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CNC Milling for Beginners: What it is and How it Works

Computer numerical control (CNC) milling is a process in which automation and milling tools work together, removing material from a workpiece and producing a high-quality finished part or product. It is called a subtractive process, as opposed to the 3D printing additive operations that have become popular over the last decade.

CNC milling operations are appropriate for different materials, including metal, wood, and plastic, although they are most popular within the metal-making industry. CNC milling involves turning and drilling, but the material is removed from a workpiece using specific milling tools to produce machined parts.

Although this article will emphasize the CNC milling process, we will touch on the different operations to compare other manufacturing processes with milling. First, let’s take a look at the basics of milling and then compare it to the second-most popular machine tool: the lathe.

What is CNC Milling?

Milling is a process where a rotary milling cutter removes material to get the desired shape. There are many types of cutting tools at a machinist’s disposal to turn raw material into a finished part. End mills and face mills are two of the most common cutting tools in the machine shop.

With these two milling tools alone, it’s possible to produce simple holes and slots or some of the most complex parts in every shape imaginable. Flat surfaces with excellent surface finishes are routine operations for the correct tool. Whether you operate a CNC milling machine or a traditional manual machine, choosing the right cutting tool is the key to success.

CNC milling machines are traditional milling machines on steroids. Milling cutters are run at higher RPMs, and feed rates are increased. Heavy duty depth of cuts are the norm, and, as you’ve probably guessed, production rates increase exponentially. The speed and versatility of CNC machining centers are incredible, while the high precision and tight tolerances of these machine tools leave many experienced machinists in disbelief.

Even though the CNC milling machine is a popular machine tool, there are other types of CNC contributing to the success of manufacturers. Some of these include automated versions of grinding, laser, plasma, electrical discharge machines (EDM), and routers. But the machine that often sits next to the machine shop’s milling machine is the lathe, and here’s how they compare:

What is the Difference Between CNC Milling and CNC Turning?

The short answer is that milling rotates the cutting tool against a fixed workpiece fastened to a movable worktable. Milling typically uses square or rectangular bar stock to produce finished parts. On the other hand, the lathe rotates the workpiece against a single-point lathe tool, and round bars are its typical raw material.

Both processes are subtractive and produce metal chips, but they use different materials, methods, and tools to make parts and components. Milling and turning are appropriate for steel, cast iron, aluminum, steel, brass, and copper. And because these manufacturing techniques can generate heat, they often require cutting fluid or coolant.

An Overview of the CNC Milling Process

CNC machining is a manufacturing process utilizing computerized controls to manipulate a CNC milling machine and milling tools to turn raw materials into custom parts and components. Although the CNC process provides various capabilities, the fundamental principles of the process remain unchanged throughout all of them. These are the stages of the process:

  • Designing the computer-aided design (CAD) model
  • Converting the CAD file to a CNC machine program
  • Preparing the CNC milling machine
  • Executing the operation

What are the Axes of Motion on a Typical CNC Milling Machine?

Machinists refer to the “axes of motion” when describing the CNC milling machine’s movements relative to an operator facing the machine and based on an X, Y, and Z coordinate measuring system. The X-axis represents movement left-to-right, the Y-axis is front-to-back, and the Z-axis is the spindle moving up and down.

5-Axis indicates that a workpiece can be machined from five sides on one setup. In addition to movements along the X, Y, and Z axes, 5-axis milling can occur using two of three rotational axes. The A, B, and C axes perform a 180° rotation around the X-axis, Y-axis, and Z-axis. A multi-axis tool is favored for complex components in the aerospace, automotive, and many other industries.

5-axis machining requires more CNC programming time, but it allows one workpiece to be machined on five sides during one operation, saving the extra setup of secondary operations. 5-axis machining increases productivity, ensures accuracy and allows for the creation of complex shapes and intricate details.

What are the Primary Types of CNC Milling Machines?

