How to choose correct turning insert

There are many parameters to consider when choosing a turning insert. Carefully select insert geometry, insert grade, insert shape (nose angle), insert size, nose radius and entering (lead) angle, to achieve good chip control and machining performance.

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• Select insert geometry based on selected operation, for example finishing
• Select the largest possible nose angle on the insert for strength and economy
• Select the insert size depending on the depth of cut
• Select the largest possible nose radius for insert strength
• Select a smaller nose radius if there is a tendency for vibration

l = cutting edge length (insert size)

RE = nose radius

Nose angle

Turning insert geometry

Turning geometries can be divided into three basic styles that are optimized for finishing, medium and roughing operations. The diagram shows the working area for each geometry based on acceptable chip breaking in relation to feed and depth of cut.

Roughing

High depth of cut and feed rate combinations. Operations requiring the highest edge security.

Medium

Medium operations to light roughing. Wide range of depth of cut and feed rate combinations.

Finishing

Operations at light depths of cut and low feed rates. Operations requiring low cutting forces.

ap

inchmm Feed fn mm inch

The above example illustrates the offer for steel—there are other options available for all material groups.

Turning wiper geometry

Use wiper inserts for improved surface finish with standard cutting data, or, maintained surface finish at substantially higher feed rate.

The -WMX wiper geometry is First Choice, and is a good starting point for most applications. When conditions change, there is always a productive alternative.

Choose a positive wiper geometry to lower forces and maintain productivity in case of vibration problems.

Choose wiper geometry as follows:

-WL: For improved chip control when moving to a lower fn/ap.

-WF: Improves chip control at a lower fn/ap. Also for lower cutting forces when vibrations occur.

-WMX: Always First Choice in the wide chip application area. Provides maximum productivity, versatility and the best results.

-WR: When a stronger edge line is needed—for example, for interrupted cuts.

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Turning insert grade

The insert grade is primarily selected according to:

• Component material (ISO P, M, K, N, S, H)
• Type of method (finishing, medium, roughing)
• Machining conditions (good, average, difficult)

The insert geometry and insert grade complement each other. For example, the toughness of a grade can compensate for lack of strength in an insert geometry.

Turning insert shape

The insert shape should be selected relative to the entering angle accessibility required for the tool. The largest possible nose angle should be selected to provide insert strength and reliability. However, this has to be balanced against the variation of cuts that need to be performed.

A large nose angle is strong, but requires more machine power and has a higher tendency for vibration.

A small nose angle is weaker and has a small cutting edge engagement, both of which can make it more sensitive to the effects of heat.

Cutting edge strength (Large nose angle)

• Stronger cutting edge
• Higher feed rates
• Increased cutting force
• Increased vibration

Less vibration tendency (Small nose angle)

• Increased accessibility
• Decreased vibration
• Decreased cutting force
• Weaker cutting edge

Turning insert size

Select insert size depending on the application demands and the space for the cutting tool in the application.

With a larger insert size, the stability is better. For heavy machining, the insert size is normally above IC 25 mm (1 inch).

When finishing, in many cases the size can be reduced.

How to choose insert size

1. Determine the largest depth of cut, ap
2. Determine the necessary cutting length, LE, while also considering the entering (lead) angle of the tool holder, the depth of cut, ap and the machine specification
3. Based on the necessary LE and ap, the correct cutting edge length, L and IC for the insert can be selected

Turning insert nose radius

The nose radius, RE, is a key factor in turning operations. Inserts are available in several sizes of nose radius. The selection depends on depth of cut and feed, and influences the surface finish, chip breaking and insert strength.

Small nose radiusLarge nose radius

• Ideal for small cutting depth
• Reduces vibration
• Weak cutting edge
• Generally better chip breaking

• High feed rates
• Large depths of cut
• Strong edge security
• Increased radial forces

Depth of cut and cutting forces

The relationship between nose radius and depth of cut affects vibration tendencies. The radial forces that push the insert away from the cutting surface become more axial as the depth of cut increases.

It is preferable to have more axial forces than radial. High radial forces can have a negative effect on the cutting action, which can lead to vibration and bad surface finish.

As a general rule of thumb, choose a nose radius that is equal to or smaller than the depth of cut.

