CNC machining is a subtractive manufacturing process in which computer-controlled machine tools remove material from a solid workpiece to create a finished component. A digital design is converted into machining instructions that control tool movement, spindle speed, feed rate, cutting depth and other production parameters.
Unlike manual machining, CNC machining follows a programmed sequence, allowing manufacturers to produce complex metal and plastic parts with a high level of consistency. The technology is widely used for functional prototypes, low-volume batches, tooling and production components.
This guide explains how CNC machining works, the differences between 2-, 3-, 4- and 5-axis machines, common machining operations, suitable materials, achievable tolerances, design guidelines, cost factors and quality-control requirements.
For production support, Samshion provides custom CNC machining services for prototypes, low-volume batches and end-use parts.
What Is CNC Machining?
CNC stands for Computer Numerical Control. In a CNC machining system, a computer controls the movement and operating conditions of a machine tool according to a prepared manufacturing program.
The process normally begins with a 3D CAD model and, when required, a 2D engineering drawing. CAM software converts the geometry into toolpaths, and a post-processor generates machine-specific NC code. This code tells the CNC controller where the cutting tool should move and how the machine should perform each operation.
Most CNC machining processes are subtractive. Material is removed from a solid block, plate, bar, tube, casting or forged blank until the required geometry remains.
CNC machining should not be confused with additive manufacturing. A 3D printer builds a part by adding material layer by layer, while a CNC machine generally creates the part by removing material. Each process has different advantages, limitations, material options and design requirements.
Although CNC machines are highly automated, skilled engineering involvement remains essential. Machinists and manufacturing engineers must still select tools, design workholding, set cutting conditions, control tool wear, verify the program and inspect the finished parts.
A Brief History of CNC Technology
The development of numerical-control machining began in the 1940s as engineers looked for a more reliable way to manufacture complex aircraft components. In 1952, work involving John Parsons and the MIT Servomechanisms Laboratory resulted in an early three-axis numerically controlled milling machine.
Early NC systems used punched cards or tape to store movement instructions. As computer technology developed, numerical control evolved into computer numerical control. Modern CNC controllers can now coordinate multiple linear and rotary axes, manage automatic tool changes, use probing systems and communicate with CAD/CAM and inspection software.
The history of the first NC milling systems is documented in MIT research on numerically controlled manufacturing.
How Does CNC Machining Work?
A professional CNC machining workflow contains more than simply loading a file and pressing a start button. The process must connect design intent, process planning, machine preparation, controlled cutting and dimensional inspection.
Step 1: Review the CAD Model and Engineering Drawing
The project begins with a 3D CAD model that defines the part geometry. Common file formats include STEP, STP, IGES and X_T.
A 2D drawing should also be supplied when the part includes:
Critical dimensional tolerances
Geometric tolerances and datum references
Thread specifications
Surface-roughness requirements
Heat-treatment requirements
Surface-finishing specifications
Inspection and documentation requirements
The manufacturing engineer reviews the files for tool access, internal corner radii, wall thickness, hole depth, workholding surfaces and features that may require multiple setups.
This Design for Manufacturability review helps identify potential production problems before material is cut.
Step 2: Create the CAM Program and Toolpaths
CAM software converts the CAD geometry into machining operations and toolpaths. The programmer selects the cutting tools, machining sequence, spindle speed, feed rate, depth of cut and entry or exit strategy.
Typical toolpaths may include:
Facing
Rough pocketing
Contouring
Slotting
Drilling
Boring
Thread milling
Tapping
Finishing passes
Chamfering and deburring
A post-processor then converts the toolpaths into NC code suited to the specific machine and controller. CAM systems can also simulate the machining process to identify collisions, excessive tool engagement and inefficient tool movement before production begins.
Autodesk’s official CAM documentation explains how CAM software generates toolpaths and machine-specific NC code for CNC equipment: CAM software for CNC machining.
Step 3: Prepare Tools, Material and Workholding
The machinist prepares the machine according to the approved process plan. The correct raw material is identified, cut to size and secured using a vise, chuck, fixture, soft jaw, vacuum table or other workholding method.
