Casting vs Machining: Choosing the Best Manufacturing Process

We hope you have an idea of the manufacturing processes used to make different components for the respective applications. These manufacturing processes include welding, powder metallurgy, forming, injection moulding, etc. Casting and machining are manufacturing processes, but used both serve different purposes in the field. This article will help you understand these processes and then how to choose one of them according to your requirements. So, let's begin with the introduction of Casting:

What Is Casting?

You see different complex-shaped parts like turbine blades, engine blocks, etc. These complex shapes are transformed into real-life applications using the casting process. In casting, the molten metal is poured into the mould of the desired shape, then solidification occurs, and the molten metal is converted into the desired shape.

Overview of Casting Process

The casting process is a manufacturing process which is mainly used for metals. This process consists of two steps, i.e. Melting and Solidification. In melting, we convert the solid metal into a liquid form, then it is poured into the mould and then air cooling for solidification. It has many types, for instance,

Sand Casting

In this type, the mould is made of sand. Knives, etc., are made using sand casting. Sand particles are strongly adhered to each other due to the binder in the sand. Molasses is commonly used as a binder in sand casting. It is used for large parts and is cost-effective.

Die Casting

In die casting, a mould made of steel is used for casting metals. This type is common for Al and Al alloys. Gears are manufactured using this casting type.

Investment Casting

In this type of casting, the moulds are made of wax. It is used for critical applications such as turbine blades of a jet. It provides a surface finish of (Ra 1.6–3.2 µm), which is considered smooth as compared to other types.

Key Benefits of Casting

The following are some key benefits of casting:

Better for Large Volumes

Since the moulds can be used to produce more than one unit, which reduces the price per unit, it is best for large-volume production.

Ability to Create Complex Geometries

Once a mould of the desired shape is made, it can be used for multiple parts. Easily complex shapes are produced through this process, i.e. pump impellers.

What Is Machining?

Machining is another common method of manufacturing components with high precision. It uses cutting operations mainly to transform raw material into the desired shape.

Overview of CNC Machining

The machining process produces different parts by removing the excessive material through cutting. There are many cutting tools available, like hacksaw, laser-cutting, CNC, etc. For high precision parts production, CNC machines are often used.

Milling

Multiple rotating cutting tools are used to remove the excessive material from the workpiece and to create the product. The milling process is mainly used to create difficult-to-design shapes.

Turning

The turning process is mainly used for cylindrical parts. The workpiece rotates, but the cutting tools remain stationary. It's one of the most common products is a shaft.

Grinding

A cutting method which produces a surface finish of Ra 0.1–0.8 µm on the bearings or moulds. It is used for surface-level cutting. Its most common use is in metallography.

Key Benefits of Machining

This section gives an insight into the key benefits of machining:

High Precision and Accuracy

The machining process,especially CNC precision machining, can achieve a tolerance of ±0.005 mm, which makes it a highly precise manufacturing process and is used for aerospace components.

Versatility in Material Options

The machining processes can be used for multiple materials such as wood, plastic and metals, which is why it is a versatile manufacturing process. For some machining projects, like metals, would divided into 2 types: hard machining and soft machining.

Types of CNC Machines Used in Machining

As we discussed earlier, CNC machines are often used for high-precision purposes. This section will make you familiar with different types of CNC machines.

Horizontal vs. Vertical CNC Machines

This table provides brief information for horizontal and vertical CNC machines.

Multi-Axis CNC Machines

The following table shows key differences between the two multi-axis CNC machines:

Casting vs. Machining: Key Differences

The main difference between casting and machining is the way of transforming raw material into the desired shape. For instance, in casting, solidification and melting are used for production. While in machining, the extra material is removed from the specimen using cutting tools. This section explores further differences with a brief explanation of these techniques:

1. Material Efficiency and Waste

The manufacturing techniques are categorised into two basic types, i.e. Formative and Subtractive. This part of the section presents information regarding formative and subtractive manufacturing:

Formative vs. Subtractive Manufacturing

Formative Manufacturing

• First raw material is melted, then poured into a mould and solidified.
• Material waste is very low because of melting. Mould flash can be remelted, so, low material waste.

Subtractive manufacturing

• Different cutting operations like grinding, milling, etc., are performed to transform a solid block into the product.
• Material waste is high because of chip formation. Higher material waste can be managed using nesting optimisation.

2. Tolerances and Precision

Tolerance and precision of the product greatly affect the life span and accuracy of that product. High tolerance and precision ensure successful usage of the product for the subjective applications. For higher tolerance and precision, the machining is highly recommended.

CNC Machining vs Casting: Tolerance Range Chart

This table shows the difference in tolerance and surface finish of machining and casting:

CNC machines provide 50x-100x more tolerance, which is why critical parts are made using machining.

3. Complexity of Part Geometry

The complexity of the part geometry is the major reason for choosing between casting and machining.

