Chapter IV: Appendix: A 99 (3)

Fig. 59 shows a cock of considerable size, which may be used for water or steam under high pressure. The plug in this example is hollow, and is prevented from coming out by a cover which is secured to the casing by four stud bolts. An annular ridge of rectangular section projecting from the under side of the cover, and fitting into a corresponding recess on the top of the casing, serves to ensure that the cover and plug are concentric, and prevents leakage. Leakage at the neck of the plug is prevented by a gland and stuffing-box. The top end of the plug is made square to receive a handle for turning it. The size of a cock is taken from the bore of the pipe in which it is placed; thus fig. 59 shows a 2-1/4-inch cock.

EXERCISE 61: 2-1/4-_inch Steam or Water Cock._--First draw the
views of this cock shown in fig. 59, then draw a half end elevation
and half cross section through the centre of the plug. Scale 6
inches to a foot.

Instead of drawing the parts of the pipe on the two sides of the
plug in the same straight line as in fig. 59, one may be shown
proceeding from the bottom of the casing, so that the fluid will
have to pass through the bottom of the plug and through one side.
This is a common arrangement.

All the parts of the valve and casing in this example are made of
brass.

XVII. MATERIALS USED IN MACHINE CONSTRUCTION.

_Cast Iron._--The essential constituents of cast iron are iron and carbon, the latter forming from 2 to 5 per cent. of the total weight. Cast iron, however, usually contains varying small amounts of silicon, sulphur, phosphorus, and manganese.

In cast iron the carbon may exist partly in the free state and partly in chemical combination with the iron.

In _white cast iron_ the whole of the carbon is in chemical combination with the iron, while in _grey cast iron_ the carbon is principally in the free state, that is, simply mixed mechanically with the iron. It is the free carbon which gives the grey iron its dark appearance. A mixture of the white and grey varieties of cast iron when melted produces _mottled cast iron_. The greater the amount of carbon chemically combined with the iron, the whiter, harder, and more brittle does it become.

The white cast iron is stronger than the grey, but being more brittle it is not so suitable for resisting suddenly applied loads. White iron melts at a lower temperature than grey iron, but after melting it does not flow so well, or is not so liquid as the grey iron. White iron contracts while grey iron expands on solidifying. The grey iron, therefore, makes finer castings than the white. Castings after solidifying contract in cooling about 1/8 of an inch per foot. Castings possessing various degrees of strength and hardness are produced by melting mixtures of various proportions of white and grey cast irons. White cast iron has a higher specific gravity than grey cast iron.

Cast iron gives little or no warning before breaking. The thickness of the metal throughout a casting in cast iron should be as uniform as possible, so that it may cool and therefore contract uniformly throughout; otherwise some parts may be in a state of initial strain after the casting has cooled, and will therefore be easier to fracture. Re-entrant angles should be avoided; such should be rounded out with fillets.

The presence of phosphorus in cast iron makes it more fusible, and also more brittle. The presence of sulphur diminishes the strength considerably.

The grey varieties of cast iron are called _foundry irons_ or _foundry pigs_, while the white varieties are called _forge irons_ or _forge pigs_, from the fact that they are used for conversion into wrought iron.

Amongst iron manufacturers the different varieties of cast iron are designated by the numbers 1, 2, 3, &c., the lowest number being applied to the greyest variety.

_Chilled Castings._--When grey cast iron is melted a portion of the free carbon combines chemically with the iron; this, however, separates out again if the iron is allowed to cool slowly; but if it is suddenly cooled a greater amount of the carbon remains in chemical combination, and a whiter and harder iron is produced. Advantage is taken of this in making _chilled castings_. In this process the whole or a part of the mould is lined with cast iron, which, being a comparatively good conductor of heat, chills a portion of the melted metal next to it, changing it into a hard white iron to a depth varying from 1/8 to 1/2 an inch. To protect the cast-iron lining of the mould from the molten metal it is painted with loam.

_Malleable Cast Iron._--This is prepared by imbedding a casting in powdered red hematite (an oxide of iron), and keeping it at a bright red heat for a length of time varying from several hours to several days according to the size of the casting. By this process a portion of the carbon in the casting is removed, and the strength and toughness of the latter become more like the strength and toughness of wrought or malleable iron.

_Wrought or Malleable Iron._--This is nearly pure iron, and is made from cast iron by the puddling process, which consists chiefly of raising the cast iron to a high temperature in a reverberatory furnace in the presence of air, which unites with the carbon and passes off as gas. In other words the carbon is burned out. The iron is removed from the puddling furnace in soft spongy masses called _blooms_, which are subjected to a process of squeezing or hammering called _shingling_. These shingled blooms still contain enough heat to enable them to be rolled into rough _puddled bars_. These puddled bars are of very inferior quality, having less than half the strength of good wrought iron. The puddled bars are cut into pieces which are piled together, reheated, and again rolled into bars, which are called _merchant bars_. This process of piling, reheating, and re-rolling may be repeated several times, depending on the quality of iron required. Up to a certain point the quality of the iron is improved by reheating and rolling or hammering, but beyond that a repetition of the process diminishes the strength of the iron.