The workpiece must be held firmly enough to resist cutting forces without deforming the part. Thin walls, delicate features and soft materials may require specially designed fixtures or reduced clamping pressure.
Cutting tools are loaded into the tool magazine and measured. The machinist then establishes the work coordinate system, tool offsets and machine datum.
For critical projects, the setup may also include:
Material-certificate verification
First-piece setup approval
In-machine probing
Tool-life monitoring
Coolant concentration checks
Runout verification
Fixture and datum inspection
Step 4: Machine the Part
The CNC controller executes the approved program. Roughing operations remove most of the unwanted material, while semi-finishing and finishing passes establish critical dimensions and surface quality.
During machining, the operator monitors chip formation, cutting sound, spindle load, coolant flow, tool wear and part stability. Automated machining does not eliminate the need for process control. Tool wear, temperature, material variation and workholding conditions can all affect the finished part.
Complex components may require several setups. Each time the part is repositioned, the machinist must re-establish its location and verify the relationship between machined features.
Multi-axis machining can reduce the number of setups by allowing the cutting tool to approach the part from additional directions.
Step 5: Inspect and Finish the Part
After machining, the part is cleaned, deburred and inspected. The inspection method depends on the drawing, tolerance level, feature type and production quantity.
Common inspection equipment includes:
Digital calipers
Micrometers
Height gauges
Pin and thread gauges
Optical comparators
Surface-roughness testers
Coordinate measuring machines
Vision measurement systems
If required, parts then proceed to heat treatment, anodizing, plating, polishing, bead blasting, passivation, powder coating or another finishing operation.
Types of CNC Machines by Axis Configuration
A CNC machine’s axis configuration determines how the cutting tool and workpiece can move relative to each other. Additional axes can improve tool access and reduce repositioning, but they also increase programming, setup and equipment requirements.
2-Axis CNC Machines
In the most common turning configuration, a 2-axis CNC lathe controls tool movement along the X and Z axes while the workpiece rotates in the spindle.
Two-axis turning is suitable for rotational parts such as:
Shafts
Pins
Bushings
Spacers
Threaded fittings
Pulleys
Sleeves
The process is fast and efficient for cylindrical geometry, but it cannot produce all off-center or multi-face features without additional tooling or another machine setup.
3-Axis CNC Machines
A 3-axis milling machine moves along the X, Y and Z linear axes. The workpiece is normally secured in one orientation while the cutting tool approaches it from above.
Three-axis machining is widely used for:
Plates
Brackets
Housings
Mold inserts
Fixtures
Pockets
Hole patterns
Flat and contoured surfaces
It is usually the most economical milling option for straightforward components. However, a part with features on several sides may need to be removed, rotated and re-aligned for additional setups.
Each new setup adds labor and creates another opportunity for cumulative alignment error.
4-Axis CNC Machines
A 4-axis machine adds one rotary axis to the three linear axes. The rotary movement may be used for indexed positioning or simultaneous machining.
Four-axis machining allows several sides of a component to be processed without manually repositioning it. It is suitable for:
Multi-sided brackets
Cam profiles
Helical features
Engraving around cylindrical parts
Repeated radial holes
Turbine and impeller features of moderate complexity
Four-axis machining can reduce setup time and improve the positional relationship between features located on different faces.
5-Axis CNC Machines
A 5-axis machining center combines three linear axes with two rotary axes. The cutting tool can approach the workpiece from many directions, making it possible to machine complex surfaces and multi-angle features with fewer setups.
Five-axis machining is often selected for:
Complex aerospace components
Impellers
Turbine components
Medical instruments
Optical equipment parts
Deep cavities
Contoured molds
Components with strict feature-to-feature relationships
There are two common five-axis strategies.
3+2 Positional Machining
The machine rotates the part or tool into a selected orientation and then locks the rotary axes while the three linear axes perform the cut.
This method improves access and reduces manual repositioning but does not move all five axes simultaneously during cutting.
