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What is CNC Milling?

What is CNC Milling?

First of all, CNC stands for Computer Numerical Control, so any machine using CNC essentially runs software programs that automate the cutting process. When we talk about CNC machining, we are talking about a wide range of machine processes, like laser cutting, plasma cutting, and press braking – essentially a mechanical cutting technology that removes material. 


A CNC milling machine, by contrast, uses drill bits to remove material and shape a stock piece, making it a subtractive fabrication method that applies the benefits of computing for automation.


To clarify, CNC Milling is just one aspect of CNC Machining, which is a broader field of engineering, and so in this article we will be placing our attention on just CNC Milling.

What are the Four Parts of the CNC Milling Process?

The milling process is a four-step system:

  1. Using CAD to design the parts
  2. Translating CAD files into code for the machines
  3. Setting up the actual machinery
  4. Producing the resulting parts

CAD Design

To get the project underway, you’ll need to use Computer Aided Design (CAD) software to generate a virtual representation of the final product. The best options on the market are:

  • Rhino3D
  • AutoCAD (most popular)
  • SolidWorks
  • Fusion 360
  • SketchList3D

Translating CAD files into code for the machines

Using these software programmes will allow you to create the ‘G code’ which is what the CNC Milling machines will read. The G Code also allows engineers to run a simulation of the cutting process in order to filter out mistakes that end up with a design that is not feasible to produce. The simulation also helps the engineers see if the machine can handle the design too. Remarkably, this code was once designed manually, with the laborious task taking much longer and slowing down the entire process.

Setting up the actual machinery

CNC machines do the cutting and milling work automatically, of course, but without an engineer or operator present to oversee and program the process, things can’t get started. The engineer has a few key responsibilities, such as attaching the milling tools to the spindle and fixing the workpiece to the worktable. Also, as machines and technology get newer and more innovative, the operators need to keep on top of these changes and remain proficient in their duties. Interestingly, as the machines get more advanced, the automation improves and the job should, in theory, become more streamlined for the operator, whilst at the same time, using newer code with an older machine can make their job more testing.


Perhaps one day, machines will be able to set up themselves, but for now, an operator is required.

Producing the resulting parts

Once the CAD and the setup are done, the engineer or operator makes one final check before starting the production process.

So, what happens next in the production process?

To talk about the production work, we can simplify by saying that a rotating drill bit makes contact with the workpiece to remove chips and produce the computed shape. This is perhaps an oversimplification, as there are in fact many different cutting methodologies, which we will share below.


Climb milling

The newer CNC milling machines, which you might be more accustomed to, opt for a process called climb milling.


Here are some benefits:

  • The cutting surface is cleaner, making tools last longer due to reduced rubbing
  • Chips fall behind the cutter instead of creating an obstruction
  • The cutting chip absorbs the heat and rub, rather than the workpiece
  • Horizontal climb milling (more on this in the next section) uses downward force, making extra clamping less necessary
  • Provides a highly precise finish and can be programmed to include threaded holes and slots
  • Can take a workpiece and make a final product in one process, rather than requiring regular tool changes and adjustments
  • Other fabrication processes find it hard to match the quality delivered by this process

Conventional milling

The old ways of the past are constantly being redesigned, disrupted, and innovated, but that doesn’t mean they are completely ineffective. However, they may not produce the same quality of results as climb milling, as you will see below:

  • The cutting chip increases in thickness and results in higher temperatures that make the workpiece harden (not ideal really)
  • There is more rubbing in friction, wearing the tool down and shortening its lifespan (increasing costs)
  • The operator must take more care to clamp tightly as displacement can be a major issue
  • Chips go upwards and sometimes land back into the cutting path, creating undesirable outcomes


Clearly, climb milling is a much better and more modern technique, but the question remains…

CNC Milling: Horizontal or Vertical?

Is horizontal milling the best?

  • A better surface finish and longer tool life
  • Much faster (3-4x)
  • Allows for more complex parts to be made
  • Fewer operations required, though also fewer available due to a skills gap


What about vertical milling?

  • Easier to get hold of and much cheaper – around 20-25% of the price of a horizontal mill
  • Better ease of use and visibility for the operators
  • More trained operators available for hire
  • Requires less physical space for setup


What are the Different Types of CNC Milling?

There are at least 7 types:

  • Surface/Plain milling
  • Form/Gang milling
  • Angular/Chamfer milling
  • Gear milling
  • Profile milling
  • Face milling

What is Surface Milling? (Plain Milling)

Surface milling or plain milling makes use of a horizontal mill, which allows for the use of wide or narrow cutters. This is typically used as a two-step procedure, the first cutter to remove the bulk of the excess material and the second, after a change of drill bit, to make a more accurate and economical finish. 


What are Form Milling and Gang Milling?

This is a special type of milling for creating beautiful surface contours, which makes use of advanced tools like convex and concave cutters to make round recesses and round edges. Gang milling is slightly more advanced and requires multiple cutters to do more intricate work.


What is Angular Milling? (Chamfer Milling)

This is a special type of milling to create chamfers and grooves, typically using a 3-axis mill and dovetail cutters. Alternatively, you can use a conical cutting head.


What is Gear Milling?

Milling in this way can produce gears, which are then treated and turned for an exceptional final outcome.


What is Face Milling?

Face milling simply uses a cutting tool with multiple teeth to produce intricate contours and a great finish. The side teeth are used to cut, whereas the teeth on the tip create a nice finish.


What is Profile Milling?

Last but not least is profile milling, which is used for making convex and concave parts. The first step is roughing, where round inserts do the removal work, then semi-finishing takes place using ball-nose end mills. The final finishing is also done with this type of mill, the help of CNC milling, and the availability of multi-axis technology.

What more is there to know?

Whether you’re a hobbyist, a professional, or part of an engineering business, diving deep into CNC machining and CNC milling can help improve your knowledge of processes and equipment. If you’re keen to learn more and advance your education in this field, please contact one of our Account Managers and start an interesting dialogue today. 


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