Understanding Machinability of Materials: Rating, Factors & Comparison

Machinability of the materials is an extrinsic property that varies from material to material. There are certain parameters that make machinability good or bad, for instance, the chemical composition. This guide explores some important information regarding machinability and its different aspects, such as how it is different from other terms like workability under the broader term of manufacturing.

What Does Machinability Mean?

Machinability is the behaviour of a material when it is brought under machining processes. If it is easy to cut and machine, its machinability is considered high. Aluminum is a common example. It is easy to cut, and all the machining processes work smoothly on it. Machinability means how easily we can machine the raw material into the desired shape.

Machinability vs Machining Properties

The difference in machinability and machining properties is described in the following table:

Is Machinability Important?

Yes, machinability is an important characteristic of materials because if a material is not machinable, it will damage the cutting tools, requiring high cutting forces and energies, so the cost of the final product will rise.

What Factors Can Affect Machinability of Materials?

Machinability of materials can be affected by many factors, for instance, chemical composition, mechanical properties, and microstructure, etc.

Chemical Composition

It is the fundamental factor that affects the machinability because every element in the chemical composition adds new properties. The classic example is steel (Fe-C). The percentage of C in Fe affects its machinability greatly. That is the reason cast iron (>2% C) is very difficult to machine compared to simple steel.

Mechanical Properties of Materials

It is an important concept with respect to machinability. The mechanical properties of materials, the high hardness of W and Ni metals, make them exhibit poor machinability. A material with good toughness and ductility is easy to machine; that's why Al is easy to machine.

Microstructure

The microstructure means how the grains are aligned. For instance, after the quenching process, grains are not arranged in any order, but pile up in martensitic microstructure, which results in poor machinability.

Materials Condition

The condition of the material means whether it is annealed or in another metallurgical state. For instance, if the material is in an annealed condition, it means its microstructure is aligned and has relatively coarse grains. It leads to easy machining. A material in the quench state will exhibit poor machining because of very fine grains.

Surface of Materials

The rough surface of the materials leads to poor machinability because of unstable tool engagement. A material with a hardened surface exhibits poor machinability because higher cutting forces are required.

How to Calculate Machinability of Materials

In fact, machinability is not an absolute value but a relative property. The machinability of materials is calculated through the following methods:

Select Reference Material

First, the reference material is selected. It is that material for which a machinability rating of 100% is assigned. AISI B1112 free-cutting steel is commonly used as a reference material.

Measure Cutting Speed

Then, the cutting speed is calculated from the spindle speed and the diameter of the tools. The maximum cutting speed of the test material is determined under the same processing conditions as the reference material.

Formula for Calculating Machinability

Formula:

Machinability Rating% =(cutting speed of test material / cutting speed of standard material )x 100

Machinability rating stands for how easily a material can be cut, drilled, or machined under given conditions.

What Materials Have Machinability?

The materials vary from each other based on their chemical composition, microstructure, and crystal structure. So, the machinability of different materials is different.

Machinability of Metals

Steel, aluminum, and copper are the common metals that are widely used around the world in different applications. But each metal of them exhibits a different level of machinability.

Machinability of Steel

There are thousands of grades of steel used in the world. Below are machinability rating of common grades of steel(Reference: AISI B1112 = 100%):

The machinability of each grade is different due to the different chemical composition. For instance, in AISI 1018, there is only 0.18% carbon, due to which it is a ductile and highly machinable grade. But, in AISI 1095, carbon is very high, which makes it a very hard metal to machine.

Aluminum and Alloys Machinability

Pure Al is easy to machine because it is soft compared to Al alloys because of their chemical composition. Machinability rate of common Al alloys (Reference: AISI B1112 = 100%):

6061 is the most widely used Al alloy due to its balanced strength and corrosion resistance. 7075 is an aerospace-grade alloy that exhibits very high strength. In Al alloys, only 1100 shows poor machinability due to its very soft nature.

Copper Machinability

Cu is used in various fields, especially in electronics due to its higher conductivity. Brass C36000 exhibits excellent machinability, but C95400 bronze is difficult to machine because of its high strength.