Simultaneous 5-Axis Machining
All five axes can move together while the tool follows a complex surface. This method is suitable for continuously changing contours and features that require controlled tool orientation.
Five-axis machining does not automatically guarantee a particular tolerance. Final accuracy depends on part size, material, geometry, workholding, machine calibration, thermal stability, tool condition and inspection strategy.
Learn more about Samshion’s 5-axis CNC machining capabilities for complex prototypes and precision parts.
CNC Axis Configuration Comparison
| Factor | 2-Axis | 3-Axis | 4-Axis | 5-Axis |
|---|---|---|---|---|
| Primary Motion | X and Z with spindle rotation | X, Y and Z | Three linear axes plus one rotary axis | Three linear axes plus two rotary axes |
| Best Suited To | Cylindrical parts | Prismatic parts | Multi-sided features | Complex contours and multi-angle features |
| Repositioning | Low for rotational geometry | Often required for multiple faces | Reduced | Frequently minimized |
| Programming Complexity | Low | Moderate | Moderate to high | High |
| Relative Machine Rate | Low | Medium | Medium to high | High |
| Main Benefit | Efficient turning | Flexible general machining | Multi-side access | Complex machining with fewer setups |
| Main Limitation | Limited geometry | Restricted approach angles | One rotary axis | Advanced programming and workholding required |
The machine with the greatest number of axes is not always the best choice. The correct process is the one that meets the drawing requirements with the lowest overall manufacturing risk and cost.
Common CNC Machining Operations
CNC Milling
CNC milling uses a rotating multi-edge cutting tool to remove material from a workpiece. The machine can perform facing, pocketing, contouring, slotting, drilling, boring and thread milling.
Milling is commonly used for non-rotational components such as brackets, housings, plates, heat sinks, molds and precision fixtures.
The final result depends on tool geometry, spindle speed, feed rate, tool engagement, material properties, rigidity and coolant strategy.
CNC Turning
CNC turning rotates the workpiece while a cutting tool moves along the required profile. It is normally used for cylindrical or rotational parts.
Turning operations include:
Facing
External and internal diameter turning
Grooving
Parting
Boring
Threading
Knurling
Drilling
Live-tool turning centers can also mill flats, cross holes and off-center features without transferring the part to a separate milling machine.
CNC Drilling and Hole Machining
CNC drilling creates cylindrical holes using a rotating drill. Depending on the accuracy and surface requirements, the hole may subsequently be bored, reamed, countersunk, counterbored or threaded.
Hole quality is affected by drill geometry, depth-to-diameter ratio, chip evacuation, tool runout and material properties. Deep holes may require special cycles, through-tool coolant or dedicated equipment.
Electrical Discharge Machining
Electrical discharge machining removes electrically conductive material using controlled electrical discharges rather than direct mechanical cutting contact.
Wire EDM uses a continuously moving wire electrode to cut profiles through a conductive workpiece. Sinker EDM uses a shaped electrode to produce cavities, ribs and other features.
EDM is useful for hardened steel, narrow slots, sharp internal details and geometries that are difficult to reach with conventional cutting tools. Because EDM is a thermal process, its parameters and finishing passes must be controlled to manage the recast layer and protect surface integrity.
Sinker EDM performance also depends on accurate EDM electrode design and manufacturing.
Materials for CNC Machining
CNC machining is compatible with a wide range of metals and engineering plastics. The best material cannot be selected by price alone. The designer must consider strength, weight, temperature, corrosion, wear, electrical properties, dimensional stability, appearance and regulatory requirements.
Compare additional grades in Samshion’s manufacturing materials guide.
Metals for CNC Machining
Aluminum
Aluminum alloys offer a favorable balance of machinability, weight, corrosion resistance and cost.
Common grades include:
Aluminum 6061 for general engineering parts
Aluminum 7075 for higher-strength applications
Aluminum 2024 for fatigue-sensitive structures
Aluminum 5052 for corrosion resistance and formed components
Aluminum 6063 for profiles and appearance-sensitive parts
Aluminum can be anodized, plated, polished, bead blasted or left as machined.
