SEED Guides Selecting Manufacturing Processes < >

This section presents information PRIMAs on the characteristics and capabilities on a number of important manufacturing processes. Each process is divided into seven categories as listed and defined below:

• Process Description: an explanation of the fundamentals of the process together with a diagrammatic representation of its operation.
• Materials: describes the materials currently suitable for the given process.
• Process Variations: a description of any variations of the basic process and any special points related to those variations.
• Economic Considerations: a list of several important points including: production rate, minimum production quantity, tooling costs, labour costs, lead times and any other points which may be of specific relevance to the process.
• Typical Applications: a list of components and parts that have been successfully manufactured using the process.
• Design Aspects: any points, opportunities or limitations that are relevant to the design of the part as well as standard information on minimum section, size range and general configuration.
• Quality Issues: standard information includes a process capability chart, surface roughness and detail, as well as any information on possible faults, etc.

As mentioned previously, a key feature of the PRIMAs is the inclusion of process capability charts for most processes. Tolerances tend to be dependent on the overall dimension of the component characteristic and the relationship is specific and largely non-linear.

The charts have been developed to provide a simple means of understanding the influence of dimension on tolerance capability. The regions of the charts are divided by two contours. The region bounded by these two contours represents a spectrum of tolerance-dimension combinations where C_pk � 1.33 is achievable. Below this region, tolerance-dimension combinations are likely to require special control or secondary processing if C_pk = 1.33 is to be realised. Note, C_pk or process capability index is a measure of process performance. If the process characteristic is a normal distribution, C_pk can be related to a parts-per-million (ppm) defect rate. C_pk = 1.33 equates to a defect rate of 30 ppm at the nearest limit. At C_pk = 1, the defect rate equates to 1300 ppm.

In the preparation of the charts it has been assumed that the geometry is well suited to the process and that all operational requirements are satisfied. Where the material under consideration is not mentioned on the maps, care should be taken. Any adverse affects due to this or geometrically driven component variation should be taken into consideration. For more information the reader is referred to references (17-19).

The information presented has been compiled from contacts in industry and from published work. As many as twenty different data sources have been used in the compilation of the individual process capability charts. Attempts have been made to standardise the data given as far as possible. Difficulties were faced in this connection since it was not always easy to obtain a consensus view.

Process Description

• Moist banded sand is packed around a pattern. The pattern is removed and molten metal poured into the cavity. Risers supply necessary molten material during solidification. The mould is broken to remove the part.

Materials

• Most metals. Some difficulty encountered in casting: lead, tin and zinc alloys, also refractory alloys, beryllium, titanium and zirconia alloys.

Process Variations

• Green sand casting: the most common and the cheapest. Associated problems are that the mould has low strength and high moisture content.
• Dry sand: core boxes are used instead of patterns. Expensive and time consuming.
• Skin-dried sand: the mould is dried to a certain depth. Used in the casting of steels.
• Patterns: one piece solid patterns are cheapest to make; split patterns for moderate quantities; match plate patterns for high volume production.
• Wooden patterns: low volume production only.
• Metal patterns: for medium to high volume production.
• Hard plastics are increasingly being used.

Economic Considerations

• Production rates of 1 to 60 pieces/hour, but dependent on size. Lead time ranges from days to several weeks depending on complexity and size of casting.
• Material utilisation is low to moderate - 20% to 50% of material lost in runners and risers.
• Both mould material and runners and risers may be recycled. Patterns are easy to make and set, and are reusable.
• Pattern material dependent on the number of castings required. Easy to change design during production.
• Economical for low production runs. Can be used for one-offs. Tooling costs are low.
• Equipment costs are low.
• Direct labour costs are moderate to high. Can be labour intensive.
• Finishing costs can be high. Cleaning and fettling are important before secondary processing.

Typical Applications

• Engine blocks.
• Manifolds.
• Machine tool bases.
• Pump housings.
• Cylinder heads.

