CNC Machining Sharp Inside Corners:Comprehensive Guide

CNC Machining Sharp Inside Corners:Comprehensive Guide-Yonglihao Machinery

What is CNC machining technology? Simply put, it refers to using a computer program to control a machine tool to complete the machining of a workpiece. It has the advantages of high precision, good repeatability, and fast processing speed, although it has been widely used in aerospace, digital electronics, defense technology, automotive, medical equipment manufacturing, and other industries, but in the production and processing process, there are still some of the CNC machinist’s headache, for example, how to efficiently process the workpiece’s sharp inner corner? Next, this article will take you to a detailed understanding of the relevant issues of CNC machining of sharp inner corners.

Table of Contents

Why is CNC machining of sharp inside corners difficult?

CNC machining is now widely used in many industries. But why is it difficult for CNC machinists to machine sharp inside corners, especially straight inside corners?

Most CNC machining tools are cylindrical. Therefore the depth of the cut is limited. When a tool cuts inside a workpiece cavity, the corners of the cavity always produce rounded corners. Change the tool’s path to 90 degrees to get a true right Theoretically, the minimum internal fillet radius obtained is equal to the radius of the cutting tool. To get a true right angle, change the tool’s path to 90 degrees. However, the path of the CNC tool is not constructed this way. You can also change the smallest tool so that the radius of the inside corner is close to a sharp shape, but this is not a sharp inside corner.

So what are the solutions to these problems? Follow us as we continue.

Difficulty of CNC machining sharp inside corners--Yonglihao Machinery

 Strategies for machining sharp inside corners

Avoidance techniques

The simplest solution is to avoid sharp corners. This means changing the corners in the product’s design to be round. It may not seem like the answer to this problem, but it is the most economical and effective solution.

Because it solves the root cause of the sharp inner corner in the CNC machining process can not be directly processed, the actual production of most of the design of the product can be changed to the workpiece inner cavity of the right angle of the CNC can be processed into smaller rounded corners, only need to be slightly adjusted to achieve, does not affect the workpiece and the inner cavity of the fit between the workpiece and the product does not affect the final function of the product to achieve. In this process, we need to pay attention to the relationship between the depth of the machined cavity and the fillet. This information will help me to select the right machining tool. The machining tool in turn ensures the machining efficiency and surface quality of the workpiece.

If a workpiece can realize the same function by avoiding the technology, please choose this strategy preferably.

CNC machining strategy for sharp internal angles--Yonglihao Machinery

Use of T-bone and dogbone fillets

When design is unavoidable, we can consider solving the problem during the machining process. This is done by adding undercuts to each sharp corner, using T-bone and dogbone fillets to remove excess workpiece material. This method is most effective when you need to assemble a part with sharp outside corners into an internal cavity. It does not cause mismatches in the workpiece assembly. It also does not affect the functionality of the assembly. It just mills away some of the material from the workpiece.

T-bone: This is simply a T-bone fillet, which is T-shaped with rounded edges and extends the cutting area in one direction. Usually, the extension of the cut is half the diameter of the tool to match the workpiece leaving enough space for assembly.

Dog Bone Fillet: So named because the shape of the sharp corner resembles a dog bone after machining. It extends the kerf in two directions, unlike the T-bone’s one direction. The machining process is a little more complicated than T-bone. However, the overall appearance is better.

EDM technology

In addition to the above CNC milling to obtain sharp inside corners we can also use EDM technology to obtain sharp inside corners. Compared with the above methods, the inner corner obtained by EDM is the closest technology to the sharp inner corner.

EDM is a very common machining technique. It uses an electrical conductivity between the tool and the workpiece, which in turn removes material by melting and erosion. In this article, we will discuss two EDM techniques: EDM molding and EDM wire cutting.

EDM molding: The process consists of an electrode and a workpiece. It is usually immersed in an insulating fluid (e.g., oil or other dielectric fluid). When the electrode and workpiece are connected to a suitable power source during machining, an electrical potential is generated between the two components. When the electrode is close to the workpiece, a dielectric breakdown occurs in the fluid, forming a plasma channel that causes a small spark to jump back and forth, which in turn melts and removes the excess of the workpiece material to achieve the desired sharp inside corner. In EDM molding, the tool is a custom electrode with a conductive function. The outer corners of the electrode can be designed to match the sharp inner corners of the workpiece’s inner cavity. This makes the processed corners the closest to sharp.

EDM Wire Cutting: This is a different technique from EDM molding. The tool used is a fine conductive wire with a diameter of less than 0.1mm, which is moved through the workpiece along the characteristic contours, cutting away excess material. Since the diameter of the fine wire is exceptionally small (less than 0.1mm), it is theoretically possible to obtain a fillet radius with an inside corner diameter equal to half of the diameter of the fine wire, which is close enough to the sharp inside corner to be acceptable, and therefore it is ideally suited for machining sharp inside corners.

However, EDM machining has some limitations. First, the workpieces machined by both methods must be electrically conductive, and second, their machining efficiency and the quality of the workpiece surface are not high. They are generally much slower than traditional machining methods, and the surface finish of the workpiece is not high. EDM wire-cutting processing of the inner cavity of the sharp inner corner can only be done through the type, with a certain depth not through the inner cavity can not use this method.