As you can see, computers control the CNC milling machine, but there are distinctions among milling machines that go beyond axis classifications. Here are the main types:

  • Vertical mill: The 3-axis vertical milling machines have a worktable, spindle, and vertical column. In some models, the table moves up and down along the z-axis to meet the fixed arm. On a vertical turret machine, the spindle is stationary, and the table moves along the x-axis and y-axis. On a vertical bed mill, the table can only move along the x-axis, while the spindle can travel along the length of the arm in the y-axis direction.
  • Horizontal mill: The CNC horizontal milling machine is similar to a vertical mill, except the spindle is oriented horizontally. While not as versatile as the vertical mill, the horizontal mill works better for longer or heavier work. A universal horizontal mill also offers the additional benefit of rotating the table around the z-axis, making it a 4-axis milling machine.
  • Knee mill: Often referred to as a Bridgeport-type vertical knee mill, the table moves up and down along the column according to an adjustable knee, and another Z-axis might come from a spindle that also moves up and down.

What are Some of the Applications of CNC Milling?

A few decades ago, milling was a manual task with human operators taking on the type of work they could handle and making the occasional error. Today, CNC milling machines perform most of the milling operations with an experienced programmer and an entry-level operator who mainly changes parts. And there are excellent reasons things have changed.

CNC milling offers high-precision parts and elevated production rates. This ability to accurately customize parts is why many manufacturers use CNC milling machines to make their products. Companies in many industries choose CNCs since CNC milling machines and lathes can mass produce their products with accuracy, speed, and repeatability.

Although machine shops use CNC milling machines and lathes to manufacture parts for industrial applications, specific industries rely exclusively on CNC milling services to process their components. Here are some of them:

  • Aerospace: The aerospace industry requires its components to be made of difficult-to-machine materials, such as Inconel, a nickel-chromium alloy that holds up under high temperatures. CNC milling plays an essential role in producing these aerospace components, allowing them to standardize this process.
  • Electronics: Electronic hardware requires precise configuration for fast and complex performance, and CNC milling offers precision engraving, depressions, and holes, some of the features of electronic parts.
  • Automotive: Auto assembly lines rely on automation to improve productivity, and CNC mills have become one of the most productive tools for automakers.
  • Agriculture: CNC milling machines have large-scale and short-term production capabilities, so unique components and general-purpose parts are ideal for these machine tools.
  • Medical: The production of medical equipment and tools, such as prostheses, benefits from the CNC milling process, as do other medical components that require detailed and unique designs.
  • Military: The defense industry uses CNC milling machines to create prototypes and mass-produce parts. Many of these machined parts have transferred to other sectors, allowing advances initially developed for the military to benefit aerospace, electronics, medical, and other industries.
  • Energy: Nuclear power plants are one example of the energy industry requiring precision parts. Hydropower and solar and wind energy suppliers use CNC milling to manufacture components to ensure continuous power generation. And the oil and gas industry often requires corrosion-resistant metals, such as 5052 aluminum, to be machined in the CNC mill.

CNC milling is fast and convenient, making it ideal for producing replacement parts. All that’s required is a machine setup since you should already have a program from the production process.

No matter which industry you are in, some type of CNC milling will fit your needs with a fast turnaround time and the same reliable quality for each piece.

What are the Main Benefits of CNC Milling?

Although there are numerous reasons why CNC milling machines are considered the most reliable machine tools for producing parts from various materials, some of the most significant are listed below:

  • Speed: CNC milling has a high production rate because the limitations on humans are removed, and the milling tool is free to move at the maximum speed.
  • Precision: In addition to speed, CNC milling machines operate unequally accurately and precisely. Since human error is eliminated from the manufacturing process, CNC milling can work in industries like aerospace and medical, where every detail must be followed without even a minor mistake.
  • Running unattended: CNC milling machines can run on autopilot, requiring little human intervention. They can operate overnight and on weekends without breaks and produce parts with consistent quality and tolerance on each piece.
  • Fewer workers: The automated process that CNC milling represents means fewer people are required to monitor and run it. With today’s employee shortage, needing only a few skilled workers eases the need to find people while reducing production costs and saving money for your clients.
  • Consistency: The automated process behind CNC milling results in unmatched consistency, even if the worker managing the tools changes. After creating a master design file, anyone can use it and have the same quality as the previous version. Even the most highly-skilled manual machinist cannot achieve this repeatability.
  • Scalable: CNC milling grows as your business grows, and you can count on it for tooling parts of all shapes and sizes.