The process of piling and rolling gives wrought iron a fibrous structure. When subjected to vibrations for a long time, the structure becomes crystalline and the iron brittle. The crystalline structure induced in this way may be removed by the process of _annealing_, which consists in heating the iron in a furnace, and then allowing it to cool slowly.

_Forging and Welding._--The process of pressing or hammering wrought iron when at a red or white heat into any desired shape is called _forging_. If at a white heat two pieces of wrought iron be brought together, their surfaces being clean, they may be pressed or hammered together, so as to form one piece. This is called _welding_, and is a very valuable property of wrought iron.

_Steel._--This is a compound of iron with a small per-centage of carbon, and is made either by adding carbon to wrought iron, or by removing some of the carbon from cast iron.

In the _cementation_ process, bars of wrought iron are imbedded in powdered charcoal in a fireclay trough, and kept at a high temperature in a furnace for several days. The iron combines with a portion of the carbon to form _blister steel_, so named because of the blisters which are found on the surface of the bars when they are removed from the furnace.

The bars of blister steel are broken into pieces about 18 inches long, and tied together in bundles by strong steel wire. These bundles are raised to a welding heat in a furnace, and then hammered or rolled into bars of _shear steel_.

To form _cast steel_ the bars of blister steel are broken into pieces and melted into crucibles.

In the _Siemens-Martin_ process for making steel, cast and wrought iron are melted together on the hearth of a regenerative gas-furnace.

_Bessemer steel_ is made by pouring melted cast iron into a vessel called a converter, through which a blast of air is then urged. By this means the carbon is burned out, and comparatively pure iron remains. To this is added a certain quantity of 'spiegeleisen,' which is a compound of iron, carbon, and manganese.

_Hardening and Tempering of Steel._--Steel, if heated to redness and cooled suddenly, as by immersion in water, is hardened. The degree of hardness produced varies with the rate of cooling; the more rapidly the heated steel is cooled, the harder does it become. Hardened steel is softened by the process of _annealing_, which consists in heating the hardened steel to redness, and then allowing it to cool slowly. Hardened steel is _tempered_, or has its degree of hardness lowered, by being heated to a temperature considerably below that of a red heat, and then cooling suddenly. The higher the temperature the hardened steel is raised to, the lower does its 'temper' become.

_Case-hardening._--This is the name given to the process by which the surfaces of articles made of wrought iron are converted into steel, and consists in heating the articles in contact with substances rich in carbon, such as bone-dust, horn shavings, or yellow prussiate of potash. This process is generally applied to the articles after they are completely finished by the machine tools or by hand. The coating of steel produced on the article by this process is hardened by cooling the article suddenly in water.

_Copper._--This metal has a reddish brown colour, and when pure is very malleable and ductile, either when cold or hot, so that it may be rolled or hammered into thin plates, or drawn into wire. Slight traces of impurities cause brittleness, although from 2 to 4 per cent. of phosphorus increases its tenacity and fluidity. Copper is a good conductor of heat and of electricity. Copper is largely used for making alloys.

_Alloys._--_Brass_ contains two parts by weight of copper to one of zinc. _Muntz metal_ consists of three parts of copper to two of zinc. Alloys consisting of copper and tin are called _bronze_ or _gun-metal_. Bronze is harder the greater the proportion of tin which it contains; five parts of copper to one of tin produce a very hard bronze, and ten of copper to one of tin is the composition of a soft bronze. _Phosphor bronze_ contains copper and tin with a little phosphorus; it has this advantage over ordinary bronze, that it may be remelted without deteriorating in quality. This alloy also has the advantage that it may be made to possess great strength accompanied with hardness, or less strength with a high degree of toughness.

_Wood._--In the early days of machines wood was largely used in their construction, but it is now used to a very limited extent in that direction. _Beech_ and _hornbeam_ are used for the cogs of mortise wheels. _Yellow pine_ is much used by pattern-makers. _Box_, a heavy, hard, yellow-coloured wood, is used for the sheaves of pulley blocks, and sometimes for bearings in machines. _Lignum-vitae_ is a very hard dark-coloured wood, and remarkable for its high specific gravity, being 1-1/3 times the weight of the same volume of water. This wood is much used for bearings of machines which are under water.

XVIII. MISCELLANEOUS EXERCISES.

The illustrations in this chapter are in most cases not drawn to scale; they are also in some parts incomplete, and in others some of the lines are purposely drawn wrong. The student must keep to the dimensions marked on the drawings, and where no sizes are given he must use his own judgment in proportioning the parts. All errors must be corrected, and any details required, but not shown completely in the illustrations, must be filled in.

EXERCISE 62: _Single Riveted Butt Joint with Tee-iron Cover
Strap._--Two views, one a side elevation and the other a sectional
elevation, of a riveted joint are shown in fig. 60. Draw these
views, and also a plan projected from one of them. Show the rivets
completely in all the views. Scale 4 inches to a foot.

