Pin Joints

Pin joints

Pin joints are among the detachable joints. There are dowel pins, fixing pins and shear pins.

Dowel pins need not transmit any forces, but only locate two workpieces relative to one another. They make assembly easier.

Retaining or fixing pins are used to transmit small amounts of force, in place of screws.

Shear pins protect easily damaged parts against overloading. They transmit the full driving force. It the force becomes excessive, however, the pin shears off and separates the two halves of the joint. Alter the fault has been eliminated, a new shear pin has to be fitted.

Types of pin

Grooved pins (Fare cheap and easy to fit. They have three rolled grooves along the surface. When hammered in, the ridges are partly forced back into the grooves to provide a firm seat in non- reamed holes. Grooved pins are used as locating and locking pins.

Split sleeves are split hollow cylinders of spring steel, with a diameter 0.2 to 0.5 mm greater than the drilled hole into which they are inserted. The hole need not be reamed, since the force

exerted by the compressed sleeve provides the desired pressure.

Straight pins are usually supplied with either tapered or spherical ends. Hardened straight pins are used as dowels. The hole for the pin must first be reamed.

Taper pins have a 1 :50 taper. They are therefore easier to insert than straight (cylindrical) pins. The hole is reamed with a taper reamer until the pin can be pressed in by hand with its head 3 to

4 mm above the rim of the hole. The pin is then driven in to its correct depth. A threaded journal or internal thread enables the pins to be pulled out of the joint again if there is no provision for driving them out from the opposite side.

10.3 Keys and splines

Keys and splines are used to connect shafts and hubs together. They prevent the hub from rotating on the shaft. They can also prevent the shaft from sliding axially.

Feather keys have a taper (inclination usually 1 ; 100). The taper generates the pressure which creates a firm joint between the parts to be connected. There are driven taper keys and sunk keys. In a driven joint, the key is forced in between the shaft and the hub. ln a sunk joint, the key is first inserted into the slotin the shaft, and the hub assembly is then driven on to the shaft over the

key. The flat key and the saddle key can only transmit fairly small forces. This kind of joint is seldom used in automobile engineering.

Plain keys have no taper. They create a pure driving joint only, with the force transmitted by the site faces of the key. The keys have an interference fit in the slot in the shaft.

Parallel keys usually have a close fit in the slot in the hub. The hub has to be prevented from sliting laterally. The sliding key permits the hub to slide laterally, for example then shift gears in a gearbox. The Woodruff key is mainly used at the ends of taper shafts. In the case of splined shafts, shaft

and keys are integral. This joint is used on highly stressed transmission components. The serrated joint is similar to the splined shaft.

Shear pins protect easily damaged parts against overloading. They transmit the full driving force. It the force becomes excessive, however, the pin shears off and separates the two halves of the joint. Alter

the fault has been eliminated, a new shear pin has to be fitted.

Types of pin

Grooved pins  are cheap and easy to fit. They have three rolled grooves along the surface. When hammered in, the ridges are partly forced back into the grooves to provide a firm seat in non reamed holes. Grooved pins are used as locating and locking pins. Split sleeves are split hollow cylinders of spring steel, with a diameter 0.2 to 0.5 mm greater than the drilled hole into which

they are inserted. The hole need not be reamed, since the force exerted by the compressed sleeve provides the desired pressure.

Straight pins are usually supplied with either tapered or spherical ends. Hardened straight pins are used as dowels. The hole for the pin must first be reamed.

Taper pins have a 1 :50 taper. They are therefore easier to insert than straight (cylindrical) pins. The hole is reamed with a taper reamer until the pin can be pressed in by hand with its head 3 to

4 mm above the rim of the hole. The pin is then driven in to its correct depth. A threaded journal or internal thread enables the pins to be pulled out of the joint again if there is no provision for driving them out from the opposite side.

10.3 Keys and splines

Keys and splines are used to connect shafts and hubs together. They prevent the hub from rotating on the shaft. They can also prevent the shaft from sliding axially.

