What is a CNC Milling Machine and how does it work? How do CNC milling machines compare to CNC Lathes? When do you need such a CNC machine tool?
Focused on milling – the process of machining using rotating tools to gradually remove material from a workpiece – CNC milling machines are a mainstay for factories around the world. These machine tools make use of a variety of cutting tools along one or more axes to remove material from a workpiece through mechanical means.
CNC milling machines are often used in a variety of manufacturing industries: from industries like aerospace, shipping, automobiles, and oil drilling / pumping and refining, to medical, FMC manufacturing, and precision engineering sectors.
Also called CNC Machining Centers, the more advanced CNC milling machines can operate along multiple-axis. These may be fitted with automatic tool changers, advanced machine coolant systems, pallet changers, and advanced software to improve the efficiency and accuracy of machining processes.
In this article, we will be looking at the following aspects of a CNC Milling Machine/ Machining Center:
- What is a CNC Milling Machine?
- How does a CNC Milling Machine work?
- CNC Milling Machine vs CNC Lathe
- What are the main components of a CNC Milling Machine?
- What are the cutting tools used by a CNC Milling Machine?
- What are the different types of CNC Milling Machines?
- How much does a CNC Milling Machine cost?
- Where can you buy CNC Milling Machines?
What is a CNC Milling Machine/ Machining Center?
First, let us look at what a CNC milling machine is.
CNC Milling Machines are machine operated cutting tools that are programmed and managed by Computer Numerical Control (CNC) systems to accurately remove materials from a workpiece. The end result of the machining process is a specific part or product that is created using a Computer Aided Design (CAD) software.
These machine tools are normally equipped with a main spindle and three-linear-axes to position or move the part to be machined. More advanced versions may have a 4th or 5th rotational axis to allow for more precise shapes of varying dimensions and sizes to be machined.
CNC milling machines / machining centers normally employ a process of material cutting termed milling or machining – the milling process involves securing a piece of pre-shaped material (also known as the workpiece) to a fixture attached to a platform in the milling machine. A rapidly rotating tool (or a series of interchangeable tools) is then applied to the material to remove small chips of the material until the desired shape for the part is achieved.
Depending on the material used for the part, as well as the complexity of the machined part, varying axes, cutting head speeds, and feed rates may be applied.
Milling is normally used to machine parts that are not symmetrical from an axial perspective. These parts may have unique curvatures or surface contours, which may require a combination of drilling & tapping, grooves, slots, recesses, pockets and holes to work on them. They may also form parts of the tooling for other manufacturing processes – for example in the fabrication of 3D moulds.
Features of Advanced CNC Milling Machines / CNC Machining Centers
In the past, milling machines were manually operated. Operators had to use a combination of machines with different tools to machine a more complex part or product. Or they had to use various settings on one machine just to complete the job.
With the advancement of technology such a CNC controls and Automatic Tool Changers (ATCs), greater efficiency, flexibility and speed can be achieved – even for more convoluted parts. The provision of digital readouts and measuring systems has also improved the accuracy of CNC machining processes.
To cater to manufacturers that require the flexibility of “High Mix, Low Volume” (HMLV) or “small batch” production, CNC machining centers can be fitted with Pallet Changers or other automation solutions to form part of a Flexible Manufacturing Cell (FMC) or Flexible Manufacturing System (FMS). This allows such machines to cater to a wide variety of machining demands and needs.
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How does a CNC Milling Machine work?
The general principle for a CNC milling machine or CNC machining center is that the part to be machined is clamped on top of the machine table. It could be clamped directly on the table itself, or held in place by a vice or fixture.
The spindle (moving section) including the cutting tool is then either vertically or horizontally positioned. In that configuration, the tool can reach various X-Y-Z positions on the work piece and commence cutting and shaping actions. As it does so, the work piece or part may either be fixed, mounted, or moved/positioned by the table in a linear direction to the spindle with the cutting tool. This allows material to be removed according to the desired shape needed for the machined part.
A CNC milling machine may either be horizontal or vertical.
You can see how this looks in the simplified diagram below.
