A Guide to Designing and Manufacturing Custom CNC Parts

CNC machining is a versatile manufacturing process capable of producing a wide range of custom parts with high precision and repeatability. It is essential for producing complex, high-precision parts across a wide range of industries, including aerospace, automotive, healthcare, and electronics. However, the pursuit of perfection in CNC machining involves more than just the machinery itself. It is an art form that requires a keen eye for design and a deep understanding of the process. In this guide, we’ll demystify the design and manufacturing of custom CNC parts. From general best practices to customized techniques for specific part operations, we’ll delve into how to shape your part design to achieve peak performance. Welcome to the innovative, fast, and precise Xtproto brand, where every guide we share is designed to advance your manufacturing excellence.

Why should the design and manufacturing of custom parts adhere to the fundamental capabilities of various CNC machines?

In CNC machining, the development of custom parts, from initial concept to final physical form, requires a precise and technologically advanced process. First, a CNC designer creates a design using advanced CAD software. This design is then converted into G-code (the instruction code for the CNC machine). According to this code, CNC machines use specialized cutting tools to methodically carve parts from solid blocks. CNC machines, such as vertical and horizontal milling machines and lathes, can operate on multiple axes. For custom manufacturing of relatively simple parts, traditional three-axis machines can manipulate parts along three linear axes (X, Y, and Z). Five-axis machining allows machining along three linear axes and two rotary axes to create more complex parts. This CNC machining process is capable of producing high-precision, complex parts from a variety of materials, including metals, plastics, and composites. Furthermore, it is fast, automated, accurate, and scalable, making it suitable for prototyping, one-off production, and mass production.

Guidelines for Designing Low-Cost CNC Custom Parts

Understanding the nature of CNC machining lays the foundation for understanding the importance of adhering to design specifications. These specifications are crucial to reducing costs while maintaining high quality and precision. The following are common guidelines for designing and manufacturing CNC machined custom parts, as summarized by Xtproto’s team of engineers and designers.

• Avoid non-planar and draft angle surfaces: Non-planar and draft angle surfaces are complex and difficult to machine, resulting in lower cutting speeds, longer machining times, and increased tool wear. Furthermore, these surfaces make it difficult to maintain consistent and precise part quality and tolerances. To avoid non-planar and draft angle surfaces in your design, use simple, planar geometry whenever possible. Use fillets and radii to soften sharp corners and reduce the number of complex surfaces. Incorporate draft angles into your design to facilitate material removal and reduce tool wear during machining.

• Increase the size of internal fillets: Internal fillets are rounded corners or transitions within a part that reduce stress concentrations and increase part strength. Increasing the size of fillets can improve machining quality and efficiency, specifically by reducing cutting forces and tool wear during machining. This improves chip removal and material flow during cutting, reduces tool breakage and premature wear, and improves surface finish and part quality.

• Add undercuts to sharp corners: Undercuts are recesses or notches in part corners that provide better tool access and improve material removal during machining. Optimize undercut design for CNC custom part machining. However, creating undercuts can be a complex and challenging task, as these effects are difficult to achieve using standard cutting tools. Furthermore, specialized tools or multi-axis machining may be required to machine undercuts. Minimizing the size and complexity of undercuts can help achieve better results. When designing undercuts, consider designing undercuts between 3 mm and 40 mm, with a maximum clearance of four times the undercut depth.

• Use standard tolerances: Standard tolerances ensure that finished CNC parts meet the required specifications and functional requirements. Unnecessarily tight tolerances increase machining costs and time. By specifying standard CNC machining tolerances, manufacturers can reduce the need for secondary operations and improve the overall efficiency of the machining process.

• Text and fonts: When creating text or letters, the tool must be able to maintain consistent width, height, and spacing throughout the entire machining process. Any variation in these factors can result in the final product not meeting design specifications. Consider the font and size of the text or letters. Text that is too small may be difficult to read or fail to meet the required specifications, while text that is too large may cause tool deflection or affect the accuracy and precision of the machining process. To address these challenges, Xtproto’s team of engineers and designers recommend several good design practices.

• Use standard fonts appropriate for the machining process.
  • Avoid overly complex or detailed fonts.
  • Specify larger font sizes.
  • Choose fonts with more consistent width, height, and spacing.
  • Carefully consider the orientation of the text relative to the workpiece.
  • Adjust the tool accordingly to maintain consistent height, spacing, and cutting speed.

