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Skeleton Pattern In Casting

Skeleton Pattern In Casting

Cope and drag pattern in the sand casting process produces casting of larger in size as compared to other patterns such as multi-piece pattern solid pattern and split pattern.

But what if we want to manufacture sand casting larger in size using the least pattern material?

This is where engineers have developed a new pattern called skeleton pattern to manufacture huge-size casting such as pump housing.

This is done by using just a "skeleton" made from wood strips instead of making a complete solid pattern for producing sand casting.

I have discussed the skeleton pattern in the casting process below with diagrams, definitions, steps for producing large-size patterns, advantages, disadvantages, MCQ and application of this pattern.

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What is the skeleton pattern in casting?

A skeleton pattern is a pattern used to make large-size casting in pit mould using a wooden frame (template) which outlines the shape of the final casting.

The purpose of a skeleton pattern is similar to the sweep pattern. Instead of making an entire pattern, pattern makers make segments/outlines of the pattern.

This saves pattern material for making a bigger-size casting as only the skeleton of the pattern is to be made and assembled.

Skeleton pattern are used for producing casting with high dimensional tolerance in low production volume.

A typical skeleton pattern in a foundry is shown in diagram (a) below. It is important to note that this pattern is not solid. This pattern is made from segments of wood pattern material assembled together to make a large-size pattern.

(a) Skeleton Pattern

This pattern is placed in the mould and sand is rammed properly as shown below in diagram (b). Care must be taken so that the pattern does not fracture during the sand ramming process. In order to do this structure of the pattern is designed in such a way that it can take heavy sand ramming force.

(b) Skeleton Pattern In Mould

The skeleton pattern is then withdrawn from the sand mould leaving the mould cavity behind in which molten metal will be poured as shown below in diagram (c) 

(c) Skeleton Pattern Mould Cavity

Once the mould cavity is read, molten metal is poured into the cavity as shown in diagram (d) below.

(d) Pouring Operation Skeleton Pattern

Once the molten metal in the mould is solidified final sand casting at a lower cost as shown in diagram (e).

(e) Skeleton Pattern Casting

The steps for producing casting using a skeleton pattern in sand casting are as follows:

• Assemble segments of wood to make the required shape of the pattern. Consider various types of pattern-making allowance in the sand casting process for this pattern.
• Place the pattern in the pit mould or the floor mould.
• Pour green sand into the pit mould or floor mould and do the sand ramming operation.
• Strike off excess sand with the strike-off bar.
• Withdraw the skeleton pattern from the mould, leaving the mould cavity behind.
• Pour molten metal into the large mould cavity.
• Allow the metal to solidify in the pit mould and lift the final large casting out.

What material is used for making skeleton patterns in casting?

Wood is a patterned material which is the most preferred material for making bigger-size casting such as pipes it is cost-effective and lightweight as compared to metal pattern material.

Skeleton Pattern In Casting Advantages

• Large-size axis-symmetrical casting such as pipes can be cast using this pattern as compared to other patterns such as sweep and cope & drag patterns. 
• The skeleton pattern is lightweight as compared to the match plate pattern etc.
• Waste of pattern material is less as only required segments of wood are assembled together to make a final pattern.
• Is a cost-effective pattern as it is an exact skeleton outline of the final sand casting.
• Pattern-making is a simple, easy-to-design and assembly process.
• Patterns can be assembled together and made as per choice having high design flexibility and tolerance.
• This pattern is the best in the green sand moulding casting process.
• Hollow casting products such as drainage pipes can be manufactured by making a wooden hollow structure and filling the clay sand between the cope and drag part of the pattern making a large core.