CNC Milling Machine Vs CNC Lathe (Turning Center)
While CNC Milling Machines are machine tools just like a CNC Lathe, both tools operate quite differently.
For a CNC milling machine (aka CNC machining center), the work piece is fixed or mounted in position using a vice or fixture while the cutting tool is manoeuvred on top of or around the piece. Material is then gradually removed using cutting tools or drills which rotate at high speed with varying feed rates along two or more axes.
In the case of a CNC Lathe (aka CNC Turning Center), however, the work piece (usually cylindrical) is mounted on a rotating chuck or on the main spindle. It is then “turned” (hence the name turning Center) or rotated along a main axis while the cutting tool located in a rotational or positioning turret would move in a parallel direction to the piece. Material is then removed using stationery cutting tools.
Thus, a CNC milling machine use a spinning tool with a stationery work piece, whereas a CNC Lathe would involve spinning the material to be worked on by a stationery cutting tool.
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What are the Main Components of a CNC Milling Machine?
So what are the different parts of a CNC Milling Machine or Machining Center? The diagram below provides a breakdown of its main components.
The frame is the main structure that supports the milling machine and helps to give it stability and rigidity. It usually comes with a base and detachable column/s.
An important part of the frame is the machine headstock where the main spindle is mounted on. The headstock, while important, is often overlooked. If the headstock is not rigid and fails to provide stability and support to the spindle, you could get vibrations and chattering during the machining operation. This could result in machining errors and a shorter lifespan to your cutting tool.
The spindle can be considered as the “heart” of a CNC milling machine. It normally comprises a rotating assembly, and a tapered section where tool holders may be positioned. The shaft of the spindle is normally where the tool is attached to, usually via a tool holder.
A motor with different levels of transmission is used to rotate the spindle.
To keep your spindle in good condition over the long-term, various forms of lubrication are used. They may include Grease Lubrications (not suitable for long durations of high speed operations), Air-Oil Lubrications (sufficient for longer cycle runs at higher speeds) or the Hwacheon Oil-Jet-Lubrication (ideal for all conditions – even extreme, long and high speed applications).
Depending on the machine type, the spindle can be vertically or horizontally positioned.
In general, CNC milling machines have X / Y / Z as well as additional rotational axis or C / A or B (subject to configuration). These can be programmed using g-code in the CNC controller.
The columns of the CNC machining center can be single (eg travelling column HiRex 4000 or also C-Frame like HiT 400 / 360; VESTA line including “B”; SIRIUS-650 / 850 / 1050) or double (SIRIUS 1250 / 2500 / L1 / L2). This depends on the level of complexity needed in the machining task.
#5 CNC Control Panel
This is the main “nervous system” of the machine tool. It contains the electronics that helps to control the different cutting actions through programming functions. The control panel has a CNC monitor and programming buttons where data and codes can be punched in. It usually also offers a manual function. For ease of machine operation, the control panel should be easily accessible and within easy reach.
#6 Tool / Tool Changers (Automatic Tool Changer or ATC)
These are either mounted at the column or separately mounted to the machine. The latter is preferred if larger tool changers are needed with 40 up to 300 different tools. Doing so not only helps to save time and effort – it also helps your operators to avoid unnecessary vibrations during operation.
#7 Tool Holders
These come in many different sizes, systems and for various applications. The standard sizes for tool holders are BT 30 to BT 40 and BT 50 (BT refers to the taper angle of the cone on the holder).
(In Europe, the term used is SK 30, SK 40 or SK 50, which has the same angle as BT holder but with an additional orientation groove at the flange. Together with a key-stone mounted at spindle nose, SK holders can be orientated in a specific position. Eg.: boring bridges, boring bars, angular milling heads etc.)
For higher rigidity and balanced fast rotations, BBT versions are recommended or the HSK System.
The BBT version is an up-graded version of the standard BT system providing double contact points between the tool holder and spindle. As such, it connects and bonds more strongly, provides significantly less vibrations as the tool holders are balanced to class Q 2.5. This improves not only the machining capabilities and results of your machine tool, but provides better surfaces to your part / mould, extends your tool life and lastly saves spindle replacements in the long run as bearings will last longer.
