CNC Machining Design Guideline: Maximizing Your Results

CNC machining is a versatile manufacturing process that can produce a wide range of parts with high precision and repeatability. It is an essential manufacturing process for producing high-precision, complex parts for various industries, including aerospace, automotive, healthcare, and electronics. Despite its many advantages, it is important to carefully follow CNC machining design guidelines to ensure that parts are produced to the highest quality standards.

By following the established CNC machining design guide, you can produce parts that meet desired specifications and production requirements and ensure high quality and repeatability. A thorough understanding of the capabilities and limitations of the machinery is required to design parts for CNC machining. This guide outlines key design considerations and guidelines to ensure the best results for CNC machined parts.

What Is CNC Machining?

CNC (Computer Numerical Control) machining is a unique method of fabricating parts from raw materials using computer-controlled machines. This subtractive manufacturing process creates finished components by removing material layers from solid blocks of materials (blank) to produce the desired shape.

In CNC machining, a part design is created in a CAD (Computer-Aided Design) program. Then the design is translated into machine-readable code (G-code) and fed into a CNC machine. The CNC machine then uses cutting tools to precisely remove material from the raw material and produce the desired part shape.

CNC machines like vertical & horizontal milling machines and lathes can operate on various axes. To create relatively simple parts, traditional 3-axis CNC machines can manipulate parts along three linear axes (X, Y, and Z ). The 5-axis CNC machining can work along the three linear axes and around two rotational axes to create more complex components.

CNC production machining is widely used in the manufacturing industry for producing high-precision, complex parts in various materials, including metals, plastics, and composites. It is popular for its speed, automation, and increased precision. It is also a scalable manufacturing process because it is ideal for prototyping, one-off production, and large-scale production.

Importance of CNC Design for Manufacturability

The design of a part is the foundation of the entire manufacturing process and is critical to the success of the finished product. Design for Manufacturability (DfM) helps to optimize the manufacturing process, making it faster, more efficient, and cost-effective. This often requires the modification of specific features that are not feasible to produce with the available equipment and materials.

A successful DfM strategy offers the following advantages:

Reduce Manufacturing Costs and Time

The design of a part plays a significant 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 ensure that the production process is optimized for speed and efficiency. This can lead to reduced cycle times, improved productivity, and a reduction in overall production costs.

Streamline Manufacturing Process Efficiently

The design of a part can also impact the efficiency of the CNC machine. A well-designed part can minimize tool wear, reduce cycle times, and increase machine utilization, improving productivity and profitability. DFM also ensures increased material utilization. By optimizing the use of materials, manufacturers can reduce costs and increase profitability.

A well-designed part will minimize material waste, which can significantly impact the overall cost of production. By considering factors such as material type, thickness, and part geometry, manufacturers can ensure that materials are used efficiently and effectively.

Avoid Fatal Design Flaws

The use of CAD (Computer-Aided Design) and CAM (Computer-Aided Manufacturing) software allows for quick and easy changes to the design of a part without the need for costly tooling changes. This flexibility will enable manufacturers to respond quickly to changing customer requirements and to make design changes as needed to improve performance, quality, or cost.

For example, they may be able to simplify tool paths, reduce the number of setups required, or optimize material utilization. Moreover, adequate design increases the possibilities of automation, reducing human errors and the need for multiple setups.

CNC Machining Design Guidelines: Cost Reduction Tips

Adhering to certain design guidelines can reduce the costs associated with CNC machining while maintaining high standards of quality and precision. In this section, we will explore various CNC design guidelines that can be incorporated into your design process to reduce the CNC machining cost.

Common Design Practices for CNC Machining

The following are essential design practices and tips to help you get the best out of the design for CNC machining:  

Choose Softer Material

One vital design practice to reduce CNC machining costs is choosing a softer material. Softer materials are typically easier to machine, resulting in faster cutting speeds and reduced tool wear. This, in turn, can result in shorter machining times and lower costs.

Additionally, softer materials are often less prone to cracking or deformation during machining, resulting in improved part quality and reduced post-machining processing time. However, it is important to consider the intended use and final application of your product, as softer materials may not be suitable for high-stress or high-wear applications.