• Part Size: CNC machine capabilities vary depending on their size and capacity. Some machines may be too small to accommodate large parts, while others may be unable to handle parts that are too small. Therefore, carefully consider part size when designing a part and select the appropriate machine accordingly. In addition to machine size, part size also affects machining speed. Larger parts take longer to machine and are more expensive to produce than smaller parts because engineers need to remove more material during machining.

• Selecting a softer grade: Softer materials are easier to machine, resulting in higher cutting speeds, less tool wear, and lower machining time and cost. Furthermore, they are less likely to crack or deform during machining, improving part quality and reducing post-processing time. However, soft materials should only be selected if the product’s intended use and end application warrant it. Minimize Tool Changes and Workholding Setups: Frequent tool changes and workholding setups during the machining cycle are time-consuming and costly. Consider minimizing tool changes and setups. Parts with similar features and geometries can be CNC machined using a single cutting tool. Setups can also be reduced by designing parts with consistent orientation or using modular fixtures that can accommodate multiple parts. Finally, multifunctional cutting tools can be used to perform multiple operations with a single tool change.

A Basic Guide to Custom Design and Manufacturing of CNC Milled Parts

• Keep Available CNC Cutting Tools in Mind: Optimizing CNC parts to reduce cost and lead time requires aligning designs with the capabilities of standard CNC milling tools. Choosing designs that conform to the dimensions and capabilities of these standard tools can significantly reduce the need for custom or specialized tooling. A practical example is the design of internal fillets. It is recommended to avoid using radii smaller than those capable of being machined with standard CNC tools. Creating such features requires using smaller tools, or even custom tools, which can increase time and cost, with the benefits likely outweighed by the actual cost. Therefore, staying within the capabilities of standard tooling is a key consideration for efficient CNC part production.

• Avoiding Sharp Internal Corners: CNC milling has inherent limitations, one of which is the inability to create sharp internal corners. This limitation stems from the rounded shape of CNC milling cutters. To address this, engineers often use fillets in their designs. The radius of these fillets needs to be at least half the cutter diameter. For example, for a 1/4-inch cutter, the minimum radius of the fillet should be no less than 1/8 inch. To address the challenges of sharp corners in parts, specific design techniques are employed. These techniques include:

• Avoiding Deep, Narrow Slots or Pockets: Good design practice dictates that the final depth of cut should not exceed the ratio determined for the material being machined. For example, for plastics, this ratio should not exceed 15 times the end mill diameter; for aluminum, it should not exceed 10 times; and for steel, the upper limit is 5 times. This is because longer tools are more susceptible to deformation and vibration, which can lead to surface defects. Furthermore, the internal fillet radius also depends on the tool diameter. If you want to CNC-machine a 0.55-inch-wide slot in a steel part using a 0.5-inch end mill, the depth should not exceed 2.75 inches. Furthermore, end mills with larger aspect ratios may be more difficult to obtain. Therefore, it is recommended to reduce the depth of the slot or feature or increase the tool diameter. Typically, a cavity depth of 4 times is designed, while the cavity width is controlled to 10 times the tool diameter, or 25 cm.

• Designing the Maximum Allowable Internal Radius: During the design phase, consider the tool size used on the CNC milling machine. Larger tools can remove more material in one pass, reducing machining time and cost. To fully utilize the capabilities of larger tools, design internal corners and fillets with the largest possible radius, ideally greater than 0.8 mm. Another tip is to make the corner radius slightly larger than the end mill’s radius, for example, 3.3 mm instead of 3.175 mm. This results in a smoother cutting path and a finer surface finish on the machined part. Typically, the inside corner radius is ⅓ times (or greater) the cavity depth.

• Choosing the Right Thickness: It’s important to note that thin part walls can present significant challenges during machining, especially when it comes to maintaining rigidity and dimensional accuracy. To avoid these challenges, design metal parts with a minimum wall thickness of 0.25 mm and plastic parts with a minimum wall thickness of 0.50 mm, as these parts are designed to withstand the rigors of the manufacturing process. Wall thicknesses of 1.5 mm (plastic), 0.8 mm (metal), 1.0 mm (plastic), and 0.5 mm (metal) are recommended.

