Fabrication and Welding Technology for Mechanical Engineering
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Fabrication and Welding8
Fabrication and Welding Technology for Mechanical Engineering
Axisymmetric Metal Sheet Forming Process: Spinning and flow turning
Metal sheet forming process involves the process of changing metal into flat and thin pieces. Spinning in metal formation process involves the process where a flat metal blank is rotated at a high speed and formed into asymmetric art with a roller which gradually forces the blank onto a mandrel, bearing the final shape of the spun part. This technique is usually used in the formation of cylinders, baskets, pails, bases, tank’s bottom, funnels and dome shapes among others. Metal formation takes different techniques into consideration depending on the nature, size and usability of the end product. Metal spinning therefore, considers the shape of the final metal sheet desired but which have axisymmetric parts. Metal spinning rely on to three major parameters to produce quality metal products. These parameters include work-piece, availability of tools and process parameters. This precisely implies that the diameter, thickness and material should be considered and be standard enough to allow production of the required output. Working tools should provide efficiency and should be mechanically effective to allow for production of quality products. In addition, the entire process should be appropriately designed to suit the needs of the desired end product. Interestingly, metal sheet forming through spinning can be of two types, that is, shear forming and conventional spinning. On the other hand, axisymmetric metal sheet forming process also involves flow turning, where parts produced have cone-like shapes which round cross-sectional area. It may appear that conventional spinning and flow-turning are similar but what needs to be understood is that the latter starts from the thickness of the sheet to the production of the end parts.
Press braking is a metal sheet forming process where the press breaking tools such as punches and dies are used to fatten or bend a metal sheet. The punching tool in this case is used to press the metal sheet into the die to reshape the metal sheet. Punch force, the amount of force required to punch on a metal sheet with respect to its thickness, is calculated as the product of shear strength, perimeter and thickness as follows:
Punch force= Shear strength × Thickness × Perimeter, which is also given as
P= σS × P× t.
In case of a round bottom tool,
P= Π × D × t × σ
The resulting outcome of press braking is bending of the metal sheet into different desired angles. Whilst we consider bending, where are two types of bending in press braking metal sheet formatting process; air bending, bottoming and coining. Air bending process entails the mechanical operation of pressing the sheet as the surface of the sheet is in contact with three points; one of the punch and the rest of the open die. The radius of the sheet depends on the size of the radius of the die. Bottoming and coining are used in situations where accurate bending is required. Task 1b
High Energy Rate Forming Process
This is the process of metal sheet formation by passing large amount of electric energy over a short period. According to Dewsnap, (1971), HERF is a technique of forming metals by subjecting them to very considerable force available when stored electrically energy is suddenly discharged. Additionally, this process helps min the formation of metals which cannot be formed through other available techniques. Some methods used in HERF techniques include electrohydraulic forming, electromagnetic and explosive forming. Unlike other metal processing techniques, we can draw from its definition that metal formation is brought about by electric discharge. This is process is not mechanical as the rest. In HERF, there is a deformation energy which is released at a high rate than in other conventional techniques. In addition to this, the velocity of deformation is usually high. When employing this metal processing technique, more than one form of metal can be produced, that is, both which can be produced or cannot be produced by other techniques. It is also cheap to use die costs as production is actually higher than in conventional methods. However, this process is quite technical and require high skilled manpower who may be very difficult to find. Secondly, this method require the use of dangerous energy producing chemicals which may have harmful impact on the health of human beings and the ecosystem in general. Too much restrictions and inflexibility of government regulation also makes this technique expensive than the conventional ones.
Concerning the various methods of HERF, electrohydraulic involves production of an electrical discharge in sparks form at the electrodes between the water medium. In this method, the energy is stored in the capacitor which replaces a chemical explosive in explosive forming. The amount of strain used affects the process. In an experiment by Rohatgi, et al., (2012), it was found that the use of die resulted in greater strain when the strain at the apex was linear and proportional in High Energy Rate Forming (p.1070). Explosive forming involves the process of forming metals through exploding and forcing components into the die. This process is more dangerous than electrohydraulic and electromagnetic. Explosive forming consumes a lot of energy and can be destructive than electrohydraulic and electromagnetic. Explosive can be used in a controlled way in the manufacture of profile metal components, (Mynors, and Zhang, 2002, p.1). It is a suitable method in production of large parts but in small quantities. In addition, the process of exploding depends on the pressure applied. Talking about electromagnetic process of metal forming, an electronic coil is used in the work piece. Then an electrical surge is initiated and sent though the coil and a magnetic field is formed. The metal sheets create their own magnetic field due to the close proximity with the coil, moving parts apart. Whilst explosive forces is used to form large parts, electromagnetic process is used to form thin metal sheets.
