Cores and Furnaces

 Cores and Furnaces

CORES

A core can be defined as a body of sand which is used to form a cavity of desired shape and size in a casting. Core are prepared separately in core boxes.

Types of cores:

Horizontal core: 

It is most common and simple type of core. It is assembled in the mold with its axis horizontal. It is supported in the mold at its both ends.

Vertical core: 

It is quite similar to a horizontal core except that it is fitted in the mold with its axis vertical.

Balanced core: 

It is used to produce a blind hole along a horizontal axis in a casting. As a matter of fact it is nothing but a horizontal core, with the exception that it is supported only on one end, the other end remaining free in the mold cavity.

Hanging or cover core: 

A core which hangs vertically in the mold and has no support at its bottom is known as hanging core. In such case it is obvious that the entire mold cavity will be contained in drag only.

MELTING FURNACES

A melting furnace is a very necessary equipment in foundry shop. It is used to melt the metal to be casted.

TYPES OF FURNACES

The main types of furnaces used in foundries for melting of various varieties of ferrous and non-ferrous metals and alloys are described as:

Crucible furnaces: 

These are the simplest of all the furnaces used in foundries. They are used in most of the small foundries where melting is not continuous and a large variety of metal is to be melted in small quantities. In these furnaces the entire melting of metal takes place inside a melting pot called crucible, which is made of clay and graphite. These furnaces can be classified as:

a. Coke fired furnaces: 

These furnaces are generally installed in a formed pit and are used for melting small quantities of ferrous metals for producing iron casting and also non-ferrous metals and alloys. They are provided with refractory lining inside and a chimney at the top. Coke is used as fuel. Broken pieces of metal are placed in the crucible. Bed coke is fired in the furnace and the crucible placed into it. Afterwards more coke is placed all around the crucible.

b. Oil and gas fired furnaces: 

These furnaces utilize oil or gas as a fuel. In fact a mixture of gas and air or oil and air is fed into the furnace which burns inside to produce the desired temperature. The furnace essentially consists of cylindrical steel shell, provided with refractory lining inside and proper passage for entry of the fuel mixture. The crucible is seated on a pad formed at the bottom.

Cupola furnace: 

For melting of cast iron in foundry the cupola furnace is used. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates. The kindling material, generally soft and dry piece of wood, is first placed over the sand bed followed by a small amount of coke charge known as bed charge. 

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The coke for this charge is put in the furnace through the charging door. The kindling material is ignited through the hole. This fire spreads slowly into the coke around the kindling material. 
Additional coke is fired until the bed charge acquires the required height. Cover plated opposite the tuyeres are opened to allow the free entry of air to aid combustion and they are left open till the entire bed charge is fully ignited. A carefully weighted proportionate amount of metal, pig iron, scrap and flux is then fired over the bed charge followed by a weighed quantity of coke. 
They are repeated in alternate layers, of course a predetermined quantity of each, until the cupola is full to the charging door. If the cupola, on account of its fixed capacity, is unable to take up the entire material to be melted at a time, the remainder is fed into it after the initial charge has been melted.

ADVANTAGES OF USING CUPOLA

1. The initial cost is comparatively lower than other types of furnaces of same capacity.
2. Operation and maintenance of the furnace does not involve too many complications.
3. Cost of operation and maintenance are comparatively lower.
4. The floor area required is hardly a fraction of that required for other furnaces of similar capacity.
5. It can be operated for a number of hours at a stretch.
6. It does not involve very complicated problems in its design which is comparatively simpler.

PRECAUTIONS IN OPERATING THE CUPOLA

1. A superior refractory lining should be used to withstand high temperature produce inside the furnace.
2. The man who fires the coke and charge should place the metal charge in the center.
3. The molten metal should be tapped out well before its level rises too high in the well.
4. The tap hole should be properly closed by means of well prepared clay bott or plug.
5. In closing the tap hole care should be taken to press the plug downwards in the hole so that the splash of the molten metal does not fall on the hands.
6. The amount of air supply should be properly controlled. An excess amount of air will result in lowering of temperature inside. 

Planning Machine

 Planning Machine

It is used to perform almost the same operations as are done on a shaper but jobs of comparatively larger size are machined on the planner. 


It is commonly used for machining large flat surfaces or machining similar jobs more in number keeping the line. The surfaces to be machined may be horizontal, vertical or inclined. Several tools can be fitted on this machine to work separately or at a time to give faster output.

CONSTRUCTION

Bed: 

It is a very large and heavy cast iron structure which is provided with cross ribs for additional strength and stiffness. In case of large planers the bed is sometimes made in two parts which are properly machined and then bolted together to form a single length of bed. 
Leveling pads or jacks are provided under the bottom of the bed to level it during its installation. The bed itself accommodates the table driving mechanisms. V-ways and sometimes flat ways are provided on the top of bed to guide and support the reciprocating table of the planer. These bed ways are straight and parallel accurately machined and scrapped. The bed ways should be constantly lubricated.

Table: 

The table or platen is also made of cast iron with an accurately machined top. It carries a box type construction provided with strengthening ribs under it in order to make it strong enough to support the heavy work over it. At its top it has longitudinal T-slots and holes to accommodate the clamping bolt and other devices. 
Under the table chip pockets are cast integral with it for collecting and removing the chips. On its side the table carries adjustable stops to reverse its motion at the end of each stroke. A common safety device i.e. large cutting tool is bolted under the table on its both sides and stops blocks are fixed on the bed on each side. 
In case of running away of the table the tool will take a deep cut into the stop block and thus absorb the whole kinetic energy of the table, bringing it to a halt without damage to machine or workman.

Housing and columns: 

These are vertical members situated on both sides in case of double housing planer and on one side only in case of an open side planer. Inside them they carry the different mechanisms for transmission of power to the upper parts of the machine. The vertical guide ways along which the cross rail slides up and down are machined at the front of columns, where side tool heads are used, they also slide vertically along the same ways.

Cross rail: 

It is a horizontal member of heavy structure which connects the two vertical housings of the machine. It provides additional rigidity to the machine. By means of elevating screws it can be moved up and down along the guide ways. 
Clamps are provided to lock the cross rail in any desired position along the columns. In order that the cross rail is moved up or down uniformly on both ends, both the elevating screws are rotated simultaneously by horizontal shaft, mounted on the top of the machine. 
Accurately finished ways are provided at the front of the cross rails for the two vertical tool heads. Inside the rail the feed rods for vertical power feed and cross feed for the tools are provided.

Tool heads: 

The planner tool heads resemble with shape tool heads in construction as well as in operation. Four tool heads can be fitted in a planer and any or all of them can be used at a time. Two tool heads can be fitted in a vertical position on the cross rails and the other two can be fitted in horizontal position on the vertical columns. 
The tool head on the cross rail can travel horizontally along the cross rail. They can also be raised or lowered moving the cross rail up and down. The side tools can move up and down along the vertical column ways. The side tools can be fed horizontally into the job or at a desired inclination. 
Both hand and power feeds can be employed for all the tool heads. The tool head can swivel on either side from its normal position and the apron can also be set at an inclination whenever needed in the same way as in shaper head. The clapper block is also hinged in similar way as in shaper so as to avoid scratching during idle stroke.

Controls: 

Various controls for governing the action of different parts of planer are systematically provided in the easy reach of the operator. The controls may be provided on a suspended pedant easily approachable by the operator. A suspended pedant usually has the controls like start, stop, automatic cut, automatic return etc. The controls are devices for quick adjustments of vertical or horizontal tool feeds, control for rapid transverse of the tool heads individually or together. These controls sometimes increases the work efficiency.