Machining Allowance In Casting
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| Machining Allowance |
Machining Allowance
What is the machining allowance in casting?
This allowance in casting is required to finish and remove rough texture on the casting surface after the casting is taken out of the mould.
Machining allowance is extra material added to the casting by making the pattern bigger so that the casting can be surface finished at the required size, shape, accuracy, tolerance and surface finish.
An extra dimension was added to the pattern to make the casting larger in size than the original casting. This extra metal on casting is used for finishing operations to achieve a smooth surface finish called machining allowance.
Machining or finishing allowance can also be defined as thickness of metal taken out of the casting surface during machining operation.
This allowance is also called the finishing and cleanup allowance and is measured in mm/surface.
Allowance becomes important while finishing ferrous metals that have scales on the skin, casting with surface casting defects, roughness, small pores and cracks.
The finishing allowance is one of the allowances among the 6 types of pattern allowance in casting.
Machining allowance in casting is required for the finishing operation such as grinding, turning, milling, honing and lapping of the job.
As shown below in diagram (a), the final casting has extra metal that will be finished later.
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| (a) Extra Metal Added To Casting Surface |
The final casting produced has less surface finish and is rough in texture.
It is more desirable to finish the surface of the casting through the finishing process.
Diagram (b) shows the finished casting and machining allowance added to the casting.
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| (b) Machining Allowance In Casting |
Finishing allowance depends on the material used and the type of surface finish to be achieved. Allowance provided on the casting is from 2mm-15mm.
Allowance should be given properly during the casting process to reduce the wastage of material.
Is machining allowance positive or negative allowance?
Machining allowance is a positive allowance as extra metal is added to the final sand casting.
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| (c) Finishing Surface of Casting Allowance |
This extra metal is then removed shown in the diagram above (c) using finishing operations such as grinding, lapping and honing giving a high surface finish to the surface of the casting in the manufacturing process.
What is the minimum allowance provided for the casting?
The minimum finishing allowance provided on the casting is 2.5mm to 4mm.
The objective of the engineers should be to keep the finishing allowance as least as possible. Allowance increases with the size of the casting.
What is the machining allowance provided for steel and cast iron?
The machining allowance for steel and cast-iron metals is shown below in the table and is as follows:
| Metal | Dimensions in mm | Allowance in mm |
|---|---|---|
| 1) Cast iron | Up to 300 | 3.00 |
| 301 to 500 | 5.00 | |
| 2) Steel | Up to 150 | 3.00 |
| 151 to 500 | 6.25 |
Generally machining or finishing allowance is not provided for casting produced from the die casting process because die casting has a higher surface finish than other casting processes such as sand casting. At most die casting can be given an allowance of up to 0.5mm.
What is the relation between machining allowance and aid handling?
After casting is produced casting needs to go through different handling and machining operations such as turning, drilling, milling, grinding and finishing operations.
These casting products need to be fixed on lathe machines, drilling machines, milling machines and grinding machines.
In order to fix these products in fixtures there is a need to provide aid for holding these parts. These extensions and the extra allowance added are called aid handling machining allowance.
Shown below are cylindrical and taper bars provided with extensions as they can properly fit on the lathe machine.
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| Aid Handling Machining Allowance |
Failing to provide proper aid on the component will cause products to slip. Apart from operations, it becomes necessary to provide sections for easy material handling large-size casting. These all allowances provided are positive machining allowances.
Machining Allowance Advantages
An important reason to provide an allowance in casting is as follows:
- Improves surface finish and quality by reducing casting surface roughness.
- Eliminates the surface defects such as pores and sand inclusions.
- Removes oxide scales, small cracks and sand layers from the surface.
- Removes machining errors and other imperfections during machining operations.
- Provides close tolerance to the casting sections.
- Improves dimensional accuracy of the casting sections achieving the exact shape of the casting.
- Provides excess strength to thinner sections to take clamping force by fixtures.
- Reduce distortion in the sections of the casting as extra metal is added to the thin sections making it thicker.
- Increases stiffness and strength of the thinner casting.
Factors For Selection Of Machining Allowance
What are the factors for the selection of machining allowance?
Machining allowance in casting depends upon the following factors:
- Type of material used for casting. (Allowance for ferrous metals is larger than for non-ferrous metals)
- Shape and size of the casting. (Allowance for large size casting is more as compared to small size casting)
- Type of casting process. (Allowance is larger for sand casting as compared to pressure die casting)
- Hand moulding vs machine moulding. (Allowance is larger for hand moulding as compared to machine moulding)
- Type of sand used in the sand casting process. (Facing sand gives a better surface finish to the casting by reducing metal penetration of molten metal in sand)
- Type of sand grains used in sand casting (Fine to medium size sand grains in sand mould promote better surface finish to the casting)
- Type of mould washes in die casting process. (Mould wash when applied to the die improves the surface finish of the final die casting)
- Casting defects such as metal penetration and sand holes.
- Characteristics and properties of the molten metal.
- Type of finishing operations and machine selected for machining operation.
- Sections of the casting are to be surface-finished.
- Design of the casting such as U-shape casting and circular shape casting will require more allowance.
- Warping tendencies of the casting parts.
- Position of the casting in the mould, the cope side of casting will require more allowance than the drag section of casting.
- For the desired finishing of the final sand casting, certain faces of casting might require more allowance compared to the other faces of casting.
- Dimensional accuracy and tolerance after machining the casting.
- Fixtures holding the final casting during the machining process.
- Deformation is caused during the heat treatment process.
- Distortion in casting and internal stress caused during the cooling process.
Machining Allowance Reduction
How can the machining allowance be reduced?
Allowance can be reduced in the casting process by taking the following steps:
- Using fine to medium-sized grain with an angular structure in the sand casting process promotes a better surface finish to the final sand casting.
- During mould making process making the sand mould is less compact.
- Using facing sand as a carbonaceous material to reduce metal penetration in the mould in a sand casting mould.
- Apply mould wash to the die before starting the die-casting process.
- Allowance can be reduced by doing finishing operations on automated machines such as CNC rather than manually operated machines.
- Better the finishing process less allowance is required.
- Reduce surface casting defects caused by sand in the mould such as sand fusion and dirt casting surface defects on the casting surface.
Excess & Low Machining Allowance Problem
The right amount of allowance is required in casting, excess or low allowance can cause the following problems.
Excess allowance can cause the following wastages:
- Wastage of molten metal, tools and casting material.
- Higher machining costs are required for machine-casting products.
- Waste of time required for machining operations.
- Overall, less productivity in the foundry.
- Excess allowance adds excess weight to the casting increasing material handling cost.
- The electrical energy consumed in the foundry for finishing operation.
- Increases unnecessary labour costs.
- Thermal deformation will take place in casting as excess metal removal during machining is time-consuming.
Less than the required allowance can cause the following problems:
- Casting surfaces will be not acutely finished.
- Casting surface defects such as scales on casting, dirt defects from mould on the casting surface, scab and drop will not be properly removed from the casting surface.
- Required tolerance will not be acutely achieved.
- The dimensional accuracy and quality of the casting surface will be low.