Minimize Tool Changes and Workholding Setups

This is one of the most critical CNC machining design guidelines that can significantly reduce the cost and lead time of your machining operations. The more tool changes and setups required during a machining cycle, the more time-consuming and expensive the process becomes. To minimize the number of tool changes and setups, consider the following design practices:

• Design your parts with similar features and geometries that can be machined with a single tool.
• Use multi-functional tools that can perform multiple operations with a single tool change.
• Reduce the required setups by designing parts with consistent orientations or using modular fixtures that can accommodate multiple parts.

Avoid Non-Planar and Draft Angle Surfaces

Non-planar and draft angle surfaces are complex and challenging to machine, which can result in slower cutting speeds, longer machining times, and increased tool wear. Additionally, these surfaces can make it more difficult to achieve consistent part quality and tight tolerance. To avoid non-planar and draft angle surfaces in your design:

• Consider using simple and flat 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 allow easy material removal and reduce tool wear during machining.

Increase the Size of Internal Fillets

This simple design practice can greatly improve the quality and efficiency of your machining operations. Internal fillets are rounded corners or transitions within a part that help reduce stress concentrations and improve the part’s strength. By increasing the size of these fillets, you can also improve the machining process by:

• Reducing cutting forces and tool wear during machining.
• Improving chip removal and material flow during cutting.
• Reducing the likelihood of tool breakage and premature tool wear.
• Improving surface finish and part quality.

Add Undercuts to Sharp Corners

Undercuts are recesses or notches in the corners of a part that allow for better tool access and improved material removal during machining. They are used to:

• Reduce cutting forces and tool wear.
• Improve chip removal and material flow during cutting.
• Reduce the likelihood of tool breakage and premature tool wear.
• Improve surface finish and part quality.

However, creating undercuts can be a complex and challenging task because they can be difficult to reach 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. The following should be taken into consideration when designing undercuts:

	Recommended
Undercut dimension	3 mm to 40 mm
Undercut clearance	4x depth

Use Standard Tolerances

In CNC machining, standard tolerances are used to ensure that the finished part meets the desired specifications and functional requirements. Unnecessary tight tolerances can increase the cost and time of machining. By specifying standard CNC machining services tolerances, manufacturers can reduce the need for secondary operations and improve the overall efficiency of the machining process.

	Recommended	Feasible
Tolerances	±0.1 mm	±0.02 mm

Text and Lettering

When creating text or lettering, the tool must be able to maintain a constant width, height, and spacing throughout the machining process. Any variation in these factors can result in a final product that doesn’t meet design specifications.

Another issue to consider is the font and size of the text or lettering. If the text is too small, it may be difficult to read or not meet the desired specifications. If it is too large, it may cause tool deflection or affect the accuracy and precision of the machining process.

To address these challenges, you can use standard fonts that are well-suited for CNC machining and avoid overly complex or fine lettering. Additionally, you can specify a larger font size and opt for a font with a more consistent width, height, and spacing. Additionally, it is essential to carefully consider the orientation of the text relative to the workpiece and adjust the tool accordingly to maintain a consistent height, spacing, and cutting speed.

Part Size

Several types of CNC machines have varying capabilities, including size and capacity. Some machines may be too small to accommodate large parts, while others may not be able to handle parts that are too small. As a result, designers need to carefully consider the part size and choose the appropriate machine accordingly.

In addition to the size of the machine, the part size can also impact the speed of the machining process. Larger parts require more material to be removed and take longer to machine than smaller parts. This means that the machining time for larger parts is often longer, and production costs are higher.

	Maximum Dimension	Minimum Dimension
CNC Milling	4000×1500×600 mm157.5×59.1×23.6 in.	4×4 mm0.1×0.1 in.
CNC Turing	200×500 mm7.9×19.7 in.	2×2 mm0.079×0.079 in.

