Casting Questions and Answers – Difference between Pattern and Casting

Answer.2. The pattern is the replica of the casting object

Explanation:-

A pattern may be defined as a model or replica of the object to be cast. Except for various allowances and core prints, a pattern exactly resembles the casting to be made. When molten metal is poured into a cavity and allowed to solidify, the result is the cast object or casting. The process of making patterns is called pattern making, the person who makes the patterns is known as a pattern maker, and the shop where patterns are made is called pattern making shop.

Answer.3 Pattern Making

Explanation:-

The pattern is a direct duplication of the object, the pattern closely confirms to the shape and size of the desired casting except for a few variations due to the essential allowances provided in the pattern. The process of making patterns is known as pattern making.

Answer.4. All of the above

Explanation:-

The master pattern is the key from which a number of duplicate metal working patterns are cast. For general purposes, a master pattern is made of pine. Other materials from which master patterns can be made include plastics, silicone rubber, gypsum cement, aluminum alloy, white metal, brass, bronze, lead, gray iron, and steel.

Answer.2. Pattern

Explanation:-

A pattern may be used for forming an impression known as mold in damp sand or other suitable moulding material. This mould is a cavity which when filled with molten metal that is allowed to solidify forms a reproduction of the pattern called casting. Since it is a direct duplication, the pattern closely confirms the shape and size of the desired.

Answer.4. Pattern allowance

Explanation:-

The patterns are not made of the exact size as that of the desired casting because such a pattern would produce undersize casting. When the pattern is prepared, certain allowances are given on the sizes specified in the finished component drawing so that the casting produced from that pattern after it has been finished and machined will conform to the specified size. This extra size given on the pattern is called pattern allowance.

Answer.4. All of the above

Explanation:-

Reasons for providing pattern allowance: Pattern allowance is provided for the following reasons: Metals shrink in size when they solidify, Castings require machining and/or other surface finishing operations, Castings tend to warp or bend during solidification.

The various types of pattern allowances are:

1. Shrinkage allowance
2. Machining or finish allowance
3. Draft allowance
4. Distortion or camber allowance
5. Rapping allowance

Answer.3. Hole

Explanation;-

A core is a hole or cavity that is desired in the casting to help reduce the amount of material removed later or to establish a wall thickness. Cores are made either from clean sand mixed with binders such as resin and baked in an oven for hardening or from ceramic products when a more refined surface finish is required. When the pattern is made, a core print is created as a place for positioning the core in the mold. After the mold is made, the core is then placed in position in the mold at the core print.

Answer.3. Shrinkage allowance

Explanation

Shrinkage Allowance: When molten metal is poured into the mould it will solidify and cool. During cooling, the metal will shrink and reduce in size. To compensate for this, the pattern is made larger than the required casting. This extra size given on the pattern for metal shrinkage is called shrinkage allowance.

Answer.4. All of the above

Explanation

All metals shrink or contract volumetrically during cooling from pouring temperature to room temperature in different phases, i.e. shrinkage in the molten stage, shrinkage owing to solidification, and shrinkage in the solid-state. The shrinkage of different metals is different due to different characteristics of metals and castings. The shrinkage of metal also depends upon the following factors.

(i) Pouring temperature of the molten metal

(ii) Design and dimensions of castings

(iii) Mould material

(iv) Method of molding.

Answer.2. Machine finishing allowance

Explanation:-

All the castings are to be machined to finish the rough surfaces and to remove excess metal. For this purpose, the pattern is made larger than the required casting. This extra size added to the pattern for machining purposes is called machining or finish allowance. In general, a machining allowance of 3 mm is given for ferrous metals, iron, and steel. For non-ferrous metals like brass and aluminum, a machining allowance of 1.5 mm is given.

Answer.4. All of the above

Explanation:-

The machining allowance is provided due to the following factors:

1. For removing surface roughness and other imperfections on the surface finish on the castings.
2. For achieving the exact dimension of casting,
3. To remove the scale formed on the surface of the casting due to various treatments, etc.
4. Characteristics of the metal whether ferrous or non-ferrous.
5. Size and shape of the casting.
6. Method of molding employed.

