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Positive Angle Inserts vs Negative Angle Inserts
When you put CNC turning and milling under observation, you’ll see how cutting insert geometry plays a critical role in the success of any machining operation. One of the most fundamental geometric properties is the insert angles, positive and negative angle inserts.
The choice you make between a positive and a negative insert will have a direct impact on cutting forces, surface finish, tool life, and overall process stability.
In this article, we provide you with a detailed comparison between positive and negative angle inserts. Read on as we explore their unique characteristics, advantages, disadvantages, and typical use applications. By the end of this piece, you will have a clear understanding of how to choose the correct insert geometry for your machining operations.
What Are Positive Angle Inserts?
Sometimes referred to as positive rake inserts, positive angle inserts are cutting tools characterized by their sharp, acute-angled cutting edge. This geometry is a fundamental characteristic that determines the performance of the insert.
An insert with a positive angle has a rake angle less than 90°, creating a sharp, angled cutting edge that neatly cuts through the material. This design is ideal for reducing cutting forces, making it suitable for machining difficult-to-cut materials like stainless steel and cast iron.
Because workpiece penetration requires less force, positive angle inserts are particularly effective for turning internal holes. They are a professional’s preferred choice for low-powered machine tools or those with poor system rigidity.
Key features of positive turning inserts include improved chip control and reduced cutting forces. Positive angle inserts can be obtained in various materials, ranging from carbide to ceramic and polycrystalline diamond (PCD). They are optimized for a wide range of machining operations, from super finishing to medium machining.
Advantages of Positive Angle Inserts
Positive angle insert boasts several significant advantages that make it an excellent choice for specific machining scenarios, typically for finesse and precision-focused applications.
Some of the advantages of positive angle inserts include:
- Low Cutting Forces: Positive inserts generate significantly lower cutting forces during operation, reducing the load on the machine spindle, tool holder, and the workpiece itself. This feature makes positive angle inserts ideal for finishing operations and machining soft materials or thin-walled components.
- Better Surface Finish: The sharper cutting edge results in a smoother, cleaner cutting action, producing a superior surface finish. This function is essential for applications that require high accuracy and can often reduce the need for secondary finishing processes.
- Reduced Vibration: Due to the engagement of the material with less force, positive angle inserts tend to produce less vibration during cutting. This leads to greater stability and precision, especially on less rigid machines.
- Improved Chip Control: The positive insert angle directs chips up and away from the cutting edge and workpiece. This helps to improve chip evacuation and reduces the chances of chip jamming, which is particularly beneficial in internal turning operations.
Disadvantages of Positive Angle Inserts
While positive angle inserts are characterized by the advantages we mentioned and even more, it also comes with some limitations. What are some of these disadvantages?
- Weaker Cutting Edge: The thin edge of a positive insert is weaker than that of a negative insert; hence, it is less suitable for heavy roughing, interrupted cuts (such as turning a part with keyways), or working abrasive materials.
- Reduced Tool Life in Demanding Applications: In high impact applications, the less robust edge can lead to increased wear. Potentially, it can reduce the overall life of the tool. Under the same conditions, a negative insert offers better outcomes.
- Single-Sided Use: As a result of the required rake angle, positive inserts can only be used on one side. They offer half the number of cutting edges of a double-sided negative insert, which can impact profitability.
Use Applications of Positive Angle Inserts
Positive angle inserts possess unique properties that make them perfect for tasks where precision and low cutting forces are non-negotiable.
Inserts with a positive lead angle are typically used for cutting softer materials, as less force is required to achieve a clean cut. They are used for machining low-strength ferrous and non-ferrous metals.
Positive angle inserts are also usually applied to high-speed steel (HSS) tools. They are great for internal boring and finishing operations on a wide range of materials, including stainless steel and high-temperature alloys.
However, we don’t recommend using positive angle inserts for heavy, high-speed roughing operations where resistance and durability are more important than surface finish.
What are Negative Angle Inserts?
A negative angle insert is the workhorse of modern turning. It is designed to provide exceptional strength, durability, and maximum metal removal. The geometry of negative angle insert is fundamentally different from that of a positive insert, as it is built for power rather than finesse.
What a negative angle insert should tell you is that the insert has a 90° angle between the flank and the top surface, creating a 0° rake angle.
This square geometry means the insert has no built-in back angle and allows both the top and bottom sides of the insert to be used as cutting edges. Negative angle inserts are an ideal choice for heavy machining and roughing operations.
Advantages of Negative Angle Turning Insert
What are the advantages of a negative angle turning point, and why is it the preferred choice for high-demand, high-production environments?
- Superior Edge Strength and Durability: Due to their thickness and 90° cutting angle, negative angle inserts are capable of withstanding higher cutting forces and are much less prone to chipping or breaking. This makes them perfectly suited for heavy roughing, interrupted cuts, and processing of hard or abrasive materials.
- Longer Tool Life in Roughing: The robust nature of negative inserts often results in significantly longer tool life, especially in high impact cutting conditions where a positive insert would fail quickly.
- Double-Sided for Cost-Effectiveness: Because they have no rake angle, negative inserts can be flipped, thus achieving twice the cutting edges on a single piece of carbide. This increases profitability and reduces setup costs in high-volume operations.
