CNC Precision Machining: Tolerance, Equipment, and Applications
High-quality machining is desirable in every industry. Normally, CNC machining processes are accurate and offer very high complexity.
There is a growing demand for precision CNC machining and for this reason, we’ve compiled this guide which breaks down precision CNC machining so you don’t have to go anywhere else.
What is CNC Precision Machining?
Precision CNC machining is a production process where a blank or workpiece is turned into a finished product by subtractive machining using machine tools that are controlled by the CNC machine and guided by the computer-aided design of the desired shape. Here subtractive machining refers to removing material from the workpiece.
Precision CNC machining relies on Computer Aided Design (CAD) for the design of the product and Computer Aided Manufacturing (CAM) to turn the part into a machinable process with instructions to the CNC equipment. Despite the automatic nature of manufacturing instructions, CNC machining still requires a skilled machinist at one stage or another.
High-precision CNC machining is desirable because it produces standardized parts which are almost identical with little to no deviation from the desired diameters, lengths, and dimensions. Precision machining services are utilized by automobile companies, defense industry where aerospace vehicles and landing gear ports are manufactured, and to develop prototypes and custom CNC machined parts.
Tolerance Of The CNC Precision Machining
CNC machining tolerances define CNC technology that is most feasible for the machined parts. Precision CNC machining equipment can reach very close tolerances of up to 0.0025 mm. However, CNC machining for such tolerances is expensive, time-consuming, and often times not needed.
Most precision machining services are comfortable offering a tolerance from ± 0.051 mm to ± 0.005 mm. Precision machining process is capable of maintaining this tolerance or achieving a much closer tolerance for tiny parts depending on the desired shapes and raw materials.
Precision machining process doesn’t have a specific tolerance and the variation comes from the manufacturing process, CNC machine, and type of tolerance measurement system.
The most common tolerance measurement systems for machined components are:
Unilateral Tolerance: Tolerance is specified for only one side of the dimension. Meaning, it can go beyond the nominal dimension on one side but stay within the exact tolerance on the other side.
Bilateral Tolerance: This is the most common tolerance and you will see it as a value with ± sign. The dimension can deviate on both sides.
Limit Tolerance: Limit tolerances specify a range of values that the dimension can take. Normally between two numbers with a degree of precision defined by the decimal places.
Common Types of Precision Machining Equipment
Precision machining is not a new process but not all machines are capable of achieving it. The most common types of precision machining equipment rely on CAM software to control the Computer Numerical Control (CNC) machines.
Although these machines are capable of doing most processes, in terms of cost minimization and production rates, it’s better to use certain machines for certain tasks.
CNC Milling Machines
CNC milling machines have a spindle that holds the cylindrical cutter that moves along the axes of the machine.
CNC mills have a variety of cutting tools that have multiple teeth. CNC milling machines are used for rectangular-shaped blanks and can be used for a variety of operations from drilling to carving slots. They are also used to shape the surface features and match ordinary tolerances before surface finishing processes.
Milling machines are commonly used to shape flat surfaces. CNC mills can perform face milling or peripheral milling. Peripheral milled features are cut by placing the cutter parallel to the workpiece.
CNC Lathes and Turning Machines
CNC lathes are able to shape cylindrical blanks by rotating the part at high speeds and driving it on the intended axes. The tool remains stationary and the chuck rotates.
Another type of CNC lathes are the Swiss-type lathes. CNC Swiss lathes are a relatively modern addition to the host of CNC machines that uses a sliding headstock and a guide bushing to support long cylindrical parts. In addition to this, Swiss-type lathes also have milling capability.
CNC turning machines are specifically used to provide some of the standard turning operations like reaming, drilling, spherical balls, pins, grooves, or tappings and tapers. Keep in mind, CNC lathes are strictly for producing machined components from cylindrical pieces.