For Drilling Parts

Optimal Hole Depth

The ideal depth of a drilled hole should balance the stability of the tool and the strength of the material being machined. Drilling too shallow can result in a weak joint and reduce the holding power of screws while drilling too deep can cause the drill bit to break or bend, leading to poor accuracy and surface finish.

To determine the optimal hole depth, you must consider the drill bit’s size, the material’s hardness and thickness, the strength required for the intended application, and the overall stability of the machine setup. Drilling the hole just deep enough to accommodate the screw or fastener is recommended, leaving some material for support. If a countersink is required, then the hole should be drilled deeper to allow for the countersink.

	Recommended	Feasible
Hole Depth	4 times the nominal diameter	40 times the nominal diameter

Distinguish Through Holes and Blind Holes

In CNC machining, understanding the difference between through holes and blind holes is important, as they both require different drilling techniques and tools. A through hole is a hole that extends entirely through the workpiece from one end to the other. This type of hole is generally easier to produce, as the drill has to enter and exit the part at opposite sides. Through holes can be used for many purposes, including fastening, mounting, and routing of electrical and mechanical components.

Blind holes, on the other hand, do not go all the way through the workpiece and stop at a specific depth. These holes are often used to create cavities, recesses, or pockets within the workpiece and are generally more challenging to produce than through holes. Blind holes require special drill bit geometries and cutting speeds to ensure that the cutting edge does not break through the bottom of the part.

Through Holes	Blind Holes
Tip 1: Determine the correct drill sizeTip 2: Maintain rigidityTip 3: Use proper cutting fluidsTip 4: Monitor drill speedTip 5: Drill in stages	Tip 1: It should be 25% longer than the needed depthTip 2: Use a center drillTip 3: Ensure sufficient hole depth above the drill tipTip 4: Reduce speed and feed ratesTip 5: Avoid reaming

Avoid Partial Holes

A partial hole occurs when the drill does not fully penetrate the material and can be caused by various factors such as the drill bit breaking, incorrect drill bit selection, or incorrect parameters such as speed, feed, and depth of cut. Therefore, you should select the right drill bit, maintain the right parameters, and use coolant to dissipate heat.

Avoid Drilling Through Cavities

While drilling, keep in mind that intersecting holes with existing cavities in parts can compromise its structural integrity. To avoid this, it’s best to position the drill points away from existing cavities. However, if the drilled hole must cross the cavity, make sure that its center axis does not intersect with it to maintain the part’s stability.

Design Standard Drill Size

Optimize your design for standard drill sizes to save time and money. Designing for common tools will make it easier for machine shops to produce your part without needing costly custom tooling. Consider using a standard drill size like 0.12 ” instead of a more precise but less common size like 0.123”. Also, try to limit the number of different drill sizes used in your design, as multiple sizes increase the time and effort required for tool changes during the machining process.

	Recommended	Feasible
Drill Size	Standard drill bit (0,12”)	Any diameter larger than 1 mm

Specify Threaded Holes

A threaded hole allows for the attachment of bolts, screws, and other threaded fasteners. Make sure to specify the correct depth of the thread so that the threaded fastener has enough engagement to hold the part together. The deeper the thread, the stronger the fastener grip.

The type of material can affect the type of thread.. Soft materials may require a shallower thread, while harder materials may need a deeper thread. When specifying threaded holes in a drawing, use clear and accurate thread callouts to ensure the correct thread standard, pitch, and depth. Ensure enough clearance for the threaded fastener to be installed and removed without binding or stripping the thread.

	Recommended	Feasible
Thread Length	3 times nominal diameter	1.5 times nominal diameter

Avoid Deep Taps

To achieve accurate and precise results, avoiding deep taps is crucial in CNC machining design. The longer the tap, the greater the risk of it vibrating and wandering while in operation, leading to imperfections in the final product. A tap that exceeds 3 times its diameter is considered deep and can pose a significant challenge.

However, in many cases, even a tap that goes 1.5 times the diameter will provide ample thread engagement, thereby eliminating the need for a deep tap altogether. Using deep taps increases the risk of tool breakage, flawed threads, and diminished precision, making it an undesirable aspect of CNC machining design.

