BRIGHTSTAR
PROTOTYPE CNC CO., LTD
+86 137 5010 5351
EN
June. 04, 2026
CNC machining is an extremely powerful manufacturing technology. It can machine everything from aluminum alloys to titanium alloys, from plastics to stainless steel, can achieve micron-level precision, and can produce extremely complex geometries.
However, CNC machining is not a universal solution. There are some features that CNC machines simply cannot machine. There are also other features that, while theoretically machinable, are so expensive and time-consuming that they are practically infeasible.
If you are a designer or engineer, understanding the limitations of CNC machining can help you avoid those "impossible to machine" features during the design phase, saving time and cost.
So, what features are impossible to CNC machine? What features, while potentially machinable, should be avoided whenever possible? This article will answer these questions in detail"
Before discussing specific features, let us understand a few basic limitations of CNC machining.
CNC end mills and drills are round rotating tools. This means they cannot machine a perfect internal sharp corner. Any internal corner will have a radius left by the tool.
CNC tools need physical space to access the workpiece. If a feature is blocked by another feature, the tool cannot reach it, and it cannot be machined.
The length of a tool is limited. A tool that is too long has poor rigidity, is prone to chatter, and cannot guarantee precision. Generally speaking, the effective cutting length of a tool should not exceed 4 to 5 times its diameter.
During CNC machining, the workpiece must be securely fixed to the worktable. If a part is too small, too thin, or has an irregular shape, the fixture may not be able to hold it reliably.
Certain materials (such as extremely hard ceramics, rubber, and very soft silicone) are not suitable for CNC machining.
The following features cannot be machined by CNC machines at all.
This is the classic limitation of CNC machining. CNC end mills are round, so any internal corner will have a radius. The radius is equal to the radius of the tool.
To machine a perfect internal sharp corner, a tool with zero radius would be required, which is impossible. Even with a very small tool, you can only approach a sharp corner, not achieve a perfect one.
Solution: Add fillets to all internal corners during design. The fillet radius should be greater than or equal to the radius of the planned cutting tool. A minimum fillet radius of 0.5 to 1 millimeter is generally recommended. If a sharp internal corner is truly necessary, consider Electrical Discharge Machining.
Undercuts are areas that are blocked by other features and cannot be accessed directly from the top. On a 3-axis CNC, undercuts cannot be machined at all because the tool cannot tilt.
On a 5-axis CNC, some undercuts can be machined by tilting the workpiece or tool, but not all undercuts are machinable. If the undercut area is too narrow or in a very hard-to-reach position, even 5-axis cannot machine it.
Solution: Avoid undercuts whenever possible during design. If an undercut is necessary, design it so that it can be accessed from some angle. Alternatively, consider splitting the part into multiple components, machining them separately, and then assembling them.
If the depth of a slot is much greater than its width, a CNC tool cannot enter. For example, a slot that is 1 millimeter wide and 20 millimeters deep. A 1 millimeter diameter tool can hardly reach a length of 20 millimeters, and even if such a tool exists, it would vibrate severely due to insufficient rigidity, making precision impossible.
A general rule of thumb: slot depth should not exceed 5 times the slot width. For small diameter tools, this ratio is even lower.
Solution: Increase the slot width, or reduce the slot depth. If a deep narrow slot is absolutely necessary, consider Electrical Discharge Machining.
CNC tools need to enter from outside the workpiece. If a part has a completely enclosed internal cavity with no opening, the tool cannot enter, and the cavity cannot be machined.
Solution: Design an opening in the cavity to allow tool entry. Alternatively, split the part into two halves, machine the internal cavity separately, and then weld or bond them together.
When drilling, the minimum hole diameter is limited by the minimum drill diameter. The common minimum drill diameter is about 0.3 millimeters. Smaller drills (0.1 millimeters or less) exist but are very fragile, easily broken, and require specialized high-speed spindles.
Even if such tiny holes can be drilled, the depth is very limited. The length-to-diameter ratio for micro drills typically does not exceed 10:1.
Solution: If extremely small holes are truly necessary, consider laser drilling or EDM drilling.