Design Aspects

• High degree of shape complexity possible. Limited only by the pattern.
• Loose piece patterns can be used for holes and protrusions.
• All intersecting surfaces must be filleted: prevents shrinkage cracks and eliminates stress concentrations.
• Design of gating system for delivery of molten metal into mould cavity important.
• Placing of parting line important i.e. avoid placement across critical dimensions.
• Bosses, undercuts and inserts are all possible at low added cost. Machining allowances are usually in the range 1.5mm to 6mm.
• Draft angle ranges from 1� to 5�.
• Minimum section typically 3mm for light alloys and 6mm for ferrous alloys.
• Sizes range from 20g to 400t in weight.

Quality Issues

• Moulding sand must be carefully conditioned and controlled.
• Most casting defects can be traced to and rectified by sand content.
• Casting shrinkage and distortion during cooling governed by shape, especially when one dimension is much larger than the other two.
• Extensive flat surfaces are prone to sand expansion defects.
• Inspection of castings is important.
• High porosity and inclusion levels are common in castings.
• Defects in castings may be filled with weld material.
• Castings generally have rough grainy surfaces.
• Material strength is inherently poor.
• Castings have good bearing and damping properties.
• If production volumes warrant the cost of a die, close tolerances may be achieved.
• Surface detail fair to moderate.
• Surface roughness is a function of the materials used in making the mould and is in the range 3.2 to 50 �m Ra.
• Not suitable for close specification of tolerances without secondary processing.
• Process capability charts showing the achievable dimensional tolerances using various materials are given on the next page. Allowances of �0.5mm to �2mm should be added for dimensions across the parting line.

Process Description

Molten metal is inserted into a metallic mould under pressure where it solidifies. The die is then opened and the casting ejected.

Materials

• Limited to non-ferrous metals i.e. zinc, aluminium, magnesium, lead, tin and copper alloys.
• Zinc and aluminium alloys tend to be the most popular.
• High temperature metals e.g. copper alloys, reduce die life.
• Iron based materials for casting are under development.

Process Variations

• Cold-chamber die casting is used for high melting temperature metals.
• Hot-chamber die casting is used for low melting temperature metals due to erosive nature of molten metal. Can be either plunger or goose-neck type.

Economic Considerations

• Rapid production rates possible, up to 200 pieces/hour.
• Lead time could run into months.
• Material utilisation is high.
• Gates, sprues, etc, can be re-melted.
• High initial die costs due to high complexity and difficulty to process.
• Production quantities of 10,000 and above are economical.
• Tooling costs are high.
• Equipment costs are high.
• Direct labour costs are low to moderate.
• Finishing costs are low. Little more than trimming operations required to remove flash, etc.

Typical Applications

• Transmission cases.
• Engine parts.
• Pump components.
• Electrical boxes.
• Domestic appliances.
• Toy parts.

Design Aspects

• Shape complexity can be high. Limited by design of movable cores.
• Bosses are possible with added costs.
• Undercuts and inserts are possible with added costs and reduced production rates.
• Wall thickness should be as uniform as possible; transitions should be gradual.
• Sharp corners, or corners without proper radii should be avoided. (Pressure die casting permits smaller radii because metal flow is aided).
• Placing of parting line important i.e. avoid placement across critical dimensions.
• Holes perpendicular to the parting line can be cast.
• Casting holes for subsequent tapping is generally more economical than drilling.
• Machining allowance is normally in the range 0.25mm to 0.8mm.
• Draft angle ranges from 0.5� to 3�.
• Maximum section = 12mm.
• Minimum section ranges from 0.4mm to 1.5mm depending on material used.
• Sizes range from 10g to 50kg. Castings up to 100kg have been made in zinc. Copper, tin and lead castings are normally less than 5kg.

Quality Issues

• Very low porosity.
• Particularly suited where casting requires high mechanical properties or absence of creep.
• The high melting temperature of some metals can cause significant processing difficulties and die wear.
• Difficulty is experienced in obtaining sound castings in the larger capacities due to gas entrapment.
• Close control of temperature, pressure and cooling times important in obtaining consistent quality castings.
• Mechanical properties are good.
• Surface detail excellent.
• Surface roughnesses in the range 0.4 to 3.2 �m Ra can be achieved.
• Process capability charts showing the achievable dimensional tolerances using various materials are given on the next page. Allowances of �0.05mm to �0.35mm should be added for dimensions across the parting line.