	Recommended	Feasible
Tap Size	0.5 times the diameter	1.5 times the diameter

For CNC Milling Parts

Keep Available Cutting Tools in Mind

Designing CNC parts with consideration for commonly available CNC milling tools can reduce cost and lead time. By using standard tools and designing to their standard sizes, you can minimize the need for custom or specialty tools.

When designing internal fillets, for example, make sure the radius is not smaller than what a standard cutting tool can handle. Otherwise, a tool change to a smaller tool would be necessary to achieve the feature, which may not be worth the extra time and expense.

Avoid Sharp Internal Corners

Designing parts with sharp internal corners is not possible with CNC milling, as round tools are used. To create a smooth transition, radiused corners must be incorporated, and the radius must be larger than the cutter’s diameter. The size of the radius should be half the diameter of the cutting tool. For instance, if a 1/4” cutter is used, the fillets must be larger than 1/8”.

To accommodate parts with sharp corners, holes can be drilled to “break” the corners, allowing sharp edges to fit within the cavity. Additionally, fillets are crucial when sloped or drafted surfaces meet vertical walls or sharp edges. Using square or ball end mills will always result in material between the wall and surface unless the surface is flat and normal to the tool.

Avoid Deep, Narrow Slots or Pockets

The final depth of cut should not exceed certain ratios based on the material being machined. For instance, with plastics, the ratio should not be greater than 15 times the diameter of the end-mill, aluminum should be no more than 10 times, and steel’s limit is 5 times. This is because longer tools are more susceptible to deflection and vibration, leading to surface imperfections.

Furthermore, the internal fillet radius also depends on the diameter of the cutting tool. If a 0.55” wide slot for a steel part is to be machined using a 0.5” end-mill, then the depth should not exceed 2.75”. Additionally, high length-to-diameter ratio end mills can be harder to obtain. Hence, it is advisable to either decrease the depth of the slot or feature or increase the diameter of the cutting tool.

	Recommended	Feasible
Cavity Depth	4 times cavity width	10 times tool diameter or 25 cm

Design the Largest Allowable Internal Radii

The cutting tool size used in CNC milling is a crucial factor to consider when designing your part. A larger cutter removes more material in one pass, reducing machining time and costs. To take full advantage of the capabilities of larger cutters, design your internal corners and fillets with the largest possible radius, preferably greater than 0.8mm.

An added tip is to make the fillets slightly bigger than the end-mill’s radius, such as a radius of 3.3mm instead of 3.175mm. This creates a smoother cutting path and produces a finer finish on your machined part.

	Recommended
Internal Corner Radius	⅓ times cavity depth (or larger)

Choose Suitable Thickness

It’s important to note that thin walls in parts can create significant challenges in the machining process, especially in terms of maintaining stiffness and accuracy of dimensions. To avoid these difficulties, it’s recommended to design walls with a minimum thickness of 0.25mm for metal components and 0.50mm for plastic parts in order to withstand the rigors of the manufacturing process.

	Recommended	Feasible
Wall Thickness	1.5 mm (plastics), 0.8 mm (metals)	1.0 mm (plastics), 0.5 mm (metals)

For CNC Turning Parts

Avoid Sharp Internal Corners

Sharp internal and external corners in a part design can be a challenge during machining. To overcome this issue, it is recommended to have radiused internal corners, providing a gradual transition for the tool to move smoothly. Alternatively, incorporating a slight angle in the steep sidewalls can eliminate sharp internal corners and simplify the machining process by reducing the number of operations required with a single tool.

Avoid Long, Thin Turned Parts

Instability is a common concern when it comes to long, thin-turned parts. The spinning part can easily chatter against the tool, leading to an imperfect finish. To combat this, consider incorporating a center drill at the end and using a center to keep the part spinning in a straight manner. This can help maintain stability and achieve a better outcome. As a guideline, it’s best to keep the length-to-diameter ratio at or below 8:1 to minimize the risk of instability during machining.

