CNC milling is a modern method of material processing that has revolutionized the manufacturing industry. The process involves the precise removal of material from a solid using rotating, computer-controlled cutting tools. CNC (Computer Numerical Control) machines execute programmed toolpaths with extreme accuracy, making it possible to create complex shapes with tolerances down to a few micrometers.
The technology is used in a wide range of industries – from automotive, aerospace and medical to electronics. The main advantages of CNC milling include high precision, repeatability and the ability to machine a variety of materials, such as metals (steel, aluminum, titanium), plastics and composites.
The process begins with a CAD (Computer Aided Design) design, which is then converted into machine code by CAM (Computer Aided Manufacturing) software. This code contains instructions for the CNC machine regarding tool movements, rotational speed, depth of cut and other machining parameters.
Despite its advanced capabilities, CNC milling is subject to certain physical and technical limitations. Cylindrical tool shapes, limited access to certain areas of the workpiece or material properties can pose challenges when producing complex parts. Understanding these limitations is key to effectively designing parts for CNC machining.
Basic geometric limitations in CNC technology
CNC milling, despite its many advantages, faces some natural barriers due to the physics of the machining process. These constraints determine what shapes can be successfully produced.
Internal radii
One of the most characteristic limitations of CNC milling is the inability to create perfectly sharp inside corners. This is due to the cylindrical shape of the cutting tools. Each internal corner will have a radius at least equal to the radius of the cutter used. For example, if a cutter with a diameter of 6 mm is used, the smallest possible inner radius will be 3 mm.
Designers often recommend using internal radii of at least 130% of the tool radius, which ensures optimal machining and reduces wear on the cutter.
Depth of pockets and holes
Another important limitation is the ratio of the depth to the width of the pocket or hole. The standard recommendation is that the depth should not exceed four times the width.
Exceeding this value can lead to:
- Problems with chip evacuation
- Increased risk of tool breakage
- Deterioration of surface quality
- Difficulties in maintaining dimensional accuracy
For deep threaded holes, the load is mainly concentrated on the first turns of the thread (up to 1.5 × nominal diameter). For this reason, threads longer than 3 × nominal diameter are often unnecessary from the point of view of joint strength.
Thin walls
Machining parts with thin walls is challenging because of:
- The risk of vibration and distortion during cutting
- The possibility of damage to the workpiece by cutting forces
- Difficulties with clamping
For metals, the minimum recommended wall thickness is about 0.8 mm, while for plastics it is 1.5 mm. These values may vary depending on the type of material, its mechanical properties and machining parameters.
Undercuts and recesses
Standard 3-axis milling has limited possibilities for creating undercuts and recesses. This is due to the rectilinear motion of the tool in three axes, which makes it impossible to reach areas under overhangs or in deep, narrow spaces.
The solution to this problem may be to use:
- 4- or 5-axis milling
- Special angle tools
- Dividing the workpiece into parts that can be machined separately
3-axis milling does not allow machining complex undercuts and recesses. The limitation is a straight tool path. In such cases, 4- or 5-axis machines or angle tools are used. Sometimes the solution is to divide the workpiece into smaller pieces and machine them separately.
Influence of tool size on workpiece machining capabilities
The size of cutting tools directly affects the range of possible shapes and the efficiency of the CNC milling process. Choosing the right tool is a compromise between accuracy, machining time and technical capabilities.
Limitations of small tools
Small cutters (less than 3 mm in diameter) allow the creation of small workpieces and smaller internal radii. However, their use comes with certain limitations.
Smaller tools are more prone to breakage and require a lower feed rate, which increases machining time. Because of their delicacy, they can cause vibration during cutting, which negatively affects surface quality.
The machining of micro-elements (less than 2.5 mm) requires specialized micro-milling machines and often special machine settings. Such operations are classified as micromilling, which is governed by slightly different physical laws than standard machining.