EXERCISE 63: _Girder Stay for Steam Boiler._--The flat crown of the
fire-box of locomotive and marine boilers is generally supported or
stayed by means of girder stays, an example of which is shown in
fig. 61. A B is the side elevation of a portion of one of these
girders. Each girder is supported at its ends by the plates forming
the vertical sides of the fire-box. The flat crown is bolted to the
girders as shown. Observe that the girders are in contact with the
crown only in the neighbourhood of the bolts. Consider carefully
this part of the design, and then answer the following questions:
(1) What objections are there to supporting the girders at the ends
only without the contact pieces at the bolts? (2) What objections
are there to having the girders in contact with the crown plate of
the fire-box throughout their whole length?

Draw the views shown in fig. 61, and from the right-hand one
project a plan. Scale 4 inches to a foot.

EXERCISE 64: _End of Bar Stay for Steam Boiler._--On page 12 one
form of stay for supporting the flat end of a steam boiler is
described. Another form of stay for the same purpose is shown in
fig. 62. A B is a portion of the end of a steam boiler. C D is one
end of a bar which extends from one end of the boiler to the
other. The ends of this bar are screwed, and when the bar is of
wrought iron the screwed parts are generally larger in diameter
than the rest of the bar. When made of steel the bar is generally
of uniform diameter throughout. In the case of wrought-iron bar
stays the enlarged ends are welded on to the smaller parts.
Welding is not so reliable with steel as with wrought iron. Write
out answers to the following questions: (1) What is the advantage
of having the screwed part of the bar larger in diameter than the
rest? (2) Why are steel bar stays not generally enlarged at their
screwed ends?

Draw the views shown in fig. 62, and project from one of them a
third view. Scale 4 inches to a foot.

EXERCISE 65: _Knuckle Joint._--Draw the plan and elevation of this
joint shown in fig. 63, and also draw an end elevation looking in
the direction of the arrow. The parts at A and B are octagonal in
cross section. Scale 4 inches to a foot.

EXERCISE 66: _Locomotive Coupling Rod Ends._--A form of knuckle
joint used on locomotive coupling rods is shown in fig. 64.

In this case two rods meet and work on the same pin, as shown at
(a) fig. 64. Draw, in addition to the views shown in fig. 64, a
plan and a vertical section through the axis of the pin. Scale 6
inches to a foot.

Would it be practicable to replace the two rods A B and B C by a
single rod working on the crank pins at A, B, and C? Give reasons
for your answer.

EXERCISE 67: _Bell Crank Lever._--Draw the plan and elevation of
the lever shown in fig. 65. Scale 6 inches to a foot.

EXERCISE 68: _Back Stay for Lathe._--Draw a plan and two elevations
of the stay shown in fig. 66. Make all necessary corrections and
show all the details in each view. Scale full size.

EXERCISE 69: _Conical Disc Valve and Casing._--Draw, half size, the
views shown in fig. 67 of the conical disc valve and casing, and
also add an elevation looking in the direction of the arrow.

EXERCISE 70: _Connecting Rod End._--The student should carefully
compare this connecting rod end (fig. 68) with those illustrated on
pages 50 and 52. The lower part of fig. 68 is a half plan and half
horizontal section, and the upper part is a half side elevation and
half vertical section. Draw these views and also an end elevation.
Scale 6 inches to a foot.

EXERCISE 71: _Engine Cross-head._--The cross-head shown in fig. 69
is for an inverted cylinder marine engine. A is the piston rod, and
B B are pins, forged in one piece with C, to which the forked end
of the connecting rod is attached. Draw the upper view with the
central part in section as shown. Make the right-hand half of the
lower view a plan without any section, and make the left-hand half
a horizontal section through the axis of the pins B B. Scale 4
inches to a foot.

EXERCISE 72: _Ratchet Lever._--The lever shown in fig. 70 is used
for turning the horizontal screw of a traversing screw jack. Draw
the two views shown, and from one of them project a plan. Scale
full size.

EXERCISE 73: _Steam Whistle._--Draw, full size, the elevation and
section of the steam whistle shown in fig. 71. Draw also horizontal
sections at A B, C D, and E F.

EXERCISE 74: _Screw Coupling for Railway Carriages._--Draw the
three views of the screw coupling shown in fig. 72. Scale 6 inches
to a foot.

If the link A is fixed, through what distance will the link B move
for two turns of the lever?

EXERCISE 75: _Loose Headstock for a 6-inch Lathe._--Two views of
this headstock are shown in fig. 73. On one of these views a few of
the chief dimensions are marked. The details, fully dimensioned,
are shown separately in figs. 74, 75, and 76.

Explain clearly how the centre is moved backwards and forwards, and
also how the spindle containing it is locked when it is not
required to move.

Draw, half-size, the views shown in fig. 73, and from the
left-hand view project a plan. Draw also the detail of the locking
arrangement shown in fig. 74.