Forging vs Machining: Key Differences Explained

Forging and machining are metal fabrication processes that are performed to transform metals into desired shapes and sizes.

But what exactly is the difference between the two? And which process is suitable for your application?

The difference between forging and machining is that forging involves heating the metal and shaping it by applying mechanical force, which improves its material properties, whereas machining is the removal of material to achieve desired shape or size resulting in the final product.

Some products such as shafts and rods are better to be forged whereas products like couplings and turbine blades are easier to create by machining.

This article discusses the difference between forging and machining in terms of properties, cost, and applications.

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Forging vs Machining: How do they Differ?

Factors	Forging	Machining
Process	Generally performed at a temperature that is above room temperature	Generally performed at room temperature
Grain Flow	Streamlining of grain flow	Normal grain flow is disrupted
Material properties	Enhanced strength and reliability	No change, but is prone to failure along the machined area
Structural changes	It fills in the material voids by causing the material to flow.	No structural changes. Material defects are intact.
Material cost	Minimizes cost by reducing scrap.	The shape is made by removing the material. So the cost is high
Shape complexity	Suitable for producing simpler shapes	Can produce any complex shape, especially when used with CNC.

Forging is a manufacturing process in which the workpiece is heated above room temperature to enhance its malleability.

On the other hand, machining is generally performed at room temperature, eliminating the need for heating the workpiece.

Apart from that, there are various other differences that you need to know before you decide on either process for making your part.

Difference in Process: Forging vs Machining

What is Forging?

Forging is a metal-forming process where the workpiece is molded into the desired shape by applying pressure.

It is broadly categorized into two methods: hot forging and cold forging.

Hot forging involves heating the material to about 75% of its melting point, or above its recrystallization temperature.

Heating the workpiece to this temperature enhances its malleability without melting the workpiece or causing any deformation due to heat.

Heating the metal beyond its recrystallization temperature can lead to a molten or semi-molten state, which compromises the structural integrity of the metal, making it difficult to shape.

Cold forging is another technique where the metal is not heated up to its recrystallization temperature and can be worked just above room temperature.

However, forging a metal without heating requires comparatively greater force and develops unwanted residual stresses in the metal workpiece.

Apart from that, cold forging prevents any microstructural changes in the material, thereby retaining its internal stresses.

Therefore, it is suitable for applications where a high surface finish is preferred over high ductility and machinability.

What is Machining? 

Machining is the process of removing unwanted material from the workpiece with the help of suitable tools, achieving the desired shape and design.

Similar to forging, machining also helps change the cross-sectional shape of a billet, but unlike forging, machining involves the removal of material.

As a result, machining can be used to produce comparatively more complex and intricate shapes, which otherwise cannot be achieved by forging.

CNC machining is best suited to serve this purpose. Almost all machining operations can be automated using computer controls.

While forging uses a die to make complex parts, machining employs various machining operations, such as drilling, turning, milling, boring, grinding, etc., to get the desired shape and surface finish.

Differences in Application: Forging vs Machining

Some products such as shafts and rods are better to be forged whereas couplings and turbine blades are better machined.

Part	Process	Reason
Connecting rod	Forging	High load capacity
Crank pin	Forging	Ease of making with hand-held tools
Shaft	Forging	High load capacity
Piston	Casting followed by machining	Intricate design and high tolerance
Wheel	Casting and forging	Easy to make on a large scale and simpler design
Pipe joints	Casting followed by machining	Requires a casting die for mass production possible
Aircraft structural panel	Forging	Need to support the frame against wind forces
Medical prosthetics	Forging	Need fine material quality. No structural defects can be afforded.
Bolts used in agricultural equipment	Forging	Need to withstand loading forces that arise from soil digging. It is also required to fasten heavy harvesting equipment to a tractor.
Couplings, threads, joints	Machining	Complex geometry.
Turbine blades	Machining	A smooth surface is required.

Forging and machining have different applications in the manufacturing industry.

While machining is ideal for applications that require a high surface finish with tight tolerance and good accuracy, forging is suitable for applications that require strength over the surface finish.