Stainless Steel
Stainless steel provides corrosion resistance, strength and durability. Common grades include 303, 304, 316 and 17-4 PH.
Some grades are easier to machine than others. Work hardening, heat generation and tool wear must be controlled through appropriate tooling and cutting parameters.
Carbon and Alloy Steel
Carbon and alloy steels offer strength, wear resistance and cost efficiency. They are widely used for shafts, tooling, fixtures, gears, industrial equipment and structural components.
Heat-treated steels may require harder cutting tools, grinding or EDM for certain critical features.
Titanium
Titanium offers a high strength-to-weight ratio, corrosion resistance and biocompatibility. It is used in aerospace, medical and high-performance engineering applications.
Titanium has low thermal conductivity, so heat remains concentrated near the cutting edge. Rigid workholding, stable tool engagement and controlled cutting conditions are important for tool life and part quality.
Brass and Copper
Brass generally machines well and is used for fittings, valves, connectors and decorative components. Copper provides excellent electrical and thermal conductivity but may require special tooling because of its ductility.
Plastics for CNC Machining
POM and Acetal
POM offers good dimensional stability, low friction and wear resistance. It is frequently used for gears, bushings, fixtures and moving mechanical components.
ABS
ABS is economical and easy to machine for prototypes, housings and general-purpose components. It offers good impact resistance but lower temperature performance than advanced engineering plastics.
Nylon
Nylon provides toughness, wear resistance and low friction. Moisture absorption and dimensional movement must be considered when tight tolerances are required.
Polycarbonate
Polycarbonate combines transparency and impact resistance. Heat generation, clamping pressure and surface scratching must be controlled during machining.
PEEK
PEEK offers high-temperature performance, chemical resistance and mechanical strength. It is used in aerospace, medical, semiconductor and other demanding applications, but material cost is significantly higher than common engineering plastics.
CNC Machining Tolerances and Surface Finish
CNC machining can produce accurate components, but no single tolerance applies to every part or feature.
Achievable tolerance depends on:
Part dimensions
Material stability
Wall thickness
Feature depth
Tool access
Workholding
Machine condition
Temperature
Surface finish
Inspection method
A tolerance that is straightforward on a small aluminum bracket may be difficult on a long, thin plastic component. Critical tolerances should therefore be evaluated feature by feature.
General tolerances may be specified for dimensions that do not have individual tolerance indications. ISO 2768-1 covers general tolerances for certain linear and angular dimensions, but it should not replace individually defined functional requirements.
Surface finish and dimensional tolerance are different specifications. A dimension controls part size or geometry, while a surface-roughness requirement controls the texture left by the machining or finishing process.
An as-machined surface may retain visible tool marks. If the application requires improved corrosion resistance, wear performance or appearance, additional treatment may be necessary. Review the available CNC machining surface finish options before finalizing the drawing.
Advantages of CNC Machining
High Repeatability
Once a stable process has been established, CNC equipment can repeat the same programmed movements across multiple components. Process control and inspection are still required to manage tool wear, material variation and temperature.
Broad Material Compatibility
CNC machining can process many metals and engineering plastics without requiring dedicated molding tools.
Complex Geometries
Multi-axis machines can produce pockets, contours, threads, compound angles and features located on several sides of a component.
Fast Design Changes
Because the process begins with digital CAD and CAM data, many design changes can be implemented without manufacturing a completely new mold or die.
Suitable for Prototypes and Production
CNC machining is suitable for functional prototypes, bridge production, customized components and repeat production. For higher quantities, dedicated fixtures and optimized toolpaths can reduce the cost per part.
Limitations of CNC Machining
Setup and Programming Cost
Every new part requires manufacturing review, programming, tooling and machine setup. These costs have a greater effect on one-off parts than on repeat production.
Tool-Access Restrictions
A rotating cutting tool must physically reach the feature being machined. Deep pockets, narrow channels, hidden features and square internal corners may require design changes or alternative processes.
