CNC Machining Explained: Process, Uses & Key Benefits

What Is CNC Machining — And Why Do So Many Industries Depend on It?

CNC machines are everywhere in the mechanical parts manufacturing business. From aerospace brackets to medical implants and automotive housings, you’re sure to find a CNC machine running when you enter a production facility. The CNC machine has existed for many years but is being utilized more and more in manufacturing facilities for very good reasons.

CNC machining, or Computer Numerical Control machining, cuts away (subtracts) some of the material from an existing block to create a completed part. The process starts with a piece of raw material (a block made of metal, plastic, or composite) and uses a CNC machine to cut the material into exact shapes by using a combination of drilling, cutting and/or milling. The motion of the cutting tool is controlled by a program that is pre-written into the machine; this is why CNC machining is so precise and repeatable from one part to the next.

In this article, we look at how CNC machining works; we will also compare CNC machining with other methods of manufacturing, including metal casting, sheet metal fabrication, and 3D printing. Finally, we will discuss when it is most practical to use CNC machining for production projects.

How CNC Machining Works

A CAD (or Computer-Aided Design) file is sourced from the beginning as a digital 3D Model of the component/instrument. The CAD file will then be converted into G-code — an instruction code given to the CNC Machine that provides tool path, cutting speed, and feed rate instructions.

The process continues using the same principles as a skilled manually operated machinist’s work; however, it produces faster results and tighter tolerances than a skilled machine operator can typically produce. The located cutting tool moves across the material (cutting tool movement is typically in three axes but can be made in complex shapes using four or five axes); this results in the removal of material with each pass.

Common Types of CNC Machining : CNC turning – is performed by rotating the workpiece and keeping the cutter mostly stationary. This method of producing parts is typically used to create cylindrical components such as pins, bushings, and shafts. CNC milling is performed by rotating the cutter and moving it over the surface of the workpiece. The milling process is used to produce flat surfaces, slots, pockets, and other more complex 3D shapes.

Electrical Discharge Machining (EDM): is the process of removing materials using electrical discharges (sparks) to erode the material away. This method works well on hard steels and for difficult-to-machine internal shapes where standard cutting tools are not effective or possible.

Drilling and Tapping: Drilling holes and cutting threads are common operations that are both essential for assembling mechanical components.

The location of methods is determined by part geometry and surface finish.

CNC Machining and Other Types of Manufacturing It can be very helpful to compare CNC machining with other techniques to better understand the capabilities of CNC machining. Each manufacturing process has advantages over the others; selecting the most appropriate process has a significant impact on cost, lead time, and the overall quality of the finished part.

CNC Machining vs. Metal Casting

Casting metals involves the process of pouring molten metal into a specific shape using a mold. This casting process allows for the material to be produced in a very close to finished shape (also known as near-net) and is an efficient method of making many complex external forms in a large quantity. The different methods of casting metal are die casting, sand casting and precision casting.

CNC machining begins with a piece of solid material and removes material from the original piece until it forms your part. CNC machines are ideal for applications where tight tolerances are required, or where the costs associated with creating a mould for the application do not justify the expense of making a mold. Many manufacturers will actually use both methods together; they will first product a rough cast version of the part and then machine the dimensions of the final product.

When you require a large number of identical components, the most logical choice is to cast the metal; whereas when you’re looking to manufacture relatively small volumes, create a prototype and/or get an item with internal features that cannot easily be produced in a mold, CNC machining would be the better choice.

Metal Casting versus Sheet Metal Fabrication

Processes that involve sheet metal, like laser cutting and CNC bending, utilize flat raw material to be cut and formed into finished parts. These processes are an ideal method for making enclosures, brackets, panels, and structural components, where you do not need the entire weight of a solid piece.

CNC (Computer Numerical Control ) machining uses solid metal blocks called billets in order to produce parts. If thin-walled parts cannot easily be made using this method, it may be more convenient and cost-effective to manufacture those parts via Sheet Metal Fabrication because they can be shaped rather than carved from solid stock.

The two approaches produce different types of parts. Machined parts are usually heavy and accurate while sheet metal parts tend to be lighter and made faster.

CNC Machining or 3D Printing Additive manufacturing (or 3D printing) makes parts by layering material instead of taking the material from an existing part. This method is perfect for prototyping various shapes and for making complex geometry like an internal lattice or conformal cooling passages that are not possible with subtractive machining methods.