Feather keys have a taper (inclination usually 1 ; 100). The taper generates the pressure which creates a firm joint between the parts to be connected. There are driven taper keys and sunk keys. In a driven joint, the key is forced in between the shaft and the hub. ln a sunk joint, the key is first inserted into the slot in the shaft, and the hub assembly is then driven on to the shaft over the key. The flat key and the saddle key can only transmit fairly small forces. This kind of joint is seldom used in automobile engineering.

Plain keys have no taper. They create a pure driving joint only, with the force transmitted by the site faces of the key. The keys have an interference fit in the slot in the shaft.

Parallel keys usually have a close fit in the slot in the hub. The hub has to be prevented from sliting laterally. The sliding key permits the hub to slide laterally, for example the shift gears in a gearbox. The Woodruff key is mainly used at the ends of taper shafts. In the case of splined shafts, shaft and keys are integral. This joint is used on highly stressed transmission components. The serrated joint (Figure 10.31) is similar to the splined shaft.

10.4 Interference-fit joints

The parts to be connected are manufactured oversize, so that pressure is built up at the joint when the parts are assembled. Interference-fit joints can transmit longitudinal and transverse forces. They are easy and cheap to produce. but can only be separated with difficulty. Depending on the method of making the interference-fit joint, we distinguish between the following types:

Longitudinal press tits. The two parts are pressed together in an axial direction when cold.

Transverse shrink fit. The outer part is heated uniformly and placed over the inner part. After cooling, a shrink lit is obtained,

Transverse press fit by expansion. The inner part is cooled down unllormly and then placed inside the outer part. Alter regaining normal temperature, an expansion joint is obtained.

Joining Techniques

Joining techniques

The principal distinction is between detachable and non detachable joints. A joint is regarded as detachable if the parts can be taken apart without destroying the connecting elements: for example, screw joints, pin joints, key joints and press-fit joints. A joint is non detachable, on the other hand, it it can only be separated by destroying the connecting elements, for example glued joints, riveted joints, soldered joints or welded joints.

1. Screw joints

Screws are mechanical elements with a wide range of uses. They are distinguished mainly by the shape ofthe screw head and the material from which they are produced.

10.1.1 Screws and nuts

Hexagonal head screws and bolts are used with nuts in through holes and without nuts in threaded holes, in which an internal thread is cut in the actual workpiece.

Studs are used if the joint is to be separated frequently, The shorter threaded end of the stud is screwed into the workpiece and tightened with a stud setter. When the threaded joint is taken apart, the stud remains in the workpiece.

Hexagon socket head screws  save space as a consequence of their cylindrical heads, which can also be countersunk. There is a special pattern of hexagon socket head screws with internal serrations. Hexagon socket head screws are suitable for screwed joints which cannot, because of their position, be tightened with an ordinary open ended wrench or ring spanner.

Body-fit bolts are used if relative movement between the workpleces must be prevented, and shear forces are high. Their shank diameter is slightly greater than the thread diameter. Anti fatigue bolts are used in screwed joints subject to continuous alternating loads, for example connecting rod big ends. lf standard bolts are subjected to continuous alternating loads in this way, they break as a result of fatigue after a period of operation, even if they are nominally of sufficient strength. In such cases, anti-fatigue bolts (neck bolts) have a much longer working life, Their shank diameter is only about 90% ofthe minor diameter of the screw thread except at the points where they contact the hole. The anti fatigue bolt is tightened with a torque wrench to the value specified by the manufacturer. This preloads it with a tensile force significantly greater than that which acts on it externally during operation. The anti-fatigue bolt can be stressed in the elastic range during operation until close to its elastic limit. Anti-fatigue bolts tightened to the specified torque retain their preload and need no additional locking devices. Their threads and those of the corresponding nuts must turn freely during assembly of the joint.

Slotted head screws and cross-recessed (Phillips head) screws can have various head patterns: half round (domed head), cheesehead, countersunk, raised cheesehead or raised countersunk.

Grub screws are screws without a head, threaded for their entire length. Depending on the application they can have various ends: coned point, full dog point, cupped point. Grub screws (setscrews) are used for the clamping or locking of hubs. bushes or bearings.

Wood screws are available with slotted, square or hexagonal heads. They are used for joints between wooden parts.

