Optimal Design and Manufacturing Solutions for Precise Machining Sharp Interior Corners

CNC machining is capable of producing parts with intricate patterns and shapes. It is precise, efficient, versatile, and CNC machining costs are comparatively lower.

However, CNC machining has a glaring limitation — machining sharp internal corners. It can be a nightmare for engineers to create square internal corners through machining. The challenge arises due to the shape of CNC bits. Common CNC cutting tools, such as end mills and drills, are cylindrical. Therefore, when cutting a 90-degree angle they create a corner radius. This term refers to the inaccessible corner geometry.

Fortunately, there are a few design workarounds that can help you avoid sharp inside corners. But what if your design absolutely needs this feature? Well, there are several manufacturing solutions that you can employ. Here is all you need to know about CNC machining sharp internal corners.

Why are sharp inside corners an issue in CNC machining?

types of milling

Before we discuss how to solve the sharp inside corner problem in CNC machining, we need to understand why this design feature poses a challenge.

The issue of sharp corners in machining comes up when cylindrical cutting tools encounter sharp inside corners such as square pockets. Due to the shape of the cutters, machining a true square corner becomes unachievable. To obtain a perfectly sharp corner, the cutting tool would need to follow a 90-degree path and CNC bits are not designed in this manner.

The minimum inside corner radii is equal to the cutting tool radius. This issue is unavoidable since rounded cutting tools are simply unable to create sharp corners. In most cases, designers and CNC machinists solve the sharp corner problem by simply avoiding sharp internal features and incorporating sharp corner alternatives. However, some applications require that you accommodate sharp right-angled corners. For example, parts used in assembling often require precise inside corners to fit a part with external sharp corners.

Alternatives to Sharp Corners

Square corners may seem like a good idea on paper, but they are a pain to machine and they’re often totally unnecessary. Here are common sharp corner alternatives that you can use instead.

Radiused corners

ow To Machine Sharp Internal Corners

Most projects can accommodate a radiused corner without affecting the part’s performance or functionality. A corner radius is usually perfectly acceptable.

In the case of assembly parts, the corners of the mating component will be rounded to ensure a perfect fit. Unless a specific reason makes inner corner radii unacceptable, we’d recommend choosing this alternative.

Big tools produce wider inside corner radii. On the flip side, they operate faster reducing the machining time and costs. With some applications, you can leave the larger internal corner radii intact. However, you can also reduce it and achieve a tiny radius using a drill bit with a smaller radius.

The tool radius is also connected to the depth of the internal pocket. Using a small tool for deep cavities can affect the quality of the part and cause fatigue on the tool. It is recommended that you adopt a corner radius of at least a third of the depth of the cavity. Also, the depth of the cavity needs to be twice or triple the tool diameter.


An easy way of eliminating internal radii from inside corners is incorporating an undercut. This is especially the case in the production of parts with joining features that need to mate fully. But what exactly is a corner undercut? In machining, it refers to a feature that removes extra material from a corner. CNC machinists usually employ two common undercut variations; T-bone and dog bone fillets.

T-bone Fillets
Dog Bone
T-bone Fillets
T-bone Fillets

Also known as a one-sided undercut, a T-bone fillet is the simplest type of undercut as it does not require additional calculations. Creating this feature involves cutting and extending one axis of the workpiece by half the diameter of the cutter. This extension allows the mating part to fit within the pocket.

Dog Bone Fillets

The main difference between T-bone and dog bone fillets is that the latter extends the cut in two axes rather than one. A dog bone fillet, otherwise known as a two-sided undercut, is more complex to machine. However, it offers better aesthetics and is therefore suitable for applications that require an excellent look.

How do Machine Sharp Internal Corners

When CNC machined parts require sharp inside corners, operators can use the following methods to create these features. These processes offer varying levels of precision.

Wire EDM

Wire EDM

Electrical discharge machining, or EDM for short, is a non-contact form of machining that uses electrical sparks to remove material from a workpiece.

As the name suggests, wire EDM utilizes a thin wire electrode. The wire and workpiece are alternatively either positively or negatively charged. As they get close to each other, a hot electric charge is generated and it erodes pieces of the workpiece.

Wire EDM is a fantastic way of machining sharp corners. It is exceptionally precise, and it can achieve a fine surface finish. That being said, you’ll have a tiny inside corner radii in the range of 0.005″ – 0.006″. Also, wire EDM is a relatively slow technique and it can be quite costly.

Sinker EDM (Ram EDM)

Instead of a wire electrode, a block of material (either graphite or copper) serves as the electrode in sinker EDM. This machining process gets its name from the fact that the electrode sinks into the workpiece.

The electrode bears one side of the charge and the workpiece carries the other side of the charge. As they get close, an electric spark jumps between them. The resulting high temperatures melt material from the workpiece. The electrode sinks into the workpiece, and you end up with a negative imprint that mirrors the electrode.

Sinker EDM excels at creating sharp inside corners. However, it requires significant machining time and it’s not exactly cheap. Sinker EDM also goes by ram EDM, cavity EDM, plunge EDM, and volume EDM.

Laser Cutting

Laser cutting

Laser cutting is a technology that uses a high-energy focused laser beam to cut different materials. It is especially suitable for creating 2-dimensional parts from plate or sheet materials.