Material Waste
Machining removes material from stock, so it may generate more waste than casting, forging or molding. Near-net-shape blanks can reduce waste for larger production quantities.
Thin-Part Deformation
Thin walls and long components can move under cutting force, clamping pressure or residual stress. Additional fixtures, staged machining and stress-relief operations may be required.
CNC Machining Design Guidelines
Use Practical Internal Corner Radii
Rotating milling tools create radiused internal corners. A larger corner radius permits the use of a larger and more rigid cutting tool, which can improve stability and reduce machining time.
Very small internal radii may require smaller tools, slower cutting conditions and additional finishing operations.
Learn how internal corner radii affect CNC milling.
Avoid Deep, Narrow Cavities
Deep cavities require long cutting tools. Tool length reduces rigidity and may increase vibration, deflection and surface variation.
Where possible:
Increase the cavity width
Reduce unnecessary depth
Use larger internal radii
Provide access from more than one side
Divide extremely deep geometry into multiple components
Maintain Adequate Wall Thickness
Thin walls can bend during cutting or clamping. The appropriate wall thickness depends on material, wall height, unsupported length, geometry and required tolerance.
Plastic parts normally require more consideration for thermal expansion and clamping deformation than rigid metal parts.
Use Standard Holes and Threads
Standard drill sizes, thread forms and tooling reduce setup time and manufacturing risk.
Avoid specifying threads deeper than the functional engagement requires. For blind threaded holes, provide enough additional depth for the drill point, tap lead and chip clearance.
Thread size, class, depth and whether the hole is through or blind should be shown clearly on the 2D drawing.
Apply Tight Tolerances Selectively
Applying a very tight tolerance to every dimension increases machining and inspection time without necessarily improving product function.
Identify the features that control:
Assembly
Alignment
Sealing
Bearing fit
Motion
Optical position
Electrical contact
Use practical general tolerances for non-critical features and tighter individual tolerances only where function requires them.
Provide Both 3D and 2D Files
The 3D model defines nominal geometry, while the 2D drawing communicates manufacturing and inspection requirements that may not be fully represented by the model.
The drawing should clearly identify:
Datums
GD&T
Critical dimensions
Threads
Surface finish
Coating
Heat treatment
Inspection level
Material and grade
What Affects CNC Machining Cost?
Material
Material cost includes more than the price of the raw stock. Hardness, abrasiveness, heat resistance, cutting speed, tool wear and scrap value all affect the total cost.
Part Geometry
Deep cavities, thin walls, small internal radii, complex contours and restricted features require more programming and machining time.
Number of Setups
Every setup requires workholding, alignment and verification. Designing the part for fewer setups can reduce labor and improve feature-to-feature consistency.
Tolerances and Inspection
Tight tolerances may require slower finishing cuts, controlled temperature, specialized measuring equipment and more extensive inspection reports.
Surface Finishing
Anodizing, plating, heat treatment, polishing and coating add production steps, transportation, masking and inspection requirements.
Quantity
Programming and setup costs are distributed across the total order quantity. Larger quantities may reduce the cost per part, but they can also justify dedicated fixtures and formal process validation.
For additional planning considerations, review the CNC production machining guide.
Common CNC Machining Applications
CNC machining is used wherever components require controlled dimensions, functional materials and reliable feature relationships.
Typical applications include:
Aerospace brackets, housings and structural components
Medical instrument and laboratory-equipment parts
Automotive prototypes and mechanical components
Robotics joints, actuator housings and end effectors
Electronics housings and heat-management components
Industrial fixtures, shafts, valves and machine components
Mold inserts, electrodes, jigs and inspection fixtures
For demanding aerospace projects, see Samshion’s aerospace manufacturing capabilities. CNC machining is also widely used in industrial equipment manufacturing and product-development projects that require rapid prototyping.
How Is CNC Machining Quality Controlled?
Quality control should begin before the machine starts cutting.