Sheet metal screws (self-tapping screws) are used to make joints with sheet metal. Their threads are similar to wood screws. As they are tightened, they cut their own thread in the sheet metal. The hole in the sheet should have the same diameter as the core of the screw. Clamp nuts (sheet metal nuts) are also frequently used. The joints are vibration resistant and can be separated any number of times. Sheet metal screws are available with slotted, cross recessed or hexagonal heads.

Self-tapping screws are surface-hardened screws. They cut their own threads as they are inseited into the core hole. They are tapered to make them easier to start. Since considerable force is required to tighten them, self tapping screws are produced only in sizes M 2.5 to M 8. They are suitable for all materials up to a tensile strength of 600 N/mm?. They should be dipped in oil before screwing in. When tightening, pressure must be applied along the axis of the screw.

Thread inserts and thread bushes are used if the thread is to be cut in a soft material, if the screwed joint has to be detached often or il the thread already cut in the workpiece has been

damaged. Thread inserts consist ot a rhomboid-form chrome nickel steel wire formed into a coil to produce an inner and an outer thread. A screw thread is cut in the core hole with a special tap. An assembly tool is used to insert and preload the thread insert. Thread bushings cut their own threads when inserted.

Nuts are also manufactured in various forms. Hexagonal nuts have a thickness otabout0.8 x d, or about0.5 x d in the case of special thin nuts.

Castle (castellated) nuts with 6 or 10 slots are used if a split pin is to be inserted as a keeper.

Cap nuts cover the screwed joint from the outside to protect the thread against damage, avoid injury and achieve a smarter appearance.

Union nuts are used for pipe joints.

Wingnuts and knurled nuts can be tightened by hand, without tools.

Slotted round and capstan nuts with metric fine pitch thread are mostly used to lock ball and roller bearings on to shafts and to adjust endplay.

Weld nuts are used in coach building. The weld nut is spot welded electrically to the component at three raised areas. A ring shaped collar centres the nut in the hole.

Property classes for screws and nuts

Steel screws are marked with the manufacturer’s symbol and the strength rating. This consists of two numerals separated by a point. The first number is one-hundredth of the minimum tensile strength in N/mm2, in our example a minimum tensile strength of 1 200 N/mm2. The second number is ten times the apparent yield point ratio (minimum apparent yield point in relation tominimum tensile strength). The product of the two numbers, e.g. 12 ><9 = 108, corresponds to 0ne tenth of the minimum elastic limit ol 1080 N/mm?.

Steel nuts are marked with the manufacturers symbol and a numeral which is one-tenth of the proof stress in N/mm2, for example 10 means that the nut can be loaded to a proof stress of 1000 N/mm2, The proof stress of the nut corresponds to the minimum tensile strength of the

corresponding screw. lf these screws are loaded up to the minimum ultimate load, then a nut must be chosen with a proof stress value at least as high as the minimum tensile strength of the screw,

10.1.2 Screw locking devices

The threads of screws used for clamping purposes are self-locking, but vibration can cause the nut or the screw to work loose, Locking devices are intended to prevent this.

Spring washers and toothed or serrated lock washers apply a load to the nut and also bite into its surface and that of the workpiece.

Convex or corrugated spring washers are also used. In the case of double nuts (locknuts, counter-nuts or check nuts), the top nut elongates the end of the screw or bolt within the elastic range as it is tightened down against the lower nut, This presses the two nuts together and prevents them from coming loose.

Spring action locknuts are hexagon nuts stamped from sheet metal. They are tightened against the main nut like a normal counter nut. As pressure builds up, their spring teeth press against the flanks of the bolt thread and prevent the main nut from loosening.

Lock washers are made from thin steel sheet. lt a wire keeper is specified, a soft steel wire is inserted through holes in the screw head and twisted to tighten it. The wire must be attached in such a way that it is stressed in tension if a screw tends to work loose.

Castle nuts can be locked very reliably by means of split pins, However, the range oi adjustment is limited by the number of slots on the nut.

Elastic stop nuts (self-locking nuts) have a firmly located polyamide ring at the end of the thread, which is pressed into the bolt threads when the nut is screwed on, and prevents the screwed joint from working loose. The friction exerted by the polyamide ring means that greater tightening torque is required. These nuts must be used only once.