With this in mind, 3D printing typically produces solid parts slower than CNC machining and usually requires post-processing to achieve quality finishes. Typically, CNC machining manufacturing methods provide superior surface quality, material properties, and dimensional accuracy compared to most functional metal parts made using 3D printing processes. Many engineering processes will utilize 3D printing to produce rapid prototypes, then transition to CNC machining processes to manufacture the final version of the part. As a result, both methods are used together more often than they compete against one another.

Commonly used materials in CNC machining

There are a variety of materials available for use with CNC machines, but aluminum is the most popular material used because it is lightweight, can be machined quickly, and has the highest machinability ratings. On the other hand, stainless steel has excellent resistance to corrosion, but slower machining speeds are required. In addition, titanium is widely used for aerospace and medical applications due to its strength and biocompatibility; however, machining is very demanding. Other common materials used with CNC machines include brass, copper, and various engineering plastics (PEEK, Delrin).

Manufacturers can influence tool wear, cutting speed, surface finish, and consequently, cost per part through their choice of the material that you use to produce a part or component. Reputable manufacturers will advise you on the most appropriate material to use for a part based on how the end product will function, rather than simply recommending the inexpensive option.

Tolerance and Surface Finish Tolerance control is one of the primary reasons why engineers use CNC machining. The standard tolerances for CNC machining are typically ±0.1mm, while precision machining tolerances can meet ≥±0.01mm or tighter in relation to critical features.

The roughness of a surface (Ra) should also be taken into account; a surface machined directly from a CNC lathe will typically have an acceptable Ra or “Roughness Average” value of about 1.6 – 3.2 µm (micro metering units), with the potential for this value to decrease considerably for mating assemblies and surfaces subjected to finish processing by grinding, lapping, polishing, etc. CNC produced components also provide an assembly relationship based on precision, allowing engineers to design the components in accordance with an assembly fit, using CNC machining, and to have complete confidence that they will function properly according to the nominal specifications provided.

When It is Appropriate to Use CNC Machining

CNC machining is not necessarily the best solution, however it can be beneficial in specific instances where:

1. Lower to medium volumes exist and the cost of tooling for castings would be too high for your project’s budget.
2. Precision tolerances are required for critical components (i.e., tolerance on bores, threads, and assembled parts).

Whether you want a prototype. A functional metal part you can have without delaying the production of a mold, you’re seeking a prototype for a complex geometric shape that can only be accessed using multiple tool angles, or perhaps hard materials (such as tool steels or titanium) where there are no other effective means of creating the part, CNC machining could play a key role in your overall production strategy. If your applications do fall into one (or more) of the aforementioned categories, then metal casting or sheet metal are typically more economical methods of producing high-volume pieces in basic shapes. Where you want precision and flexibility over per unit cost at high volumes, CNC machining will have a persistent role in your overall production strategy.

A Note on Choosing a Manufacturing Partner

The quality of any CNC machined component is determined by many variables, including the CNC equipment used, the programming method, and the operator’s experience with that equipment. While tolerances that appear to be simple illustrations can often be achieved with proper fixturing, tool selection, and process control.

Look for documented quality control processes and material certificates when evaluating a supplier’s ability to provide inspection report sharing to establish their experience in metal casting manufacturing as well as sheet metal and 3D printing services all in one facility so that you know what manufacturing process option will work best for your metal casting project before selecting an option after an evaluation.

CNC machining has been proven effective in various industries and offers a high degree of flexibility and precision. CNC machining can accommodate materials and tolerances beyond what other methods can provide, and it also works well within production processes that include traditional metal casting methods, sheet metal fabrication, and 3D printing. Helpful External Resource A thorough academic overview of machining and manufacturing fundamentals can be found at MIT’s OpenCourseWare on Manufacturing Processes; use this as a good resource if you want a deeper understanding of the theory behind machining and manufacturing processes.

When deciding on which process or combination of processes to use, Engineers and Procurement teams take into consideration factors such as part geometry, intended type or amount of material, volume of parts produced and allowable tolerances. Understanding how CNC Machining compares to other types of manufacturing allows for better understanding of how they can use this information to make decisions regarding manufacturing processes.

If you would like to talk about different manufacturing processes for custom metal components, Xuxiangtai Metal  can provide you with advice about whether CNC machining, casting, sheet metal, or additive manufacturing methods would suit your needs as an industrial OEM.