Machinability of Plastics

Mostly, plastics are soft, which they are hard to machine because they cannot withstand machining forces. But some engineered plastics are machinable.

PEEK Machinability

PEEK is a thermoplastic and engineered polymer. It provides high stiffness and dimensional stability, making it easy to machine because cutting forces do not deform the structural integrity.

Nylon Machinability

Polyamide (PA), also known as nylon, is a thermoplastic polymer and exhibits good mechanical properties. Compared to PEEK, it is difficult to machine because it is difficult to maintain dimensional stability because of its moisture-absorbing nature. It is very easy to cut.

PVC Machinability

Polyvinyol Chloride polymer shows good properties. It is very easy to machine and maintains better dimensional stability than nylon. Thermal degradation is a limit, though.

Polycarbonate Machinability

It is a heat-sensitive and ductile polymer and is easy to machine. But its ductile nature leads to smearing and chip control issues.

Polypropylene Machinability

Polypropylene is easy to cut, but its machinability is challenging because of thermal and mechanical sensitivity. It's such sensitivity leads to burrs and dimensional errors.

What Materials Don't Have Machinability?

Many materials offer poor machinability. These materials include brittle ceramics, glass, and gemstones. The brittle nature of these materials is the fundamental reason for their poor machining because they do not exhibit plastic deformation but fail catastrophically.

Machinability and Workability: What Differences?

Although machinability and workability both come under the broader term of manufacturing, fundamentally, they are different from each other.

Purpose

• Converting a raw material into the final desired product through the machining processes is called machinability.
• If a raw material is converted into the final product only through plastic deformation, it is called workability.

Processes

For machining, different CNC machining operations:

• Turning
• Milling
• Facing
• Drilling

They are used to achieve high precision in dimensions and geometries. For instance, CNC precision turning is used to machine shafts. For workability, the processes used to turn the raw material into its desired shape include:

• Sheet forming
• Forging
• Extrusion

Measurement

Machinability is measured using:

• Tool life
• Surface finish
• Cutting forces
• Chip formation
• Power consumption

Workability can be measured using:

• Tensile test
• Compression test
• Bend test
• Forming limit diagram

Material Factors

Different material factors, such as microstructure (fine grains result in uniform cutting), thermal conductivity affects heat accumulation, high hardness results in poor machinability, etc. The factors such as ductility, yield strength, strain hardening, etc., affect workability.

Differences between Machinability and Formability

In general, both machinability and formability are properties of a material. However, machinability is related to material removal, and formability relates to material deformation.

Definition: Machinability vs Formability

Machinability means the level of ease to machine a raw material from cutting to its final shape. Formability is the ability of a material to get into the final shape without failure or cracking, only through plastic deformation.

Primary Processes

Machinability is defined through different machining processes, which include CNC turning, CNC milling, CNC facing, CNC drilling, etc. Formability is defined by those processes in which force is used to deform the materials into the final shape. These processes are bending, stamping, and deep drawing.

Key Factors: Machinability vs Formability

These factors affect machinability:

• Microstructure of the material: distorted grain structure =>low machinability
• Chemical composition: alloying elements, a higher percentage of C results in poor machining
• Surface condition: corroded surface requires higher cutting forces
• Hardness is in inverse relation to machinability

Formability of a material is affected by:

• Thickness: the higher the thickness, the more difficult to deform
• Strain hardening results in higher forces
• The higher the yield strength, the higher the forces required to start plastic deformation

Typical Applications

In applications, such as:

• Threading and hole-making
• Aerospace components
• Finishing operations
• Automotive parts

To achieve higher precision, machinability is required.

Formability is required in:

• Forging
• Rolling
• Extrusion
• Wire drawing
• Sheet metal forming

CNC Machining: Good Machinability vs Poor Machinability

Let's understand good and poor machinability with respect to the CNC machining:

Cutting Force

For material with good machinability, like Al, lower cutting forces are used compared to a material with poor machinability, like Tungsten.