4.3 Injection Moulding

Process Description

• Granules of unpolymerised material are heated and then forced, under pressure into the die cavity. Components produced have characteristic sprues left on surface.

Materials

Mostly thermoplastics, but thermosets, composites and elastomers can be processed.

Process Variations

• Reaction injection moulding: two reactive fluids are forced under pressure into the mould producing a thermoset part (chemical reaction is irreversible).

Economic Considerations

• Production rates are high, cycle times of 10 to 60 seconds typical.
• Thermoset parts usually have a longer cycle time.
• Lead times can be several weeks due to manufacturing of complex dies.
• Material utilisation is good. Scrap generated in sprues and risers.
• If material permits, gates and runners can be reused resulting in low material loss.
• Flexibility limited by dedicated dies, die changeover and machine set-up times.
• Economical for high production runs - typically 10,000+.
• Tooling costs are high. Dies are usually made from tool steel.
• Equipment costs are moderate to high. Direct labour costs are low.
• Finishing costs are low - little trimming required.

Typical Applications

• High precision, complex plastic components.
• Electrical parts.
• Fittings.
• Containers.
• Bottle tops.
• Housings.
• Tool handles

Design Aspects

• Very complex shapes and intricate detail possible.
• Pockets, holes, bosses and minor re-entrant features common.
• Radii should be as generous as possible.
• Uniform section thickness should be maintained.
• Marked section changes should be tapered sufficiently.
• Living hinges and snap features allow part consolidation.
• Thread forms also possible.
• Placing of parting line important i.e. avoid placement across critical dimensions.
• Inserts may be moulded in (e.g. metallic inserts for electrical conduction).
• The damping force required is proportional to the projected area of the moulded part.
• Draft angle ranges from less than 1� to 3� typical, depending on section depth.
• Maximum section, typically = 13mm.
• Minimum section = 0.4mm for thermoplastics, 1mm for thermosets.
• Sizes range from 10g to 25kg in weight for thermoplastics, 6kg maximum for thermosets.

Quality Issues

• Thick sections can be problematic.
• Care must be taken in the design of the running and gating system, where multiple cavities are used to ensure complete die fill.
• Unsuitable for the production of narrow necked containers.
• Control of temperature (material and mould) is critical, also injection pressure/speed, condition of resin, dwell and cooling times.
• Adequate clamping force is necessary to prevent the mould from flashing.
• Thermoplastic moulded parts usually require no de-flashing: thermoset parts often require this operation.
• Excellent surface detail obtainable.
• Surface roughness is a function of the die condition. Typically, 0.2 to 0.8�m Ra is obtainable.
• Process capability charts showing the achievable dimensional tolerances using various materials (see key) are given on the next page. Allowances of approximately �0.1mm should be added for dimensions across the parting line. Note, Charts 1, 2 & 3 are to be used for components which have a major dimension greater than 5Omm and large volume production typically.

Process Description

• An appropriate quantity of the raw, unpolymerised plastic is introduced into a heated mould which is subsequently closed under pressure, forcing the material into all areas of the cavity as it melts. Similar to closed die forging of metals.

Materials

Mainly thermosets, but also some composites, elastomers and thermoplastics.

Process Variations

• Flash-type: shallow parts; more material lost.
• Semi-positive (partly positive, partly flash): used for closer tolerances work and when the design involves marked changes in section thickness.
• Positive: high density pans involving composite sheet and bulk moulding compounds or impact-thermosetting materials.
• Cold-moulding: powder or filter (often refractory) is mixed with a binder, compressed in a cold die and cured in an oven.