Disadvantages of negative angle turning insert
While their strength is a major advantage, negative angle turning inserts have compromises that make them unsuitable for certain applications. These disadvantages are a direct result of their blunt and powerful geometry.
- High Cutting Forces: A negative insert penetrates the material rather than cutting it, generating significantly higher cutting pressures. This requires a powerful and very rigid machine tool to handle the load without causing vibration or chatter.
- Increased Heat Generation: Higher cutting forces and friction generate more heat in the cutting zone. This can increase tool wear and have a negative impact on the surface of some sensitive materials.
- Poor Performance in Finishing: The blunt cutting action does not promote the production of a fine surface finish. They are generally ineffective for light finishing passes where low cutting forces are required.
- Unsuitable for Softer/Gummy Materials: High pressure can cause problems when machining soft, rubbery materials like aluminum, often leading to built-up edge and poor chip control.
Use Applications of Negative Angle Inserts
The strength and durability of negative inserts do not remove applications. They are the first choice for any operation where material export speed and toughness are the primary requirements.
Negative inserts are the best and most durable solution for standard roughing and turning applications, because their solid shapes and thickness allow for greater depths of cut and higher feed rates.
They are excellent for external turning of steel, cast iron, and other hard materials on powerful and rigid CNC lathes. The most common negative inserts are CNMG (80-degree rhomboid), DNMG (55-degree diamond), and WNMG (80-degree trigon).
For heavy roughing work, SNMG inserts with a 45-degree lead angle are an excellent choice, as they allow even greater depths of cut.
Difference Between Positive angle Inserts and Negative Angle Inserts
While both are used in turning operations, choosing between a positive and a negative insert will radically alter the machining process. Their contrasting geometries determine different strengths, weaknesses, and ideal use cases.
To make an informed decision, it’s essential to understand these key differences side by side. The following table provides a clear comparison of their most important characteristics.
Feature | Positive Angle Inserts | Negative Angle Inserts |
Geometric Definition | Edge angle is less than 90° (has a clearance angle). | Edge angle is 90° (has a 0° clearance angle). |
Edge Strength | Lower. The sharp edge is more prone to chipping. | Higher. The thick, blunt edge is very strong and durable. |
Cutting Forces | Low. Requires less power and creates less pressure. | High. Requires a powerful and rigid machine. |
Primary Application | Finishing, internal boring, machining soft materials. | Heavy roughing, interrupted cuts, machining hard materials. |
Number of Cutting Edges | Single-sided (fewer edges per insert). | Double-sided (twice the edges per insert). |
Vibration Tendency | Low. Ideal for less rigid setups or thin workpieces. | High. Can cause chatter on machines with low rigidity. |
Surface Finish | Excellent. Produces a smooth, high-quality finish. | Poor to fair. Not suitable for finishing passes. |
Ideal Materials | Stainless steel, high-temp alloys, non-ferrous metals. | Carbon/alloy steels, cast iron, hard materials. |
How do I Choose the Correct Turning Insert?
Now that you understand the differences, to make the right choice you need to analyze your specific application. There are many parameters that you must consider when choosing the right turning inserts, and that includes careful selection of geometry, grade, shape and size of the insert.
Making the critical choice of which turning insert helps you to achieve good chip control and optimum machining performance depends significantly on the following key factors:
1.Material Being Machined:
If you’re working with hard ferrous materials like steel and cast iron, negative inserts are usually the preferred option for roughing due to their incredible strength.
However, for finishing passes or for machining of non-ferrous materials like aluminum and brass, positive inserts are most ideal. The reason for this is that they provide a better surface finish with lower cutting forces.
2.Machine Tool Capabilities:
The capabilities of your equipment are crucial. High-power, high-rigidity machines with improved stability can handle the high cutting forces generated by negative turning inserts.
On the other hand, for low horsepower machines, lower rigidity machines, or if you’re using long, thin boring bars, we recommend using positive inserts. This is because they produce less cutting force and vibration.
3.Type of Operation (Roughing vs. Finishing):
This is a huge determining factor in choosing the right turning insert. For heavy roughing where you need to quickly remove a large amount of material, the longer tool life of a negative insert makes it most ideal.
For finishing operations where quality of the surface is the primary objective, the crisp, clean cutting action of a positive insert is best.
4.Tooling and Setup Rigidity:
Consider the entire setup of your operation. Tasks that involve internal drilling with a long overhang are subject to vibration.
For these kinds of tasks, it is advisable to use a positive turning insert, as it helps to minimize cutting forces and maintain stability, regardless of the material.
Conclusion
Choosing between negative and positive turning inserts is a fundamental decision that directly impacts the outcome of your machining operations. Ultimately, there isn’t a single perfect option for every use case; there’s just the right tool for the particular task you’re carrying out.
What turning insert you eventually choose is a representation of the fundamental compromise between the resistance and convenience of negative inserts and the finesse and precision of positive inserts.
Ready to improve the performance of your projects? We’re here to serve your best interest at Sundi Precision Tools.
Explore our catalog to discover a wide range of cutting tools designed for peak performance. After that, reach out to our team of seasoned experts for consultations and to get an instant quote for your project.
