CNC metal machining is a process that uses computers to control machines to cut and shape metal parts very precisely. Good design is vital in CNC (Computer Numerical Control) machining because it affects how well the parts are made, how much they cost, and how long they take to produce. When a design is done well, it can save material, speed up the machining process, and ensure the parts are strong and work correctly. This article will explain the basics of CNC metal machining and how to create the best designs.
Understanding CNC Metal Machining
The basic principles of CNC machining involve programming a computer to guide the machine's movements, ensuring accurate and consistent results. Common metals used in CNC machining include aluminum, stainless steel, steel, and brass, each chosen for its specific properties like strength, weight, and resistance to corrosion.
There are several CNC machining processes. The common ones include milling, turning, and drilling. CNC milling involves cutting away material from a workpiece using rotating tools. CNC turning spins the workpiece while a cutting tool shapes it, and CNC drilling creates holes in the metal. Combining these processes makes it possible to create complex and detailed metal parts for various industries.
Key Design Considerations for CNC Machining
Several key considerations must be considered when designing for CNC machining of metal materials.
Material selection is crucial because different metals have unique properties and levels of machinability. For example, aluminum is lightweight and easy to machine, while steel is stronger but more challenging to handle.
Choosing the proper material depends on the specific needs of the part, such as strength, weight, and resistance to wear or corrosion.
Tolerance and accuracy are also crucial. Tighter tolerances mean higher precision but can increase production costs. Designers must balance precision and budget, ensuring that parts are accurate enough to meet their intended uses without being unnecessarily expensive.
Surface finish requirements impact both the design and the machining process. A smoother surface finish might be needed for aesthetic reasons or to reduce friction, but achieving this can require additional machining steps and increase costs. Designers should specify the surface finish based on the function and appearance of the part.
Part complexity is another major factor. More complex designs can be harder and more expensive to manufacture. Designing for manufacturability means creating parts that are functional but also easy and cost-effective to machine. Simplifying designs where possible can help reduce machining time and costs while maintaining the necessary performance and quality.
Best Practices for Metal CNC Machining Design
There are no specific industry standards for designing metal parts for CNC machining, as CNC machine and tool manufacturers continuously improve their technological capabilities. Therefore, we have summarized the recommended and feasible values for the most common features when machining metal parts using CNC.
#1 Cavities and Grooves
Typical Cavity Depth: 4 times the cavity width.
The cutting length of end mills is limited (usually 3-4 times their diameter). When the depth-to-width ratio is low, issues such as tool deflection, chip evacuation, and vibration become more significant. Limiting the cavity depth to 4 times its width ensures good results.
If greater depth is needed, consider designing parts with variable cavity depths.
Deep Cavity Milling: Cavities deeper than six times the tool diameter are considered deep cavities. Specialized tools can achieve a diameter-to-cavity depth ratio of up to 30:1 (e.g., using a 1-inch diameter end mill, the maximum depth would be 30 cm).
#2 Internal Edges
Vertical Corner Radius: Recommended ⅓ x cavity depth (or larger).
Using the recommended internal corner radius ensures the use of appropriate diameter tools and aligns with the suggested cavity depth guidelines. Making the corner radius slightly above the recommended value (e.g., by 1 mm) allows the tool to cut along a curved path rather than a 90° angle. It is preferred as it results in a higher-quality surface finish. Adding a T-shaped undercut instead of decreasing the corner radius if straight 90° internal corners are needed.
Bottom Radius: Recommended 0.5 mm, 1 mm, or no radius; any radius is feasible.
The lower edge of end mills is either flat or slightly rounded. Other bottom radii can be machined using ball-end tools. Using recommended values is a good design practice since machinists prefer them.
#3 Thin Walls
Minimum Wall Thickness for Metal Parts: Recommended 0.8 mm or 0.5 mm.
Reducing wall thickness decreases material stiffness, thus increasing vibration during machining and reducing achievable precision.
#4 Holes
Diameter: Recommended standard drill sizes; any diameter larger than 1 mm is feasible.
Holes can be machined using drills or end mills. It's better to use standard drill sizes. Reamers and boring cutters are used for holes requiring high tolerances. For diameters less than ⌀20 mm, standard sizes are recommended.
Maximum Depth: Recommended 4 x nominal diameter; 10 x nominal diameter is typical; 40 x nominal diameter is feasible.
Non-standard diameter holes must use end mills. Therefore, the maximum cavity depth limit is applicable, and the recommended maximum depth value should be used.
Specialized drills (minimum diameter 3 mm) can machine holes deeper than typical values.
Drill-machined blind holes have a conical bottom with an angle of 135° angle, while end-milled holes machined are flat.
In CNC machining, both through holes and blind holes are equally viable options, depending on the specific requirements of the project.
