What is CNC machining?

What is CNC machining and how does it work? Learn the basic principles and mechanics of this subtractive manufacturing process, as well as its main advantages and limitations.

CNC machining is the most widely used subtractive manufacturing technology today and is an extremely flexible and robust way of producing custom metal and plastic parts. Using CAD models, CNC machines precisely remove material from a solid block using a variety of cutting tools.

Overall, CNC machining produces parts with tight tolerances and impressive material properties. It's suitable for one-off and low-to-medium volume production (up to 1,000 parts) because of its high repeatability. However, it has more design limitations than 3D printing, partly due to the subtractive nature of the technology.

In this introductory guide, we provide an overview of the basic principles of the technology and how these relate to its key advantages and limitations. We also explain the key differences between the two main CNC machine setups: milling and turning.

How does CNC machining work? Let's talk about milling and turning #

The two main types of CNC (Computer Numerical Control) machining systems are milling and turning. Due to the characteristics of each type of machine, milling and turning are each uniquely suited to producing different geometries.

Let's break down how parts are produced using these two different machine setups.

How does CNC milling work? #

CNC milling is the most popular CNC machine architecture. In fact, the term CNC milling is often used as a synonym for CNC machining. CNC milling machines use rotating cutting tools to remove material from a part mounted on the machine bed.

Most CNC milling systems have 3 linear degrees of freedom: the X, Y and Z axes. More advanced systems have 5 degrees of machining freedom via rotation of the bed and/or tool head (A and B axes). 5-axis machines can produce parts with high geometric complexity and can eliminate the need for multiple machining operations.

Here is an overview of how a CNC milling machine turns a CAD model into a custom part.

• The CAD model is turned into G-code, a set of commands for the CNC machine to follow.
• The blank or workpiece is shaped by cutting a block of material to size, either mounting it directly on the bed or using a vice.
• Precision is key when placing and aligning the workpiece to attain accurate manufactured parts. Special metrology tools such as touch probes can aid in positioning and alignment.
• Specialised cutting tools, rotating at high speeds (thousands of RPM), remove material from the block. Initially, the machine rapidly removes material to create an approximate shape with lower accuracy. Subsequently, several higher accuracy passes are taken to produce the final component.
• In the event that the model has features that cannot be reached by the cutting tool in a single setup, the operator needs to invert the workpiece and repeat these steps.

After machining, it is necessary to deburr a milled part. Deburring is the process of manually removing small defects from a finished part caused by material deformation during machining, typically found on sharp edges. For instance, blemishes left by a drill exit on the far side of a through hole must be removed.

Then, it is important to examine the critical dimensions of the part if tolerances were noted in the technical drawing. After completing this step, your part is ready to be used or processed further. Post-processing for CNC-machined parts (whether milled or turned) offers many possibilities, so we advise brushing up on your knowledge and taking your skills to the next level.

How does CNC turning work? #

CNC turning machines utilise fixed cutting tools to remove material from a component, affixed to a rotating chuck. This method is ideal for producing parts with symmetry along their central axis. Turned parts are usually produced quicker and at a reduced cost than milled parts.

Usually, CNC turning systems, or lathes, are utilised to manufacture cylindrical parts. Advanced multi-axis CNC turning centres, complete with CNC milling tools, are capable of producing non-cylindrical parts. These machines integrate the high productivity of CNC turning with the capabilities of CNC milling and can manufacture a vast range of geometries with rotational symmetry, for instance, camshafts and radial compressor impellers.

Here is a summary of how a CNC turning machine produces components.

• The operator produces G-code from a CAD model and loads the machine with a cylinder of stock material.
• The part begins to rotate at high speed, whilst a stationary cutting tool traces a profile, gradually removing material until the desired geometry is achieved.
• Internal cutting tools and center drills can be utilised to cut holes along the centre axis of the workpiece.
• Should there be a need to flip or move the part, the process will need to be repeated. Otherwise, once you have finished reducing the material, the section should be ready for utilisation or additional post-processing.

Since the distinction between CNC milling and turning systems can often be unclear, the remainder of this guide will concentrate primarily on CNC milling since it is the more commonly employed manufacturing technique.