Economic Considerations

• Production cycle times from 20 to 600 seconds typical.
• Cycle time is restricted by material handling: each cavity must be loaded individually.
• The greater the thickness of the part, the longer the curing time.
• Multiple cavity mould increase production rate. Mould maintenance is minimal.
• Time required for polymerisation (curing) depends mainly on the largest cross-section of the product and the type of moulding compound.
• Lead times may be several weeks according to die complexity.
• Material utilization is high (no sprues or runners).
• Flexibility is low. Differences in shrinkage properties reduces the capability to change from one material to another.
• Production volumes are typically 1,000+, but can be as low as 100 for large parts.
• Tooling costs are generally high.
• Equipment costs are moderate to high
• Direct labour and finishing costs are generally low. Flash removal required.

Typical Applications

• Dishes.
• Handles.
• Container caps.
• Electrical components and fittings.

Design Aspects

• Shape complexity is limited to relatively simple forms.
• Moulding in one plane only.
• Holes, protrusions, pockets and minor re-entrant features are possible.
• Inserts can be moulded in to achieve special properties.
• Thin walled parts with minimum warping and dimensional deviation may be moulded.
• When moulding materials with reinforcing fibres, fibre directionality is maintained enabling high strength to be achieved.
• Placing of parting line important i.e. avoid placement across critical dimensions.
• A draft angle of greater than 1� required.
• Maximum section, typically = 25mm.
• Minimum section - 0.25mm.
• Sizes range from several grammes to 15kg in weight.

Quality Issues

• Variation in polymer charge results in variation of part thickness.
• Air entrapment possible.
• Internal stresses are minimal.
• Dimensions in the direction of the mould opening and the product density will tend to vary more than those perpendicular to the mould opening.
• Flash moulds do not require that the quantity of material is controlled.
• Tumbling may be required as finishing process because of flash.
• Surface roughness is a function of the die condition. Typically, 0.2 to 0.8�m Ra is obtained.
• Process capability charts showing the achievable dimensional tolerances using various materials are given on the next page. Allowances of approximately �0.1 mm should be added for dimensions across the parting line.

Process Description

A plastic sheet is softened by heating elements and pulled under vacuum an to the surface form of a cold mould and allowed to cool. The part is then removed.

Materials

• Most thermoplastics.
• The material to be processed should exhibit high uniform elongation.

Process Variations

• Moulds are usually made of cast aluminium or aluminium-filled epoxy.
• Top and bottom heating elements.
• Top heading elements only.

Economic Considerations

• Process cycle times range from 10 to 60 seconds.
• Lead times of a few days typical, but can be weeks.
• Material utilisation is moderate to low: unformed parts of the sheet are lost and cannot be directly recycled,
• Multiple moulds may be used.
• Set-up times and change-over times are low.
• Production volume trends vary from small batches (10) to high volume, 1,000+.
• Tooling costs are low to moderate, depending on complexity.
• Equipment costs are low to moderate, but can be high if automated.
• Labour costs are low to moderate.
• Finishing costs are low.

Typical Applications

• Open plastic containers.
• Panels for non-heavy fixtures.
• Pages of Braille text.
• Vending cups.
• Packaging.
• Automotive parts.
• Electronic enclosures.
• Bath tubs.

Design Aspects

• Shape complexity limited to mouldings in one plane.
• Open forms of constant thickness without re-entrant angles.
• Bosses and inserts not possible.
• Parts with openings or holes cannot be formed.
• Corner radii should be large compared to thickness of material.
• Draft angles of 1� or greater recommended.
• Maximum section - 3mm.
• Minimum section - 0.05mm to O.5mm, depending on material used.
• Sizes range from 25mm2 to 7.5m x 2.5m in area.

Quality Issues

• Control of temperature, clamping force and vacuum pressure are important if variability is to be minimised.
• It multiple moulds are used it is necessary that there is sufficient distance between cavities to avoid flow interference.
• Excessive thinning can occur, particularly at sharp corners.
• Surface detail fair.
• Surface finish is good and is a related to the condition of mould surface.
• Dimensional tolerances range from �0.25mm to �2mm and are largely mould dependent.
• No parting lines.

Process Description

• A hot hollow tube of plastic (parison) is extruded or injected downwards and then caught between two halves of a shaped mould which closes the top and bottom of the tube. Hot air is blown into the parison, expanding it until it uniformly contacts the inside contours of the cold mould. The part is allowed to cool and is then ejected.

