There are commonly 5 types of pattern allowances in casting. They are;
- Shrinkage allowance
- Machining allowance
- Draft allowance or taper allowance
- Rapping or shaking allowance
- Distortion or camber allowance
But first of all, let me tell you few things about the pattern allowances.
- The size of the pattern should not be kept exactly as the size of the desired casting, because such exact sized pattern would produce undersized casting.
- So when the pattern is prepared, certain allowances are given on the pattern which is beneficial to get the required dimensions of the final casting.
Now let’s dive right into each of the pattern allowances.
#1) Shrinkage allowance
You already know that if a metal cools down, then it shrinks in size.
So when the molten metal is poured in the mould cavity, it occupies the entire space of the mould cavity. And when this molten metal solidifies, it will shrink.
So the size of final casting will become less than the size of the mould cavity.
In order to compensate for this shrinkage, the pattern and the mould cavity should be made larger than the required casting.
The amount of compensation which is given for the shrinkage is known as shrinkage allowance.
But here I want to tell you one thing that the amount of shrinkage varies from metal to metal.
It also depends on shape of the casting, thickness of casting, temperature of casting, mould strength, etc.
So if you want to give shrinkage allowance, then you can give the allowance as described in the table below. These are the allowances which are obtained from past experiences.
Shrinkage allowance for important casting metals.
|Type of metal||Amount of shrinkage (in %)|
|Grey cast iron||0.55 % – 1.00 %|
|White cast iron||2.10 %|
|Malleable cast iron||1.00 %|
|Manganese steel||2.60 %|
|Brass||1.30 % – 1.55 %|
|Bronze||1.05 % – 2.10 %|
|Aluminum alloys||1.30 % – 1.60 %|
#2) Machining allowance
If machining is required to be done on the casting, then the size of the casting should be kept more so that machining operations can easily be performed on the casting piece.
In other words, the parts of the pattern where machining is required to be done, those parts are made extra thick. And this type of pattern allowance is known as machining allowance.
There are few reasons for providing machining allowance on the pattern.
- The casting piece gets oxidized inside the mould cavity and this oxidation also takes place during its heat treatment. Because of this reason, the scale formation takes place on the surface of the casting, and hence for removing the scale, machining is needed to be done.
- The slag formation, roughness of the surface, dirt and other imperfections requires machining.
- Machining allowance is given so that the exact dimensions can be achieved on the casting by doing machining operations.
- Machining allowance is provided to get the smooth surface finish on the casting.
Now there are also a few other factors which tell us how much extra thickness should be provided to the pattern.
- Machining methods to be used (grinding, boring, turning, milling, etc). Grinding operation removes less metal as compared to that of turning or milling.
- Characteristics of metal (whether metal is ferrous or not, whether metal is hard or soft, etc).
- Method of casting used for the manufacturing of castings. For example, sand casting requires more machining, while die casting requires less machining.
- Shape and size of castings. For example, there is more warpage in longer castings. Hence more allowance is given for longer castings. Also the thicker sections take more time to solidify, which results in accommodation of more impurities. And these thick sections ultimately require more machining.
- Degree of finishing required. For example, if a casting requires very good surface finish, then it should be given more machining allowance.
The table mentioned below shows the standard machining allowance for different metals and alloys.
|Types of metals and alloys||Machining allowance|
|Cast irons||Large size castings (>1000 mm)||10.0 mm|
|Medium size castings (< 150 mm)||3.0 mm|
|Cast steels||Large size castings (>1000 mm)||12.0 mm|
|Medium size castings (< 150 mm)||4.3 mm|
|Non-ferrous materials||Large size castings (>1000 mm)||5.0 mm|
|Medium size castings (< 150 mm)||1.5 mm|
#3) Draft allowance or taper allowance
There are high chances that the edges of the mould gets damaged when a pattern is removed from the mould.
Draft allowance is a type of pattern allowance in which a taper is made on the vertical faces of the pattern.
By making the taper section on the vertical plane, it becomes easy to remove the pattern from the mould.
For some patterns, the draft is expressed in degrees, while in some patterns, draft is expressed in mm/meter.
The amount of draft required on the pattern depends upon the following factors.
- Shape of casting
- Depth of casting
- Moulding method
- Moulding material
The draft, which is given on the vertical side of the pattern, generally ranges from 5 to 30 mm per meter.
In precision casting, the draft of around 3 to 6 mm per meter is given.
#4) Rapping or shaking allowance
Before removing the pattern from the mould, it is slightly shaken so that it gets easily removed from the mould.
The size of the mould increases due to such small shaking of pattern.
In order to compensate for this increase in size, the patterns should be initially made smaller in size than the actual required size of castings.
Generally this Rapping or shaking allowance is ignored for the small size castings. But for the bigger castings, this allowance is required to be calculated.
#5) Distortion or camber allowance
Some castings tend to bend, twist or distort because of their shape, size or the type of metal.
This distortion also occurs due to different cooling speeds of the molten metal in the mould cavity.
Distortion or camber occurs due to uneven shrinkage of different parts of the casting.
By expecting the amount of warpage, the patterns are made by giving the allowance of such warpage. This allowance is also known as camber allowance.
Let me explain this to you with an example.
The image shows that the U shaped casting gives diverging legs when it cools down.
So in order to compensate for this, the pattern is made with the converged legs. So that finally when the casting is formed, the legs will be obtained straight.
The warpage required to be given on the pattern depends on the metal as well as thickness of the casting.