Casting for Complex Shapes

For complex shapes, Casting (formative manufacturing) is used. This technique can easily produce hollow structures, organic contours and intricate internal channels. Casting is used to make engine blocks in the automotive industry.

Machining for Intricate Details and Tight Tolerances

Although casting excels in producing difficult and intricate shapes, machining provides unparalleled intricate details and tight tolerances. CNC machines are used for making components such as robotic joints, structural supports, etc., due to a high level of precision.

4. Surface Finish and Aesthetics

It means that casting can be best for producing difficult shapes, but machining is used for high tolerance, etc.

Post-Processing Techniques

For better surface finish, we use different post-processing techniques such as sandblasting, plating, anodising, etc., for both cast and machined parts. These processes make the part able to resist corrosion and wear.

Is Casting Cheaper Than Machining?

There are many factors which make any technique cheaper than others. These factors include the application of that product, unit/volume production and the level of precision. The technique which satisfies these factors is considered cheaper. Let's dive into these factors:

Manufacturing Cost Comparing

This table provides a cost comparison of different parameters of casting and machining:

According to this table, casting is way cheaper than machining for mass production. While machining is more cost-effective in prototyping and achieving high tolerances.

Precision CNC Machining for Hard Metal Custom Projects

To achieve high tolerance and precision, CNC machining outperforms casting in the case of multiple materials. Hard metal custom projects, such as turbine blades of Inconel 718, are machined using CNC machines. Another example is a Titanium (Ti-6Al-4v) impeller.

Choosing the Right Process Based on Production Volume

This section of the article presents comprehensive information regarding choosing between casting and machining techniques.

When to Choose Casting for High Volume Production

• Ideal for Mass Production Runs

Casting is an excellent, cost-effective technique for mass production. For instance, it can produce 1000+ identical parts, or a mould can be used for 50,000 car wheels/month.

• Cost Benefits in Large Quantities

If you produce 10,000 parts through casting, 60-80% cost of single unit drops. Secondly, material waste is lower.

When to Choose Machining for Small Batch or Custom Parts

• Low Setup Costs for Prototyping

For small production or prototyping, machining is best. For instance, if you make 10 prototype gears, it will cost you only 1500$.

• Flexibility for Custom or Low-Volume Production

There are many perks of machining, like you can use different materials within a batch, you can change the design of the parts, etc.

Material Considerations in Casting vs. Machining

Material consideration is an important criterion in choosing machining or casting. This section will help you choose a suitable machine according to the material.

Material Options for Casting

Material options for casting are limited due to melting points, reactivity and fluidity during pouring.

Metals and Alloys Commonly Used in Casting

Casting works best for materials like aluminium due to excellent fluidity, steel due to high strength, cast iron due to ductility and zinc due to a lower melting point.

Limitations in Material Choices for Casting

• Highly reactive materials are not often cast because they can form undesired compounds in molten form.
• High melting points, such as Tungsten material, are not cast.

Material Options for Machining

Machining is a versatile manufacturing technique which can work easily on the most difficult materials, such as Tungsten, plastics and even composites.

Precision Control Over Material Properties

Machining provides precise control over the material properties as the following table shows.

The major reason is that there are no microcracks or voids in machined parts as compared to cast parts.

Prototyping: Casting or Machining?

Which prototype is better: machined or cast? Let's figure out in this section:

Advantages of Casting in Prototyping

If you cast your prototype, these are the advantages:

1. Complex Geometry Testing
2. Material Property Verification
3. Cost-effective for large prototypes

Advantages of Machining in Prototyping

In case of machined parts, these are the advantages:

1. Higher speed and flexibility
2. Material versatility
3. Precision validation

Limitations of Casting and Machining

Every manufacturing technique comes with some limitations, no matter how much it is advanced.

Limitations of Casting

The two main limitations of the casting are briefly discussed here:

Limited Precision and Surface Finish

Through casting, we can achieve ±0.25mm tolerance and 3.2-25µm Ra surface finish. Due to this, we often perform post-processing operations after casting.

High Material Waste for Small Runs

In small runs, material waste is 20-30%, which is considered a limit to using casting for small runs.

Limitations of Machining

And in machining, there are two limits for not using the machining manufacturing technique:

High Tooling and Labour Costs for High Volumes

Tools often break during machining hard materials, and you need 5- 10x more labour for machining parts.

Material Waste in the Material Removal Process

50-60% material waste in the form of chips. Only 10% is recycled.

Conclusion

Casting and machining are both manufacturing techniques which are used according to the requirements. If you cost-effectively need mass production, you should go for casting. In case of casting, you can't choose any material with high reactivity or a melting point. If you need small-level production or high precision and accuracy, and tight tolerance, then machining is the right option for you. In machining techniques, you can face issues of high costs due to tool wear and more labour.

FAQs

Is die casting cheaper than CNC?

It depends on the volume of production. In case of more than 5000 parts, die casting is cheaper.

Casting vs machining strength, which is stronger?

Cast materials generally exhibit higher strengths than a machined part.