1.4 General Procedure in Machine Design
In designing a machine component, there is no rigid rule. The
problem may be attempted in several ways. However, the general
procedure to solve a design problem is as follows :
1. Recognition of need. First of all, make a complete statement
of the problem, indicating the need, aim or purpose for which the
machine is to be designed.
2. Synthesis (Mechanisms). Select the possible mechanism or
group of mechanisms which will give the desired motion.
3. Analysis of forces. Find the forces acting on each member
of the machine and the energy transmitted by each member.
4. Material selection. Select the material best suited for each
member of the machine.
5. Design of elements (Size and Stresses). Find the size of
each member of the machine by considering the force acting on the
member and the permissible stresses for the material used. It should
be kept in mind that each member should not deflect or deform than
the permissible limit.
6. Modification. Modify the size of the member to agree with
the past experience and judgment to facilitate manufacture. The
modification may also be necessary by consideration of manufacturing
to reduce overall cost.
7. Detailed drawing. Draw the detailed drawing of each component and the assembly of the
machine with complete specification for the manufacturing processes suggested.
8. Production. The component, as per the drawing, is manufactured in the workshop.

welding process

Welding Processes
The welding processes may be broadly classified into the following two groups:
1. Welding processes that use heat alone e.g. fusion welding.
2. Welding processes that use a combination of heat and pressure
e.g. forge welding.
These processes are discussed in detail, in the following pages.
10.4 Fusion Welding
In case of fusion welding, the parts to be jointed are held in position while the molten metal is supplied to the joint. The molten metal may come from the parts themselves (i.e. parent metal) or filler metal which normally have the composition of the parent metal. The joint surface become plastic or even molten because of the heat from the molten filler metal or other source. Thus, when the molten metal solidifies or fuses, the joint
is formed.
The fusion welding, according to the method of heat generated, may be classified as:
1. Thermit welding, 2. Gas welding, and 3. Electric arc welding.
10.5 Thermit Welding
In thermit welding, a mixture of iron oxide and aluminium called thermit is ignited and the iron
oxide is reduced to molten iron. The molten iron is poured into a mould made around the joint and
fuses with the parts to be welded. A major advantage of the thermit welding is that all parts of weld
section are molten at the same time and the weld cools almost uniformly. This results in a minimum
problem with residual stresses. It is fundamentally a melting and casting process.
The thermit welding is often used in joining iron and steel parts that are too large to be manufactured
in one piece, such as rails, truck frames, locomotive frames, other large sections used on steam
and rail roads, for stern frames, rudder frames etc. In steel mills, thermit electric welding is employed
to replace broken gear teeth, to weld new necks on rolls and pinions, and to repair broken shears.
10.6 Gas Welding
A gas welding is made by applying the flame of an oxy-acetylene or hydrogen gas from a
welding torch upon the surfaces of the prepared joint. The intense heat at the white cone of the flame
heats up the local surfaces to fusion point while the operator manipulates a welding rod to supply the
metal for the weld. A flux is being used to remove the slag. Since the heating rate in gas welding is
slow, therefore it can be used on thinner materials.


10.7 Electric Arc Welding
In electric arc welding, the work is prepared in the same manner as for gas welding. In this case
the filler metal is supplied by metal welding electrode. The operator, with his eyes and face protected,
strikes an arc by touching the work of base metal with the electrode. The base metal in the path of the
arc stream is melted, forming a pool of molten metal, which seems to be forced out of the pool by the
blast from the arc, as shown in Fig. 10.1. A small
depression is formed in the base metal and the
molten metal is deposited around the edge of this
depression, which is called the arc crater. The slag
is brushed off after the joint has cooled.
The arc welding does not require the metal
to be preheated and since the temperature of the
arc is quite high, therefore the fusion of the metal
is almost instantaneous. There are two kinds of
arc weldings depending upon the type of electrode.
1. Un-shielded arc welding, and
2. Shielded arc welding.
When a large electrode or filler rod is used for welding, it is then said to be un-shielded arc welding.
In this case, the deposited weld metal while it is hot will absorb oxygen and nitrogen from the atmosphere.
This decreases the strength of weld metal and lower its ductility and resistance to corrosion.
In shielded arc welding, the welding rods coated with solid material are used, as shown in Fig.
10.1. The resulting projection of coating focuses a concentrated arc stream, which protects the globules
of metal from the air and prevents the absorption of large amounts of harmful oxygen and nitrogen.
10.8 Forge Welding
In forge welding, the parts to be jointed are first heated to a proper temperature in a furnace orforge and then hammered. This method of welding is rarely used now-a-days. An electric-resistance welding is an example of forge welding.
In this case, the parts to be joined are pressed together and an electric current is passed from one part to the other until the metal is heated to the fusion temperature of the joint. The principle of applying heat and pressure, either sequentially orsimultaneously, is widely used in the processesknown as *spot, seam, projection, upset and flash
welding.