A brief overview of CNC machining parameters. #

The machine operator determines most machining parameters by generating G-code. The primary aspects we wish to address are the size and precision of the CNC machine.

CNC machines possess a relatively spacious build area, particularly when compared to 3D printers. CNC milling systems have the capability to machine parts with dimensions of up to 2,000 x 800 x 100 mm (78" x 32" x 40"), whilst CNC turning systems can machine parts with a diameter of up to Ø 500 mm (Ø 20").

CNC machining enables precise and accurate manufacturing of parts with tight tolerances. In fact, CNC machines can achieve tolerances of less than half the diameter of an average human hair (± 0.025 mm or 0.001”).

If you do not specify the tolerance in the technical drawing, then an operator will typically machine the part with an accuracy of 0.125 mm (.005’’). The operator in this case will follow ISO2768.

What are the most frequently utilised cutting tools in CNC machining? #

To generate a diverse range of geometries, CNC machines utilise a variety of cutting tools. The following are a few of the most prevalent devices employed for milling.

The flat head, bull head and ball head tools are utilised for milling slots, grooves, cavities and other vertical surfaces. Due to their distinctive geometric capabilities, they can machine numerous types of features. Moreover, ball head tools are frequently applied in 5-axis CNC machining to manufacture surfaces that involve curvature and freeform geometries.

Drills, on the other hand, are the most widely utilised tool for boring holes efficiently. You can locate all of the standard drill sizes here. To produce holes with atypical diameters, a plunging flat-headed tool (which follows a helical path) is an option.

Slot cutters are designed with a shaft diameter that is lesser than the diameter of their cutting edge and can generate T-slots and undercuts by eradicating material from the sides of a vertical wall.

Thread taps are employed to form threaded holes. Accomplishing threading requires exact management of the tap's rotational and linear speed. Machine shops often still rely on manual tapping.

Face milling cutters remove material from large, flat surfaces. Due to their larger diameter compared to end milling tools, they require fewer passes to machine sizable areas. This reduces the total machining time required to produce parts with flat surfaces. Operators usually perform a face milling step during the machining cycle to prepare the dimensions of the block.

You'll discover a comprehensive assortment of cutting equipment implemented in CNC turning, encompassing all your machining requisites such as face cutting, threading and groove cutting.

CNC machined parts with intricate designs: what are the design limitations? #

Despite the freedom CNC machining provides, not all geometries can be manufactured with turning and milling machines. Unlike 3D printing, more intricate designs will result in higher machining costs. This is because more complex parts require additional steps.

The primary limitations associated with CNC machining stem from the specific geometry of each cutting tool. The tool's geometry determines a component's radii. As most CNC cutting tools are cylindrical and have a limited cutting length, sharp corners pose significant challenges. As most CNC cutting tools are cylindrical and have a limited cutting length, sharp corners pose significant challenges. As most CNC cutting tools are cylindrical and have a limited cutting length, sharp corners pose significant challenges.

Additionally, accessing tools is a notable limitation when it comes to CNC machining. For instance, 3-axis systems can only handle a certain level of part complexity. If you intend to design for a 3-axis machine, all part features will only be accessible directly from above. 5-axis systems provide exceptional flexibility, as the angle between the component and the device can be altered to gain access to more difficult to reach regions of the workpiece.

Additionally, CNC machines may face challenges when working on parts with delicate walls or other intricate details. Thin walls are susceptible to vibration, which may cause breakage during turning or milling. It is advisable to design metal parts with a minimum wall thickness of 0.8 mm and plastic parts with a thickness of 1.5 mm to mitigate such issues.

Understanding how complex you can design your component for various types of machines, as well as what limitations to bear in mind, is paramount to guaranteeing that your components are produced as intended and to the standard of quality you desire. To learn more about how design can save you a substantial amount of time and money on CNC machining, refer to this article.

What are the features of CNC machining? #

A major advantage of CNC machining is its ability to consistently create durable parts from a vast range of materials. CNC machines can work with virtually any engineering material.

Unlike 3D printing, parts produced through CNC machining possess fully isotropic physical properties that exactly match the properties of the bulk material they were machined from.