Positive or negative turning insert style

A negative insert has an angle of 90° (0° clearance angle), while a positive insert has an angle of less than 90° (for example, 7° clearance angle). The illustration of the negative style insert shows how the insert is assembled and tilted in the holder. Some characteristics of the two insert types are listed below:

Positive turning insert

• Single sided
• Low cutting forces
• Side clearance
• First Choice for internal turning and for external turning of slender components

Clearance angle

Negative turning insert

• Double and/or single sided
• High edge strength
• Zero clearance
• First Choice for external turning
• Heavy cutting conditions

Clearance angle

Entering angle for turning

The entering angle, KAPR (or lead angle, PISR), is the angle between the cutting edge and the feed direction. It is important to choose the correct entering/lead angle for a successful turning operation. The entering/lead angle influences:

• Chip formation
• Direction of cutting forces
• Cutting edge length in cut

Large entering angle (small lead angle)

• Forces are directed toward the chuck. There is less tendency for vibration.
• Ability to turn shoulders
• Higher cutting forces, especially at the entrance and exit of the cut
• Tendency for notch wear in HRSA and case-hardened workpieces

Small entering angle (large lead angle)

• Increased radial forces directed into the workpiece will cause a tendency for vibration
• Reduced load on the cutting edge
• Produces a thinner chip = higher feed rate
• Reduces notch wear
• Cannot turn a 90° shoulder

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Choosing the appropriate turning insert requires a delicate balance of numerous key parameters. Good chip control and machining performance of these metal lathe tools are influenced by a variety of factors, including:

• Insert geometry
• Grade
• Shape
• Size
• Nose radius
• Entering angle

Each of these factors significantly influences the overall result. The geometry of the insert should be matched to the type of operation—whether it’s finishing, medium, or roughing—which directly affects how well the insert performs.

Material properties also significantly influence the choice of turning inserts. Characteristics like hardness, toughness, and thermal conductivity must be considered to select an insert that complements the machined material. Insert size, too, is important and should be chosen based on the depth of cut and cutting length required. Comprehending these parameters enables you to make informed decisions that boost your machining efficiency.

Let's discuss the various Turning Insert Parameters and how to choose inserts based on these parameters.

Cutting Edge Length

Select the Cutting Edge Length based on the depth of cut.  Deeper cuts (and higher MRR's) require larger inserts.

Selecting the appropriate cutting edge length requires consideration of the tool holder’s entering (lead) angle and machine specifications. This guarantees the tool’s accuracy and efficiency all through the machining process.

Nose Radius

The nose radius of a turning insert plays a major role in reducing vibration and enhancing the workpiece’s surface finish. A larger nose angle provides increased strength, but it also requires more machine power. Additionally, it has a higher tendency for vibration. Conversely, a smaller nose angle is less strong but reduces vibration and cutting force.

Matching the nose radius to the depth of cut helps minimize vibration tendencies. A smaller radius, such as 0.4mm or 0.2mm, is recommended for finishing operations to avoid vibration and achieve a better surface finish. Adopting a smaller nose radius also contributes to better chip control and less vibration.

Nose Angle

Nose Angle is largely a function of the insert's shape.  Larger Nose Angles result in stronger inserts.  Select the largest possible Nose Angle to maximize strength.  You need to ensure that the Nose Angle will clear all the features of the part you are machining.

Turning Insert Nose Angles, courtesy of CADEM...

Depth of Cut

The depth of cut significantly influences tool life, cutting forces, and overall machining efficiency. Deeper cuts can reduce the number of passes required, but they also increase cutting forces, which can potentially reduce tool life. On the other hand, a shallower depth of cut tends to prolong tool life by reducing stress on the tool. In the world of machining, finding the right balance between these light depths and deeper cuts is crucial for optimal performance.

High feed rates and large depths of cut require a larger nose radius for strong edge security. Reducing the cutting depth while increasing the feed rate can enhance chip control and reduce vibration during machining.

Material Considerations

The material being machined plays a critical role in the selection of turning inserts. Each material, whether it’s steel, stainless steel, high-temp alloys, or cast iron, has unique properties that affect how it interacts with the cutting tool. The choice of turning inserts must balance these material properties with the features of the cutting tool to achieve optimal performance.

Different materials require specific types of inserts to handle their hardness, strength, and thermal properties. Understanding these requirements helps in selecting the right inserts that will enhance machining efficiency and extend tool life.

Steel and Stainless Steel

Machining steel and stainless steel requires careful selection of turning inserts. For steel, coated carbide inserts are often recommended due to their durability and ability to handle higher speeds. Ferritic stainless steels machine similarly to traditional steels, necessitating inserts with chipbreakers designed for general steel turning.

Austenitic stainless steels, however, require inserts with high positive rakes and aggressive chipbreaker designs to manage segmented chips and high cutting temperatures. For high-speed turning of stainless steels, a cobalt-enriched carbide grade is recommended. For medium-speed turning, an M-25 grade carbide insert offers a good balance of toughness and resistance to varying cutting forces.