dmm-1 3rd question

Advantages and Disadvantages of Welded Joints over Riveted Joints
Following are the advantages and disadvantages of welded joints over riveted joints.
Advantages
1. The welded structures are usually lighter than riveted structures. This is due to the reason,
that in welding, gussets or other connecting components are not used.
2. The welded joints provide maximum efficiency (may be 100%) which is not possible in
case of riveted joints.
3. Alterations and additions can be easily made in the existing structures.
4. As the welded structure is smooth in appearance, therefore it looks pleasing.
5. In welded connections, the tension members are not weakened as in the case of riveted joints.
6. A welded joint has a great strength. Often a welded joint has the strength of the parent
metal itself.
7. Sometimes, the members are of such a shape (i.e. circular steel pipes) that they afford
difficulty for riveting. But they can be easily welded.
8. The welding provides very rigid joints. This is in line with the modern trend of providing
rigid frames.
9. It is possible to weld any part of a structure at any point. But riveting requires enough
clearance.
10. The process of welding takes less time than the riveting.
Disadvantages
1. Since there is an uneven heating and cooling during fabrication, therefore the members
may get distorted or additional stresses may develop.
2. It requires a highly skilled labour and supervision.
3. Since no provision is kept for expansion and contraction in the frame, therefore there is a
possibility of cracks developing in it.
4. The inspection of welding work is more difficult than riveting work.

lets start with our hod sirs subject

dmm-1 1st mid first answer

1.3 General Considerations in Machine Design
Following are the general considerations in designing a machine component :
1. Type of load and stresses caused by the load. The load, on a machine component, may act
in several ways due to which the internal stresses are set up. The various types of load and stresses are
discussed in chapters 4 and 5.
2. Motion of the parts or kinematics of the machine. The successful operation of any machine
depends largely upon the simplest arrangement of the parts which will give the motion required.
The motion of the parts may be :
(a) Rectilinear motion which includes unidirectional and reciprocating motions.
(b) Curvilinear motion which includes rotary, oscillatory and simple harmonic.
(c) Constant velocity.
(d) Constant or variable acceleration.
3. Selection of materials. It is essential that a designer should have a thorough knowledge of
the properties of the materials and their behaviour under working conditions. Some of the important
characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion,
ability to cast, welded or hardened, machinability, electrical conductivity, etc. The various types
of engineering materials and their properties are discussed in chapter 2.
4. Form and size of the parts. The form and size are based on judgement. The smallest practicable
cross-section may be used, but it may be checked that the stresses induced in the designed
cross-section are reasonably safe. In order to design any machine part for form and size, it is necessary
to know the forces which the part must sustain. It is also important to anticipate any suddenly
applied or impact load which may cause failure.
5. Frictional resistance and lubrication. There is always a loss of power due to frictional
resistance and it should be noted that the friction of starting is higher than that of running friction. It
is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces
which move in contact with others, whether in rotating, sliding, or rolling bearings.
6. Convenient and economical features. In designing, the operating features of the machine
should be carefully studied. The starting, controlling and stopping levers should be located on the
basis of convenient handling. The adjustment for wear must be provided employing the various takeup
devices and arranging them so that the alignment of parts is preserved. If parts are to be changed
for different products or replaced on account of wear or breakage, easy access should be provided
and the necessity of removing other parts to accomplish this should be avoided if possible.
The economical operation of a machine which is to be used for production, or for the processing
of material should be studied, in order to learn whether it has the maximum capacity consistent with
the production of good work.
7. Use of standard parts. The use of standard parts is closely related to cost, because the cost of standard or stock parts is only a fraction of the cost of similar parts made to order.The standard or stock parts should be used whenever possible ; parts for which patterns are already in existence such as gears, pulleys andbearings and parts which may be selected from regular shop stock suchas screws, nuts and pins. Bolts and studs should be as few as possible toavoid the delay caused by changingdrills, reamers and taps and also to decrease the number of wrenches required.
8. Safety of operation. Some machines are dangerous to operate, especially those which are
speeded up to insure production at a maximum rate. Therefore, any moving part of a machine which
is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It
is, therefore, necessary that a designer should always provide safety devices for the safety of the
operator. The safety appliances should in no way interfere with operation of the machine.
9. Workshop facilities. A design engineer should be familiar with the limitations of his
employer’s workshop, in order to avoid the necessity of having work done in some other workshop.
It is sometimes necessary to plan and supervise the workshop operations and to draft methods for
casting, handling and machining special parts.
10. Number of machines to be manufactured. The number of articles or machines to be manufactured
affects the design in a number of ways. The engineering and shop costs which are called
fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If
only a few articles are to be made, extra expenses are not justified unless the machine is large or of
some special design. An order calling for small number of the product will not permit any undue
expense in the workshop processes, so that the designer should restrict his specification to standard
parts as much as possible.
11. Cost of construction. The cost of construction of an article is the most important consideration
involved in design. In some cases, it is quite possible that the high cost of an article may immediately
bar it from further considerations. If an article has been invented and tests of hand made samples have
shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum
of money in the design and development of automatic machines to produce the article, especially if it
can be sold in large numbers. The aim of design engineer under allconditions, should be to reduce the manufacturing cost to the minimum.
12. Assembling. Every machine or structure must be assembled as a unit before it can function. Large units must often be assembled in the shop, tested and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection.

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