For example, hand tools such as spanners, are generally made by forging.

This is because removing the material from a metal block to shape the workpiece will render the hand tool weak and prone to failure under load.

On the other hand, forging compresses the material, increasing its density, and making it suitable for high-load applications.

Difference in Machines and Tools Required

Forging involves hammering a workpiece to mold it, while machining involves using sharp tools to remove the material from a workpiece.

Therefore, there are various differences in the type of tools used for forging and machining.

Tools Required for Forging

The simplest form of forging requires only an anvil to place the metal piece, a hammer to shape, and a tong to hold the metal workpiece.

Apart from this essential equipment, forging also uses fuller and flatter.

A Fuller is a curved or shaped tool used to create an indentation. This process is also called forming operation in forging.

Applying force on the hot metal results in the flowing of metal in a perpendicular direction along the fuller, resulting in the formation of the desired shape.

Another forging tool, called a flatter is also used to flatten the surface of the workpiece.

Although these simple tools can get you started with your forging projects, complex and large-scale forging requires additional tools such as a power hammer and dies.

Tools Required for Machining

Machining operations can cut through the metals to achieve the desired shape.

A lathe is a universal machine and one of the oldest ones. It is commonly used for asymmetric machining operations, like turning, boring, grinding, threading, etc.

Whereas, a milling machine is suitable for symmetric and asymmetric operations like milling, drilling, contouring, engraving, peripheral milling, thread milling, tapping, etc.

These machines are generally available in the form of CNC mills and CNC routers.

While CNC routers are suitable for DIY applications due to their low cost, CNC mills are suitable for machining tough metals.

Differences in Ease of Use

Traditional forging tools are accessible for DIY jobs. However, industrial-scale drop forging machines can weigh around 50 tons, which is not suitable for small-scale and DIY jobs.

However, pneumatic forging machines are comparatively smaller and weigh around 200 Kg, with a capacity to deliver a load of 16 Kg, making them ideal for the small-scale production of light material parts.

Although hydraulic presses for small-scale industries cost around $1500, they are suitable only for cold forging.

Therefore, forging is not ideal for DIY applications, except for the forging of very small workpieces.

On the other hand, machining is accessible to DIY jobs and small-scale industries. A good lathe costs around $300-$3000.

Apart from that, entry-level CNC machines such as CNC routers and CNC mills can cost around $500 - $3000, making them best suited for DIY machining applications.

Differences Based on The Grain Flow

Grain flow determines the internal structure of the metal workpiece, which affects its mechanical properties.

Forging involves hammering and shaping the workpiece which changes the grain flow without breaking it.

This ensures uniform mechanical properties throughout the workpiece and provides high strength.

On the other hand, machining involves removing material from the workpiece, thereby breaking the grain flow and rendering the workpiece weak along the machined area.

Therefore, a machined workpiece is comparatively more prone to failure under load than a forged workpiece of similar size and geometry.

Although forged parts provide superior build quality, it is important to perform the operation with utmost care to avoid the forging defects from creeping in and weakening the part.

Differences in Mechanical Properties Affected

Forging involves hammering the material and forcing the metal to flow, filling in all the voids and cracks present in the workpiece.

Apart from that, heating the workpiece removes any moisture content present in the material, thereby ensuring that no void is left in the material.

The absence of micro-cracks reduces the risk of failure due to creep and fatigue while enhancing strength and durability.

Furthermore, heating the workpiece above its recrystallization temperature and then allowing it to cool refines the grain structure and eliminates any residual internal stress.

This further enhances its mechanical properties and improves the ability of the material to withstand strong forces.

On the other hand, machining does not alter the internal structure of the workpiece, instead, affects the surface of the material by removing a layer of the material.

The application of strong cutting forces imparts internal stresses in the material which leads to the failure of the workpiece under load.

Differences in Material Consumption

Forging conserves the material, while machining results in the wastage of material.

To understand this, let us consider an example of transforming a billet of diameter D1 and length L into a workpiece with length L and diameter D2.