Avoid Thin Walls

When turning parts, it’s important to be mindful of the amount of material being machined away. Over-machining can cause undue stress on the part, while thin walls can result in decreased stiffness and difficulty in maintaining tight tolerances. As a guideline, the wall thickness of turned parts should be kept at a minimum of 0.02 inches to ensure stability and accuracy during the manufacturing process.

	Recommended	Feasible
Wall Thickness	1.5 mm (plastics), 0.8 mm (metals)	1.0 mm (plastics), 0.5 mm (metals)

Limitations That Affect CNC Machining Design

CNC machining is a highly precise and efficient method for producing complex parts and components. However, like any manufacturing process, certain limitations must be considered when designing CNC machining parts. Understanding these limitations can help to ensure that the final product meets the required specifications and that the production process is as efficient and cost-effective as possible.

The limitations to CNC machining design include the following:

Tool Capabilities

When it comes to CNC machining, one aspect that often poses a hurdle is an ability to reach and precisely machine features with a large depth-to-width ratio. The tool capabilities and access play a significant role in determining what shapes can be effectively machined. The tool must approach the workpiece from above to remove material, making it difficult to reach and machine intricate features.

For instance, deep cavities may require tools with extended reach to reach the bottom, which can increase machine chatter and a reduction in accuracy. The size, shape, travel distance, and other factors contribute to the limitations of CNC machining and can impact the final product’s precision.

Tool Shape

When it comes to CNC machining, one vital aspect to consider is the geometry of the cutting tool. A large majority of CNC cutting tools have a cylindrical shape and limited cutting length, which affects the final cut and the shapes that can be achieved.

For example, the internal corners of a workpiece will always have a radius, even if the cutting tool used is extremely small. This is because the tool’s geometry is transferred onto the machined part as the material is removed. The cylindrical shape and restricted cutting length of common CNC cutting tools, such as end mill tools and drills, also limit their ability to machine certain features.

Tool Stiffness

In CNC machining, the cutting tool is typically made from a material such as carbide, tungsten, or similar material with superior properties compared to the workpiece. Despite these materials’ high-performance characteristics, tool deflection can still occur and be a major source of deviation in the design and final results.

While working with general tolerances may not present a problem, the slight deflection of the tool can become a significant issue in extremely precise jobs with tight tolerances. The deviation caused by tool deflection can restrict the design possibilities and compromise the accuracy of the final product.

Workpiece Stiffness

CNC cutting tools are known for their exceptional stiffness and high-performance characteristics. However, some workpiece materials can prove to be a challenge, particularly if they have their own superior mechanical properties.

The stiffness of the workpiece can result in vibrations and deflections that negatively impact the accuracy and precision of CNC machining operations. The precision and accuracy achievable with a stiff workpiece can vary, making it challenging to meet tight tolerances.

Workpiece Shape

The stability and success of a CNC machining process largely depend on the shape of the workpiece. The geometry of the workpiece is a critical factor that determines the number of processes required and the overall viability of the design. In some cases, complex geometries may require reorientation during machining, even on multi-axis machines. These may reduce the overall efficiency of the process.

Workholding

Stiffness is crucial in machining as it ensures smooth and accurate operations. A weak link in the “chain of stiffness” comprised of the machine, tool, part, and fixture can lead to vibrations and reduce precision. Any part movement during machining leads to inconsistent results and deviates from the design tolerances. A poor setup results in low accuracy and lacks precision, as each machined part will differ from the others.

Material Selection Guide for CNC Machining

Material selection is an essential aspect of the CNC machining design guide. The material’s properties will affect the machinability, cost, and overall quality of the finished part. When selecting CNC machining materials, you must consider machinability, mechanical properties, cost, availability, and environmental impacts.

Metals

Metals are known for their high strength and durability, making them ideal for use in CNC machined parts that will be subject to high stress and heavy loads. They also have good machinability, heat & corrosion resistance, and they are highly versatile in producing components for different applications.

Some of the common metals used in CNC machining include:

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

Plastics

Plastics are widely used in CNC machining due to their low cost, lightweight, and ability to be molded into complex shapes. Some plastics also have good chemical resistance, making them ideal for use in parts that will be exposed to harsh chemicals or corrosive environments.