Tool reach
The length of the cutting tool determines the maximum depth to which machining can be performed. Typically, the effective cutting length of a cutter is 3-4 times its diameter.
Longer tools are subject to:
- Increased vibration
- Deviations from the set path
- Deterioration of surface quality
- Faster wear
When designing deep pockets, it is important to consider that the working space of a CNC machine may be further limited by the tool length. Even if the theoretical range of motion of the Z-axis allows machining a deep workpiece, the actual depth may be limited by the maximum length of the available tool.
Impact on machining time
Tool size significantly affects machining time and cost:
- Larger cutters remove material faster, but cannot create small parts
- Smaller cutters allow greater precision, but increase machining time
- Changing tools during the process (e.g., from larger to smaller) requires additional time
Optimal tool selection involves using the largest cutters possible for coarse material removal, and then gradually switching to smaller tools for a finer finish. This approach balances machining time with the quality of the final product.
Constraint compensation strategies
Modern CAM software offers advanced machining strategies to help optimize the use of different tool sizes:
- Residual mach ining – automatically detecting areas that have not been machined with a larger tool and finishing them with a smaller one
- Adaptive cutting paths– adapting the tool path to the geometry of the workpiece, which reduces the load on the tool
- Plunge optimization – controlled plunge of the tool into the material, extending the life of the cutter
CNC milling services at CNC Partner
CNC Partner is a company specializing in precision CNC metalworking. The company offers comprehensive CNC milling services with the highest quality and accuracy.
Advanced machine park
The company has a state-of-the-art machinery park, which allows it to carry out even the most demanding projects.
The equipment includes:
- GF Mikron VCE 1600 Pro – working area: 1700 x 900 x 800 mm
- GF Mikron VCE 800 – working field: 800 x 500 x 540 mm
- AVIA VMC 800 V – working field: 1000 x 550 x 600 mm
- AVIA VMC 650 V – working area: 800 x 550 x 600 mm
The varied dimensions of the working fields allow CNC Partner to process small parts and larger industrial parts.
Interesting fact: CNC Partner’s largest milling machine, the GF Mikron VCE 1600 Pro model, allows the machining of parts exceeding 1.5 meters in size, enabling projects for heavy industry and aerospace.
Material specialization
CNC Partner is distinguished by the wide range of materials it processes.
The company specializes in milling:
- Aluminum of various grades (PA4/6082, PA6/2017, PA9/7075, PA11/5754, PA13/5083), which are used in the aerospace, automotive and medical industries. Each aluminum grade requires a specific approach to processing due to differences in hardness, machinability and mechanical properties.
- Structural steels of the S235 and S355 types, commonly used in the railroad, automotive and construction industries. Machining steel requires the proper selection of tools (usually carbide) and cutting parameters.
CNC Partner machines various grades of aluminum and steel, matching the tools to the properties of the material. The company makes parts for the aerospace, automotive, medical and construction industries. The selection of parameters depends on the hardness and machinability of the metal. Milling is carried out precisely and according to project requirements.
Comprehensive range of services
In addition to CNC milling, the company offers a range of complementary metalworking services:
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CNC turning – carried out on a HAAS SL-30THE machine tool with a passage fi 76 mm, a maximum turning diameter fi 482 mm and a turning length of 864 mm
-
Wire electrical discharge machining (WEDM) – carried out on two GF CUT 300SP machines with a working area of 550 x 350 x 400 mm
-
CNC grinding – performed on a JUNG grinding machine with a working area of 2000 x 1000 mm
Such a variety of technologies allows complex projects to be carried out comprehensively without the need for subcontractors, which translates into shorter lead times and better quality control.
Problems with machining complex internal shapes
Machining complex internal shapes is one of the biggest challenges in CNC milling technology. Limitations arise from both the physical characteristics of the process and the design of the machines and tools.
Challenges with internal channels
Creating complex internal channels faces a number of difficulties.