CNC can machine standard threads (metric, imperial, pipe threads, etc.), but cannot machine arbitrary shape threads. If a non-standard thread is required, a CNC thread mill cannot machine it.
Solution: Use standard threads. If a non-standard thread is necessary, consider custom thread mills (expensive), or use other processes.
When wall thickness is less than 0.5 millimeters (for metal) or 1 millimeter (for plastic), CNC machining becomes very difficult. Thin walls vibrate easily during machining, leading to poor surface quality, inaccurate dimensions, or even part scrap.
Solution: Increase wall thickness, or add temporary support ribs. If thin walls are absolutely necessary, consider using vacuum chucks or low-melting-point alloy fixtures.
Intersecting holes are two or more holes that meet inside a part. In CNC machining, drilling intersecting holes is feasible, but the burrs at the intersection are extremely difficult to remove. These burrs may be in inaccessible locations and cannot be removed by conventional deburring methods.
Solution: Avoid intersecting holes whenever possible during design. If unavoidable, consider electrochemical deburring or high-pressure fluid deburring.
The following features can theoretically be machined by CNC, but the cost is extremely high and the time is very long. They should be avoided in practical production.
CNC can drill holes, but when the hole depth exceeds 10 times the diameter, problems arise. The drill will wander, causing poor hole position and straightness. Chip evacuation is difficult, which may cause the drill to jam or break. Coolant cannot reach the cutting area easily, and the drill overheats.
For holes with a depth-to-diameter ratio exceeding 20:1, gun drilling is typically required. This is a specialized deep hole machining process, and not all CNC shops have this equipment.
Solution: Try to keep hole depth within 5 to 10 times the diameter. If deep holes are necessary, confirm whether your supplier has gun drilling equipment.
CNC machining can achieve precision of ±0.005 millimeters or even higher. But the tighter the tolerance, the higher the cost, the longer the machining time, and the higher the scrap rate. When tolerance requirements go below ±0.002 millimeters, ultra-precision machining equipment, temperature-controlled environments, and specialized inspection equipment are needed, and costs multiply.
Solution: Only use tight tolerances where functionally necessary. Use standard tolerances elsewhere. If extremely high precision is truly required, consider grinding or lapping.
CNC milling and turning can produce fairly smooth surfaces (Ra 0.2 to 0.4 micrometers), but cannot achieve a perfect mirror finish. The tool always leaves tiny marks. A true mirror finish requires polishing.
Solution: If a mirror finish is not required, the CNC machined surface is usually sufficient. If a mirror finish is required, perform mechanical polishing or electropolishing after CNC machining.
CNC can machine hardened steel (such as quenched steel), but requires special tools (ceramic or CBN) and extremely rigid machine tools. Machining speeds are very slow, tool wear is high, and costs are extremely high.
Solution: Perform rough machining while the material is in the annealed state, leaving a small amount of stock. After heat treatment, perform finish machining. This significantly reduces machining difficulty and cost.
CNC can machine very small parts, but small parts require small tools. Small tools are fragile, easily broken, and slow. Small parts also require specialized fixtures, making them difficult to secure. Inspection of small parts is also challenging.
Generally speaking, features smaller than 1 millimeter are very difficult to reliably machine with CNC.
Solution: If parts are very small, consider Electrical Discharge Machining, laser machining, or micro-machining. Alternatively, design multiple small parts as a single assembly, machine them, and then cut them apart.
Some materials have extremely high hardness, such as tungsten carbide, ceramics, and hardened tool steels. These materials cannot be machined with conventional CNC tools. Diamond tools or CBN tools are required, and machining speeds are extremely slow.
Solution: Use grinding or Electrical Discharge Machining instead of CNC milling. Or choose a more machinable material during design.
Elastic materials such as rubber and silicone deform during CNC machining and cannot hold dimensions. The cutting force of the tool causes the material to bend and stretch, leading to inaccurate dimensions.
Solution: Use compression molding or 3D printing instead of CNC machining.
Very soft plastics (such as polyurethane) tend to produce burrs during machining, making it difficult to obtain a clean surface.