Conclusion

Computer Numerical Control (CNC) milling has benefited manufacturing industries by providing them with the technology for high production rates, greater accuracy, quality finishes, and the ability to work with various materials. The number of axes now available on CNC machine tools ensures that very few tasks will be beyond the grasp of designers and programmers.

With improved product quality control, standardization, repeatability, and precision, CNC machining with CAD technology can automate the milling, cutting, and shaping of many parts, components, and products.

About Peter Jacobs

Peter Jacobs is the Senior Director of Marketing at CNC Masters, a leading supplier of CNC mills, milling machines, and CNC lathes. He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general. You can connect with him on his LinkedIn.

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MX Software – Easy to Use, Easy to Learn – Included with your machine purchase
The MX software is designed to work seamlessly with your CNC Masters machine. It is made to work with Windows PC – desktop, laptop, or an all in one – on standard USB. Use it on Windows 8 or 10 64-bit operating systems.
No internal conversion printer/serial port to USB software or additional conversion hardware is used with the MX.

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2. Clutter Free Interface
The MX is engineered for the CNC MASTERS machine so you do not have to fiddle with a detailed complicated configuration that can be overwhelming. Just load in the MX and start machining!2. Clutter Free Interface
The MX is engineered for the CNC MASTERS machine so you do not have to fiddle with a detailed complicated configuration that can be overwhelming. Just load in the MX and start machining!

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3. Features Tour and Tutorials Included
The Features Tour will give you a quick run-down on all the features the MX can do for you. The Tutorials are easy to follow even for the first time CNC machinist.
Feel free to download the MX on any of your computers. We recommend downloading the MX along with your CAD and CAM software there at the comfort of your office computer to generate your tool path programs. You don’t need to be hooked up to the machine either to test your program in simulation mode.

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4. Navigate and Edit Your Program through the MX interface with Ease
With a few clicks of the mouse or using touch screen technology, you can easily navigate through the MX interface importing saved programs into the Editor from the File drop down menu. Using standard windows features to edit your program you can then lock the Editor Screen to avoid accidental editing, and if you need to insert a line in the middle of a program, just click on [ReNum] to re-number your tool path list.
You can create a program or import CAM generated G-code tool paths into the Editor
The X Y and Z W arrow jog buttons are displayed from the point of view of the cutter to avoid confusion when the table and saddle are moving. You can also adjust your spindle speed and coolant control while jogging each axis.

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5. Feed Hold – Pause in the Middle of your Program
Feed Hold lets you pause in the middle of a program. From there you can step through your program one line at time while opting to shut the spindle off and then resume your program.
You can also write PAUSE in the middle of your program and jog each axis independently while your program is in pause mode.

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6. Hot Keys
Hot Keys is an alternative method to easily control your machine using your hard or touch screen keyboard. One can press P to pause a program, press S to turn Spindle On, G to run a program, Space Bar to Stop, J to record your individual movements one line at a time to create a program in teach mode.

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7. Pick Menu – for conversational mode programming
Write FANUC style G-codes directly into the Editor or select commands off the [Pick] menu and write your tool path program in conversational mode such as what is written in the Editor box. You can even mix between conversation commands and G-codes in the same program.

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8. Pick Menu List of Options
Use commands such as MOVE, SPINDLE ON/OFF, COOLANT ON/OFF, PAUSE, DELAY, GO HOME…. to write your tool path programs in conversational mode.

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9. Draw the Tool Path to verify it before pressing Go
Hit Draw to view your tool path program drawing, check out its run time, or even simulate the tool path in 3D mode. This can be helpful to quickly verify your program before running it. You can also slow down or speed up the drawing or simulation process.
You can also hit Go within the Draw Window itself to verify the cutter’s position on the machine. The current tool path will be highlighted and simultaneously draw out the next path so you can verify what the cutter will be doing next on the program.

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10. Run each tool path independently to study its movement
1. Run the machine on Trace mode. You can run each tool path independently, one line at a time to study the tool path movement on the machine to verify the position of the application and if any fixture/vise is in the way of the cutter’s path.

2. You can also verify your program by clicking on the Trace and Draw buttons together. This will allow you to view each tool path independently one line at a time in the Draw Window.