Some common plastics used in CNC machining are:

• Acetal (POM)
• Nylon
• Polycarbonate (PC)
• Acrylic (PMMA)
• Polyphenylene Oxide (PPO)
• Polyethylene (PE)

Surface Finishes Selection for CNC Machining

Surface finish is a crucial consideration in CNC machining as it can affect the final product’s appearance, functionality, and durability. Several surface finishing options are available to complete CNC-machined parts to meet aesthetic and functional requirements.

They include the following:

As Machined

This is the raw surface finish that results from the CNC machining process. The surface of an as-machined part typically has a finish similar to 125 µin Ra, although tighter tolerances can be achieved by requesting a finer finish of 63, 32, or even 16 µin Ra. The surface may have visible tool marks, and the finish may not be uniform.

Bead Blasting

For a sleek, matte texture, bead blasting is a great option. This process involves propelling fine glass beads at the machined part’s surface in a controlled manner. The resulting finish is smooth and uniform. Different materials, such as sand, garnet, walnut shells, and metal beads, can be utilized depending on the desired outcome and the purpose of the bead blasting, whether it’s for cleaning or as a pre-treatment for further surface finishing.

Anodizing (Type II or Type III)

Anodization is a versatile and popular surface treatment for CNC machined components, offering superior resistance to corrosion, increased hardness, wear resistance, and improved heat dissipation. It’s widely used for painting and priming due to its high-quality finish. At RapidDirect, we offer two forms of anodization: Type II, known for its corrosion protection, and Type III, which provides an additional layer of wear resistance. Both processes can be tailored to produce a range of color finishes to suit your specific needs.

Powder Coating

The powder coating process is a highly effective way to protect machined parts from wear, corrosion, and elements. In this method, a special type of powdered paint is applied to the part’s surface, and then it is subjected to high heat in an oven. This process creates a long-lasting, protective coating with a multitude of color options to choose from. Whether you need a classic or bold look, powder coating provides a versatile and durable solution for your machined parts.

Custom

These surface treatments are tailored to meet specific design requirements and aesthetic preferences. These finishes can range from simple color changes to complex textured patterns. Custom finishes are essential for improving machined parts’ appearance, durability, and performance and can be important in creating a unique brand identity.

Start Your CNC Machining Project in 3 Steps

RapidDirect is your reliable CNC machining service provider committed to delivering exceptional results that meet international standards. With ISO9001:2015 certification, our CNC machining services ensure high quality parts that meet your specifications.. In addition, our cutting-edge digital manufacturing platform offers a seamless experience for customers looking to get instant quotes for their CNC parts.

With a combination of automation and expert knowledge, our platform streamlines the design process and ensures that each part meets our customers’ specifications. We take pride in providing a comprehensive DFM experience that anticipates any potential manufacturing challenges, ultimately delivering top-quality results in the shortest turnaround time possible.

Start your CNC machining project in just 3 simple steps:

Upload Your Technical Drawing

The first step is to create a detailed technical drawing of your part. This design should include all the critical dimensions, features, and surface finishes you need for your part. Then you can export the drawing to a CAD file format (STEP, STP, STL, IGES) using CAD software. You can then simply upload the CAD design on our online quotation platform.

Get Instant Quote

Our instant quotation platform lets you get a detailed price breakdown within a few minutes. It is simple, straightforward, and convenient. The instant quote also comes with a free, detailed DFM analysis report to help you improve your part’s design.

Start Manufacturing

Once you review to quote and confirm every design specification, our expert technicians will begin your CNC machining project to bring your design to life. In our platform, you can track specific production processes to get vital insight into your production efficiency.

Conclusion

Guide to designing for CNC machining is a critical aspect of the manufacturing process that should not be overlooked. By focusing on the part’s design, you can significantly improve the quality and efficiency of the manufacturing process, reduce costs, and increase profitability. To ensure the best results, it is best to work with experienced CNC machining professionals who have the knowledge and expertise to design parts that meet the highest standards. Contact RapidDirect today to begin your CNC machining project.