What is Shrink Rule and Why Used in Pattern Making?

Hello, friends today we are going to discuss what is shrink rule as well as why it is used while making a pattern.

The shrink rule is a special type of rule which is used to ease pattern making.  

The shrink rule looks like a simple rule, just the graduation on it is bigger than a simple rule because shrinkage allowance is added to each division of the shrink rule.

It is used while making a pattern or core box.

So, before knowing the what is shrink rule first of all we have to know what is a pattern and what is shrinkage allowance so that we can better understand the uses of the shrink rule.

So without wasting time let's get started

What is Pattern?

To create a product in the casting process, a pattern is used to form a mold cavity in the sand.

Pattern materials can be made from wood, metal, plastic, compound, or wax.

What is Shrinkage Allowance

Shrinkage is defined as the reduction during the cooling or solidification process.

The magnitude of shrinkage varies from material to material, but every material has to shrink.

To avoid this, the pattern is made larger than the required size of the casting product.

Then the difference between the actual pattern size and the required casting size is known as the shrinkage allowance.

What is Shrink Rule?

This type of steel rule is like a standard rule.  The only difference is that its inch or mm markings are slightly larger than the standard rule.  

If the shrinkage allowance is 10 mm per meter of length, the 30 cm shrinkage scale will be 3 mm longer than the normal measuring scale.

Different shrink rules are used for different pattern materials.

It is also known as contraction scale or shrinkage scale or pattern makers scale.

This scale provides a ready-made method of assigning shrinkage allowances to a pattern.

This type of rule is mostly used by pattern makers. 
Shrink rule is mostly made of steel or wood.
shrink rule
Shrink Rule

There are following some materials shrinkage allowance given in the table in a per mm as well as per feet.

Materials Allowances (mm/100 mm) Allowances (inch/feet)
Cast Iron             1.0            1/8
Iron             2.1            1/4
Brass             1.6           3/16
Copper             1.6           3/16
Aluminium             1.6           3/16

Why Shrink Rule Used in Pattern Making?

When the molten metal is filled in the mold that the pattern maker makes, it is red hot.

 After filling the mold, when the metal cools, it shrinks a bit.  

In this way, when the pattern is made of some larger size, it will make the mold of the same size and after the metal part is formed, when it cools down and shrinks, it will become almost the right size.  

This type of rule is specified according to its length and shrink allowance.  

The Shrink Allowance is printed above this rule.

This scale eliminates the trouble of adding this allowance every time a dimension is set. 

Thus, a lot of time and labor is saved and chances of mistakes are eliminated.

So friends here you have to know what is shrink rule and why these rules are used in pattern making.

I hope you all enjoy this topic.

Thank You.

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