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11. Counters display in Inches or Millimeters – Continuous Feed
1. When running a program, the counters will display a “real-time” readout while the machine is in CNC operation without counting ahead of the movement.
2. The current tool path is highlighted while the machine is in operation without causing slight interruptions/pauses as the software feeds the tool path to the machine. The MX internally interprets a program ten lines ahead to allow for “continuous machining” avoiding slight interruptions as the machine waits for its next tool path command.
3. “Run Time” tells you how long it takes to run your tool path program.

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12. Use the “Go From Line” command to start in the middle of your program
If you ever need to begin your program from somewhere in the middle of it, use [Go From Line] which you can find under Tools. The Help guide will walk you through how to position the cutter without losing its position on the machine.

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13. Exact Motion Distance without over-stepping on an axis while jogging
Use “Relative ON” to enter a specific coordinate to jog any of your axes to an exact location without having to write a program. It’s like using “power feed” but easier. You can jog an exact distance on any of the axes without needing to keep the key pressed down and mistakenly over-step the movement releasing your finger too slowly off the jog button.
Let’s say you need to drill a hole exactly 0.525” using the Z. So you enter 0.525 in the Z box. Next, adjust the JOG FEED RATE slider for the desired feed rate. Then “click once” on the +Z or -Z button to activate the travel. In this case you click once the -Z button first to drill the hole exactly 0.525”. Then click once on the +Z button to drive the axis back up 0.525”.

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14. Teach Mode – Jog Input
You can create a tool path program by storing each point-to-point movement by simply jogging an axis one at a time. Click on either of the Jog Input buttons to store each movement on the Editor Screen. You can then add Spindle ON, feed commands, and press GO to run the new program as needed. This is a great feature to help you learn to create a program by the movements you make on the machine without necessarily writing out an entire program first.

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15. Override on the fly to adjust the Jog Feed to Rapid or the Spindle Speed during the middle of a program
1. Jog Feed and Rapid with Override: You can adjust feeds using the slider from slow minimum 0.1″ per minute to a rapid of 100″ per minute of travel. You can even micro-step your jog as low as 0.01”/min. The [-][+] buttons allow you to fine tune feeds in 5% increments while the program is in motion.
2. Spindle Speed with Override: You can adjust speeds using the slider from a slow minimum RPM to the max RPM according to the machine setup. The [-][+] buttons allow you to fine tune feeds in 5% increments while the program is in motion.

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16. Adjust Counters using Pre-Set if you cannot begin the program from 0.00
In a situation where you cannot begin your cutter at it’s 0.00 location, you can “Pre-Set” directly into the counters by typing in your beginning coordinate. You can press Go from here to run your program. You can also “zero all” or “zero” your counters independently. With one click of the [Return to 0.0] button, all axes will travel back to its respective 0.0 on the machine.

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17. Set and Save your 0.00 position for future runs
Set and save your 0.00 position on the machine. These coordinates will be recorded as the first line of the program in the Editor Screen. Should you desire to return to this program at a later date, you only have to click on the Set Zero Return button. This will command the machine to automatically jog each axis to its saved “set” 0.00 position according to the recorded coordinates at the first line of the program.

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18. Create a “Home” position to clear your application and run multiple times
Let’s say you need to machine one application times 100 pieces. This usually requires a jig to retain that physical 0.00 position. But in this case, you want the program to end with a clearance of the axes to easily switch out the next piece of stock and start again. With Save Home, you have the ability to save this offset (home) position while still retaining your Set Zero position where the machine will mill your part out. Pressing [Save Home] will record this new position under the Set Zero line in your program.
Pressing [Go Home] will jog your axes back to your “saved home” position where you originally pressed the Save Home command. You can also input GO_HOME from the Pick Menu as its own tool path in your program. At the completion of your program the axes will end at your Home position. Replace your part, then press [Return to 0.0] button to allow the axes to return to its zero position, and press Go to start your next run.

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19. Disable the axis motors to manually hand crank each axis into place
Easily de-energize the axis motors by clicking [Disable Motors] to crank each axis by hand, and then press [Reset Control] to re-energize the axis motors.

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20. Change up to 30 tools with compensation, and store your tool offsets for other programs
The MX supports…

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21. Use the optional ATC rack up to 8 tools for milling, drilling, and rigid tapping applications
The CNC Masters Automatic Tool Changer Rack and Tools (US Patent 9,827,640B2) can be added to any CNC Masters Milling Machine built with the rigid tapping encoder option. The tutorial will guide you through the set-up procedure using the ATC tools.