#5 Threads
Minimum Thread Size: Can be M2; M6 or larger.
Internal threads are cut with taps, and external threads with dies. Taps and dies can cut threads down to M2.
Machinists prefer CNC threading tools as they seldom break the threads while tapping. These tools are available for threads up to M6.
Minimum Thread Length: 1.5 times the nominal diameter; recommended 3 times the nominal diameter.
Most of the load on threads is carried by the first few teeth, which can bear up to 1.5 times the nominal diameter. Therefore, threads longer than triple the diameter are unnecessary.
For threads in blind holes to be tapped, such as all threads smaller than M6, leave an unthreaded length equal to 1.5 times the nominal diameter at the bottom of the hole.
When CNC threading tools can be used (i.e., threads larger than M6), the hole can be threaded along its entire length.
#6 Small Features
Minimum Hole Diameter: The best practice is 2.5 mm (0.1 inches) or 0.05 mm (0.005 inches).
Most machining vendors can precisely process cavities and holes using tools using a diameter smaller than 2.5 mm (0.1 inches). Anything below this limit is considered micro-machining. Machining such features (where the physical changes in the cutting process fall within this range) requires specialized tools (micro drills) and expert knowledge, so it is recommended to avoid them unless necessary.
#7 Tolerances
Standard: ±0.125 mm (0.005 inches)
Typical: ±0.025 mm (0.001 inches)
Feasible: ±0.0125 mm (0.0005 inches)
Tips for Reducing Machining Time and Costs
Reducing machining time and costs is important for making CNC manufacturing more efficient. One good way to do this is by designing parts that need less material to be cut away, which makes the process faster and wastes less material.
Also, cutting down on time when the machine isn't working, like during setup and tool changes, can make things go quicker. Using tools that can do more than one job at a time, like drilling and milling together, helps save time and reduces the number of tool changes needed.
Talking with machinists early in the design process is beneficial because they can advise on how to make the parts easier to produce and avoid problems. By following these tips, designers can create parts that are cheaper and faster to make.
Common Mistakes to Avoid in CNC Design
When designing for CNC machining, it is vital to avoid common mistakes to ensure a smooth and cost-effective process.
Overly Tight Tolerances: Unnecessary Precision
Specifying extremely tight tolerances means that the parts need to be machined with very high precision.
It often requires more time, specialized tools, and advanced machining techniques, which can drive up costs and extend production times.
Only specify tight tolerances that are necessary for the function of the part. For non-critical dimensions, looser tolerances can save time and money.
Ignoring Material Properties: Machinability Issues
Different materials have different properties that affect how easily they can be machined.
Choosing a material without considering its machinability can lead to problems such as excessive tool wear, longer machining times, and poor surface finishes.
The solution is to select materials known to be machinable and suitable for the intended application. Consult with machinists or material experts if unsure.
Complex Geometries: Difficult and Costly to Machine
Designing parts with intricate shapes and features can increase the difficulty of machining them.
Complex geometries often require specialized tools, multiple setups, and longer machining times, resulting in higher costs.
So, simplify the design wherever possible. Avoid unnecessary complexity and design parts with features that are easily accessible and machined by standard tools.
Lack of Consideration for Tool Access and Fixturing
Tool access refers to the ability of the cutting tool to reach all areas of the part, while fixturing involves securely holding the part in place during machining.
A design without consideration of tool access and fixturing may result in difficult-to-machine parts. It may require multiple setups, increasing the risk of errors and extending production times.
The best practice is to design parts with clear tool paths and ensure that all features can be accessed by the cutting tool. Also, consider how the part will be held in place during machining to avoid the need for complex fixturing solutions.
Conclusion
In conclusion, designing for CNC metal machining requires careful consideration of several key factors to ensure efficiency and cost-effectiveness. Thoughtful design is crucial as it not only streamlines the machining process but also reduces costs and enhances the quality of the final product. By applying these tips, designers can significantly improve machining outcomes, leading to better performance and satisfaction in manufacturing projects.
RFQs of CNC Metal Machining
1. What metals can be CNC machined?
Almost all metals can be CNC machined.
The common ones are aluminum, stainless steel, steel, brass, and copper.
Some special metals are hard for machining, like titanium and some steel. When machining these materials, the tools are easy to break. The tolerances cannot be guaranteed. It's vital to find the right vendor to do the tough job.
2. What finishings are available for CNC-machined metal parts?
The common finishings for CNC metal machining parts are deburring, sanding, polishing, anodizing, powder coating, blackening, electroplating, etc.
3. What is the average lead time of CNC metal machining?
It depends on the quantity, material, and finishing. For one-off CNC metal parts, it usually takes at least four days.