Curved or helical channels are virtually impossible to produce with standard CNC milling methods. This is due to the fact that cutting tools travel in straight lines or curves in specific planes. Making a curved hole would require that the tool be able to change direction inside the material, which is physically impossible.
For channels with variable cross sections or complex geometry, tool access can be severely limited or impossible. Standard cutters are unable to reach all areas of such a channel.
Interesting fact: For some complex internal channels, alternative manufacturing methods are used, such as 3D printing or electrical discharge machining (EDM). With EDM, an appropriately shaped electrode can create complex internal geometries without requiring the tool to physically access all areas.
Problems with internal cavities
Machining internal cavities with complex shapes poses the following problems:
- Difficulty draining chips from deep cavities
- Limited tool access to corners and nooks
- Risk of collision between the tool holder and the workpiece
- Need for special machining strategies
For cavities that are more than four times as deep as their width, effective machining becomes much more difficult. This involves problems with cooling, chip evacuation and maintaining tool stability.
Limitations in creating sharp edges
Creating sharp inside edges is one of the fundamental limitations of CNC milling:
- Each inside corner will have a radius at least equal to the tool radius
- Smaller radii require smaller tools, which increases machining time
- Very small radii (less than 1 mm) increase the risk of tool breakage
For applications requiring perfectly sharp inside corners, alternative methods such as EDM or waterjet cutting may be necessary.
Limitations associated with tool access to hard-to-reach areas
Tool access to all areas of the workpiece is a key limitation in CNC milling technology. This problem is particularly evident when machining complex geometries with deep recesses, undercuts or workpieces that require machining at different angles.
Undercuts and overhanging features
Undercuts pose a particular challenge for standard 3-axis milling machines.
In traditional 3-axis milling, the tool moves perpendicular to the machining plane, making it impossible to reach areas under overhangs or requiring machining from underneath. Features such as dovetail grooves, angled undercuts or internal protrusions are difficult or impossible to machine without special solutions.
Standard solutions to this problem include:
- Changing the orientation of the part during machining (requires additional fixtures)
- Dividing the model into parts that can be machined separately
- Use of special angle tools (limited effectiveness)
Undercuts require a non-standard approach, since the 3-axis milling machine cannot reach hard-to-reach areas. Machining such parts forces the workpiece to be repositioned, the use of angle tools or the division of the model. Each of these solutions increases working time and requires precision. In many cases, a 5-axis machine is a better choice.
Deep pockets and holes
Machining deep pockets and holes faces the following limitations:
- The maximum machining depth is limited by the length of available tools
- Long, slender tools are prone to vibration and deformation
- Difficulties with chip removal from deep holes
- Cooling problems in deep cavities
For holes with depth-to-diameter ratios greater than 10:1, special deep-hole drills or partition drilling techniques are often used to provide better chip evacuation and cooling.
Impact of tool geometry on accessibility
The shape and dimensions of a cutting tool directly affect the ability to reach hard-to-reach areas:
Standard cutters have a limited length-to-diameter ratio, which affects their rigidity. Typically, the effective cutting length is 3-4 times the tool diameter. Exceeding this value increases the risk of vibration and deformation.
Tool holders can interfere with the workpiece, restricting access to deep cavities or areas close to high walls. This problem is particularly evident when machining small pockets of depth.
Strategies for minimizing access problems
Modern design approaches take into account tool access limitations:
- Avoiding deep, narrow pockets in favor of wider, shallower geometries
- Designing with machining direction in mind
- Adding wall slopes in deep cavities (min. 0.5° per side)
- Considering corner radii corresponding to available tools
Designing parts with milling in mind increases efficiency and reduces the risk of errors. Wider pockets, inclined walls and corner radii that match tools make machining easier. A well-chosen shape reduces working time and improves surface quality. Consideration of milling direction eliminates the need for complex solutions.