The initial blank diameter.
The safe drawing ratio for the first draw.
Safe drawing ratio= initial blank diameter/punch diameter
Punch diameter= 1.3 σt π t [D – d]= 1.3 × 450mpa× 0.9(243.361 – 115) = 67, 582.07mm2
Therefore, safe drawing ratio;
=243.361/675.8207 = 0.36
The blank diameter for the first draw.
Blank diameter for the first draw= 0.36 × 243.361 = 87.61
The draw ratio for the second stage draw.
Draw ratio= 1.3 × 450mpa × 0.4 (243.361- 87.61) = 36,445.7
An approximate value, in tonnes, for the press capacity required for the first draw.
Press capacity= π ×P ×t × σs
Press capacity= 1.3 × 87.61 × 0.4 × 450Nmpa= 51,251.85= 51.251 tons
Joining methods for sheet Metals
Self-secured joints involve the use of solders, pop-riveting and welding in fabrication. There are a couple of Self-secured joints forming such: riveting, seaming joints and welding. Seaming joints are those which are formed by use of solders which have a low melting point. These are usually done through hand-held mechanical and electrical devices. Welding on the other hand, is the process of joining metals. This is done through the use of the weld puddle which is heated until into changes to molten state then when it cools strongly joins metals together. This requires high electrical energy since it has high melting point. Welding can be gas welding or arc welding. The other type of self-secured is riveting. This is the process of joining metals permanently which was traditionally used in the construction of railways. They were used to join railway.
Patented sheet metal captive fastener
This is a machine that is used to join metals together using small pieces of metal sheets. This type of metal joining is becoming common and is subject to technological advancement. Metals are joined using this method are not permanently joined. Techniques applicable in this case may include use of bolts, clinching or using screws. Other techniques include rivets, thread lockers, spring-steel fasteners, and molded-in-threads as mechanical fastening. Sheet metal captive fasteners are majorly used in the appliance manufacturing industries for its cost effective approach. However, the use of sheet metal captive fastener depends on the thickness of the sheet and the nature of the product part to be formed. In most cases, thin metal sheets are usually joined using fasteners.
This involves joining of metal parts using non mechanic approaches. This techniques replaces the use of mechanical tools to join metal parts. What happens in this case is that metal sheet are designed in such a manner that they have parts that allow for joining. This is a temporary form of joining metal parts. Like in assembling some parts, no use of screws, rivets or welding are used. This technique is cheap than the above mentioned ones.
Application method is part of fabrication method used in joining metal. An example of this is brazing. Brazing is the process of joining metals by passing a filter metal into the joints. This filler metal does not have a high melting point thereby causing no structural and mechanical issues while joining the metals. Many metal parts especially in the car manufacturing firms use this technique. Brazing is also common in welding.
The following procedures are applicable in bolting process
There are a number of fastening sequence used in bolting. One is the angle-controlled fastening sequence. This is where the highest yield point is reached by maintaining maximum accuracy. This maximum accuracy is achieved if the joints are pre-tightened and then force applied to bring surfaces together. There is a minimum tension forced applicable here and so therefore, no any tightening tool or force will be required. Another type of fastening sequence is yield-point controlled fastening sequence. In this case, the fastener yield point is reached and the tightening torque doesn’t linearly increase. Thirdly, Torque yield-controlled tightening occurs when the tighter is tightened to torque. Fastening sequence depends on the shape of the metal because tightening bolts causes what is called bolt crosstalk. The following procedure is general to tightening bolts.
Preparing the Joint
The joints need to be mobilised and lubricated before bolting is done. Lubrication is very important because it reduces friction and enhance. Additionally, the joint is prepared by cleaning and removing dusts and other particles on the flange faces.
The surfaces that form the joint point should be aligned as designed. There are different alignments of joints and differ in terms of accuracy and tightening. Alignment ensures that all the metal parts being joined do not forcefully join.
Ensure that nuts and screws are numbered and grouped
Different joints will require different types of nuts and other joining things. Therefore, it is imperative that they be named or numbered to avoid confusion and ensure time is saved.
The next step is joining parts according to the alignment above. Joining surfaces will help identify areas where bolts are required in order to use them. Joining is similar to assembling the parts.
