12 Key Lessons Learned in CNC Machining
To fully utilize the capabilities of CNC machining, designers must design according to specific manufacturing rules. However, this can be challenging because specific industry standards do not exist. In this article, we have compiled a comprehensive guide to the best design practices for CNC machining. We have focused on describing the feasibility of modern CNC systems and have disregarded the associated costs. For a guide to cost-effectively design parts for CNC, refer to this article.
CNC Machining
CNC machining is a subtractive manufacturing technique. In CNC, different cutting tools that rotate at high speeds (thousands of RPM) are used to eliminate material from a solid block in order to create a part based on a CAD model. Both metals and plastics can be machined using CNC.
CNC machining offers high dimensional accuracy and tight tolerances suitable for both high-volume production and one-off jobs. In fact, it is currently the most cost-effective method for producing metal prototypes, even when compared to 3D printing.
CNC Main Design Limitations
CNC offers great design flexibility, but there are certain design limitations. These limitations are related to the basic mechanics of the cutting process, mainly to tool geometry and tool access.
1. Tool Shape
The most common CNC tools, such as end mills and drills, are cylindrical and have limited cutting lengths. As material is removed from the workpiece, the shape of the tool is replicated on the machined part.
For instance, this means that internal corners of a CNC part will always have a radius, regardless of the size of the tool used.
2. Tool Calling
When removing material, the tool approaches the workpiece directly from above. This cannot be done with CNC machining, except for undercuts, which we’ll discuss later.
It’s a good design practice to align all features of a model, such as holes, cavities, and vertical walls, with one of the six cardinal directions. This is more of a suggestion than a restriction, especially since 5-axis CNC systems offer advanced work holding capabilities.
Tooling is a concern when machining parts with features that have a large aspect ratio. For instance, reaching the bottom of a deep cavity requires a specialized tool with a long shaft, which can reduce end effector stiffness, increase vibration, and reduce achievable accuracy.
CNC Process Design Rules
When designing parts for CNC machining, one of the challenges is the absence of specific industry standards. This is because CNC machine and tool manufacturers are continually enhancing their technical capabilities, thus broadening the range of what can be achieved. Below, we have provided a table summarizing the recommended and feasible values for the most common features found in CNC machined parts.
1. Pockets and Recesses
Remember the following text: “Recommended Pocket Depth: 4 Times Pocket Width. End mills have a limited cutting length, usually 3-4 times their diameter. When the depth-to-width ratio is small, issues such as tool deflection, chip evacuation, and vibration become more prominent. To ensure good results, limit the depth of a cavity to 4 times its width.”
If you need more depth, you might want to think about designing a part with variable cavity depth (see the image above for an example). When it comes to deep cavity milling, a cavity is classified as deep if its depth is more than six times the diameter of the tool being used. Special tooling allows for a maximum depth of 30 cm with a 1-inch diameter end mill, which equals a tool diameter to cavity depth ratio of 30:1.
2. Inside edge
Vertical corner radius: ⅓ x cavity depth (or greater) recommended
It is important to use the suggested inside corner radius values for selecting the right size tool and to adhere to the recommended cavity depth guidelines. Slightly increasing the corner radius above the recommended value (e.g., by 1 mm) enables the tool to cut along a circular path instead of at a 90° angle, which results in a better surface finish. If a sharp 90° inside corner is needed, consider adding a T-shaped undercut rather than reducing the corner radius. For floor radius, the recommended values are 0.5 mm, 1 mm, or no radius; however, any radius is acceptable. The lower edge of the end mill is flat or slightly rounded. Other floor radii can be machined using ball-end tools. Adhering to the recommended values is a good practice as it is the preferred choice for machinists.
3. Thin Wall
Minimum wall thickness recommendations: 0.8 mm (metal), 1.5 mm (plastic); 0.5 mm (metal), 1.0 mm (plastic) are acceptable
Reducing the wall thickness decreases the stiffness of the material, leading to increased vibrations during machining and reduced achievable accuracy. Plastics have a tendency to warp due to residual stresses and soften due to increased temperature, therefore, it is recommended to use a larger minimum wall thickness.
4. Hole
Diameter Standard drill sizes are recommended. Any diameter greater than 1 mm is feasible. Hole-making is done with a drill or end cnc milled. Drill sizes are standardized in metric and imperial units. Reamers and boring tools are used to finish holes that require tight tolerances. For diameters less than ⌀20 mm, it is advisable to use standard diameters.
Maximum depth recommended 4 x nominal diameter; typical 10 x nominal diameter; feasible 40 x nominal diameter
Non-standard diameter holes should be machined using an end mill. In this scenario, the maximum cavity depth limit is applicable, and it is recommended to use the maximum depth value. If you need to machine holes deeper than the typical value, use a special drill with a minimum diameter of 3 mm. Blind holes machined with a drill have a tapered base with a 135° angle, while holes machined with an end mill are flat. In CNC machining, there is no specific preference between through holes and blind holes.