CNC Drilling Machines
Drilling normally does not require high precision and most vertical drilling machines are hand-operated. However, when producing holes of close tolerances and identical dimensions, it is important to have an automated numerically controlled machine.
Apart from cylindrical holes of the desired diameter, CNC drilling includes broaching, drill presses, screw machine drills and chucking reamers.
These holes are normally drilled for screws, car parts, and plastic assemblies. For instance, the speaker holes in your phone’s metal chassis are made from precision CNC drilling machines.
Electrical Discharge Machines (EDM)
Electrical Discharge machines remove metal by continuously sparking and producing heat on the metal. This process is called Spark machining, It works by applying a DC electric current to the electrode and the metal part below, developing an electric field.
The tiny metal parts suspended in the electrode form at the center of this field and because they are electrically conductive, these tiny metal impurities heat up and the plasma torches the material removing metal.
EDM use a wire or a sinker die. Wired EDMs use the principle of electrode wire burning away the material while sinker die EDMs use an electrode that is a positive image of the desired cavity.
How Are CNC Machines So Precise?
Precision refers to consistency and repeatability. Broadening that definition to CNC machining work we can say precision for a CNC machine refers to the variation in similarity of the parts produced. Bear in mind, for most CNC this variation is less than 0.1 mm.
This means if you were to take two parts made by the same CNC machine you can expect a difference of 0.1 mm in some dimensions. For precision CNC machines this difference is even less and the parts can be presumed identical.
CNC machines are so precise because they are largely operated by numerical codes. The instructions guide the machine to move x mm in the vertical and x mm in the horizontal direction rotating the cutting tool at such and such speeds. With such guidance, it is hard to make an error.
On top of this, the movement on the axes is measured in real-time using linear encoders. Plus, there are built-in calibrators that account for any mechanical deviation and calibrate the machine. Furthermore, for extreme cases of tolerances, the temperature and humidity levels are controlled for optimal performance.
Application of CNC Precision machining
Aerospace
CNC precision machining is extensively used in the aerospace industry to manufacture critical components with high accuracy and close tolerances. It is employed in the production of engine parts, turbine blades, landing gear components, structural components, and more.
Automotive
CNC precision machining plays a crucial role in the automotive industry for manufacturing engine parts, transmission components, suspension systems, brake components, and other critical parts. It ensures high precision and repeatability, resulting in improved performance and reliability of automotive systems.
Medical Devices
Precision CNC machining produces complex components for medical devices like surgical instruments, implants, prosthetics, orthopedic devices, dental components, and diagnostic equipment. Its high accuracy ensures the safety and functionality of these devices.
Electronics
CNC precision machining is involved in the production of electronic components, such as connectors, housings, heatsinks, and PCBs (Printed Circuit Boards). CNC machines are capable of creating precise features, intricate patterns, and fine details required in the electronics industry.
Defense and Military
CNC precision machining is crucial for manufacturing components used in defense and military applications. It is used for producing firearms, weapon systems, optical devices, armored vehicle components, and other defense-related equipment that require high precision, reliability, and durability.
Industrial Equipment
CNC precision machining is widely used in the production of various industrial equipment, including pumps, valves, gears, bearings, shafts, and machine tool components. CNC machines enable the manufacturing of complex geometries, ensuring efficient and reliable operation of industrial machinery.
Advantages Of Precision CNC Machining Services
Better Quality
The most obvious advantage of precision CNC machining is the improved quality of the product. CNC precision machines can reproduce parts with minuscule deviations that appear identical to the naked eye. In addition to this, the surface finishing is much better than standard machined parts.
Tighter Tolerances
It is possible to achieve unmatched tolerances with CNC precision machining. CNC machining operations can achieve tolerances of ± 0.00254 mm. This makes precision CNC desirable for applications where fitting issues can arise.
Tighter tolerances also ensure all produced parts fit into the assembly and are not rejected by quality control. High precision produces identical parts.
