What Is CNC Milling? A Basic Guide
- Written by: Gavin Leo
- Updated By: CoCo
- Published:
- Updated
Most of my career has been spent working near a buzzing CNC machine with other machinists in a very active machining environment, watching metal chips fly while trying to achieve micron-level accuracy with our precision CNC machining techniques.
The term “precision CNC machining” is essentially the process of achieving tight tolerances with actual production parts, rather than laboratory specimens, using computer numerical controlled (CNC) machining methods, which can often be achieved down to ±0.005 mm.
This article will explain, in simple terms, to designers, engineers, and buyers alike, what is happening inside the machine shop door and what you need to know as a buyer or supplier of precision-machined components, so that your components are delivered when needed, meet their intended specifications, and are priced reasonably.
What’s CNC Precision Machining?
CNC precision machining is extremely precise; therefore, there are nearly zero errors. The 3D CAD model drives the tool path that removes material only where required.
It will be easier to have a higher degree of precision when compared to a “rough” operation in terms of dimensions, surface finishes, and geometric features.
When you use a fixture that doesn’t move and take light finish cuts on your part, all of your parts will be aligned perfectly so they will fit together as a unit without having to adjust by hand.
What is Accuracy？
Accuracy in machining defines how well your product meets the specifications you were given. For example, a shaft with a diameter of 10.000mm, but actually measuring at 10.001mm would be acceptable.However, a shaft measuring 10.200mm would not meet the requirements for accuracy.
In addition, a manufacturing process is considered accurate when the measurement of a part remains relatively close to its nominal dimension (the desired size), which will allow the part to perform as expected once it is incorporated into a larger assembly or system.
What is Precision?
Precision is a measure of the consistency of a manufacturing process. The precision of a manufacturing process is defined by the repeatability of the measurement of parts produced on the shop floor.
For example, if 200 parts have a measured diameter of 9.98mm, this indicates that the manufacturing process is very precise, even though the measured diameters are less than the nominal dimension of 10.00mm.
The ability of a manufacturing process to produce products that consistently fall within the specified limits (tolerance) demonstrates the precision of the manufacturing process. Therefore, precision is demonstrated by consistent measurements.
Why is precision CNC machining important in manufacturing？
Precision CNC machining is “the difference between ‘a part that looks right’ and ‘a part that keeps a production line running all year long.’
With tight geometric control over dimensions and geometry, assemblies fit together smoothly and precisely; bearings operate at lower temperatures; seals do not leak; and the operator does not need to “work” the parts into place using files or sandpaper.
On many of the jobs I’ve worked on, this means literally meeting the Geometric Dimensioning & Tolerancing (GD&T) callouts on the print, i.e., true position in bolt circles, flatness on sealing surfaces, and perpendicularity in bores – not just dimensionally close on a few basic measurements.
This may seem simplistic, but when applied across a production line, it determines whether you’ll experience continuous production or continuous downtime.
Stabilizing the manufacturing process with precision CNC machining is what ensures that the entire production process remains as stable as possible. Even the slightest deviation from specifications will require adjustments to be made in other areas, such as grinding, coating, or assembly.
If you do not control for thermal expansion, tool deflection, and machine stiffness, then your product will become out of specification due to prolonged cycle times. One faulty characteristic can potentially render an entire assembly unusable.
The ability to maintain predictability in downstream operations and reduce overall cost is why precision machining is used throughout all of manufacturing.
Finally, there is the issue of safety and compliance. Critical dimensions and surface finishes in industries such as aerospace, medical devices, and automotive are directly related to performance specifications and certifications.
If an engineered product does not meet these specifications, it cannot be shipped, regardless of its appearance. It is tight, repeatable, precision CNC machining that allows companies to sign off their inspection reports with confidence, knowing that the products being shipped from the dock will function as intended by the design engineer.
CNC Precision Machining Process: Step by Step
This is the basic, typical flow of how I do things when I am running a CNC Precision Machining Process in my shop:
Design CAD Drawing
Almost all of our jobs will begin with a 3D CAD drawing made using standard computer-aided design (CAD) software.
Before we even think about cutting metal, I review the job to ensure the datums, features, and tolerances are reasonable and also that the CAD model represents how we plan to physically hold the part on the machine.
Design for Manufacturing
I then walk through the CAD model as if I were going to physically hold it in my hands. I think about how we will physically reach inside the part with the tools, what the wall thickness will be, whether there are enough fillet radii between surfaces, and whether we have adequate clamping surfaces.