Washers improve the nut contact surface and reduce the danger of the nut seizing when tightening or loosening. inclined or spherical washers can be used to compensate for contact faces which are not parallel.

10.1.3 Screw tightening tools

Screwdrivers with a plain blade are used for slotted head screws. To prevent damage to the screw head, the screwdriver blade must be of the correct width and thickness. It must be flat and not ground to a wedge shape. For the various types of cross recesses screws there are special patterns of screwdriver.

The leverage exerted by a spanner or wrench should be sufficient to obtain an adequately firm screwed joint after tightening, with no risk of overstressing or stripping the thread. This can occur if the spanner is extended by attaching a length of tube to its handle, in which case the thread may well be damaged beyond repair.

Spanners should always be an exact fit on the bolt head or nut. lf the spanner is too large, the bolt head or nut will be damaged and will tend to become round. so that even the correct size of spanner no longer fits; furthermore, a slipping spanner could cause an accident.

Torque wrenches indicate the amount of torque being exerted on a suitably calibrated scale. If the values specified by the automobile manufacturer, for instance, are adhered to, all threaded connections on the vehicle will be tightened correctly.

Screws and nuts should not be tightened with too much force or they may be damaged; if they are too loose, on the other hand, they could fail to grip and work loose.

Most torque wrenches can be set to a definite value. lf this is exceeded, the wrench slips and therefore cannot overtighten the joint. There are many plug-in heads and inserts for torque wrenches, particularly for tightening various sizes of hex bolts and nuts.

Tuban punya teknologi pengolahan air, bener gak ya ?

Tuban punya teknologi pengolahan air, bener gak ya ?

           

Tuban merupakan salah satu kabupaten yang terdapat di propinsi jawa timur, Luas wilayah Kabupaten Tuban 183.994.561 Ha, dan wilayah laut seluas 22.068 km². Letak astronomi Kabupaten Tuban pada koordinat 111^o 30' - 112^o 35 BT dan 6^o 40' - 7^o 18' LS. Panjang wilayah pantai 65 km. Ketinggian daratan di Kabupaten Tuban bekisar antara 0 - 500 mdpl. Sebagian besar wilayah Kabupaten Tuban beriklim kering dengan kondisi bervariasi dari agak kering sampai sangat kering yang berada di 19 kecamatan, sedangkan yang beriklim agak basah berada pada 1 kecamatan.

Cita-cita masyarakat tuban salah satunya adalah bisa memenuhi kebutuhan sehari-hari tanpa ada hambatan. Akankah cita-cita tersebut tercapai apabila masyarakatnya saja kesulitan untuk mendapatkan air bersih. Oleh karena itu, terdapat beberapa kecamatan di tuban yang kesulitan mendapatkan air bersih saat musim kemarau melanda.

Kita ambil contoh :

1.      Kecamatan Semanding

2.      Kecamatan Grabagan

3.      Kecamatan Bangilan

4.      Kecamatan Kenduruhan

Daerah-daerah diatas merupakan daerah yang rawan kesulitan mendapatkan air bersih apabila musim kemarau melanda di wilayah kabupaten tuban.

Beberapa tindakan yang harus dilakukan adalah melakukan pengembangan teknologi yang bergerak dalam penyediaan air bersih.

Ø Daerah yang mempunyai letak geografis dekat dengan laut, kita bisa melakukan tindakan dengan cara mengolah air laut menjadi air tawar atau air jernih yang layak minum.

Teknologi yang perlu kita kembangkan :

·      Pengolahan air laut  menjadi air bersih

Metode Reverse Osmosis merupakan suatu sistem pengolahan air dari yang mempunyai konsentrasi tinggi menjadi air tawar yang mempunyai konsentrasi agak rendah (encer) dikarenakan adanya tekanan osmosis. Penerapan sistem ini lebih efisien dan dapat diandalkan karena air melewati membrane semi permiabel yang kerapatannya 0,0001 mikron. Tahapan-tahapanya : Filtrasi menggunakan filter gerabah – filter karbon aktif menggunakan kayu bakau (membrane alami) untuk menyaring air.

Ø Daerah yang mempunyai letak geografis dekat dengan sungai, kita bisa melakukan tindakan dengan cara mengolah air sungai yang kotor dan keruh menjadi air untuk keperluan domestik  seperti MCK.