Tool Wear

Tool wear is high in the case of materials with poor machinability, like Ni-based alloys, because of their hardness.

Chip Formation

Chip morphology is different for materials with good and poor machinability. Segmented or discontinued chips are produced in the case of high machinability, and vice versa.

Surface Finish

Due to ease of machining, materials with good machinability produce a smooth surface finish easily compared to materials with low machinability.

Cutting Speed

The materials with good machinability allow higher cutting speed, while the materials that are difficult to machine can only allow lower cutting speed to maintain the life of cutting tools.

Machining Requirements

Standard CNC tools can be easily used for materials with good machinability, while materials with poor machinability require specialized tools for machining.

How to Improve Machinability?

Although machinability is a relative property of materials, it can still be improved from a machining perspective by using the following tips:

Choose Materials with Good Machinability

Materials are mostly selected based on the applications. For some applications, two different materials can be used. In such cases, materials with good machinability should be selected. For instance, for the mass production of fasteners, free-cutting steel can be used instead of low-carbon steel.

Adjust Cutting Parameters Properly

Cutting parameters change with respect to the machinability of materials. For instance, higher cutting speed, lower feed rate, etc., are preferred for difficult-to-machine materials because of some specific requirements.

Select Appropriate Cutting Tools

Appropriate tools should be used based on the machinability of materials. For instance, for hard or low-machinability materials, special tools, such as diamond-coated tools, are used.

Improve Chip Control

Machinability can be improved by improving the chip control, such as by forcing the chip to fracture or curl tightly instead of flowing plastically.

Modify Material Composition

The machinability can also be improved by alloying. For instance, Sulfur is added to steel to improve its machinability. S reacts with Mn and forms MnS precipitates, which break the chips efficiently and enhance machinability.

Apply Heat Treatment Before Machining

The microstructure of the material changes with heat treatment. The classic example of this scenario is the formation of martensite when steel is quenched in water from the austenitic state. Then, to make it machinable, tempering heat treatment is performed.

Can Part Design Affect Material Machinability?

The design of a part has a great influence on its machinability. In simple words, with difficult designs, a greater amount of machining is required.

Influence of Part Design on Material Machinability

The design of a part affects its machinability through these things:

Geometries of Parts

Geometry controls the machinability of a part through thickness, feature accessibility, surface continuity, corners, and internal radii, etc. The common example is uniform thickness, which results in stable cutting, so better machinability.

Tolerance Requirement

Tighter tolerances in the design of a part require higher machining, so the machinability of that part is reduced.

Surface Quality Requirement

In the case of the higher surface finish, higher machining is required.

How to Improve Machinability by Optimizing Part's Design

By optimizing the design, the machinability of the part can be improved:

Use Appropriate Wall Thickness

Do not add unnecessary, very thin walls in the structure because they result in increased machining and reduced dimensional accuracy.

Increase Corner Radii

Sharp corners in the designs are stress concentrators, so they should be avoided. Minimal tool wear can be achieved by reducing the sharp corner radii.

Reduce Deep Pockets and Cavities

Deep pockets and cavities are sometimes made within the parts to reduce their weight. But unnecessary pockets and cavities make machining of that part difficult.

Conclusion

Machinability of the materials is a relative property which vary from material to material. Some important factors that affect it include chemical composition, microstructure, surface finish, heat treatment, and design of the part. To measure machinability, the cutting speed of the subjective material is compared with that of a reference material. Workability, formability, and machinability are relevant terms in manufacturing, but the process to convert the raw material into the final product is different. Through the optimization of heat treatment and design, machinability can be improved.

FAQ

Is manufacturing and machining the same thing?

Manufacturing is a broader term that includes every process that turns raw material into its final shape. Machining converts the raw material into the desired shape through cutting processes specifically.

What materials are good for machinability?

These materials include Al, Cu, low-carbon steel, stainless steel, etc., because they offer appropriate strength and toughness, and exhibit good machinability.

What is the hardest material to machine?

Ceramics, such as Alumina, are the hardest to machine materials because they are hard and brittle.