It is during this time that I may suggest some minor changes to the CAD model so the original designers’ intent is maintained; however, the physical machining of the part is improved for repeatable results.
Programming
Using CAM software, I create the CNC program; I establish work offsets, select the cutting tools, and develop tool paths for both the roughing and finishing operations.
I use the material properties to optimize feed rates and spindle speeds, simulate the path, and visually inspect for collision or chattering problems prior to the first chip being generated.
Rough Machining
We typically clamp the raw material to the machine table, face the raw material, and then perform the majority of the removal of material from the raw material while leaving an allowance of approximately 0.002 to 0.005 inches on the critical surfaces.
When performing the rough machining operation, I am primarily concerned with maintaining sufficient rigidity of the setup and ensuring proper chip evacuation. I am not overly concerned at this point with achieving precise dimensional accuracy.
Finish Machining
After completing the rough machining operation, we perform the finishing operation. We use sharper cutting tools, take multiple light cuts, control the amount of coolant supplied to the cutting zone, and control the speed of the spindle.
During this phase, we focus on cleaning up the remaining material from the previous machining operation, ensuring that the radii are acceptable, and inspecting that the mating surfaces and hole locations meet the print specifications.
Tolerance Inspection
After the final operation has been completed, we measure the critical dimensions on the finished part using micrometers, bore gauges, and possibly a Coordinate Measuring Machine (CMM).
The measurements taken are documented so that we can track any dimensional deviations over time and prevent such deviations from resulting in scrap.
Surface Roughness Inspection
If the surface roughness of a part requires verification, we utilize a surface roughness measuring instrument to verify the surface roughness measurement requirements (Ra or Rz) for the sealing or sliding surfaces.
In cases where surface roughness inspection is not required, I will still perform a “fingernail” test to assess the acceptability of the finish by comparing it to a roughness standard block.
If the finish does not pass the fingernail test, it is likely that it does not meet the acceptable surface roughness standards.
What is Difference Bewteen Precision Machining and Common CNC Machining Process？
To produce components with tolerancing of ±0.1mm, we utilize standard CNC milling for rapid production of quality products.
To produce components to a precision tolerance, however, we have an additional level of precision machining capability available; we are able to design custom fixtures to match the print, monitor our tools for wear and tear, make very light cuts in our material, and check our critical features at regular intervals.
All these aspects (tight tolerances, heat control, surface finish, and GD&T) are important to achieve the desired fit, avoid potential leak issues, and minimize scrap during final product assembly.
Types of Precision CNC Machines
CNC Milling Machines
I use a 3-5 Axis Milling Center for most of my Precision Work. This type of equipment enables me to square blocks, create pockets, machine 3D Surfaces, and keep extremely close tolerance positions.
If you have good tooling and setup, you can maintain a tolerance of ±0.01mm on Critical Faces for Prismatic Parts and Flat Sealing Surfaces
CNC Turning and Lathes
For round components such as shafts and bushings, I prefer to use CNC Lathes. I can count on the CNC Lathe to keep its roundness in check, and with a light pass, it is easy to obtain an accurate dimension and surface finish.
Roundness and runout can be maintained in the few-micron range when the CNC lathe has been set up correctly. Additionally, live tooling allows me to make additional features (such as flats or cross-holes) at the same time that the part is being made.
CNC Drilling Machines
CNC drilling machines are ideal for parts requiring multiple holes or repeating patterns, such as manifolds and brackets.
The consistency in locating the hole to the required depth is a major advantage of using CNC drilling machines.
Tool changers also allow for peck cycles and correct drill bit selection. In terms of efficiency and stability, when compared to milling, CNC drilling machines are an excellent choice for production drilling.
Electrical Discharge Machining
For difficult-to-machine materials and/or those requiring complex geometries, I utilize electrical discharge machining (EDM).
For example, wire EDM can create sharp corner geometries while sinker EDM can remove material from blind areas that cannot be accessed by standard cutting tools.
Since there are no cutting forces, thin-walled parts remain parallel, and precise tolerances can be maintained on fragile features.
CNC Laser Machines
I choose to utilize CNC Laser Machines for cutting thin sheet metals and 2D profile shapes due to their ability to cut all types of metals (aluminum, stainless, etc.) with speed and accuracy, leaving no debris on the cut edge and requiring very little deburring.
The machines allow me to create blanks from which I can bend parts or complete additional machining processes.