Teknologi yang perlu kita kembangkan :

·           Pengolahan air sungai menjadi air bersih :

·         Metode pengolahan air melalui serbuk biji kelor

Serbuk kelor kita  masukkan ke dalam air sungai, dengan takaran 3-5 gram serbuk kelor untuk 1 liter air sungai. Kemudian aduk selama 10-15 menit lalu partikel-partikel kotoran di air akan mengendap dengan endapan sebuk kelor. Setelah itu terciptalah air jernih dan bisa diminum. Untuk menghilangkan bau kelor yang timbul maka pada saat pengadukan kita tambahkan bungkusan arang yang sedemikian rupa.

·         Metode pengolahan air PT. Holcim Indonesia Tbk

1.      Air sungai kita pompa dengan water pump

2.      Kita endapkan pada kolam namanya “sand basin”

3.      Kita hisap air menuju kolam penampungan air melalui water pump, atau bahasa plant-nya adalah Reservoir.

4.      Setelah itu, air kita alirkan ke wadah tempat penambahan tawas, untuk menjernihkan air. Takaran tawas yaitu 25 kg untuk 4000 liter air sungai.

5.      Selanjutnya masuk kedalam water tank, didalamnya terdapat proses pengadukan air dengan baling-baling atau agigator

6.      Setelah itu masuk kedalam media filter. Media filternya apa saja sich ?

Terdapat pasir, ijuk, serabut-serabutan, kerikil dan karang. Keluaran dari media filter air sudah dalam kondisi bersih tapi dimungkinkan terdapat kuman.

7.      Proses selanjutnya air masuk lagi ke dalam kolam penampungan air atau reservoir dan ditambahkan kaporit untuk membunuh kuman yang masih terkandung dalam air. Takaran kaporit yaitu 20 cangkir kaporit untuk 4000 liter air.

8.      Kemudian air yang telah diberi kaporit siap ditampung ke dalam water tank dan distribusikan untuk keperluan air domestic plant seperti MCK.

Cita-cita yang diimpikan masyarakat tuban tidak mustahil akan terwujud apabila pemerintah setempat mau atau terdorong untuk mencoba teknologi baru pengolahan air yang ada di Tuban melalui beberapa ide yang telah di urai secara garis besarnya saja diatas. Pertimbangan yang matang adalah modal utama untuk terjadinya perubahan.

Casting

Casting

To produce castings, liquid (molten) metal is poured into moulds. After the metal has solidified, a replica of the workpiece based on the mould pattern is obtained. The work is carried out in the foundry. The mould is produced in the moulding shop. The mould-maker uses a pattern made from the workpiece lor this purpose. The casting is fettled (cleaned up and projecting flash or moulding sand removed) in the

casting cleaning shop.

Patterns

Patterns are made from wood, metal or plastics. For the larger patterns, air dried, dense and knot-free softwood is used; for smaller, more complex patterns hardwood is preferred. Metal patterns, made from cast iron, steel, brass or light metal, are of course more expensive, but have a longer life.

ln pattern making, allowance has to be made for the fact that casting metals contract or shrink on cooling. The pattern is therefore larger than the casting by the amount lost in shrinkage. At points to be machined later, the pattern-maker must add 0.5 to 10 mm. To avoid damaging the mould when taking out the pattern, this is tapered to make it 2% to 5% smaller internally and permit easy withdrawal. The finished wooden pattern is painted with pattern-maker’s lacquer.This gives it a smooth surface and protects it against moisture. The colours for painting are standardised. Patterns for cast iron are painted red, those for cast steel blue and for light metals green; core marks are shown ln black and faces to be machined in yellow.

Mould

We distinguish between sand casting and chill casting. The sand mould can be used only once. and is destroyed when removing the casting. For chill casting, the permanent steel or cast iron mould can be used repeatedly. Castings produced in this way have a clean surface and an accurate form. Fast cooling in the steel mould makes the surface particularly hard and wear-resistant. In order to obtain a hollow casting, acore must be placed in the mould. This is produced in a two-pan or more complex die (core box).

Flask moulding.

