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Learn The Milling Insert ISO Code

Bob Pybus

Technical Support Team Leader


If you've ever ventured into the world of milling, you've likely come across the term "ISO code" in reference to milling inserts. But what exactly does this code mean? What information does it convey? Understanding the milling insert ISO code is crucial for selecting the right tool for your milling operations and achieving optimal results.

Whether you're a seasoned machinist looking to expand your knowledge or a newcomer to the world of milling seeking clarity, this guide is here to demystify the milling insert ISO code. We'll explore how this code provides vital information about the insert's geometry, material, and cutting characteristics. By the end, you'll be equipped with the knowledge to decode and interpret these codes, empowering you to select the perfect milling inserts to optimise your machining processes.


1. Insert Shape

The first section of the milling insert ISO code is all about shape and style.

It begins with a letter that indicates the insert's shape, such as R for round, S for square, T for triangular, D for diamond-shaped, or C for rhombic.

This letter gives a quick visual cue about the overall form of the insert. By looking at the first letter of the milling insert ISO code, machinists can get an initial understanding of the insert's shape, which plays a significant role in determining its specific applications and cutting capabilities.

A. Diamond 85-88° (Rhombic)

C. Diamond 80° (Rhombic)

D. Diamond 55° (Rhombic)

H. Hexagon

L. Rectangle 90°

O. Octagon

R. Round

S. Square

T. Triangle

V. Diamond 35­° (Rhombic)

W. Trigon 80°


2. Relief Angle

Section 2 of the milling insert ISO code refers to the relief angle of the insert.

The relief angle for a milling insert is of paramount importance in achieving efficient and successful machining operations.

It plays a crucial role in chip formation, tool life, cutting forces, and surface finish. Understanding the influence of the relief angle and selecting the appropriate one can greatly enhance machining performance, productivity, and the quality of the finished product.

A. 3°

B. 5°

C. 7°

D. 15°

E. 20°

F. 25°

G. 30°

N. 0°

P. 11°


3. Tolerance

In section 3, the tolerance of the milling insert can be determined.

Tolerance refers to the allowable variation in dimensions or measurements of a manufactured part. In the context of milling inserts, the tolerance class specified in section 3 of the ISO code helps determine the level of accuracy and consistency of the insert's dimensions.

The tolerance of a milling insert is important for several reasons. Firstly, it ensures a proper fit and compatibility with the toolholder, promoting stable and secure clamping during machining. Secondly, precise tolerances contribute to dimensional accuracy, allowing for consistent and reliable machining results.

Additionally, tight tolerances enable interchangeability within a tooling system, minimising downtime. They also impact tool life and performance, as well as surface finish and accuracy.

d: Inscribed circle

t: Thickness

m: Refer to figure


Tolerance on C, E, H, M, O. P, R, S, T, W Insert Shape (Exceptional Case)

Tolerance on D Insert Shape (Exceptional Case)


4. Cross-Section Type

Section 4 of the ISO code refers to the cross-section type of the milling insert.

The cross-section type of a milling insert refers to the shape of its cutting edge when viewed from a perpendicular angle. It influences the insert's cutting action and performance.

Common cross-section types include square, round, triangle, rhombic, and pentagonal. Machinists should consider the cross-section type when selecting inserts to ensure optimal cutting capabilities and chip evacuation for their specific machining tasks and materials.

5. Cutting Edge Length / Diameter IC

Section 5 provides details about the size or cutting-edge length of the milling insert.

The cutting-edge length of a milling insert is an important factor that directly affects the insert's cutting performance and efficiency.

A longer cutting-edge length allows for a larger contact area between the insert and the workpiece, resulting in increased productivity and improved material removal rates. It enables the insert to engage with a greater surface area of the material, reducing the number of passes required to complete a machining operation.

Therefore, selecting the appropriate cutting-edge length is crucial in achieving optimal cutting performance, maximising productivity, and ensuring cost-effective milling operations.

Metric System

Inch System


• Use 1/32" unit for a insert having a smaller I.C under 1/4"

• Use 1/8" unit for a insert having a larget I.C over 1/4"



6. Thickness

Section 6 sheds light on the thickness of a milling insert.

The thickness of an insert is crucial for its strength and stability during the cutting process.A thicker insert excels under heavy loads, enhancing performance and minimizing the risk of cutting edge chipping.

Typically, double-sided (negative) inserts possess greater thickness compared to single-sided (positive) inserts.

Therefore, selecting the appropriate thickness is essential for achieving optimal cutting performance, productivity, and the desired quality of the machined components.


7. Nose Radius

Moving on to section 7, we encounter information about the insert’s nose radius.

The nose radius of a milling insert holds significant importance in achieving precise and efficient machining operations whilst being able to apply a radius to your cut. Smaller radius tend to be more suitable for finer cuts/finishing whilst larger radius are better for heavy metal removal due to the strength of the insert corner.

The radius can also impact the insert's cutting forces, chip control, tool life, and surface finish. Careful consideration of the appropriate nose radius based on the specific machining requirements and materials is crucial for achieving optimal performance, tool longevity, and surface finish in milling operations.


8. Edge Preparation

Section 8 of a milling insert ISO code typically provides information about the edge preparation of a milling insert.

The edge preparation of a milling insert refers to the intentional modification of the cutting edge before its use in machining operations. It involves applying specific treatments or coatings to enhance the insert's performance and durability.

By carefully selecting and applying the appropriate edge preparation techniques, machinists can achieve improved machining performance, productivity, and tool longevity, while also maintaining high-quality surface finish and dimensional accuracy.


9. Hand

The hand of a milling insert refers to the orientation or direction of the insert's cutting edge and its corresponding shape.

It determines whether the insert is designed for right-hand (clockwise) or left-hand (anti-clockwise) rotation during milling operations.

Using an insert with the correct hand orientation is crucial for achieving efficient and precise machining results.


10. Chip Breaker

Section 10 showcases the chip breaker of the insert.

The chip breaker of a milling insert refers to a specially designed geometry on the insert's face and cutting edge that helps control and shape the formation of chips during the machining process.

It plays a crucial role in chip control, reducing the risk of chip clogging, tool jamming, and built-up edge formation.

The presence of a well-designed chip breaker is crucial for ensuring smooth and reliable machining operations.















Understanding the milling insert ISO code is like deciphering a secret language that holds the key to successful milling operations and tool selection.

Each section of the code provides valuable insights into the insert's shape, dimensions, tolerance, and material grade.

By unravelling the meanings behind each section, machinists can confidently select the appropriate milling inserts, ensuring compatibility with their machining setup and achieving desired outcomes in terms of performance, precision, and tool longevity.

Armed with this knowledge, you're ready to decode the milling insert ISO code and unlock the potential of your milling endeavours.

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