5. Threads
The minimum thread size is M2. It is recommended to use M6 or larger threads. Internal threads are created using taps, while external threads are created using dies. Taps and dies can both be used to create M2 threads. CNC threading tools are widely used and preferred by machinists because they reduce the risk of tap breakage. CNC threading tools can be used to create M6 threads.
Thread length minimum 1.5 x nominal diameter; 3 x nominal diameter recommended
The initial few teeth bear most of the load on the thread (up to 1.5 times the nominal diameter). Thus, threads larger than three times the nominal diameter are unnecessary. For threads in blind holes made with a tap (i.e. all threads smaller than M6), add an unthreaded length equal to 1.5 times the nominal diameter to the bottom of the hole.
When CNC threading tools can be used (i.e. threads larger than M6), the hole can be threaded through its entire length.
6. Small Features
The minimum recommended hole diameter is 2.5 mm (0.1 in); a minimum of 0.05 mm (0.005 in) is also acceptable. Most machine shops can accurately machine small cavities and holes.
Anything below this limit is considered micromachining. CNC precision milling such features (where the physical variation of the cutting process is within this range) requires specialized tools (micro drills) and expert knowledge, so it is recommended to avoid them unless absolutely necessary.
7. Tolerances
Standard: ±0.125 mm (0.005 in)
Typical: ±0.025 mm (0.001 in)
Performance: ±0.0125 mm (0.0005 in)
Tolerances establish the acceptable limits for dimensions. The achievable tolerances depend on the part’s basic dimensions and geometry. The values provided are practical guidelines. In the absence of specified tolerances, most machine shops will use a standard ±0.125 mm (0.005 in) tolerance.
8. Text and Lettering
The recommended font size is 20 (or larger), and 5 mm lettering
Engraved text is preferable to embossed text because it removes less material. It is recommended to use a sans-serif font, such as Microsoft YaHei or Verdana, with a font size of at least 20 points. Many CNC machines have pre-programmed routines for these fonts.
Machine Setup and Part Orientation
A schematic diagram of a part that requires multiple setups is shown below:
Tool access is a significant limitation in the design of CNC machining. To reach all surfaces of a model, the workpiece has to be rotated multiple times. For instance, the part shown in the image above needs to be rotated three times: twice to machine the holes in the two primary directions and a third time to access the back of the part. Each time the workpiece is rotated, the machine has to be recalibrated, and a new coordinate system must be defined.
Consider machine setups when designing for two main reasons:
1. The total number of machine setups affects cost. Rotating and realigning the part requires manual effort and increases total machining time. If a part needs to be rotated 3-4 times, it’s usually acceptable, but anything beyond this limit is excessive.
2. To achieve maximum relative position accuracy, both features must be machined in the same setup. This is because the new call step introduces a small (but non-negligible) error.
Five-Axis CNC Machining
When using 5-axis CNC machining, the need for multiple machine setups can be eliminated. Multi-axis CNC machining can manufacture parts with complex geometries because it offers two additional axes of rotation.
Five-axis CNC machining allows the tool to always be tangential to the cutting surface. This enables more complex and efficient tool paths to be followed, resulting in parts with better surface finishes and shorter machining times.
However, 5 axis cnc machining also has its limitations. Basic tool geometry and tool access restrictions still apply, for example, parts with internal geometry cannot be machined. Additionally, the cost of using such systems is higher.
Designing Undercuts
Undercuts are features that cannot be machined with standard cutting tools because some of their surfaces are not directly accessible from above. There are two main types of undercuts: T-slots and dovetails. Undercuts can be single-sided or double-sided and are machined with specialized tools.
T-slot cutting tools are basically made with a horizontal cutting insert attached to a vertical shaft. The width of an undercut can vary between 3 mm and 40 mm. It is recommended to use standard dimensions (i.e., whole millimeter increments or standard fractions of inches) for the width because the tooling is more likely to already be available.
For dovetail tools, the angle is the defining feature dimension. 45° and 60° dovetail tools are considered standard.
When designing a part with undercuts on the inside walls, remember to add enough clearance for the tool. A good rule of thumb is to add space between the machined wall and any other inside walls equal to at least four times the depth of the undercut.
For standard tools, the typical ratio between the cutting diameter and the shaft diameter is 2:1, limiting the depth of cut. When a non-standard undercut is required, machine shops often make their own custom undercut tools. This increases lead time and cost and should be avoided whenever possible.
T-slot on interior wall (left), dovetail undercut (center), and one-side undercut (right)
Drafting Technical Drawings
Please note that some design specifications cannot be included in STEP or IGES files. 2D technical drawings are required if your model includes one or more of the following:
Threaded holes or shafts
Tolerated dimensions
Specific surface finish requirements
Notes for CNC machine operators
Rules of thumb
1. Design the part to be machined with the largest diameter tool.
2. Add large fillets (at least ⅓ x cavity depth) to all internal vertical corners.
3. Limit the depth of a cavity to 4 times its width.
4. Align the main features of your design along one of the six cardinal directions. If this is not possible, opt for 5 axis cnc machining services.
5. Submit technical drawings along with your design when your design includes threads, tolerances, surface finish specifications, or other comments for machine operators.
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