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Understanding How CNC Milling Machines Actually Work in Practice

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Believe it or not, but the origins of CNC milling lie in Ancient Egyptian technology.

Around 4000 BC, people living in Egypt used rollers and bearings to move heavy materials like hand-carved stones over long distances. This led to the invention of the first drilling machine, though it was made of wood—unlike today’s metal machinery.

Over the centuries, other geniuses like artist Leonardo Da Vinci, machine tool maker Henry Maudslay, engineer William Sellers, and more had a hand in crafting various iterations of the complex, computer-operated CNC milling machines that make so many of the products we use today.

Are you interested in learning more about CNC machining for cutting high-grade alloys or other manufacturing applications? Then read on!

What Is a CNC Milling Machine?

First up, what does “CNC” even stand for? Well, that one’s easy: it stands for Computer Numerical Control. And in that ultimately simplistic name lies the function of this machine.

An operator programs a computer to use a rotating cylindrical cutting tool to take out material from an object, say, a sheet of metal. There are many CNC tools, also called bits, that offer the user flexibility in terms of what kinds of holes, slots, cuts, shapes, and dimensions they want to make. You can also configure the machines to work horizontally or vertically.

There are numerous brands of CNC machines on the market today, and though they may be constructed slightly differently, they all work on similar principles. Without going into too much detail, other core parts of a CNC machine include a shaft, large worktable, spindle, frame, axis, controller (that computer we talked about before), lubrication and coolant system, and housing for the entire machine.

CNC milling can handle various materials, from metal and fiberglass to wood, glass, and plastic, making the technology popular with parts and product developers in almost every industry.

CNC Milling Process Explained

CNC milling cutters move along multiple axes and turn in various directions. This gives the user great flexibility in the parts they can create. And this means a lot of pre-planning before you can even get to the stage of using the CNC machine.

Finding a CNC Manufacturer

First, determine whether you want to work with a local machine shop or source off-shore. If you’re unsure which option is best for your project, contact manufacturers in several locations.

Next, decide on whether you want a spot or blanket order. Spot orders, if the manufacturer allows them, are beneficial for testing but are often more expensive. Likewise, consider whether you prefer to enter a long or short-term agreement for similar reasons.

When assessing machine shops, consider their credentials carefully. Are they ISO certified or ITAR registered (for shops in the USA), what do they do to assure quality assurance, and can they work with all types of alloys and materials?

If the supplier falls flat in any of these areas, cross them off your list.

Creating a CAD Model

Before you even get near a CNC milling machine, you need to create a digital model of the part or product you want to manufacture.

CAD (computer-aided design) models are used in various industries to create 2D or 3D digital models. They allow designers to build and visualize a model to scale, zoom in or out on it, and manipulate all aspects of it—all in a virtual environment. The model could be anything, from a boat’s propeller to the floor plan of a company’s new headquarters.

In this application, however, you’ll be creating a part for CNC milling. It’s crucial to get the dimensions of the CAD model correct, or the final milled part will come out wrong or be unusable—an error that could cost you or your company a lot of money.

Converting CAD to CNC

The computer in a CNC machine doesn’t recognize the CAD file format. So you need to use CAM software to convert the CAD file into a format the CNC machine program will understand.

This CNC-compatible file provides the instructions that tell the machine how to move across the work surface and where to cut and drill. It can also be used by the machine shop you’re working with to determine the pricing of your project.

There’s one last step you need to consider before handing over your CNC-compatible file. Do you want to prepare a 2D technical drawing? It’s highly recommended to include this in your file parcel because it will help the machinist understand:

  • The dimensions, features, and functions of the part
  • What machining operations they need to perform
  • The original instructions, in case of error or dispute

Setting up the CNC Machine

With the CAD file converted to a CNC-compatible format, the technical drawing made available to the machinist, and the part material sourced, everything’s ready for the actual milling process to begin.

Before anything else can happen, the operator needs to fix the workpiece—the material that will turn into the part laid out in the CAD design—securely to the milling machine’s worktable. The operator then selects the necessary CNC milling tools and bits, attaching them to the spindle.

Depending on the instructions laid out by the client in their designs, a CNC machine can operate various rotating, multi-point cutting, milling, and drilling tools on a vertical or horizontal axis.

Once everything is set up on the machine, all that’s left to do is run the CNC program. The operator simply presses start, and the machine begins the milling process.

Let’s Get Milling!

Unbelievably, the cutting tools inside a CNC milling machine rotate at speeds up to 15,000 RPM. Very fine cutting tools, called microtools, can spin up to an astonishing 43,000 RPM. They move so quickly that they can get through an average-sized, averagely complex workpiece in mere minutes.

So what is actually happening inside a CNC machine when it’s running? The machine completes three auctions simultaneously:

  • Carefully feeds the workpiece toward the cutting tool
  • Moves the cutting tool through the workpiece
  • Regulates the interaction of the workpiece and the cutting tool

The tools themselves work by chipping off tiny pieces of material until the line reaches the rough form determined by the pre-programmed instructions. Then milling tools move in to refine the line and form. If a part is complicated, it may need to go through multiple machining setups before it’s completed.

Milling is typically completed on an already machined workpiece—the basic shape of the part has already been cut from the source material. This is because milling is mainly used for finishing, also known as secondary processes, giving a piece definition, producing features, or cutting holes and slots.

Types of CNC Milling

There are many milling machines available today, modified to suit almost every parts manufacturing process or need.

Some of the more commonly available CNC milling machines include:

  • Horizontal mills
  • Vertical mills
  • Turret mills
  • Bed mills
  • 3, 4, or 5-Axis mills

You will select the machine type depending on the detail, shape, material, and requirements of the final product you’re attempting to manufacture.

For example, horizontal mills are very versatile, making them the most widely used mill machine available. They are typically used to create simple, sturdy items like axles or gears. On the other hand, Turret mills are best suited to working in a small area, so they’re used to create finer detail—think toy parts or name plaques.

Which Industries Use a CNC Milling Machine?

These machines have opened up a world of possibilities for companies seeking custom parts. In the old days, product or machine parts had to be cut literally by hand using saws, hole punches, and other manual tools. CNC machinery saves time, uses resources more efficiently, and operates with greater precision than ever before.

Industries using CNC technology include:

  • Oil and gas industry: parts for rigs, pipelines, and refineries
  • Medical industry: devices, electronics, instruments, implants
  • Electronics industry: RF shielding, housings, insulation, casings
  • Transportation industry: ship, truck, and train parts and equipment
  • Military and defense industries: weapons components
  • Marine industry: boat and engine parts
  • Aerospace industry: airplane parts, antennae, RF suppression

Companies like Illinois-based Atlas Tool Works, helmed by President Zach Mottl, provide these industries with the technology they need. They turn their engineering ideas into real-life products. In many cases, the tooling is completed off-shore in places like Taiwan, Germany, and, of course, America.

CNC Milling: It’s Easier to Understand Than You Think

Whatever your reasons for finding a CNC milling partner, it’s best to take your time researching your options. While there is undoubtedly excellent CNC capability here in the US, don’t be afraid to head off-shore.

Places like Taiwan and Germany have high standards and competitive pricing. And with supply chains now firmly global, transportation times are surprisingly short—COVID notwithstanding.

For more insight into the critical manufacturing technologies of our day, head back to our blog and browse our other articles.


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