What about the HSK System? It is lighter (good for HSC machining), shorter (good for faster tool change), and stronger than the BT system (a HSK-A63 holder is 2.5 times stronger than a BT 50 holder). The HSK System also offers excellent balancing (Class Q 2.5). Due to the strong and powerful clamping from inside to outside, the bonding between the tool holder and the spindle is ideal for heavy duty operations, heavy cuts or extremely high-speed operations at the same time.
Both the BBT and HSK versions offer far stiffer and more rigid bonding between the tool holder and spindle as these systems come with double contact clamping (i.e. between the caper / cone and front spindle nose surface).
The table provides a solid base to clamp the work piece directly on, and can be used to mount fixtures or vice to hold the piece in place. Most of the tables use T-slots for easy clamping of vice, fixture or part.
On Horizontal CNC milling machines, pallets are also available with Tap-holes. These allow greater flexibility in moving different work pieces to be machined. Increasingly, magnets are also being used for easy, fast and secured clamping. These should preferably be built into the machines table to avoid the loss of Z-axis height.
(See Hwacheon’s SIRIUS UM+ as an example.)
#9 Coolant Tank
Most CNC machining centers have a coolant tank to help supply coolant to the cutting surface or the spindle with tool during machining action. This helps to lengthen the life-span of the machine and its parts. Beyond this, the coolant will also remove heat generated by the machining action, and hence keep temperatures under control.
To hold a sufficient amount of coolant for cutting operations, the rule here is “the more, the better.” More coolant helps machine operators to avoid the warming up of coolant.
Generally, the CTS system or Coolant Through Spindle is recommended for deeper drilling operations (deeper than 4 x diameter) or when one is machining deeper cavities in mould & die applications. The standard should be 30bar with options of up to 70bar in coolant pressure. Only very specific applications would require even higher coolant pressures.
Attention needs to be given if tools with larger coolant hole diameters are used. Such machining needs may require a High Pressure – High Flow Rate Pump to be used.
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What are the Milling Tools that CNC Milling Machines use?
In general, CNC milling machines can deploy a wide range of cutting and shaping tools. These tools help to remove material by virtue of how they move within the machine or directly from the cutter’s shape.
These may include the following:
- End Mills, Flat or Ballnose, Straight or Form Tools.
- Face Mills
- Drills, U-Drills, Spiral Borer.
- Tool Holders and Hydraulic Tool Holders, Shrink-Fit or Press-Fit holders.
What are the Different Types of CNC Milling Machines?
There are many different variations and types of CNC Milling Machines – from simple two to three axes machines to multi-axis CNC machining centers. Generally speaking, the main variations are either Vertical or Horizontal machining centers.
Here are some of the types that you may find in the market:
- Horizontal Milling Machines
- Vertical Milling Machines
- Bed Mill
- Turret Mill (for manual machines only)
- Travelling Column
- Double Column
- Drill & Tap
- Five Axis Machining Centers
- Double Spindle (VESTA-500 & VESTA-650T)
When you consider the right CNC milling machine for your machining needs, it is useful to consider your specific requirements in terms of your machined part dimensions, complexity of its shape and design, need for accuracy, as well as the machine longevity and stability.
C-Frame Vertical Machining Centers
Vertical Machining Centers (VMCs) can normally take the shape of a C-Frame, which is a traditional design for a VMC. These are limited to a certain machine size – beyond this, the overhang of the spindle and overall machine floor space required may become too big.
Double Column Machining Centers
Double Column machining centers are normally used for larger parts and machine sizes. However, smaller sized high precision milling machines may also be designed with a double column to improve their rigidity and accuracy. (Examples here include Hwacheon’s SIRIUS UM+ / UL+ / UX / 1250 / 1350 / 1750 / 2500 and L1 / L2)
Machines that are made for milling simple parts with lower accuracy standards are normally much lower in price (and features) compared to those used for mold & die applications which require the highest accuracy and production standards.