EDM technology strategy in CNC machining of acute internal angles--Yonglihao Machinery

Manual Cutting

When T-bone and dogbone fillets cannot be used in the CNC machining process, it is inevitable to produce radius-rounded corners after the workpiece is finished. At this point, we must hand-cut to finish. We will use hand tools to cut, grind, and polish the inner corner. This is the way to get the sharp inner corner. Some common tools include files, sandpaper grinding, etc.

This type of processing is very labor-intensive and time-consuming and is only used after machine processing is completed when the sharp inner corner of the workpiece is particularly important.

Influence of materials on the machining of sharp inside corners

In CNC machining, material choice is critical. It affects the quality of sharp inside corners. Types of materials generally used in CNC machining are metals, plastics, and composites.

Aluminum, steel, brass, and titanium alloys are commonly used in CNC machine tools. These metals are common for their hardness, strength, durability, and corrosion resistance. They also use ABS, Delrin, and nylon, which are easy to machine and inexpensive. Common composite materials include carbon fiber-reinforced polymers (CFRP) and fiberglass. CFRP has a good strength-to-weight ratio. Fiberglass has strength, flexibility, and corrosion resistance.

Material affects machining sharp inside corners. It affects them through hardness, melting point, wear, and ductility.

The impact of material hardness: the higher the material hardness, the processing of the sharp inner corner of the parameters of the required processing tools, the higher the processing difficulty, this time the need for the higher hardness of the special rigid tool processing.

The influence of the melting point: in the processing of plastics, especially need to pay attention to the lower melting point. At this time, you need to pay attention to adjusting the temperature of the high and low, to prevent the deformation or melting of the workpiece. The quality of sharp internal corners can be ensured by adjusting the tool chip speed, and feed rate and increasing the coolant.

The impact of wear: in the processing of carbon fiber materials, due to its existence of abrasive will lead to the processing of a sharp inner corner of the tool blunt. At this time, the use of specialized processing tools to ensure machining accuracy.

Influence of material ductility: When machining sharp corners, it is essential to consider the ductility characteristics of the material for the machining accuracy of rounded corners. It also prevents deformation of the part and reduces internal forces.

Choosing the right machining material to meet the sharp internal angle machining-Yonglihao Machinery

Optimizing Machined Inside Angles with Design for Manufacturability (DFM)

DFM is a design for manufacturing methodology. It aims to optimize the manufacturing of products. This is done to reduce costs, improve quality, and reduce production time.

Before machining sharp inside corners for production, on the one hand, DFM can be used to first assess the complexity of the design of the sharp corners and to determine whether the designed sharp corners are relevant to the functional realization of the part. If rounded corners instead of sharp ones don’t hurt the final product’s quality, we can avoid sharp corners before CNC machining. On the other hand, the DFM is utilized to evaluate the magnitude of the impact of incorporating sharp corners on the machining cost. Adding sharp corners raises part costs. So, we can avoid or replace sharp corners with round ones. We can also consider using alternative machining processes — T-bone rounding or dog-bone rounding.

Cost and Efficiency Analysis of Machining Sharp Inside Corners

CNC machining of sharp inside corners of workpieces is not a particularly good design. Sharp inside corners are difficult to machine. They significantly increase production costs and time. Therefore, avoid sharp inner corner designs if they are not necessary.

For most projects, rounded corners, T-bone rounded corners or dog-bone rounded corners can be used instead of sharp corners to realize the corresponding functions. If the workpiece needs a sharp corner, you can use other methods to make it. These include EDM, hand cutting, and laser cutting.


Currently, in CNC machining, the above describes several common methods for machining pointed inside corners. If you need to machine CNC sharp inside corners, please contact us. Yonglihao Machinery has rich experience and expertise in CNC machining of sharp inside corners. We can provide a complete solution for your needs.


First, use DFM to evaluate whether the product must be machined with sharp corners before machining. If the sharp corners can be replaced with rounded corners, change the design. If they cannot be replaced, consider machining with EDM.

Mainly: 1. Select a sharp tool diameter as small as a possible tool, which is conducive to ensuring the accuracy and surface finish of the sharp corner.

2. The use of cutting fluid during processing. Cutting fluid can play a role in regulating the temperature of the tool, cleaning up the material debris, lubrication extending the life of the tool, and so on.

3. Processing tools to perform several times to take the knife. Processing the tool multiple times to take the knife can improve the accuracy of the sharp corner and surface finish.

EDM cutting achieves sharp corners by using a fine wire electrode. The wire and the workpiece are alternately positively or negatively charged. When they are nearby, they generate a thermal charge which then erodes the part, thus completing the sharp corners.

There are two main aspects:

1. Sharp corners in CNC machining can lead to stress points in the workpiece. They also lead to more tool wear and machining risks.

2.Sharp corners increase machining costs and cycle times. Therefore, do not CNC machine to sharpen inside corners if you do not have to.

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