Answer.2. Draft allowance

Explanation:-

When a pattern is withdrawn from a mold. There is always some possibility of damaging the edges of the mold. This likelihood is greatly reduced if vertical surfaces of a pattern are tapered slightly (2 to 3° to vertical surfaces) inwards. This slight inward taper on the vertical surfaces of a pattern is known as draft allowance. It is provided on both internal and external surfaces.

Answer.1. Distortion allowance

Explanation

Distortion or Camber Allowance:- Sometimes the castings because of their size, shape, and type of metal, tend to warp or distort during the cooling period. This is due to uneven shrinkage, uneven metal thickness, or to one surface being more exposed than another, casting it to cool more rapidly. To overcome this distortion the shape of the pattern is thus bent in the opposite direction. This feature is called distortion or camber allowance.

For example, a casting shaped like a letter U will be distorted with the legs diverging, instead of parallel. For compensating this condition, the pattern is made in such a way that the legs converge but, as the casting cools, the legs straighten and remain parallel. Distortion allowances range from the standard finish allowance up to 20 mm when large castings are considered.

Answer.4. All of the above

Explanation:-

Distortion or warpage of casting during cooling takes place due to:

(i) Uneven shrinkage of different parts of a casting

(ii) Poor design of casting

(ii) Irregular shape of casting

(iii) Unequal thickness of different arms

(iv) Different rates of cooling.

(v) One surface being more exposed than another or different rates of cooling.

Answer.4. The metal used in Casting

Explanation

Casting is one of the important manufacturing processes in which the starting material is heated sufficiently to transform it into a liquid state. Such molten metal is poured into the mold cavity and allowed to solidify. The object after solidification is taken out from the mold. To manufacture a wide variety of products of desired shape and size. The department of the workshop where castings are produced is known as a foundry. The requirements of producing a casting are:

(1) Preparation of a pattern (replica of the desired casting).

(2) Preparation of mold or cavity with the help of pattern.

(3) Melting the metal.

(4) Pouring the molten metal into the mold.

(5) Allowing the molten metal to solidify.

(6) Breaking the mold to retrieve the casting then its cleaning and inspection etc.

The quality of casting and the final product will be affected to a great extent by the planning of the pattern maker. The important considerations of a pattern maker to properly plan the pattern for the desired results are:

(i) Appearance and surface finish of the casting to be produced;

(ii) Number of castings to be produced;

(iii) Metal to be cast;

(iv) Method of withdrawal of pattern;

(v) Size and shape of the casting to be produced.

Note:- Metal used in casting is a concern till its melted and molted metals fills the cavity. This does not affect the final product physically, once the metal has solidified.

Answer.1. Number of casting produced

Explanation

There are several materials commonly used in the construction of patterns. Modern pattern making employs the use of materials that can be easily shaped and are durable. The type of pattern material chosen depends upon the following factors :

(a) The design of casting,

(b) The number of castings to be produced,

(c) The type of casting and molding process used in foundry

d) The degree of accuracy and surface finish required.

Answer.3. To make holes and cavities

Explanation

Casting is sometimes required to be made hollow internally or having few holes, cavities, projections, undercuts or pockets on its external surface. This is achieved by using cores during casting or by subsequent machining of the casting. Usually, the large hollows in the castings are achieved by cores. Contours of the core are exactly similar to the cavities, pockets or holes it is supposed to create in the casting.

Core: Core is a sand mass used to make a cavity or a hole in a casting. The shape of the core is similar to the required cavity in the casting made. The core is made using core sand in core boxes.

Answer.4. All of the above

Explanation

The selection of a pattern depends upon the number of castings required and their geometrical structure.

A solid pattern is a wooden pattern, also called one piece pattern because of its constructional feature. It is used when the shape of castings to be produced is not complex and also their numbers are small.

A split pattern is used for complex castings not obtainable from solid patterns. The pattern is split along the parting line into two parts, one for cope and another for drag.

A gated pattern is useful for small size castings to be produced in large numbers. It consists of multi cavity molds which are fed by a single runner.

Answer.1. Bent in the opposite direction

Distortion or Camber Allowance

Sometimes the castings because of their size, shape, and type of metal, tend to warp or distort during the cooling period. This is due to uneven shrinkage, uneven metal thickness, or to one surface being more exposed than another, casting it to cool more rapidly. To overcome this distortion the shape of the pattern is thus bent in the opposite direction. This feature is called distortion or camber allowance.