A Basic Guide to Custom Design and Manufacturing of CNC Turned Parts

• Avoid Sharp Internal Corners: Sharp inside and outside corners in part designs can create challenges during machining. To address this, it’s recommended to:

• Avoid long, thin turned parts: Stability is a common problem with long, slender turned parts. Rotating parts can easily rub against the tool, resulting in surface imperfections. To address this, refer to CNC design tips and incorporate a center drill at the end and use the center to keep the part rotating straight. Maintain a length-to-diameter ratio of 8:1 or less to minimize the risk of instability during machining.

• Avoid thin walls: During CNC turning operations, be mindful of the amount of material lost. Overworking can cause excessive stress on the part, while thin walls can reduce rigidity and make it difficult to maintain tight tolerances. As a guideline, turned parts should have a wall thickness of at least 0.02 inches to ensure stability and accuracy during manufacturing. Wall thicknesses include 1.5 mm (plastic), 0.8 mm (metal), 1.0 mm (plastic), and 0.5 mm (metal).

A Basic Guide to Custom Design and Manufacturing of CNC Drilled Parts

• Optimal Hole Depth: The ideal drill depth balances tool stability and the strength of the material being machined. Drilling too shallow can weaken joints and reduce screw holding power; drilling too deep can cause the drill bit to break or bend, affecting accuracy and surface finish. To determine the optimal hole depth, you must consider the drill bit size, the hardness and thickness of the material, the strength required for the intended application, and the overall stability of the machine setup. It is recommended to drill just deep enough to accommodate the screw or fastener, leaving some material for support. If a countersink is required, the hole should be drilled deeper to accommodate the countersink. The recommended hole depth is 4 times the nominal diameter (40 times the nominal diameter).

• Distinguishing between through holes and blind holes: Understanding the difference between through holes and blind holes is important, as they require different drilling techniques and tools. A through hole is a hole that passes through the entire workpiece from one end to the other. Because the drill bit must enter and exit from opposite sides of the workpiece, through holes are generally easier to machine. Through holes are suitable for fastening, mounting, and routing electrical and mechanical components. Blind holes, on the other hand, do not penetrate completely through the workpiece, but instead stop at a specific depth. They are suitable for creating cavities, grooves, or recesses within a workpiece and are generally more challenging to create than through holes. Blind holes require specialized CNC drill bits and cutting speeds to ensure the cutting edge does not penetrate the bottom of the part.

Through Holes

Blind holes

• Avoid partial holes: Partial holes occur when the drill bit fails to fully penetrate the material. Partial holes can occur due to a variety of reasons, including drill bit breakage, improper drill bit selection, or incorrect settings for speed, feed, and depth of cut. Therefore, you should select the appropriate drill bit, maintain the correct parameters, and use coolant to dissipate heat.

• Avoid drilling through cavities: When drilling, be aware that holes that intersect existing cavities in a part may compromise its structural integrity. You can avoid this by positioning the drill point away from the existing cavity. However, if a hole must pass through a cavity, a common working practice is to ensure its center axis does not intersect the cavity to maintain part stability.

• Design for Standard Drill Sizes: Optimize your design for standard drill sizes to save time and money and make it easier for machine shops to produce your parts without expensive custom tooling. Consider using a standard drill size, such as 0.12″, rather than a more precise but less common size, such as 0.123″. Also, try to limit the number of different drill sizes used in your CNC design, as multiple sizes increase the time and effort required to change tools during machining. Drill Sizes: Standard drills (0.12″), any diameter greater than 1″  mm.

• Designated threaded holes: Threaded holes can be used to attach bolts, screws, and other threaded fasteners. Be sure to specify the correct thread depth to ensure that the threaded fastener has enough engagement to hold the parts together. The deeper the threads, the stronger the grip of the fastener. The type of material affects the type of thread. On the one hand, soft materials may require shallower threads. On the other hand, harder materials may require deeper threads. When specifying threaded holes in drawings, use clear and accurate thread callouts to ensure the thread gauge, pitch, and depth are correct. Make sure there is enough clearance to install and remove threaded fasteners to avoid thread jamming or slippage. The thread length is 3 times the nominal diameter and 1.5 times the nominal diameter.

