amount of waste material is produced. Mould must have a 15 degree taper, no sharp edges or undercuts, fillet radii on corners, smooth finish - must be exact shape and finish of final product. Mould cheaper to produce than moulds for injection moulding or other, and thus more cost efficient on shorter runs. |
| Extrusion | Creates long (possibly indefinitely long) products with a certain cross section. Similar to injection moulded, heated up and forced through a die of desired cross section. Hollow sections produced using a 'floating pin'. Outside called 'bushing', pin inside called 'mandrel'. Can have centre or side feed. Converging or diverging end of die. Continuous or semi-continuous. E.g. guttering, uPVC window frames. |
| Rotational Moulding | Produces hollow one-piece components. Exact amount of polymer loaded into mould so no waste. Mould has polymer put in and is closed. Then heat applied to mould and mould rotated in tumbling action until all polymer melted and stuck to mould wall. Mould water or air cooled till plastic component is fully formed. Then mould opened and component removed. Can make extremely large products with NO SEAMS. Corners are stress free and walls uniform thickness. Metal inserts can be included in moulding and the colour is included. Needs very little surface finishing. However, lower volume production, labour intensive compared to injection moulding and long cycle times limit batches to between 500 and 10000. |
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