Materials

• Most thermoplastics. PETP most commonly used.

Process Variations

• lnjection blow moulding: small parts with intricate neck detail.
• Extrusion blow moulding: more applicable to asymmetrical parts, integrated handles possible.
• Multiple parisons: this requires close control since uneven parisons produce waste.
• Parisonless blowing: similar to dip-coating followed by expansion into the mould.
• Stretch blow moulding: the simultaneous axial and radial expansion of a parison, yielding a biaxially orientated container.
• Pressure moulding with an inert gas (compare with vacuum forming).

Economic Considerations

• Production rates between 100 and 2,500 pieces/hour, depending on size.
• Lead times are a few days.
• Integration with extrusion process to produce parison provides continuous operation.
• There is generally little material waste, but can be high with some geometries.
• Flexibility is limited since moulds are dedicated.
• Set-up times and change-over times are relatively short.
• Production volumes of up to 10,000,000, but also suitable for quantities as low as 1,000.
• Tooling costs are moderate to high.
• Equipment costs are moderate to high.
• Direct labour costs are low: one operator can manage several machines.
• Finishing cots are low: trimming only.

Typical Applications

• Hollow plastic parts with relatively thin walls.
• Bottles.
• Bumpers.
• Ducting.

Design Aspects

• Complexity limited to hollow, well rounded, thin walled parts with low degree of asymmetry.
• Asymmetrical mouldings e.g. off-set necks are possible with movable blowing spigots.
• Threads, inserts and undercuts all possible.
• Corner radii should be as generous as possible.
• Placing of parting line important i.e. avoid placement across critical dimensions.
• Holes cannot be moulded. Draft angles not required.
• Maximum section = 6mm. Thick sections may need cooling aids (carbon dioxide or nitrogen).
• Minimum section - 0.25mm.
• Sizes range from 12mm in length to volumes up to 3m3.

Quality Issues

• Poor control of wall thickness, typically �50% of nominal.
• Creep and chemical stability of product are important considerations
• Residual stresses e.g. non-uniform deformation, may relax in time causing distortion of the part.
• Good surface detail and finish possible.
• The higher the pressure the better the surface finish of the product.
• A process capability chart showing the achievable dimensional tolerances is given on the next page. Allowances of approximately �0.1 mm should be added for dimensions across the parting line.

4.7 Hot Forging

Process Description

• Hot metal is formed into the required shape by the application of pressure or impact forces using a press or hammer in a single or a series of dies.

Materials

• Mainly carbon, alloy and stainless steels, aluminium, copper and magnesium alloys. Titanium alloys, nickel alloys, high alloy steels and refractory metals can also be forged.
• Forgeability of materials important, must be ductile at forging temperature. Relative forgeability is as follows, easiest to forge first: aluminium alloys, magnesium alloys, copper alloys, carbon and low alloy steels, stainless steels, titanium alloys, high alloy steels, refractory metals and nickel alloys.

Process Variations

• Closed die forging: series of die impressions used to generate shape.
• Open die forging: hot material deformed between a flat punch and die. Shape and dimensions largely controlled by operator.
• Upset forging: heated metal stock gripped by dies and end pressed into desired shape.

Economic Considerations

• Production rates from 1 to 300 pieces/hour, depending on size.
• Production is most economic in the production of symmetrical rough forged blanks using flat dies. Increased machining is justified by increased die life.
• Lead times are typically weeks.
• Material utilisation moderate. Scrap loss depends on amount of subsequent machining required.
• Economical quantity is approximately 1000, depending on size and complexity. Can be 100 for large parts. However, more economical for high production volumes 10,000+.
• In the case of open die forging: lower material utilisation; machining of the final shape is necessary; slow production rate; low lead times; can be used for one-offs; high usage of skilled labour.
• Tooling costs are high.
• Equipment costs generally high.
• Direct labour costs are moderate. Some skilled operations required.
• Finishing costs are moderate. Removal of flash, cleaning and fettling important for subsequent operations.

Typical Applications

• Connecting rods.
• Crankshafts.
• Axle shafts.
• Airframe components.
• Tool bodies.
• Levers.