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22. Use the optional Rigid Tapping Wizard without the need for tapping head attachments
When you order your CNC Masters machine, have it built with the optional rigid tapping encoder. You can take any drill cycle program and replace the top line with a tapping code created by the wizard to tap your series of holes up to 1/2” in diameter.

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23. Use the optional Digital Probe to scan the profile and/or pockets of your fun/hobby type designs to write your tool path program and machine out a duplicate of your original design To “surface” scan an object, you can program the probe along the X or Y plane. The stylus will travel over the part starting on the left side front corner of the object and work its way to the end of the part on the right side. Depending on how the stylus moves, it will record linear and interpolated movements along the X, Y, and Z planes directly on the MX Editor.
To “pocket” scan an object containing a closed pocket such as circles or squares, the scan will start from the top front, work its way inside of the pocket, and scan the entire perimeter of the pocket.
Under the Setup of the MX software you will find the Probe Tab which will allow you to calibrate and program your probe. Your “Probe Step”, “Feed”, and “Data Filter” can also be changed on the fly while the probe is in the middle of scanning your object.

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24. Use work offsets G54-G59 for nesting applications
The work offsets offer you a way to program up to six different machining locations. It’s like having multiple 0.0 locations for different parts. This is very useful especially when using sub-routines/nesting applications.

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25. Create a Rectangular Pocket / Slot with our selection of Wizards to help you build a tool path program
The Cycle Wizards for the mill or lathe makes it easy to create a simple tool path without needing to use a CAD and CAM software.
On this Wizard, the Rectangular Pocket / Slots, can be used to form a deep rectangular pocket into your material or machine a slot duplicating as many passes needed to its total depth.

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26. Create a Circular Pocket Wizard
Input the total diameter, the step down, and total depth and the code will be generated.

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27. Do Thread Milling using a single point cutter Wizard

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28. Cut a gear out using the Cut Gear Wizard with the optional Fourth Axis

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29. Create a Peck Drilling Program in Circular or Rectangular Patterns
Using the Circular or Rectangular Drilling Wizards, you can program the machine to drill an un-limited series of holes along the X and Y planes. Program it to drill straight through to your total depth, use a high-speed pecking cycle, or deep hole pecking cycle. You can program the cut-in depth and return point for a controlled peck drill application to maximize chip clearance.

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30. The MX interface can easily be interchanged from Mill Mode to Lathe Mode
Use this interface for your CNC Masters Lathe. It contains all the same user-friendly features and functions that comes in Mill Mode. Simply go to the Setup page and change the interface.

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31. Use Tool Change Compensation or the optional Auto Tool Changer Turret if your application requires more than one tool in a single program
You can offset the length and angle of each tool and record it under Tools in your Setup. The program will automatically pause the lathe’s movement and spindle allowing you to change out your tool, or allowing the optional ATC Turret to quickly turn to its next tool and continue machining.
On the MX interface, you also have four Tool Position buttons. Select your desired T position, and the auto tool post will quickly turn and lock itself to that position.

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32. Use the Lathe Wizard Threading Cycle to help you program your lathe’s internal or external threads in inches or metric

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33. Use the Lathe Wizard Turning / Boring Cycle to help you program simple turning and boring cycles without having to go through a CAM or writing a long program with multiple passes

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34. Use the Lathe Wizard Peck Drilling Cycle to help you program your drill applications or for face grooving

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35. Facing / Grooving / Part Off Cycle Wizards – with Constant Surface Speed
These cycles can be used with Constant Surface Speed allowing the spindle speed to increase automatically as the diameter of the part decreases giving your application a consistent workpiece finish. With CSS built into the wizard, there is no need to break down the cycle into multiple paths and multiple spindle speed changes.

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36. This is our list of supported G and M codes which can be found under Tools > G Code/ M Code List in the MX
If you plan to use a third-party CAM software to generate your tool path program, use a generic FANUC post processor and edit it to match our list of codes. As an option, we also sell Visual mill/turn CAM software which comes with a guaranteed post processor for our machines to easily generate your tool path programs based on your CAD drawings.

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37. Our pledge to you…

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