Assembly aids are tools which help in ensuring consistency in joining parts together. The assembling procedure is also crucial to employ in joint formation. Fabrication of the product to allow the mating of product surfaces during alignment is required. Secondly, a torque wrench should be used. In case of heavy products, then machines can be used to bring surfaces together. An example of assembly aid techniques include tack bolting, where the joints are joined using a bolt but temporarily just to help in the alignment.
The next procedure is fastening the bolt. This involves the process of affixing the bolt into the metal at the joint. Fastening sequence is the manner and direction of affixing the bolt at the joint. This considers the yield point.
The correct sequence should be used in tightening the bolt. The specified bolt torque should be considered I this process.
Methods of tightening high strength friction grip bolt
There are two methods of tightening high strength friction bolt. These are turn-off-the-nut method and calibrated wrench method. Turn-off-the-nut method involves the process where the bolt or bolt of a fastener is rotated based on the length while preventing the other nut from moving or rotating. Therefore, one of the nuts will remain unturned. On the other hand, calibrated wrench method is concerned with tension created by the nuts and threads.
Structural section are joined through various joining techniques depending on the thickness and shape of the structure. The significance of joining structures is due to its economic benefits and varying functions of parts. One of the commonly applied techniques is the welding. Welding technique involves the use of highly heated molten substance to permanently join metal. Welding provide sufficiently high quality welded point structure formation and properties Fedoseeva and Yazovskikh, 2016, p.1). During welding, In fusion welding, the material that is used to join the parts is heated to melt and several processes occur. Firstly, oxy fuel gas welding process is aimed at providing a flame by burning oxygen to the substance. This is the process of burning fuels in different proportions so as to produce flames. Secondly, arc welding follows. This is the process where the heating and melting of the joining substance coalescence from an electric arc. The electric arc generates a lot of heat when the electrode action is stimulated. Thirdly, resistance welding is where the heat and pressure work together to achieve coalescence. The heat results from the arc welding process. Moreover, diffusion and friction welding are also part and parcel of the process. What needs to be remembered is that welding involves majorly electrical tools.
Another joining technique is bolting. Bolting is where nuts and screws are used to join parts. Bolting is also common in providing temporary joints. It is mechanical method that do not produce much harmful stress. During bolting, the flanges are aligned and lubricated together with the bolt torque in order to minimise tension. The bolt strength should be equal around the material surface and be assigned correct tension. Fastening of bolt follows and this depends on the yield-point, tension caused and sequence of bolts. This brings about yield-point controlled fastening sequence and torque yield-controlled surfacing sequence. While appreciating that, bolt tightening is also an area of interest. Tightening also depends on fastening sequence of the bolts.
Another important joining technique is riveting. Riveting is old techniques which was used before bolting came into place. As an important part in riveting, finding the base plate area that will allow acceptable level of stress. For a column and base plate connection, the forces are usually transmitted to the base given that end column and the base are connected and can be joined. Slab bases allow for direct compression but resist forces in situations where columns need not to have gussets. Under axial compression, the minimum thickness of the base plate is given by:
W= Uniform pressure from below the slab base
tf= flange thickness
Metal cutting is an important process in the manufacturing process. It entails a number of factor-oriented methods which are usually based on cost effectiveness and quality. Whilst metal cutting holds an important position in the manufacturing process, mechanical and electrical demands of factor input have necessitated the need to dig deep into research and development to enhance quality and required standard. There are three major cutting methods in welding; Oxy-fuel cutting, Plasma arc cutting and Laser Beam cutting.
The use of flam in the manufacturing industry has been a common practice for a number of decades. The importance of suing flames in cutting metal has not only gained much scientific attraction, but has also gathered uncountable benefits in the manufacturing process. Oxy-fuel is one of the commonly applied flame-cutting technology. This process involves the combustion of oxygen under certain conditions. The type of fuel used in this process is Acetylene which is the most suitable due to its ability to replace heat lost by the metal through conduction. Additionally, Acetylene is also able to allow cutting process faster especially on thinner plates. A number of factors are considered when using Oxy-fuel cutting method. One is that the nature of material used should have their temperatures below their melting point. Additionally, there should be intense pre-heating flame to maintain steel temperature.