Although they will not be as productive as a mill, CNC Laser Machines are ideal for producing high-quality, flat, patterned parts in a short amount of time.
Precision CNC Machines Selection Guide
| Machine type | Tolerance range (typical) | When to choose it |
| CNC Milling Machines | ±0.01 – ±0.05 mm | Prismatic Machining Parts (Flat Sealing Faces) – Bolt Patterns (Complex Parts Match CAD Model). A Good General Choice for Me When Tight Tolerances are Required on Multiple Features of One Part. |
| CNC Turning and Lathes | ±0.005 – ±0.02 mm | Rotational Parts (Concentricity, Roundness, Bearing Fits, etc.). My First Pick for High Precision Diameters. |
| CNC Drilling Machines | ±0.05 – ±0.10 mm | Manifolds, Brackets, Plates With Many Holes and Threads Which Must Repeat Across Each Part. I Use These When Hole Location and Depth Consistency Is More Important Than Surface Detail Across a Batch of Parts. |
| Electrical Discharge Machining (EDM) | ±0.002 – ±0.01 mm | Using EDM (Electro Discharge Machining) to make tool steels and mold parts that have hardened metal, very acute angles internally, very thin walls, and also a lot of complexities in their shape. |
| CNC Laser Machines | 0.10 – ±0.20 mm | For sheet-metal blanks and 2D profiles where speed and economy are more important than tight tolerances, using laser cutting as an economical method to cut raw sheets before bending, welding, or secondary-precision machining. |
What Materials Are Suitable for CNC Precision Machining?
First of all when I look at an entirely new part, I always make sure to check its material before the dimensions (other than those that relate to its thickness).
The machining characteristics of certain alloys are designed specifically for precision machining using Computer Numerical Control (CNC) machines, while other alloys seem to be fighting you every step of the way.
Metal
Material selection has a significant impact on precision machining. The aluminum alloys 6061 and 7075 have excellent cutting properties and dimensional stability, making them ideal for prototype development as well as production of aircraft components.
While stainless steel alloys 304 and 316 are generally slower to cut, they provide superior corrosion resistance, and therefore are used in applications such as medical equipment and components subject to corrosive environments.
Many hardened tool steels require Electrical Discharge Machining (EDM) or additional light finishing processes to avoid excessive distortion.
Additionally, brass and copper alloys have predictable machining properties and are commonly utilized in valve components, connectors, and electronic components.
Plastic
Material selection is critical because cutting plastic materials varies significantly compared to cutting metal materials.
Acetal (POM/Delrin) and certain types of nylon will provide a tighter tolerance than most other materials and produce a cleaner edge.
High-performance polymeric materials such as PEEK are suitable for applications requiring high temperatures or chemical exposure; however, they require sharp cutting tools to minimize distortion.
Materials that have softer properties, such as ABS, polycarbonate, and UHMW, can be prone to flexing and burring; therefore, it is necessary to use lower cutting pressures on these materials.
Applications Of Precision CNC Machining Services
Precision CNC machining is present wherever the cost of failure is high, or wherever failure would be dangerous to people, and/or the environment.
Precision CNC machining was utilized in the aerospace field where I worked; we were machining brackets, housings, small engine parts, and even landing-gear parts, where every gram of material is critical to ensure that all holes line up as per the CAD model to allow assembly of the aircraft.
We have also completed machining of satellite components and complex parts for defense projects, where the quality of the Inspection Report will typically be considered equal to the quality of the part itself.
On the Medical side, the precision-machined parts I see are used in Surgical Instruments, Orthopedic Implants, and Fluid-Handling Blocks.
These areas require biocompatibility of the material, smooth surface finish, and tight tolerance of mating features to prevent binding of the assembly or leakage.
While many of these jobs involve short runs, they all require a level of Process Control and Documentation of the machining process.
Automotive and Industrial customers utilize precision CNC machining for Valve Bodies, Pump Housings, Transmission Components, and Fixtures that are required to maintain Production Lines.
The primary advantage of utilizing precision CNC machining for these types of jobs is tighter tolerances, better repeatability, less rework, shorter set-up time, and increased production.
I have also seen extensive use of precision CNC machining in the Electronics, Robotics, and Automation Industries for Heat Sinks, Sensor Mounts, Frames, and Custom Tooling that must fit around other hardware with minimal clearance.
Everywhere Designers attempt to increase Performance, Reduce Weight, or Create Smaller Assemblies, Precision CNC Machining is one of the solutions that can accomplish those objectives.