CNC machining mainly deals with metals for both prototyping and larger production runs. Machining plastics is typically more challenging due to their lower stiffness and melting temperatures, although we do recognize the usefulness of CNC machining functional prototypes out of plastic before commencing larger-scale production runs with injection molding.

How much do CNC machining materials cost? #

The cost of CNC machining varies significantly due to the plethora of materials available. Each material carries a different price tag, and the physical attributes of these materials have an impact on the overall machining cost.

For metal parts production, aluminium 6061 is the most cost-effective option, with a rough bulk cost of £18 for a blank measuring 150 x 150 x 25 mm. ABS is the most affordable choice, priced at around £13 for a blank of the matching size. Regarding how the ease of machining impacts expenses, stainless steel serves as a proper example. It is far harder than aluminium and therefore more challenging to machine, ultimately driving the total cost up.

Here is a detailed summary of the most favoured materials available on the V1 platform and their key features.

Material	Characteristics	Cost comparison
Aluminum 6061	Good strength-to-weight ratio, excellent machinability, low hardness	$
Stainless Steel 304	Excellent mechanical properties, resistant to corrosion & acid, relatively difficult to machine	$$$
Brass C360	High ductility, excellent machinability, good corrosion resistance	$$
ABS	Excellent impact resistance, good mechanical properties, susceptible to solvents	$$
Nylon (PA6 & PA66)	Excellent mechanical properties, high toughness, poor moisture resistance	$$
POM (Delrin)	High stiffness, excellent thermal & electrical properties, relatively brittle	$$

Post-processing and surface finishes for CNC machined parts #

Parts that come directly off the CNC machine will generally have visible tool marks, which may not be desirable depending on the requirements for the part. There are many post-processing methods that can be used to improve the surface appearance of a part and increase its wear resistance, corrosion resistance, and chemical resistance. Anodizing, bead blasting, and powder coating are all viable finishing methods for custom machined parts.

As this is a more general overview, we won’t go in-depth on post-processing and surface finishes for CNC machining here. You can explore the most common techniques and finishes for CNC machined parts in this handy explainer.

The Benefits and Limitations of CNC Machining #

CNC machining is an effective manufacturing technique for prototyping and small-batch production, but it has certain advantages and disadvantages.

Accuracy and Repeatability

The excellent precision and repeatability of CNC machining makes it ideal for applications with tight tolerances like aerospace and automotive. Both milling and turning can produce parts to high specifications.

Material Versatility

Most engineering materials, including metals and plastics, can be readily machined by CNC due to their isotropic properties. This suits CNC machining to a wide range of applications.

Cost-Effectiveness

For short production runs of up to 1,000 metal parts, CNC machining is a highly cost-effective manufacturing process. It allows fast and economical prototyping and small batch manufacturing.

Geometry Limitations

Highly complex geometries can be challenging or impossible to produce via CNC machining due to its subtractive nature. This limits design freedom compared to additive techniques.

High Initial Investment

The startup cost for CNC machining is substantially higher than digital techniques like 3D printing. This makes it less accessible for low-cost prototyping.

Extended Lead Times

With typical lead times around 10 days, CNC machining is slower than 3D printing, which can produce parts in 2-5 days. This makes it less suitable for rapid iteration.

Expertise Required

Effective CNC machining requires skilled technicians to program, set up and operate the equipment. This expertise raises the barriers to adoption.

In summary, CNC machining is optimized for precision machining of engineering materials in small batches, but faces limitations in part geometry, cost, speed and accessibility. The process necessitates upfront investment and expertise.

What are V1's guidelines for CNC machining? #

Let's analyze the essential parameters to take into account while manufacturing custom parts made of metal or plastic using CNC machining.

Key CNC parameter	What V1 says
Dimensional accuracy	Typical: ± 0.125 mm (.005’’) Maximum: ± 0.02 mm (.0008’’)
Minimum wall thickness	Metals: 0.75 mm (0.030") Plastics: 1.5 mm (0.060")
Maximum build size	Milling: 2000 x 800 x 100 mm (78’’ x 32’’ x 40’’) Turning: Ø 500 mm (Ø 20’’)

Frequently Asked Questions #