A controlled CNC workflow may include:
Contract and drawing review
Material verification
Manufacturing process planning
Tool and fixture verification
First-article inspection
In-process dimensional checks
Final inspection
Surface and appearance verification
Documentation and traceability
The inspection plan should reflect the risk and function of the component. Simple dimensions may be checked with calibrated hand tools, while complex geometries and tight positional requirements may need CMM inspection.
Material certificates, dimensional reports, Certificates of Conformance and other records should be agreed before production begins.
Review Samshion’s quality assurance and inspection systems for additional information about inspection equipment and process control.
How to Choose a CNC Machining Supplier
A capable supplier should be evaluated on more than the quoted price.
Before placing an order, confirm:
Available milling, turning and multi-axis capability
Experience with the selected material
Maximum practical part dimensions
Tolerance and surface-finish capability
Inspection equipment
Material traceability
Quality-system certification
Engineering and DFM support
Prototype and production capacity
Communication and change-control procedures
Available surface treatments
Protection of drawings and intellectual property
A clear quotation should identify material, quantity, manufacturing process, surface finish, lead time and any assumptions or exclusions.
CNC Machining FAQs
What does CNC stand for?
CNC stands for Computer Numerical Control. It describes machine equipment controlled through programmed numerical instructions rather than direct manual movement.
Is CNC machining a subtractive process?
Most CNC milling, turning and drilling operations are subtractive because they remove material from a workpiece. Some modern CNC-controlled systems can also integrate additive, welding or inspection functions.
What is the difference between CNC milling and CNC turning?
In CNC milling, the cutting tool rotates while the workpiece is normally held in place. In CNC turning, the workpiece rotates while a cutting tool shapes the external or internal profile.
Milling is generally suited to prismatic parts, while turning is suited to cylindrical parts. Multi-tasking machines can combine both processes.
What is the difference between 3-axis and 5-axis CNC machining?
A 3-axis machine moves along the X, Y and Z axes. A 5-axis machine adds two rotary axes, giving the tool access to more sides and approach angles.
Five-axis machining is useful for complex geometry and fewer setups, but three-axis machining is often more economical for straightforward components.
What materials can be CNC machined?
Common materials include aluminum, steel, stainless steel, brass, copper, titanium, ABS, POM, nylon, polycarbonate and PEEK. Material selection should be based on mechanical, environmental, regulatory and cost requirements.
What files are required for a CNC quotation?
A STEP, STP, IGES or X_T file is normally preferred for 3D geometry. Supply a PDF or DWG drawing when the part includes critical tolerances, threads, GD&T, surface finish, heat treatment or inspection requirements.
Also provide the required material, quantity and surface treatment.
What tolerances can CNC machining achieve?
There is no single tolerance that applies to every CNC-machined part. Capability depends on material, size, geometry, workholding, feature accessibility, machine condition and inspection method.
Critical requirements should be identified on a 2D drawing and reviewed before production.
Can CNC machining be used for one prototype?
Yes. CNC machining is suitable for functional prototypes because it can produce components from production-grade materials without requiring a molding tool.
Prototype pricing is affected by programming and setup because these costs are distributed across only one or a few parts.
Does every CNC-machined part need surface finishing?
No. Many functional parts can be used with an as-machined finish. Additional finishing is selected when the part requires improved appearance, corrosion protection, wear resistance, electrical properties or a specified surface texture.
How can CNC machining cost be reduced?
Use standard materials and tooling, simplify non-functional features, increase internal corner radii, avoid unnecessary deep cavities, apply tight tolerances only to critical dimensions and design the part for fewer setups.
A DFM review before production is often the most effective way to identify cost-saving opportunities.
Start Your CNC Machining Project
Successful CNC machining begins with clear design intent, suitable material selection, realistic tolerances and a controlled manufacturing plan.
Send Samshion your 3D CAD model, 2D drawing, material, quantity and finishing requirements. Our engineering team can review manufacturability, recommend the appropriate machining process and prepare a project-specific quotation.
Upload your files to request a CNC machining quote and DFM review.