Skeleton Pattern Disadvantages

• Difficult to produce complex shapes which have sharp corners, edges and circular designs. These shapes are manufactured in the secondary manufacturing process and appropriate machining allowance should be given to the pattern.
• Skilled labour is required to join segments of patterns to form one single skeleton pattern.
• Not suitable for mass production because solidification time cannot be reduced as the casting is pit moulded or floor moulded.
• Not suitable for all types of pattern material. Wood is the most preferred pattern material considering the cost of other materials such as metal patterns is high.
• Careful handling is required to avoid the pattern being damaged as the pattern is not solid.
• Secondary manufacturing processes such as machining and finishing require heavy machining allowance to be added to the pattern material.
• Hard ramming of the sand can damage the pattern as the pattern design is not solid and this pattern is just an outer structure of the final casting to be produced.
• Hand moulding is necessary when using this pattern as compared to other patterns such as gated patterns where the process is automatic. 

Skeleton Pattern Application

Examples of casting produced from skeleton pattern are as follows:

• Turbine casting.
• Valve outer bodies.
• Steam valves for boilers.
• Big pump housing.
• Pipe L- bends and T-bends.
• Big industrial solid metal boxes.
• Hollow pipes for carrying industrial gas.
• Industrial oil pipes.
• Boiler outer casing.

Skeleton Pattern MCQ / Quiz

1) Which of the following pattern produces the largest casting in the sand casting process?

a) Boxed pattern.
b) Split pattern.
c) Skeleton pattern.
d) None of the above.

Answer - c) The Size of the mould cavity produced by this pattern is larger than any other casting mentioned above in terms of length, width and thickness.

 

2) Which of the following sand is used for producing casting using a skeleton pattern in the sand casting process.

a) Greensand.
b) Loam sand.
c) Clay sand.
d) All of the above sands.

Answer - d) This pattern is placed in a mould having a higher moisture percentage (green sand) with a mixture of clay sand and loam sand. 

 

3) Mass production of patterns in higher volumes is not possible using this pattern because of the following reasons.

a) The solidification/cooling cycle of the casting is longer as molten metal is at a higher quantity.
b) Heavy-weight casting is difficult to withdraw from the mould.
c) The pouring rate of the casting is slow to control turbulence in molten metal.
d) All of the above points are correct and related to this pattern and casting process.

Answer - d) At the start of the casting process molten metal is poured in the green sand mould cavity. The pouring rate is low avoiding molten metal turbulence considering the mould is made from green sand or skin-dried sand. The solidification rate of molten metal in mould and pattern withdrawal time are two other steps in the process which make the process slow reducing the output of the foundry for large-size casting produced by skeleton pattern.   

 

4) The machining allowance required for this pattern is higher because of the following reasons.

a) No control over the process parameters such as solidification time, pouring rate, dimensional tolerance of mould cavity etc
b) Pattern has a weak structure and can deform during the sand ramming process due to the force.
c) Ramming is done manually and ramming operation is dependent on the skills of the operator.
d) All the above points are the right reasons to give higher machining allowance to the pattern as compared to other types of patterns in the casting process.

Answer - d) Is a self-explanatory option/answer related to machining allowance and reasoning.

 

5) Which of the following pattern does not have a moving part (pattern is static while producing mould cavity) in the casting process?

a) Skeleton pattern.
b) Segmental pattern.
c) Sweep pattern.
d) None of the above.

Answer - a) This pattern is static and does not have a moving part as we see in the other pattern such as sweep and segmental where patterns revolved around its axis to carve the cavity in the sand mould.

Skeleton Pattern In Casting Summary

The skeleton pattern is one of the patterns among 17 types of pattern in casting.

A skeleton pattern in the foundry is used to produce an outline of the large final casting by assembling strips of wood reducing the cost and wastage of material for producing simple large casting.

In order to make casting such as large water pipes, the split skeleton pattern is designed into two halves (cope and drag) along a parting line and assembled together using screws and adhesives.

If the same pattern is to be produced with a complete pattern the cost of pattern production will be uneconomical that is why wooden strips are used and casting is manufactured in loam mould.

The skeleton pattern is similar to the boxed-up pattern with a major difference being that the box pattern can only make a box shape while the skeleton pattern can make a more complex shape than the box pattern.

A skeleton pattern also should not be confused with lagged up a pattern where outer structures are used for producing only objects such as columns and cylinders.