High Temp Alloys and Hardened Materials

High-temp alloys and hardened materials demand turning inserts that can withstand high wear resistance and cutting forces. TiAlN-coated carbide grades like KC510M are suitable for machining aluminum and high-temp alloys due to their high wear resistance. Advanced coatings like AlTiN PVD provide additional wear resistance and longer tool life.

Inserts with tough carbide grades and sharper geometries result in lower cutting forces and increased reliability of the cutting edge. For machining high-temp alloys, an ultrafine-grain carbide substrate is ideal. Using higher cutting speeds for harder materials and tougher inserts can reduce cycle times and improve efficiency.

Cast Iron

When machining cast iron, selecting the appropriate insert grade is crucial for optimal performance. Ceramic grades like K060 are suitable for finishing soft cast irons and steels up to 35 HRC. The choice of grade must balance the need for durability with the ability to maintain a sharp cutting edge.

Understanding the specific requirements of cast iron machining helps in selecting inserts that can handle the high compressive forces involved, ensuring efficient and precise machining.

Performance Enhancements

The right turning inserts can significantly enhance overall machining performance by reducing wear and increasing efficiency. CNC lathes, for instance, can automatically adjust the rpm as the cutting tool traverses different diameters on the workpiece, optimizing efficiency. This adaptability is crucial for maintaining productivity in various machining conditions.

Performance enhancements also entail choosing suitable inserts to enhance chip control, decrease cutting forces, and prolong tool life. Each of these factors contributes to a smoother, more efficient machining process.

Improved Chip Control

Achieving good chip control is essential for maintaining a high-quality surface finish and preventing damage to the workpiece and tool. Selecting inserts with positive wiper geometry can help lower forces and maintain productivity in the presence of vibration problems. A cutting-edge angle close to 90° can also help reduce vibration, thereby improving chip control.

Using a finishing chip breaker with a small radius is crucial for achieving proper chip control and avoiding poor surface finishes. Good chip control results in smoother operations and extends the life of both the insert and the machine.

Lower Cutting Forces

Lower cutting forces are essential for reducing tool deflection and improving machining accuracy. Some ways to achieve lower cutting forces include:

• Selecting inserts with sharp cutting edges
• Choosing inserts with positive geometries
• Using positive turning inserts, which have a clearance angle of less than 90°

Implementing these strategies can help reduce cutting forces and improve machining accuracy.

Using wiper inserts can not only improve surface finish but also allow for higher feed rates, enhancing overall productivity. Such optimizations contribute to smoother and more efficient turning operations, reducing the wear and tear on the tools.

Enhanced Tool Life

Maximizing tool life is a primary goal in any machining operation. Selecting inserts with a tougher carbide grade and geometries that reduce premature chipping or breaking is key to achieving this. Proper selection of cutting conditions, such as minimized depth of cut and appropriate feed rate, can significantly extend tool life.

Utilizing coolant effectively can also play a critical role in extending tool life and improving surface finish. By maintaining optimal cutting conditions and regularly monitoring tool performance, machinists can ensure that their tools last longer and perform better.

Specialized Applications

Various machining tasks necessitate specialized turning inserts to tackle particular challenges. For interrupted cuts, inserts need to possess increased toughness and wear resistance to endure frequent alterations in cutting forces. Specialized inserts with reinforced edges and unique geometries are often designed to handle these demanding conditions more effectively.

Finishing and roughing operations also have unique requirements. Finishing operations profit from inserts with smaller nose radii and sharp cutting edges for an enhanced surface finish. In contrast, roughing operations require inserts with longer cutting edges and larger nose radii to handle deeper depths of cut and higher feed rates.

Interrupted Cuts

Interrupted cuts, such as turning over bolt circles, holes, slots, or keyways, introduce a risk of premature or inconsistent breakdown of the cutting edge. To mitigate this risk, the following types of inserts are recommended:

1. Round inserts: These are the strongest option and provide excellent stability.
2. Square inserts: These offer good stability and are a reliable choice.
3. C type inserts: These have strong cutting edges and can withstand high cutting forces, making them suitable for tough materials or interrupted cutting conditions.

Using these types of inserts can help ensure long tool life and consistent performance in challenging cutting situations.

Selecting the right insert for interrupted cuts ensures that the tool can handle the repeated impacts without degrading quickly, thereby maintaining part quality and operational efficiency.