Flask moulds are normally rectangular cast iron frames with handles. They have pins and lugs so that the relative positions of the top and bottom flasks are accurately located on assembly. The sand mould is built up by the moulder, usually with the aid of a multi section pattern. He places one half of the pattern on the pattern board (Figure 9.30), positions the empty, frame shaped bottom flask moulds on it and fills it with sand. The sand is tamped firmly down, and the mould reinforced by inserting wires and pins. The bottom flask is then rotated, the second half of the pattern placed on the first half and the top flask aligned correctly. After the sprue and risers have been inserted, sand is lamped in and the top flask is lifted. The pattern is removed with great care and a core is inserted if needed. When the top and bottom flasks are re assembled, a cavity corresponding to the shape of the workpiece is produced.

Moulding machines are used only for mass production, to simplify the physically onerous work of the moulder and to save time.

Melting and casting

The metals are melted in suitable furnaces, and their quality modified or improved as necessary. The cupola furnace, also referred to as the shaft furnace, is mainly used in the iron foundry. It consists of a shaft 6 to 9 m high and about 1.5m in diameter. The molten metal is collected in the forehearth, run off into ladies, brought to the mould and poured into it. Casting stops when the riser is full. After having cooled sufficiently, the mould is destroyed and the casting taken out and cleaned.

In the casting cleaning shop the runner, the risers and the casting seams are removed by knocking off with hammers and chisels, sawn off or ground away. On steel castings, oxy-acetylene flame cutting

may even be used. The core is then driven out, and the surface of the casting cleaned with steel wire brushes, or by sand blasting, waterjet, emery wheel etc.

Casting processes

Pressure die casting. Non-ferrous heavy and light metal alloys are often mass-produced by the pressure die casting process. ln this casting process, the molten material is forced at high pressure into a

steel mould. Even the smallest cavities of the precision-machined steel moulds are filled densely and effectively. The accuracy andsurface quality are so high that the term "finish casting" is often applied to these items. Only the sprue and casting seams need to be removed. Casting pressure is between 20 and 3000 bar. A distinction is made between the cold-chamber and hot chamber processes.

Forging

Forging

Materials suitable for forging can be formed, mostly when hot, by applying heavy blows or pressure. The forming process causes a plastic change to take place in the solid material. ln contrast to workpieces machined from solid, forgings have a cohesive libre flow. Their structure is dense and their strength correspondingly high, which makes them suitable for items such as crankshafts and connecting rods. Forging is often cheaper than machining.

Forgeabllity of materials

Metals can only be forged if their plasticity (elongation) increases when they are heated; their strength decreases at the same time. The most important forgeable metals are steel, aluminium and its alloys, copper, brass and bronze. Cast iron cannot be forged, since it does not become ductile when heated.

The lower the carbon content of steel, the better it can be forged. Low carbon steels, in other words, are the most suitable for forging. The sulphur content makes the steel brittle when red-hot and leads to crack formation. Too high a phosphorus content makes the steel brittle when cold, so that workpieces tend to fracture when cold formed. Sulphur and phosphorus are therefore to be regarded as harmful components in a forging steel. When forging unalloyed steels, comply with the manufacturer's guidelines.

Forging steel

There are initial and final forging temperature limits which should always be complied with (note the manufacturer‘s recommendations). lf forming takes place within the specified temperature range, the

structure becomes dense and fine, and the strength is high, The lower the steel's carbon content, the higher the forging temperature must be. lf the workpiece is heated beyond the initial temperature and held at this temperature for some time, the steel will become overheated.

Overheated steel is coarse grained and brittle. The coarse grain can be eliminated by normalising. lf steel is heated until sparks appear, it is burnt and must be regarded as permanently unusable. Forged workpieces, particularly those of high-alloy steel, should be allowed to cool as slowly and uniformly as possible, to avoid internal stresses. Each glowing colour corresponds to a different temperature range, so that the colour of the workpiece is a good guide to the temperature it has reached.

Smith’s hearth

A health is used to heat small workpieces. A caking sulphur-free coal (smithy peas) or, in special cases, charcoal which is completely free from sulphur, should be used as fuel. To heat larger workpieces, closed forging furnaces heated by gas or oil are used.