Design Aspects

• Complexity is limited by material flow through dies.
• Deep holes with small diameters are better drilled.
• Drill spots caused by die impressions aid drill centralisation.
• Locating points for machining should be away from parting line due to die wear.
• Markings are possible at little expense on adequate areas that are not to be subsequently machined.
• Care should be taken with design of die geometry since cracking, mismatch, internal rupture and irregular grain flow can occur.
• Good practice to have approximately equal volumes of material both above and below the parting line.
• Placing of parting line important i.e. avoid placement across critical dimensions.
• Corner radii and fillets should be as large as possible to aid hot metal flow.
• Avoid abrupt changes in section thickness as this causes stress concentrations on cooling.
• Inserts and undercuts are not possible.
• Machining allowances range from 0.8mm to 6mm, depending on size.
• Drafts must be added to all surfaces perpendicular to the parting line, draft angles range from 5� to 10� depending whether internal or external features.
• Minimum section = 3mm.
• Sizes range from 10g to 250kg in weight.

Quality Issues

• Good strength due to tough grain structure alignment.
• High fatigue resistance.
• Hot material in contact with the die too long will cause excessive wear, softening and breakage.
• Die wear and mismatch may be significant.
• Surface roughness and detail may be adequate, but secondary processing usually employed to improve the surface properties.
• Surface roughnesses is in the range 1.6 to 25�m Ra are obtainable.
• Process capability charts showing the achievable dimensional tolerances for closed die forging using various materials are given on the next page. Note, the total tolerance is divided +213, -113. Allowances of +0.3mm to +2.8mm should be added for dimensions across the parting line and mismatch tolerances range from 0.3mm to 2.4mm, depending on part size. Tolerances for open die forging range from �2mm to �50mm, depending on size of workpiece and skill of the operator.

Process Description

• Various processes under the heading of cold forming tend to combine forward and backward extrusion to produce near net shaped components by the application of high pressures and forces.

Materials

• Any ductile material at ambient temperature, including: aluminium, copper, zinc, lead and tin alloys, and low carbon steels. Also alloy and stainless steels, nickel and titanium alloys processed on a more limited basis.

Process Variations

• Impact extrusion: similar to cold extrusion but cold billet is plastically deformed by a single blow of the tool. Can be forward or backward extrusion (Hooker process).
• Cold forming: can be forward, backward or a combination of both.
• Hydrostatic extrusion: metal forced through die by high fluid pressure. Used for high strength, brittle and refractory alloy.
• Can incorporate other processes such as, cold heading, drawing, swaging, sizing and coining to produce complex parts at one station.

Economic Considerations

• Production rates up to 2,000 pieces/hour.
• Lead times are usually weeks.
• High utilisation of material (95%). Possible material cost savings over machining can be high. Near elimination of heat treatment and machining requirements.
• Most suited to high production volumes, 100,000+. Can be economical for quantities down to 5,000, depending on complexity of part.
• Most applications are in the formation of symmetrical parts with solid or hollow cross sections.
• Tooling costs are high.
• Equipment costs are high.
• Direct labour costs are low.
• Finishing costs are very low.

Typical Applications

• Fasteners.
• Tool sockets.
• Spark plug bodies.
• Gear blanks.
• Collapsible tubes.

Design Aspects

• Complexity limited. Symmetry of the part is important: concentric, round or square cross-sections typical. Limited asymmetry possible.
• To avoid mismatch of dies, every effort should be made to balance the forces, especially on unsymmetrical parts.
• Length to diameter ratios of secondary formed back extruded parts may approach 10:1; forward extrusion unlimited.
• Any parting lines should be kept in one plane and placement across critical dimensions should be avoided.
• Can be used to process two materials simultaneously to produce parts such as steel coated copper electrodes.
• Inserts are not recommended.
• Undercuts are not possible.
• Draft angles not required.
• Maximum section ranges from 0.25mm to 22mm depending on material for impact extrusion. No limit for cold forming.
• Minimum section ranges from O.O9mm to 0.25mm depending on material.
• Sizes range from �1.3mm to �150mm depending on cold formability of material being processed.

