# Contraction rules



## Democritus (23 Jan 2021)

I have seen ‘contraction rules’ for sale on used tool sites. I imagine that they must have been used in metal work to measure/check changes in dimensions with fluctuations in temperature as the metal is worked. Can someone give me a simple explanation of how they were used.?


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## Droogs (23 Jan 2021)

Sorry, I thought it was a grammar question


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## Democritus (23 Jan 2021)

Droog
So you don’t know? ‘Don’t ‘ is a grammatical contraction, but I don’t think it is to those types of contraction that these particular rules apply.


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## toolsntat (23 Jan 2021)

When a pattern maker makes a mould for casting in metals, he makes it bigger by a specified amount, which is determined by the type of metal, to allow for it shrinking in size as it cools.
The appropriate contraction rule is used during the making.
Cheers Andy


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## Democritus (23 Jan 2021)

Cheers Andy,
So the contraction rule is used in the making of the mould, and then used to check that the finished item has contracted by the amount expected from the type of metal?
D


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## TFrench (23 Jan 2021)

Democritus said:


> Cheers Andy,
> So the contraction rule is used in the making of the mould, and then used to check that the finished item has contracted by the amount expected from the type of metal?
> D


You use the rule to make the pattern slightly larger than the dimensions you're aiming for, so when the metal cools and shrinks it comes out at the correct size.


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## marcros (23 Jan 2021)

dont mix the rules up, or your bookcase will be 10% too large, and when you check it on said ruler you wont be able to work out why it doesn't fit.


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## Democritus (23 Jan 2021)

Thank you, Marcros.


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## Democritus (23 Jan 2021)

TFrench said:


> You use the rule to make the pattern slightly larger than the dimensions you're aiming for, so when the metal cools and shrinks it comes out at the correct size.


Thanks TFrench. Are there reference tables showing the expansion/contraction of different metals at varying temperatures?
D


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## TFrench (23 Jan 2021)

Google thermal expansion coefficient, or casting contraction rates. Lots of stuff out there.


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## Jelly (24 Jan 2021)

Democritus said:


> Thanks TFrench. Are there reference tables showing the expansion/contraction of different metals at varying temperatures?
> D



There are tables which give good general indications, but my metallurgist friends are of the opinion that for large, complex geometry or near net shape castings it's best practice to ask the foundry who will cast the piece(s) to specify the allowances to be made in the patterns.

The standards for cast metals allow for a degree of variation in composition which can result in measurable shrinkage differences, but a given foundry will usually excercise tight control over the chemistry of their melt to achieve conditions which best suit their process (in terms of pour temperature and methods, moulding techniques, etc.) and will know what shrinkage they would expect for a given design.


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## Democritus (24 Jan 2021)

Thanks TFrench and Jelly.
Lots of good information. One further question; Do they still use contraction rulers or has technology rendered them obsolete?
D


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## xy mosian (24 Jan 2021)

I had a good friend who was a pattern maker. He had a rule with double contraction. 
He often had patterns to make with muliple indentical parts. His answer was to have these parts cast, that was the first contraction. Those parts would become parts of the second, final pattern. That was the second contraction. It took me some time to get my head around that.
Of less interest is the fact that he apprenticed in Halifax, making patterns for large woodworking machines. He recalled seeing old patterns from a time when some machines has been steam driven, belts of course.
xy.


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## Jelly (24 Jan 2021)

Democritus said:


> Thanks TFrench and Jelly.
> Lots of good information. One further question; Do they still use contraction rulers or has technology rendered them obsolete?
> D


I get the impression that it would depend what pattern shop you walked into.

The one I'm most familiar with, has casting engineers who will work out the shrinkage as part of the specification process when they get customer drawings, and will use that to produce a drawing or 3D model of the required pattern to send to the pattern shop.

But there are two or three pattern shops near me who remain very traditional in their approach, doing jobbing work direct from customer drawings or even physical parts, where a contraction rule would not be at all out of place.


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## Democritus (24 Jan 2021)

Thanks very much, xy and Jelly. Great information.
UKWorkshop has members who have a wealth of knowledge and experience; a tremendous resource for everyone to draw on.
Thanks once again to everyone .
D


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