Travelling Column Machining Centers
Travelling Column Machining Centers (like Hwacheon’s Hi-Rex 4000 / 5000 / 6000) are CNC milling machines where the table and work piece are stationary and do not move. All the three linear axes movements made by the travelling column which includes the spindle as well as the cutting tool. These machines are specially designed for a very long but slimmer (Y-axis direction) parts or to allow for multiple-clamping applications of the workpiece.
Double/ Multi-Spindle Machining Centers with Pallet Changers
Often equipped with Pallet Changers, vertical or horizontal double or multi-spindle machining centers are top-end CNC milling machines. These are made to achieve optimal yields in productivity and are designed for the mass production of complex parts.
The use of automatic pallet changers helps to reduce down times while increasing productivity and efficiency. They can also be deployed as part of your factory automation process (read this article for more.)
Multi-Axis CNC Machining Centers
Most CNC milling machines come in the standard 3-axis up to 5-axis for machining very complex shapes (ie mold & die) or complex parts which need to be positioned in various angles. With multiple axis, the machining on up to five faces are provided in a single setting.
High Speed and High Capacity Machining Centers
Also known as HSC machines, high speed machining centers can be used to achieve not only the highest surface finishes for the machined part, but to handle tools with extremely small diameters – as little as 0.030 mm in diameter or smaller.
Similarly, high capacity machining centers are designed and built to cater for extreme tough materials. Such materials include Heat Resistant Stainless Steel, Inconel or Titanium – just to name a few. (Hwacheon Horizontal Machining Centers such as H6 & H8 as well as our Vertical Machining Centers such as VESTA B-series and the larger SIRIUS line of machine are designed to handle these without problem.)
Machines with Box Guide Ways or Linear Roller Guide Ways
Depending on your factory’s need, machining centers with different guide ways may be needed. CNC milling machines with Box Guide Ways can normally provide greater stability and damping – these help to minimize any vibration or movement. If faster and more dynamic movements are required, the use of large dimensioned Linear Roller Guide Ways can be considered.
Ultimately, in choosing the right CNC milling machine for your factory, you need to lend the utmost importance to the overall machine’s mechanical design. The spindle needs to be only be powerful but provide high torque and sufficient RPM for your machining needs.
Beyond this, you should ensure that your machine has an effective lubrication and cooling system. It should also be temperature safe, and have the ability to avoid heating up – overheating will result in temperature growths and deformations during extended machining cycles.
Configure your own machine combination.
What are the Costs of CNC Milling Machines?
The price of a CNC milling machine or machining center varies significantly. They can be below USD 50,000 to several millions for one unit, depending on the different components and complexity that goes into the unit.
This very much depends on the following key considerations:
- What standard or application is the machine used for?
- How many axes will the machine have? 3 axis or 5 axis
- What is the size of the machine needed (depending on the machined part)?
- Do you need a pallet changer – automatic or manual?
- What is the spindle power and Revolutions Per Minute (RPM) needed?
- What is the degree of accuracy required?
- What are the materials that will be used for machining?
- What are the CTS systems, size and CNC control systems needed?
- Do you need to manufacture only for mass production or will custom manufacturing be needed?
For example, if a machine needs to cater to a size of 2 metres in X and 1.5 m in Y dimensions with an 8,000 RPM spindle and which is good enough to reach 50-75 micron accuracy, it may cost up to USD 200,000. The same-sized machine to achieve 10-15 micron accuracy will cost USD 350,000 to USD 450,000, while a similar-sized machine that can achieve 3-5 micron in accuracy – aka a “Mother Machine” – will cost upwards of USD 1.5 million and above.
This simple illustration shows how accuracy alone can affect the price of your CNC machining center.
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Where can You Buy CNC Milling Machines?
There are many companies that deal in CNC milling machines and machining centers. As South Korea’s leading machine tool manufacturer, Hwacheon Machine Tools offers a wide range of CNC milling machines and 5-axis machining centers that are available in different shapes, sizes and configurations to meet your needs.