Best Practices: Designing for CNC Milling

Follow these tips to save money and improve your CNC milling designs.

Intelligent design is one of the most effective ways to achieve precision and cost savings in CNC milling.

This article shows various best practices 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 Design Guidelines: Tool Diameter Considerations

1. Your design should be for standard cutter shapes and sizes rather than unique, 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 more significant than the tool radius permits de-milling without entirely stopping the cutter at the corner, resulting in better surface quality.

3. When a small, flat surface is required (e.g., for sealing, a 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 requires expensive machinery. Avoid that by using round corners and undercut. Avoid sharp internal corners due to the limitations of round-cutting tools.

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 must be machined with cutting tools with larger diameters, affecting the internal edges’ fillets. We recommend a design with a cavity width of four times 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 a cavity depth larger than 1/3 x.

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. Avoid thin walls to reduce vibrations and maintain machining accuracy.

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. Use CNC threading tools for creating consistent and reliable threads. CNC threading tools are preferred by machinists for their durability and effectiveness in reducing tap breakage, especially for thread sizes M6 and larger.

14. Leave space for the tool. Design undercuts with widths in whole millimetre increments or a standard inch fraction. A custom cutting tool must be created for undercuts with non-standard dimensions.

Tool Geometry and Access

Tool geometry and access are critical considerations in CNC machining design. Most CNC cutting tools have a limited cutting length and a cylindrical shape, which can influence the design of internal corners and cavities. The geometry of the cutting tool is transferred to the machined part, resulting in internal corners with a radius rather than sharp edges. Ensuring proper tool access is also essential, as the cutting tool must reach all workpiece surfaces. Features that cannot be accessed from the top angle may require specialized tooling or additional machining operations, which can increase costs and complexity. By designing with tool geometry and access in mind, you can optimize the CNC machining process and achieve better results.

Internal Edges and Cavities

Internal edges and cavities are crucial design considerations in CNC machining. The vertical corner radius should be at least one-third of the cavity depth to ensure a smooth surface finish. A good rule of thumb is to add a radius of 130% of the milling tool radius to internal vertical edges. When designing cavities, aim for a depth-to-width ratio of 3-4:1 to avoid issues such as tool hanging, deflection, and chip evacuation. Consider using a variable cavity design for deeper cavities to maintain machining efficiency and part quality. By adhering to these guidelines, you can minimize machining challenges and produce high-quality CNC machined parts.

Material Selection and Preparation for CNC Machining

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. Here are some key considerations for material selection and preparation:

• Material properties: When selecting a material for CNC machining, consider its strength, hardness, ductility, and thermal conductivity. These properties will influence the machining process and the performance of the final part.

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

• Surface finish: Consider the desired surface finish and select a material to achieve it. Different materials will yield different surface finishes, so choosing one that meets your specific requirements is essential.

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

Choosing the Right Materials for CNC Machining

Choosing the right materials for CNC machining is crucial to ensure the success of your project. Materials with good machinabilities, such as aluminium, copper, and steel, are preferred due to their balance of performance and ease of machining. When selecting a material, consider its strength, hardness, ductility, and thermal conductivity to ensure it can withstand the machining process and meet the required specifications. Proper material preparation is also essential; this includes cleaning the material to remove contaminants, deburring to eliminate sharp edges, and applying a rust inhibitor to prevent corrosion. By carefully selecting and preparing materials, you can enhance the quality and performance of your CNC machined parts.

Design for Manufacturability (DFM) with Tight Tolerances

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

• Tool access: Ensure that the design allows easy 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 tight tolerance to avoid issues like bit breakage, especially when using longer drill bits that can lose stiffness and precision. Tighter tolerances in CNC turning result in improved surface finishes and precise measurements, which are crucial for achieving high-quality designs. Balancing precision with manufacturability is essential to minimize machining time and costs.

• 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.

Understanding DFM Principles

Design for manufacturability (DFM) is a design approach considering the manufacturing process during the design phase. By applying DFM principles, designers can optimize their designs for CNC machining, reducing production costs and improving product quality. Key considerations include ensuring easy tool access, minimizing the need for complex tooling, and specifying tolerances achievable with CNC machining. Tight tolerances can increase machining time and costs, so balancing precision with manufacturability is vital. Effective communication between the designer, machinist, and customer is also essential to ensure that all stakeholders are aligned and that the project is successful. By embracing DFM principles, you can streamline the CNC machining process and achieve better outcomes.

Preparing Technical Drawings and Quotes

Preparing technical drawings and quotes is essential 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 produce the part accurately.

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

• Communication: Ensure clear communication between the designer, machinist, and customer. Effective communication is key to a successful CNC machining project, ensuring 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.

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 needs and constraints.

If you want to maximize CNC milling, we’re ready to help. Use the MakerVerse platform to source high-quality parts, and our experts can answer any design or material questions.