Forging tools

Forging is usually carried out on IHS anvil. This rests on a wooden base which helps to absorb shock and vibration. The face of the anvil is hardened, and contains a round and a square hole. These holes are used to insen auxiliary tools such as the anvil chisel for cutting off, the filing block for rounding, the beak iron forcutting off and bending and various dies for the production of plugs. Forging tongs have various jaw shapes, and are used to hold the hot workpieces while forging. The forging hammer weighs 1 kg~2 kg. For more extensive stretching operations the smith works together with a hammer man, who stands to one side of him and uses the cross pane sledgehammer. A forging vice is used to clamp workpieces. There are forging machines to produce larger workpieces. The twomain categories are machine hammers and presses.

Forging operations

In free forging, the smith obtains the desired workpiece shape by means of hand hammers, machine hammers and forging presses, using simple tools.

ln drop forging, the workpiece is formed in a die to a high level of dimensional accuracy, with the aid of a die forging hammer or press. Die-forged workpleces are of high strength. Forged workpieces which are later to be machined by chip-removing methods include a machining allowance. This is about 3 mm per face on smaller items and 5 to 10 mm for larger ones.

By stretching, the length ol the workpiece is increased; its cross section decreases accordingly. Stretching is carried out with the peen of the cross pane sledge hammer or on the anvil beak, then finished off on the face of the anvil.

By flnishing, forged workpieces can be given a smoother surface.

ln stepping, a sharp·edged step is obtained at the transition from a smaller to a larger cross-section.

In upsetting, the cr0ss-section is heated and thenincreased, so that the length decreases.

When parting off, part of the workpiece is separated from the remainder. A notch is first produced with the cold chisel or anvil chisel, and the workpiece is then bent over the edge of the anvil and struck.

ln punching, the workpiece is struck with a drift over a perforated plate or hole.

In forge welding or pressure welding, the workpleces are welded together at high temperature when they are in a pliable state, by hammering or pressing.

Mass produced parts of high strength can be produced by hammering or pressing in forging dies, and lettled in a deburring die.

Sheet Metalworking

Stretcher-levelling of sheet

Sheet is normally supplied flat, that is to say in a stretched condition. However, it can become dented or bent in transit or in storage. These flaws can be eliminated by stretching the sheet again. Dents are removed by stretching the material at the edge of the dent. This is done with hammer-blows, working in a spiral pattern round the dent from the inside to the outside. It a sheet has several dents, they can be reduced to a single dent by stretching the material, which is then straightened. Dents can also be removed by flame straightening.

Warped sheets always rest on the shorter diagonal. By stretching along this line (hammer-blows), the material is elongated and the sheet will lie flat again (Figure 9.15). Wavy sheet edges can be straightened again by stretching the material from the edge towards the middle, if the change of shape is not too great. The blows should be light, and the hammer itself should not be too heavy in order to avoid renewed stretching. Stretching sheet metal, particularly large areas, calls for considerable experience. On soft materials (brass, light metal), only wooden mallets or rubber faced hammers should be used. Distorted sheet metal can often be straightened easily with the aid of stretching hammers. The hammer head is ground to a diamond pattern. The resulting slight points penetrate a small distance into the metal, relieve stresses and flatten the material. Workpieces or materials can also be flattened in the press, using rough levelling tools.

Removing dents from vehicle body panels

Frequently damaged body panels are usually designed for easy replacement. This applies particularly to mud guards (front and rear sidepanels). Slight damage can be rectified, although this needs much experience. The precautions applicable to the stretching of sheet metal basically apply here too. Steel bodies are produced from thin deep drawn sheet, which acquires additional strength in the deep drawing process. Dents caused by impact can be pressed out without difficulty if the sheet has stretched only slightly. It the change in shape is too great, however, each blow applied to the dent will only magnify it. The area surrounding the dent should therefore be stretched if possible. The dent is often shrunk by flame straightening. ln many cases the dented area is cutout, a replacement section welded in and the surface reworked until it is again acceptable. Deforrned areas can also be cut, straightened and welded together again. By using dozers (Figure 9.17), body realignment jigs and frames and hydraulic compression and tension rams, bodyshells can be straightened again quickly, easily and extremely accurately.