Several equipment’s used in Oxy-fuel cutting include cutting nozzles and cutting torch. The cutting nozzles equipment are designed to have several ports and a central annulus port. These several ports are usually filled with accurate Oxygen gas proportion in the pre-heating process. The central port is the point through which oxygen is released for pre-heating. On the other hand, cutting torch equipment has valve which controls oxygen supply during preheating. It is commonly used in welding. In order to ensure accuracy in cutting, cutting devices are used. These include radius guide circle-cutting attachment, bevel guide and small circle-cutting attachment.
Oxy-fuel cutting techniques
Cutting technique for thick metals
Ensure that there is adequate supply of fuel since cutting thick metal sheets require a lot of flame. The heat the metal until the red heat is seen. Use the cutting lever and wait for the cutting reaction to occur and draw the blowpipe.
Cutting techniques for painted or galvanised Material
Ensure the surface is clean. Install fume extraction in case of adequate ventilation condition. Incline the nozzle to help in cutting.
Light and adjusting the cutting flame vs. general cutting techniques
This technique involves lighting and adjusting the cutting flame and general cutting techniques. For lighting and adjusting cutting flame technique takes care of the positioning of the gas regulator and oxygen regulator so as to ensure normal pressure is maintained. The fuel gas is lit as the flame is adjusting to prevent overheating. The heating valve is opened to allow flow of oxygen into the equipment until a fine white inner cone flame is observed. The flame is then adjusted to favourable condition. On the other hand, general cutting techniques are techniques which are common across various other flame-cutting techniques. This technique encompasses procedures such as cleaning the surface and the working area. Then the blowpipe is held in an appropriate position and the nozzles held 4 mm above the surface of the plate. Penetration into the plate surface should be achieved and then the cutting flame is turned off once cutting is done.
Cross carriage cutting machine
This is a type of machine which has been developed for small and medium scale fabrication process. The machine is automatic and cuts metal sheets with 4mm minimum thickness into required shapes. It is cost effective and efficient.
Arc Plasma Cutting
Plasma cutting is a cutting method which uses heat and a constricted arc formed between the workpiece and electrode. According to Nemchinsky and Severance, 2006). What we know and what we do not know about plasma arc cutting. Journal of Physics D: Applied Physics, 39(22), p.R423., dependence of the cyclic cathode erosion on the rate of current increase, double arcing and the role of insulating inclusions at the nozzle orifice on double arcing, dross formation and the shape of the kerf are the most important area of interest in arc plasma cutting (p.423).Arc plasma cutting process equipment and consumables include machine and hand-held torch, swirl ring which spins the cutting gas, electrode to allow electric transmission, shields, tip and retaining cups.
Plasma arc gouging is a technique of plasma arc cutting but for gouging it uses different arc tip in order to cause a reduction in the constriction of the arc. A low arc velocity is produced due to the reduction in arc constriction. The troch arc is usually set at an angle between 30 and 45 degrees from the surface base.
Mechanical cutting Techniques
Cutting by Shear
This involves using sharp cutting equipment without using heat in burn and melt. A shear is a two blade or two cutter tool that operates on the same principle as a conventional pair of scissors (Lainetti, 2011, p.5). There are several shearing machines that differ in terms of material thickness requirement. For instance, treadle shear cutter is used in cutting thin metal sheets with thickness of about 2mm. This machine is fast and economic in sheet cutting. Rotary shear machine is high-torque machine suitable for shredding bulky metal materials. It is economic, highly available and flexible.
Punching machines are usually used to flatten surface plates. The formula for calculating punch force is given as follows:
Punching Force= Perimeter × Thickness × Shear strength
I case of a round tool
A rectangular tool with dimension of 20mm x 40mm, the material is 4mm thick Stainless Steel T316L.
Perimeter, P= 2 x (20 + 40) = 120mm = 0.12m
The material thickness, t = 4mm = 0.004m
The shear strength of Stainless steel T316 = 482.63 Mpa = 4.921487 x 10^7 Kg/m^2
The Punching Force therefore,
F = 0.12 x 0.004 x ( 4.921487 x 10^7 ) = 23, 623.1376kg = 23.62 tons.
Chip forming process
Chip formation process depends on the type of cheap formation considered. There are for mechanisms of chip formation which include segmented chip formation, lamellar chip formation, discontinuous and continuous chip formation. Taking into consideration continuous chip formation, there are five zones of the formation process. In the primary shear zone deformation of plastic takes place here. The work piece is deformed in the secondary shear zone resulting from friction. Near the cutting edge, a stagnant zone develops as the separation of work piece takes place. In the preliminary shear zone, deformation of other small plastics takes place and determines the penetration of the plastic deformation.
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