• Avoid deep taps: Another key tip for getting precise results is to avoid tapping too deep. The longer the tap, the greater the risk of vibration and deflection during operation, leading to defects in the final product. A tap depth exceeding three times its diameter can pose significant challenges. However, in many cases, even a 1.5x diameter tap will provide enough thread engagement to eliminate the need for a deep tap. Using deep taps increases the risk of tool breakage, thread defects and loss of accuracy, so when using CNCNot ideal in machining design. Faucet size 0.5 times the diameter 1.5 times the diameter

Limitations Affecting CNC Custom Part Design and Manufacturing

When designing parts for CNC machining, it’s important to be aware of certain limitations. Understanding these limitations is crucial to ensuring the final product meets the required specifications while maintaining an efficient and economical production process.

• Tool Capability: A major challenge in CNC machining is the tool’s ability to reach and accurately machine features with large aspect ratios. Tool performance and reachability also play a significant role in determining workpiece shape and the difficulty of reaching and machining complex features. For example, a deep cavity may require a tool with extended travel, such as a CNC threading or drilling tool, to reach the bottom. This increases machine vibration and reduces accuracy. Therefore, tool size, shape, travel distance, and other factors constitute major design limitations for CNC machining and can affect the accuracy of the final product.

• Tool Shape: Another factor you must consider is the geometry of the cutting tool. Most cutting tools have a cylindrical shape and a limited cutting length, which affects the final cut and its shape. For example, even with a very small cutting tool, internal corners of the workpiece always have a radius. This is because the tool’s geometry is transferred to the machined part during the material removal process. The cylindrical shape and limited cutting length of common CNC cutting tools, such as end mills and drills, also limit their ability to machine certain features.

• Tool stiffness: In CNC machining, CNC machine and tool manufacturers use carbide, tungsten, or similar materials, which offer superior performance compared to the workpiece. Despite the superior performance of these materials, tool deflection can still occur, which can be a major cause of design and result variation. While this may not be a problem when working within normal tolerances, even slight tool deflection can become a significant issue in high-precision work with tight tolerances. Deviations caused by tool deflection can limit design possibilities and affect the accuracy of the final product.

• Workpiece stiffness: Cutting tools with excellent stiffness and high performance may not be suitable for certain workpiece materials with excellent mechanical properties. Workpiece stiffness can cause vibration and deformation, negatively impacting the accuracy and precision of CNC machining. The accuracy and precision achievable with rigid workpieces can vary, making it difficult to meet tight tolerances.

• Workpiece Shape: The stability and success rate of CNC machining depend heavily on the shape of the workpiece. Workpiece geometry is crucial because it determines the number of steps required and the overall feasibility of the design. In some cases, even on multi-axis machines, complex geometries may require repositioning during machining, resulting in reduced productivity.

• Workholding: Rigidity is crucial in machining because it ensures smooth and accurate operation. Weak links in the “rigidity chain” consisting of the machine tool, tool, part, and fixture can lead to vibration and reduced accuracy. Any part movement during machining results in inconsistent results and deviations from tolerances. Poor setups can lead to low accuracy and lack of precision because every machined part is unique.

The Importance of Custom CNC Part Design for Manufacturability

The design of machined parts is fundamental to the entire manufacturing process and is crucial to the success of the final product. Design for Manufacturability (DFM) helps optimize the manufacturing process, making it faster, more efficient, and more cost-effective. This often requires modifying specific features that cannot be produced with existing equipment and materials.

• Reducing Manufacturing Costs and Time: Part design plays a crucial role in determining the efficiency and speed of the manufacturing process. By considering factors such as tool selection, cutting parameters, and machine capacity, manufacturers can optimize production processes for greater speed and efficiency. This can also shorten cycle times, increase productivity, and lower production costs.

• Efficient and Streamlined Manufacturing: The efficiency of CNC machining is directly influenced by the characteristics of the part being machined. When part configurations reduce tool wear and cycle times, they can improve machine utilization, thereby increasing productivity and profitability. In addition to applying Design for Material Management (DFM) principles, maximizing material utilization is crucial for reducing costs and increasing profits. Efficient material use is crucial for lowering overall production costs. By carefully selecting the right material and considering characteristics such as thickness and compatibility with the target geometry, manufacturers can more effectively utilize material, thereby minimizing waste and optimizing production costs.