Technology solutions for overcoming the limitations of CNC milling
Despite the inherent limitations of CNC milling, there are a number of technological solutions to overcome or minimize them. Modern approaches combine advanced machinery, specialized tools and innovative machining strategies.
Multi-axis milling
Four- and five-axis milling represent a breakthrough in the ability to machine complex shapes.
Additional axes of rotation allow the tool to be positioned at different angles relative to the workpiece. This makes it possible to reach areas inaccessible with traditional 3-axis milling, such as undercuts, angled surfaces or complex spatial geometries.
5-axis milling allows the entire workpiece to be machined at a single fixture, eliminating repositioning errors and reducing production time. This technology is particularly valuable for manufacturing parts with complex, organic shapes, such as turbine blades, medical implants and injection molds.
Interesting fact: Advanced 5-axis milling uses a simultaneous machining technique, where all axes move simultaneously, tracing complex contours. This allows the tool to maintain an optimal angle relative to the surface being machined, which significantly improves the quality of the finish.
Specialized tools
Developments in tool technology offer new opportunities to overcome limitations:
- Variable-geometry cutters – reduce vibration and enable faster machining
- Extended reach tools – allow deeper cavities to be machined
- Micro milling cutters – allow the creation of finer details and smaller radii
- Ball mills – ideal for machining curved surfaces
- Angled milling cutters – make it easier to access undercuts and inclined surfaces
Modern tools increase milling capabilities and reduce working time. Ball milling cutters work well on curved surfaces, while micro milling cutters work on fine details. Extended reach models reach deep recesses. Angled tools make it easier to work undercuts. Variable blade geometry reduces vibration and improves surface quality.
Hybrid manufacturing methods
Combining different technologies overcomes the limitations of a single method.
The combination of CNC milling and EDM enables the creation of complex internal geometries. EDM makes it possible to produce sharp interior corners, deep slots and complex shapes that are impossible to achieve through milling alone.
The integration of additive technologies (3D printing) with CNC machining combines the freedom of shaping characteristic of 3D printing with the dimensional precision of milling. This hybrid method is particularly useful for manufacturing parts with internal cooling channels, voids or complex internal structures.
Advanced machining strategies
Modern CAM software offers advanced strategies that maximize CNC milling capabilities:
- Trochoidal machining – allows efficient material removal with less tool loading
- Adaptive cutting paths – adapts machining parameters to the local geometry of the workpiece
- Residual machining – automatically detects areas that need to be completed with a smaller tool
- Machining simulation – allows detection of potential problems before actual machining begins
Combining these strategies with the right choice of tools and cutting parameters allows CNC milling to greatly expand its capabilities, pushing the limits of what can be done.
Summary
CNC milling, despite its many advantages, is subject to certain limitations due to the physics of the cutting process, tool geometry and machine design. Key limitations include the inability to create perfectly sharp inside corners, the difficulty of machining deep pockets, or problems with tool access to undercuts and overhanging features.
The size and geometry of cutting tools directly affect the range of shapes that can be made. Smaller tools allow the creation of finer details, but increase machining time and are more prone to breakage. Long, slender tools allow deeper cavities to be machined, but at the expense of stability and accuracy.
Modern technological solutions such as multi-axis milling, specialized tools and hybrid manufacturing methods overcome many of these limitations. Advanced machining strategies implemented in CAM software further expand the capabilities of CNC milling.
Awareness of the limitations of CNC technology and knowledge of the available solutions are essential for the effective design of parts to be machined. Taking these factors into account at the design stage allows optimization of the manufacturing process, reduction of costs and avoidance of feasibility problems.
Companies such as CNC Partner, with their advanced machinery and wide range of services, are able to meet even the most demanding challenges by offering comprehensive metalworking solutions.
Sources:
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- https://gab.wallawalla.edu/~ralph.stirling/classes/engr480/examples/nvx/NVX/Helpful Docs/CNC_Machining_The_Complete_Engineering_Guide.pdf
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