Quality Issues

• Inside shoulders require secondary processing to ensure flatness.
• Cold working offers valuable increase in mechanical properties, including extended fatigue life.
• Concentricity of blank and punch important in providing uniform section thickness
• Supply of lubrication (commonly phosphate based) to the die surfaces is important in providing uniform material flow and reduce friction.
• Small quantities of sulphur, lead, phosphorus, silicon, etc, reduces the ability of ferrous metals to withstand cold working.
• Surface cracking: tearing of the surface of the part, especially with high temperature alloys, aluminium, zinc, magnesium. Control of the billet temperature, extrusion speed and friction are important.
• Pipe or fishtailing: metal flow tends to draw surface oxides and impurities towards centre of part. Governing factors are friction, temperature gradients and amount surface impurities in billets.
• Internal cracking or chevron cracking: similar to the necked region in a tensile test specimen. Governing factors are the die angle and amount of impurities in the billet.
• Surface detail excellent.
• Surface roughnesses in the range 0.1 to 1.6 �m Ra are obtainable. Process capability charts showing the achievable dimensional tolerances are given on the next page.
• Dimensional tolerances for non-circular components are at least 50% greater than those shown on the charts.

Process Description

• Various processes are used to form cold rolled sheet metal using die sets, formers, rollers, etc. The most common processes are: deep drawing, bending, stretch forming and roll forming.

Materials

• All ductile metals available in cold rolled sheet form, supplied as blanks, flat or coiled.
• Most commonly used metals are: carbon steels, low alloy steels, stainless steels, aluminium alloys and copper alloys. Also, nickel, titanium, zinc and magnesium alloys are processed to a lesser degree.

Process Variations

• Mechanical drives: faster action and more positive displacement control.
• Hydraulic drives: greater forces and more flexibility.
• Deep drawing: forming of a blank into a closed cylindrical or rectangular shaped die. lncorporating an ironing operation improves dimensional tolerances.
• Bending: deformation about a linear axis to form an angled or contoured profile.
• Stretch forming: sheet metal is clamped and stretched over a simple form tool.
• Roll forming: forming of sheet metal into complex sections using a series of rolls.
• Can incorporate initial sheet metal shearing operations.

Economic Considerations

• Production rates are high, up to 3,000 pieces/hour on small components.
• High degree of automation possible.
• Cycle time is usually determined by loading and unloading times for the stock material.
• Lead times vary, up to several weeks for deep drawing; could be just hours for processes like bending.
• Material utilisation is moderate to high. Bending and roll forming do not produce scrap directly.
• Production quantities should be high for dedicated tooling, 10,000+.
• Economical quantity range from one for bending to 5,000 for deep drawing.
• Tooling cost moderate to high, depending on component complexity.
• Equipment costs vary greatly; low for simple bending machines, moderate for roll forming machines and high for automated deep drawing and ironing presses.
• Labour costs are low to moderate depending on degree of automation.
• Finishing costs are low. Trimming and cleaning may be required.

Typical Applications

• Cabinets
• Mounting brackets.
• Electrical fittings.
• Cans.
• Machine frames.
• Automotive and aerospace components.
• Structural members.
• Kitchen appliances and utensils.

Design Aspects

• Complex forms possible: several processes may be combined to produce one component, or a series of operations used to progressively form the part.
• Working envelope of machine and uniform thickness of sheet can restrict design options.
• No inserts or re-entrant angles.
• Beading: edge of sheet bent into cavity of die. May be used to remove sharp edges.
• Hemming: edge of sheet folded over. May be used to remove sharp edges.
• Minimum bend radii are a function of material and sheet thickness.
• Minimum sheet thickness = 0.1 mm.
• Maximum sheet thickness: drawing - 12mm, bending - 25mm. roll/stretch forming - 6mm.
• Sizes range from �2mm to �60Omm for deep drawing and 1Omm to 1.5m width for roll forming.