The machining process to produce this workpiece involves step-turning the workpiece and removing the material until the diameter of the block is reduced to D2.

This removal of material takes place in the form of chips which are difficult to reuse as they require an elaborate process of melting and casting into a metal block.

On the other hand, the forging operation involves passing the workpiece through a pair of rollers or using drop forging to alter its shape by molding it.

This reduces the diameter of the workpiece while increasing its length. The excess length can be cut by using a cutting tool and reused without the need for an elaborate recycling process.

Difference in Cost between Forging and Machining

The cost of forging and machining depends on the task at hand. Small-scale forging processes using handheld equipment are less costly.

Generally, basic forming tools cost around $25, with an additional cost of around $100-$200 for dies, depending upon the shape and size of the die.

However, industrial forging tools cost much higher due to their heavy mass and rigid construction.

In large-scale industrial applications, the high initial cost of forging equipment is compensated by large batches of production.

On the other hand, machining tools such as lathe and CNC routers can start from around $300 for DIY applications and go as high as $15K depending on the size, capability, and configuration of the machine.

Apart from the initial cost, there are various factors such as the type of cutting tools, the speed of machining, the material wastage, etc., that determine the machining cost. 

A slow cutting speed leads to low productivity, and achieving high cutting speed requires powerful CNC machines with a constant flow of cutting fluid, thereby increasing the cost.

How Do Forging and Machining Compare to Casting?

Factor	Casting	Forging	Machining
Grain flow	No grain flow	Streamlined	Machining cuts the grain flow
Metal heating	Up to the melting point	Up to recrystallization temperature	No heating required
Complexity of parts	High	Less	Better than casting when used with CNC
Fatigue failure	Yes	No	Yes
Creep protection	Vulnerable to creep	Good protection	Vulnerable
Tensile strength	Not satisfactory	Good	Better than casting, lesser than forging
Surface finish	Not good	Better than casting	Better than forging
Cost	High	Less	Depends on the batch size

Casting is used to make heavy industrial equipment, such as engine cylinders, turbine blades, etc.

Forging and machining are subsequent steps aimed at producing the final shape with the necessary accuracy.

Unlike forging and machining, casting does not require high compressive or cutting forces to shape the material, instead, the molten metal is poured into the mold and allowed to take the desired shape.

This eliminates the risk of developing internal stress in the material. However, the melting of the workpiece disrupts the grain structure and enhances brittleness.

As a result, cast workpieces are not suitable for applications that involve high fatigue or cyclic load.

The surface finish of a cast workpiece depends upon the finishing of the mold. A sand mold will result in a rough grainy surface, while a steel mold will produce a smooth surface finish.

However, the surface finish produced by forging and machining is far superior to casting.

Furthermore, casting provides the ability to produce highly intricate patterns, which other cannot be produced by forging or machining.

Therefore, making casting is suitable for applications where producing intricate patterns is valued over the strength and rigidity of the part.

Final Thoughts 

Forging and machining are two entirely different fabrication processes.

While forging is suited for producing parts with extreme strength, machining is ideal for producing complex geometries.

Generally, drop forging is best suited for DIY applications where high strength is required, whereas CNC milling is recommended to achieve high accuracy and surface finish.

Furthermore, machining is comparatively more versatile as it consists of different operations such as turning, threading, knurling, etc., whereas forging is limited to simple shapes.

Frequently Asked Questions (FAQ)

Can steel be forged?

Yes, Steel can be forged. Low carbon steel can be cold forged, and high carbon steel is hot forged. Cold forging improves the strain hardenability of low-carbon steel whereas hot forging improves its impact strength and grain structure.

Can a machined part be forged?

Yes, a machined part can be forged to improve its mechanical properties. Through forging, the strength of the part can be enhanced but, in turn, compromises its tolerance and dimensional accuracy.

Is titanium suitable for forging?

Yes, titanium is best suited for forging as it is difficult to machine due to its material properties of high tensile strength and low thermal conductivity.