Laser cutting is a viable solution for machinists looking to create sharp corners. Since the laser beam has a diameter, this technique technically does not create true square corners. However, the corner radius is so small that it’s negligible.

This manufacturing technique is capable of achieving tight tolerances. However, for a smoother surface finish, machinists may need to conduct post-processing operations.

3D Printing

3D printing, also known as additive manufacturing, allows you to create physical products by depositing material, typically layer by layer.

Traditional manufacturing methods start with a solid workpiece and remove material to create the desired part. However 3D printing takes a different approach. This process lays down layers of material (usually in liquid or powdered form) and then fuses them together.

3D printing is capable of machining a variety of geometries including square inside corners. Common 3D printing techniques used in this process are laser sintering and binder jetting.


Broaching is a manufacturing technique that uses a toothed tool, known as a broach, for material removal. There are two types of broaching; linear and rotary. Linear broaching is the more popular technique. It is used when making squares through holes in thinner plates.

Broaching is an efficient production method that can process varied geometries. However, this operation can be costly making it suitable for mass production of products rather than creation of a few units.

Manual Cutting

In the absence of machines, it is possible to leverage manual skills and create internal corners. For most people, this process is not the first choice since it’s certainly dated. Manual cutting involves using hand tools, such as files and chisels, to square up corners.

Unsurprisingly, this process is time-consuming and more susceptible to human error. However, with the right technical expertise, it’s possible to achieve accuracy. Manual cutting is commonly used by hobbyists rather than manufacturers and product teams.

Design Tips for Machining Sharp Inside Corners

There are a few tips and tricks that you can use to enhance the quality of machined sharp corners. They include;

  • Using a sharp cutting tool. CNC cutting tools need to be very sharp. This helps create clean and precise corners.

  • Use a cutting fluid during the machining operation. Cutting fluids play a role in regulating the temperature of the cutting tool, cleaning debris, lubrication, and prolonging tool life. Regardless of the machining method, cutting fluid is indispensable, and it increases the efficiency of the process.

  • Perform multiple passes. A simple way of ensuring precision and improving surface finish when machining sharp corners is using multiple passes.

Material Considerations for CNC Machining Sharp Corners

In this section, we’ll cover how material types and characteristics impact a part’s internal sharp corner radius and the machining operation in general.

CNC Machining materials

CNC machining is a versatile fabrication technique that can create precise parts using different materials. Common materials that are suitable for CNC machining can be grouped into metals, plastics, and composites.


Metal parts have applications in virtually every industry. Different metals have varying properties. For example, machinists prefer steel and titanium for tasks that require strength and durability. Copper is known to be highly conductive, ductile, and malleable. Another commonly used metal is aluminum which is lightweight and corrosion-resistant.


Plastics are also widely used in the production of parts. Their desirable characteristics include being lightweight, affordability, and versatility. Some plastics are also exceptionally high-strength. Commonly machined plastics include acrylic, ABS, polycarbonate, teflon, polyethylene, acetal, and polypropylene.


A composite material consists of two dissimilar materials each with distinct physical and chemical properties. This creates a material that is specialized for a specific task. For example, fiberglass, a plastic and glass fiber composite, is strong, lightweight, and corrosion-resistant. Similarly, carbon fiber reinforced polymers, which are carbon fiber and plastic composites, are lightweight, good conductors of electricity, and they have a high strength-to-weight ratio.

How Does Material Type Affect CNC Machining Internal Corners?

Now that we’re familiar with the different CNC machining materials, let’s discuss how the material choice impacts the process of creating internal, 90-degree corners.

Material Hardness

Hard materials such as titanium, tungsten carbide, and hardened steel can be challenging to machine. In this scenario, creating precise inside sharp corners requires specialized, rigid tools that offer precision. Machinists also need to adjust the cutting parameters to suit the specific material being machined.

Material Ductility

Choosing a material with enough ductility allows for smooth and efficient sharp corner machining. The right material option prevents deformation and stress.

Melting Point

When working with plastics, you need to consider their melting points, especially if they are on the lower end. When creating an inside corner on such plastics, you need to regulate the temperature to prevent excessive heating and deformation. A few solutions that you can utilize include regulating the tool cutting speed, feed rate, and using cooling fluids.


Some composites, such as carbon fibers, have abrasive properties. Machining sharp corners on such materials results in tool degradation. This creates the need for specialized tools that help operators achieve high standards of precision.

Layer Orientation

Composites such as sandwich panels, fiber-reinforced polymers, and laminated plates have a layered structure. This can affect precision when creating square corners. CNC machinists need to factor in the layer orientation and choose suitable cutting tools and parameters.


There you have it! We hope that this article addresses your concerns about avoiding and machining sharp internal corners.

To sum it up, sharp internal features such as 90-degree corners can be a pain and expensive to machine. Further, in most cases, sharp inside corners are not really necessary.

For most projects, sharp corners can be substituted with radiused corners, T-bone fillets, or dog bone fillets without compromising functionality. If your project actually requires precise square corners, you can use manufacturing methods such as wire EDM, sinker EDM, laser cutting, and 3D printing.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.