Best Practices: Designing for CNC Milling

When it comes to CNC milling, one of the most effective ways to achieve precision and cost savings is through intelligent design.

This article shows various best practices that designers can adopt to optimize their designs for CNC milling. These practices revolve around standard cutter shapes and sizes, creating manufacturing preferences, bevels or chamfers, and much more. 

Understanding CNC Milling

CNC milling is a subtractive manufacturing process that employs computerized controls and rotating multi-point cutting tools to progressively remove material from the workpiece and produce a custom-designed part or product.

CNC machines are capable of handling complex geometries and multi-axis machining, which allows for the manufacture of intricate designs and eliminates the need for multiple setups. This process works with various materials, including metals, plastics, glass, wood, and ceramics. Like any manufacturing method, designing parts suitable for this process is essential to maximize the benefits.

New to CNC milling? Here’s an overview of the technology, including popular applications and materials.

CNC Milling Design Guidelines

1. Your design should be for standard cutter shapes and sizes rather than special, non-standard designs. Slot widths, radii, chamfers, corner shapes, and overall forms should conform to cutters available off the shelf rather than those requiring specialized fabrication.

Why is this important? Mainly because specialized form-relieved cutters are costly and difficult to maintain.

2. Designs should permit manufacturing preference as much as possible to determine the radius where two milled surfaces intersect or where profile milling is involved. This allows standard sizes (R=1,5; 3; 6; 2; 4 or 5 mm, for example) and leaves the tolerance open for any cutter (max. R5 mm).

Allowing a radius bigger than the tool radius permits de-milling without entirely stopping the cutter at the corner results in better surface quality.

3. When a small, flat surface is required (e.g., for sealing, bearing surface, or a bolt-head seat perpendicular to a hole), the design should permit spotfacing –the section of a part where a fastener sits flat. This is quicker and more economical than face milling.

4. When outside surfaces intersect, and a sharp corner is not desirable, the product design should allow a bevel or chamfer rather than rounding. Face mills may create bevels and chamfers, whereas rounding requires a form-relieved cutter and a more precise setup, both of which are most costly to maintain. Use angles such as 45°, 60° and 30°.

5. Round inside corners. Machining squared pockets require expensive machinery. Avoid that by using round corners and undercut.

6. A design that avoids the necessity of milling at parting lines, flash areas, and weldments extends cutter life.

7. As with other surface-machining processes, the most economical designs require the fewest separate operations. Surfaces in the same plane or at least in the same direction and parallel planes are preferred.

8. Allow your geometry to stack together. Parts with similar geometries on at least one side can also use the same machine setup.

9. Avoid deep pockets. Deeper cavities need to be machined with cutting tools with larger diameter affecting the fillets of the internal edges. We recommend a design with four times the cavity width and 25mm or ten times the tool diameter.

10. Design large internal edges in deep cavities. For internal vertical edges, the larger the fillet, the better. Edges on the floor of a cavity should be either sharp or have a 0.1 mm or 1 mm radius. We recommended larger than 1/3 x cavity depth.

11. Avoid tall features in your designs. Decreasing the wall thickness reduces the stiffness of the workpiece, increasing vibrations and lowering the achievable tolerances. We recommend 0.8 mm (min. 0.5 mm) for metal and 1.5 mm (min. 1 mm) for polymers.

Furthermore, tall features are difficult to machine accurately, as they are prone to vibrations. Consider the overall geometry of the part: rotating the part by 90° degrees during machining changes the aspect ratio. We recommend a height smaller than four times the minimum width.

12. Add clearance on the undercut of internal faces. The clearance should be four times the depth.

13. Leave space for the tool. Design undercuts with a width of whole millimeter increments or a standard inch fraction. For undercuts with non-standard dimensions, a custom cutting tool must be created.

Material Selection and Preparation

When it comes to CNC machining, selecting the right material is crucial for achieving the desired results. The material selection process involves considering factors such as the part’s intended use, the required mechanical properties, and the machining process itself. Here are some key considerations for material selection and preparation:

• Material properties: Consider the material’s strength, hardness, ductility, and thermal conductivity when selecting a material for CNC machining. These properties will influence the machining process and the performance of the final part.

• Machinability: Choose materials that are easy to machine, such as aluminum, copper, and steel. These materials typically offer a good balance between machinability and performance, making them ideal for a wide range of applications.

• Surface finish: Consider the desired surface finish and select a material that can achieve it. Different materials will yield different surface finishes, so it’s important to choose one that meets your specific requirements.

• Material preparation: Ensure that the material is properly prepared for machining. This includes cleaning the material to remove any contaminants, deburring to eliminate sharp edges, and applying a rust inhibitor to prevent corrosion. Proper preparation can significantly improve the quality of the CNC machined parts..

Design for Manufacturability (DFM)

Design for manufacturability (DFM) is a design approach that considers the manufacturing process during the design phase. Here are some key considerations for DFM:

• Tool access: Ensure that the design allows for easy tool access and minimizes the need for complex tooling. This can help reduce machining time and costs while improving the quality of the final part.

• Material selection: Choose materials that are easy to machine and minimize the need for specialized tooling. This can help streamline the machining process and reduce costs.

• Tolerances: Specify tolerances that are achievable with CNC machining and minimize the need for additional processing. Tight tolerances can increase machining time and costs, so it’s important to balance precision with manufacturability.

• Surface finish: Design the part to achieve the desired surface finish and minimize the need for additional processing. This can help reduce costs and improve the overall quality of the CNC machined parts.

Preparing Technical Drawings and Quotes

Preparing technical drawings and quotes is an essential step in the CNC machining process. Here are some key considerations:

• Technical drawings: Create detailed technical drawings that include dimensions, tolerances, and material specifications. These drawings provide the necessary information for the machinist to accurately produce the part.

• Quotes: Provide accurate quotes that include the cost of materials, labor, and overhead. Accurate quotes help ensure that the project stays within budget and avoids unexpected costs.

• Communication: Ensure that there is clear communication between the designer, machinist, and customer to avoid misunderstandings and errors. Effective communication is key to a successful CNC machining project, ensuring that all stakeholders are on the same page.

Getting Started with CNC Milling

Designing for CNC milling goes beyond just creating a part or product that fits its function. It’s about understanding the milling process, knowing what the machines can do, and optimizing your design for the best outcome.

Following these best practices can significantly improve production efficiency, reduce costs, and enhance product quality. However, remember that every project is unique. While these guidelines provide a strong foundation, always consider your project’s specific needs and constraints.

If you want to make the most of CNC milling, we’re ready to help. Use the MakerVerse platform to source high-quality parts. Our experts can help you with any design or material questions.