Faster Production
CNC can work effortlessly on various materials from wood to plastics and metals. With the instructions loaded into the CNC machine, the process is automated and doesn’t require laborious man hours. Faster turnaround times lead to quicker production and smaller costs.
Easy Prototyping
Most prototypes start their journey in the wood shop or are produced by plastics. Injection molding offers a prototyping solution for complex shapes but is expensive and time-consuming.
Precision CNC machining can easily form intricate geometry and quickly make improved designs by changing the g-code. Computer numerical machines can work on various materials alike at a much faster rate.
Reliable
CNC machined parts hardly fail quality control because the entire process is automated. CAD and CAM make sure the dimensions, processes and order of machining is correct. There are no human errors or manual processes which increases the reliability of precision CNC machining.
Best Practices for CNC Precision Machining
Materials election
Precision CNC machine shop work with a wide range of materials to produce machined parts with high accuracy and quality. The choice of material depends on the specific requirements of the part, including its function, desired properties, and environmental considerations.
Common metals used in CNC precision machining
Aluminum (5052, 6061, 6061-T651, 7075, etc.)
Stainless Steel (201, 301, 304, 316, 410, 420, etc)
Steel (1008, 1020, 1045, etc)
Copper
Brass
Bronze
Titanium
Exotic Metal Alloys
Common plastics used in CNC precision machining
ABS
PMMA
PC
POM
PA(Nylon)
Programming
Programming a CNC machine involves determining the machining type, sequencing tasks, adjusting parameters like feed rates and tool numbers, and converting the information into a numerical code (G-code or M-code) using CAM software.
Processing
The operation sequence is also part of the process stage, but many times parts are transferred between CNC machines for specialized tooling. Such processes can be precision drilling, and precision milling after CNC turning operations.
Surface Finishing
Surface finishing is an essential step in manufacturing, ensuring that parts meet desired texture, finish, and precision.
CNC milling may achieve close tolerance material removal, but surface finishing processes such as grinding and lapping, are commonly employed to enhance the polish and achieve the desired surface characteristics.
Coatings like chromate coating, anodizing, powder coating, and painting are forms of surface finishing used to improve the aesthetics and functionality of parts.
Tips for Precision CNC Machining Services
When machining metal or plastic parts on CNC it is important to maximize the life of your part. This not only means selecting the right materials but also using tested methods and making the design as machinable as possible.
Simplify your design
Try to design features that are easily machined and can perform the task required. For example, avoid angled sides if straight edges can be used and avoid placing holes close to each other. There are many design adjustments that can be made before machining.
Avoid tight tolerances
While CNC machines can produce really tight tolerances, the parts available to you might standardized. For example, you could design a cam shaft with your specifications and uneven tolerances, but finding the use of that shaft in an engine block might be difficult.
Fillet
Avoid sharp edges. Inside or outside edges can act as stress concentration points. Add a fillet radius to avoid stress concentrations.
Threading
There is a list of instructions for thread making, but the most important one is to have it match the bolts available easily. Do not thread the entire length of the hole and complicate the machining.
Avoid Thin Walls
Depending on your CNC operations the removal rate can be different. As such, you should maintain the minimum wall thickness necessary to avoid stress. In most cases this is 0.8 mm for metals and 1.5 mm for plastics.
Design parts that can be made from a single tool
Machining operations can be performed using a single tool. This is ideal as changing tools takes time and manual labor which can increase the production costs. Designing tools whose machining can be accomplished by minimum tool changes should be the standard.
Summary
Precision CNC machining uses milling machines, lathes, drills and a few other precision machines like EDM to shape a workpiece into its final design. The added advantage of achieving tighter tolerances in any manufacturing process makes it premium and of a higher quality grade.
They are mostly used in applications where high precision and small tolerance is needed from applications in defense systems to housing watch components. The process of precision machining is similar to standard CNC but there is a huge difference in the quality of the final CNC machined part.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.