Finishing Operations

Finishing operations require precision and a smooth surface finish, which can be achieved by using inserts with smaller nose radii and sharp cutting edges. Wiper inserts are especially useful in achieving improved surface finish with standard cutting data. They can also help maintain surface finish at much higher feed rates. V type inserts, with their 35° cutting edge, provide smooth cutting action, reducing vibration and chatter, resulting in improved surface finish and dimensional accuracy.

Common finishing inserts include:

• CCMT
• DCMT
• VCMT
• VBMT

Each shape is designed to meet the specific needs of finishing operations. Positive inserts are preferred for finishing as they create less cutting force and allow for lower depths of cut.

Roughing Operations

Roughing operations involve the removal of large amounts of material, requiring inserts that can handle deeper depths of cut and higher feed rates. Longer cutting edges are ideal for roughing as they allow for deeper cuts, reducing the number of passes required. Larger nose radii provide increased strength and enable higher feed rates, making them preferable for roughing operations.

Negative inserts, such as SNMG with a 45-degree approach angle, are often used for roughing due to their ability to handle the rigors of heavy cutting. These inserts offer the durability and cutting edge strength needed for efficient roughing operations.

Practical Tips for Correct Turning

Securing optimal turning performance extends beyond merely choosing the right insert. Practical tips for achieving this include:

• Choosing the proper insert size and geometry
• Modifying machining parameters
• Monitoring tool wear
• Ensuring effective chip evacuation to avert damage to both the workpiece and the insert.

These tips help machinists achieve better results and extend the life of their tools, ultimately leading to more efficient and cost-effective operations.

Correct Turning Insert

Selecting the suitable turning insert size and geometry, considering application needs and available space, is crucial. The insert size should be chosen carefully to match the operation’s requirements and the available space for the cutting tool. Insert geometry should also be selected based on the type of operation—larger point angles for roughing and smaller ones for finishing.

Selecting the correct turning insert ensures that the tool performs optimally, reducing wear and improving the overall quality of the machined parts.

Adjusting Parameters

Modifying machining parameters like the entering angle, feed rate, and cutting speed is vital to attain peak performance. The entering angle between the cutting edge and the feed direction should be carefully selected to influence chip formation and cutting forces. A smaller entering angle can reduce the load on the cutting edge and produce a thinner chip, allowing higher feed rates.

Feed rates should be adjusted to match the material hardness and insert type, optimizing cutting performance and tool life. By fine-tuning these parameters, machinists can enhance efficiency and ensure high-quality results.

Monitoring Wear

Frequent inspection of inserts for signs of wear, like chipping or deformation, is essential for upholding high machining quality. Monitoring surface finish quality can serve as an indicator of insert wear and performance. Signs of vibration can also indicate insert wear, prompting timely replacements to prevent poor machining quality.

Implementing a tool monitoring system to track insert wear patterns and predict tool life accurately helps in proactive maintenance, reducing downtime and improving overall efficiency.

A digital microscope can greatly enhance wear monitoring of your turning inserts.

Summary

Selecting the right turning inserts involves considering multiple factors, including insert shape, grade, cutting edge length, nose radius, and depth of cut. Understanding the material properties and machining conditions is crucial for optimizing performance and extending tool life. Enhanced chip control, lower cutting forces, and longer tool life can be achieved by making informed choices and adjusting machining parameters appropriately.

By following the practical tips and guidelines provided, machinists can ensure optimal turning performance, leading to more efficient and cost-effective operations. With the right knowledge and tools, achieving precision and efficiency in machining is well within reach.

Frequently Asked Questions

What factors should I consider when selecting a turning insert?

When selecting a turning insert, it's important to consider insert geometry, grade, shape, size, nose radius, and entering angle to achieve optimal performance and tool life.

How does the material being machined affect insert selection?

The properties of the material being machined, such as hardness and thermal conductivity, determine the appropriate grade and geometry of the turning insert to be used.  It's important to select inserts tailored to the specific material being worked on to optimize performance and tool life.

Why is chip control important in turning operations?

Chip control is important in turning operations because it prevents damage to the workpiece and tool, and improves surface finish and machining efficiency.

What are the benefits of using wiper inserts?

Wiper inserts improve surface finish and allow for higher feed rates.  This contributes to smoother and more efficient turning operations.  Ultimately, they offer enhanced performance and productivity.

How can I extend the life of my turning inserts?

To extend the life of your turning inserts, select inserts with tougher carbide grades, adjust cutting conditions (get proper feeds and speeds and even consider being more conservative with rpms and feed rates), use coolant effectively, and regularly monitor tool wear.  These strategies can significantly improve the longevity of your turning inserts.

For more information, please visit High Feed Milling Inserts.