• Avoiding Fatal Design Flaws: Integrating CAD and CAM software into the manufacturing process provides significant design flexibility when modifying part specifications. This adaptability is crucial for adapting to rapidly changing customer demands or making adjustments to improve performance, quality, or cost-effectiveness. This flexibility enables various process optimizations. For example, manufacturers can streamline toolpaths, reduce the number of required setups, or improve material usage efficiency. This approach also allows for increased production automation, reducing the need for human error and repetitive setups.

Material Selection Guide for Custom CNC Part Manufacturing

Material selection is a crucial aspect of this CNC design guide, as the properties of CNC machined materials impact the machinability, cost, and overall quality of the finished part.

Metals: Metals are strong and durable materials suitable for manufacturing CNC machined parts subject to high stress and heavy loads. They also offer good machinability, heat and corrosion resistance, and are versatile, allowing them to be used to produce parts for a wide range of applications. Some common CNC metals include:

• Aluminum
• Steel
• Stainless steel
• Brass
• Copper
• Titanium

Plastics: Plastics are popular in CNC machining due to their affordability, light weight, and ease of forming into complex shapes. Furthermore, the chemical resistance of certain plastics, such as polypropylene (PP) and polyetheretherketone (PEEK), makes them ideal for manufacturing parts used in harsh chemicals or corrosive environments. Some common CNC plastics include:

• Polyoxymethylene (POM)
• Nylon
• Polycarbonate (PC)
• Acrylic (PMMA)
• Polyphenylene oxide (PPO)
• Polyetheretherketone (PEEK)
• Polyethylene (PE)

Surface Finish Options for Custom CNC Part Manufacturing

The surface finish of the final product affects its appearance, function, and durability. Common surface finish options for CNC machined parts include:

• As-machined: This is the original surface finish produced by the CNC machining process. The surface finish of the machined part is typically Ra 125 microinches, but tighter tolerances can be achieved by requesting finer finishes such as Ra 63 microinches, 32 microinches, or even 16 microinches. The machined surface may have visible tool marks, and the finish may be uneven.

• Sandblasting: Sandblasting is an excellent choice for creating a smooth, matte finish. This process sprays fine glass beads onto the surface of the machined part in a controlled manner. The resulting surface is smooth and uniform. Sandblasting can utilize different materials, such as sand, garnet, walnut shells, and metal beads, depending on the desired effect and the purpose of the sandblasting, whether it’s cleaning or a pre-treatment for further surface finishing.

• Anodizing (Type II or Type III): Anodizing is a versatile and popular surface treatment for CNC machined parts, offering excellent corrosion resistance, increased hardness, wear resistance, and improved heat dissipation. Its high-quality surface finish makes it suitable for painting and priming. Xtproto offers two anodizing processes: Type II, known for its corrosion protection, and Type III, which provides an additional wear-resistant layer. Both processes can also be customized to produce a range of color finishes.

• Powder coating: The powder coating process is an effective way to protect mechanical parts from wear, corrosion, and the elements. In this process, a special powder coating is applied to the part surface and then baked in an oven at high temperatures. This process creates a durable protective coating that is available in a variety of colors. Whether you’re looking for a classic or bold look, powder coating offers a versatile and durable solution for your mechanical parts.

• Custom: These surface finishes can be customized to meet specific design requirements and aesthetic preferences. Surface finishes range from simple color changes to complex textured patterns. Custom surface finishes are crucial for enhancing the appearance, durability, and performance of machined parts and are essential for creating a unique brand image.

Contact Xtproto to transform your custom part design into a CNC machined part

Get the best results from CNC machining with the right service, and Xtproto is your trusted CNC machining service provider, committed to delivering exceptional results that meet international standards. Our CNC machining services are ISO9001:2015 certified, ensuring high-quality parts that meet your specifications. Furthermore, our advanced digital manufacturing platform provides a seamless experience for customers seeking instant quotes for custom CNC parts. Our platform streamlines the design-to-production process, leveraging automation and expertise to ensure every part meets customer specifications. We pride ourselves on providing comprehensive DFM (design, manufacturing, and fabrication) experience that anticipates potential manufacturing challenges, ultimately delivering the highest quality results in the shortest possible turnaround time.