Quality Issues

• Bending and stretch forming are limited by the onset of necking.
• The limiting drawing ratio (blank diameter/punch diameter) is between 1.6 and 2.2 for most materials. This should be observed where drawing takes place without progressive dies, otherwise excessive thinning and tearing could occur.
• Variations in stock material thickness and flatness should be controlled.
• Other problems include: spring-back (metal returns to original form) and wrinkling during drawing (comparable with forcing a circular piece of paper into a drinking glass). eliminated by adjustment of blank holder force.
• Surface detail is good.
• Surface roughness is approximately that of the stock material for forming processes.
• Process capability charts showing the achievable dimensional tolerances for a number of processes are given on the next page.

Process Description

• The removal of material by chip processes using sequenced or simultaneous machining operations on cut to length bar or coiled bar stock. The stock can be automatically or manually fed into the machine.

Materials

• All metals (mostly free machining), some plastics, elastomers and some ceramics.

Process Variations

• Manually operated machines include: bench lathes (can machine non-standard shape parts) and turret lathes (limited to standard stock material).
• Automatic machines: fully or semi-automated. Follow operations activated by mechanisms on the machine.
• Automatic bar machines: used mainly for the production of screws and similar parts. Single spindle, multiple spindle and Swiss-types are available.
• CNC machines: movement and control of tool, headstock and saddle are performed by a computer program via stepper motors.
• Machining centres: fully automated, integrated turning, boring, drilling and milling machines capable of performing a wide range of operations.
• Extensive range of cutting tool geometries and tool materials available.

Economic Considerations

• Production rates range from 1 to 60 pieces/hour for manual machining, 10 to 1,000+ pieces/hour for automatic machining.
• Load times vary from short to moderate.
• Material utilisation is poor. Large quantities of chips generated which can be recycled.
• Flexibility is low to moderate for automatic machines: change over and set-up times can be many hours. Manual machines are very flexible,
• Economical quantity is 1,000+ for automatic machines and production volumes of 100,000+ are common. Manual and CNC machining are commonly used for small production runs, but can also be economic for one-offs.
• Tooling costs are moderate to high for automatic machines, low for manual.
• Equipment costs are high for automatic/CNC machines. Moderate for manual machining.
• Direct labour costs are high for manual machining, low to moderate for automate/CNC machining.
• Finishing costs are low. Only cleaning and deburring required.

Typical Applications

• Any component with rotational symmetrical elements requiring close tolerances.
• Non-standard shapes requiring secondary operations. Shafts.
• Screws and fasteners.
• Transmission components and engine parts.

Design Aspects

• Complexity limited to elements with rotational symmetry.
• Little opportunity for part consolidation.
• Can perform many different operations in a logical sequence on the same machine.
• Potential for linking with CAD very high.
• Reduce machining operations to a minimum (for simplicity and lower cycle time).
• Fillet corners and chamfer edges where possible to increase tool life.
• Holes should be drilled with a standard drill point at the bottom for economy.
• Required number of full threads should always be specified.
• Leading threads on both male and female work should be chamfered to assure good centring in machinery.
• Special attachments make auxiliary operations possible, for example, drilling and milling perpendicular to the length of the work. Some special machines allow larger pieces but then operations are restricted.
• Sizes range from �0.5mm to �2mm for manual and CNC machining.
• Automatic machines usually have a capacity of less than �6Omm.

Quality Issues

• Machinability of the material to be processed is an important issue with regards to: surface roughness, surface integrity, tool life, cutting forces and power requirements. Machinability is expressed in terms of a 'machinability index'. (Machinability index for a material is expressed as a percentage based on the relative ease of machining a material with respect to tree cutting mild steel which is 100% and taken as the standard.)
• Multiple set-ups can be a source of variability.
• Selection of appropriate cutting tool, coolant, feed rate, depth of cut and cutting speed with respect to material to be machined is important.
• Regular inspection of cutting tool condition and material specification is important for minimum variability.
• Surface detail good to excellent.
• Surface roughness values in the range 0.05 to 25 �m Ra are obtainable.
• Process capability charts showing the achievable tolerances for turning/boring (using conventional and diamond tipped cutting tools) are given on the next page. Note, the tolerances on these charts are greatly influenced by the machinability index for the material used.