Solution: Use sharp tools, high cutting speeds, or choose other processes.
Here are some design principles to help you avoid "impossible to machine" features.
The fillet radius should be at least 0.5 millimeters. Larger is better if possible. The larger the fillet, the larger the tool diameter can be, the faster the machining, and the lower the cost.
If an undercut is necessary, ensure there is enough space for the tool to enter. Consider the tool's approach angle and path during design.
Hole depth should not exceed 5 to 10 times the hole diameter. If deep holes are necessary, use gun drilling and allow sufficient machining time.
Metal wall thickness should not be less than 0.8 millimeters, and plastic wall thickness should not be less than 1.5 millimeters.
Feature size should not be less than 0.5 millimeters. If smaller features are necessary, consider micro-machining or Electrical Discharge Machining.
During design, imagine how the tool would approach each feature. If a feature is blocked by another feature, redesign it.
If you are unsure whether a certain design is machinable, consult your CNC machining supplier during the design phase. They can identify potential problems early.
When a feature cannot be machined by CNC, consider the following alternative processes.
EDM removes material through electrical erosion and does not rely on physical contact of a tool. It can machine internal sharp corners, deep narrow slots, tiny holes, and very hard materials. The disadvantages of EDM are slow speed and high cost.
Wire EDM uses a charged thin wire to cut conductive materials. It can machine very fine features and achieve extremely high precision. However, wire EDM can only machine through shapes, not blind holes or blind slots.
Laser cutting can machine very small holes and fine contours. However, it is mainly suitable for thin sheet materials.
Grinding can achieve extremely high precision and surface finish. It is suitable for machining hardened materials and surfaces requiring precision fits.
Additive manufacturing can machine internal channels, lattice structures, and other complex shapes that CNC cannot. However, the precision and surface finish of 3D printing are typically not as good as CNC.
For high-volume parts, casting and injection molding can produce complex shapes that are difficult for CNC. However, tooling costs are high, making them unsuitable for small batches.
Q: Can CNC machine internal sharp corners?
No. CNC end mills are round, so any internal corner will have a radius. If an internal sharp corner is needed, you can design a relief groove, or use Electrical Discharge Machining.
Q: Can CNC machine threaded holes?
Yes. CNC can machine standard threaded holes using taps or thread mills. Non-standard threads require custom tooling.
Q: How small of a hole can CNC machine?
The common minimum drill diameter is about 0.3 millimeters. Smaller holes can be made with micro-drills or EDM drilling, but at higher cost and greater risk.
Q: Can CNC machine hardened steel?
Yes, but it is difficult and expensive. It is recommended to perform rough machining before hardening, and only perform a small amount of finish machining after hardening.
Q: How can I tell whether my design is machinable?
The best way is to consult a professional CNC machining supplier. Send your drawings and 3D models to Brightstar, and we can provide a free DFM review.
Q: Do you offer alternative processes?
Yes. In addition to CNC machining, Brightstar also offers EDM, wire EDM, surface finishing, and other services. If some features of your part are not suitable for CNC, we can recommend the most appropriate combination of processes.
CNC machining is a very powerful manufacturing technology, but it is not a universal solution. Understanding its limitations can help you design parts that are easier to machine, lower in cost, and higher in quality.
Remember a few key points:
CNC cannot machine perfect internal sharp corners. Add fillets.
CNC struggles with deep narrow slots and extremely deep holes. Control the depth-to-diameter ratio.
CNC cannot machine undercut features that are blocked.
CNC is not suitable for extremely hard, extremely soft, or elastic materials.
Very small features and extremely tight tolerances will significantly increase cost.
Considering manufacturability during the design phase can avoid the dilemma of "impossible to machine" later. If you are unsure whether your design is feasible, Brightstar's professional team is always ready to provide DFM review services.
Ready to Design Machinable Parts?
Whether you need design advice or CNC machining services, Brightstar can help you avoid "impossible to machine" features.
Email Amy: amy@brightstarprototype.com
Call or WhatsApp: +86 13750105351
Send us your CAD files and drawings for a free DFM review and machinability analysis.
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