CNC Machines | Milling (Machine Centers)

Many different types of CNC machines are used in industry – the majority of them are CNC machining centers and CNC lathes. They are followed by wire EDM, fabricating machines and machines of special designs. Although we focus of this website is on the two types that dominate the market,many general ideas can be applied to other CNC equipment.

CNC Machines – Milling

Description of CNC milling machines is so large, it can fill a thick book all by itself. All machine tools from a simple knee type milling machine up to a five axis profiler can be included in this category. They vary in size, features, suitability for certain work, etc., but they do all have one common denominator – their primary axes are X and Y axes – and for this reason, they are called the XY machines.
In the category of XY machines are also wire EDM machine tools, laser and water jet cutting machines, flame cutters, burners, routers, etc. Although they do not qualify as milling type machine tools, they are mentioned because the majority of programming techniques applicable to milling can be applied to these machine types as well. The best example is a contouring operation, a process common to many CNC machines.
A milling machine can be defined:

Milling machine is a machine capable of a simultaneous cutting motion, using an end mill as the primary cutting tool, along at least two axes at the same time!

This definition eliminates all CNC drill presses, since their design covers positioning but not contouring. The definition also eliminates wire EDM machines and a variety of burners, since they are capable of a contouring action but not with an end mill. Users of these machine tools will still benefit from the many subjects covered here. General principles are easily adaptable to the majority of CNC machine tools. For example, a wire EDM uses a very small cutter diameter, in the form of a wire. A laser cutting machine uses laser beam as its cutter, also having a known diameter but the term kerf is used instead. The focus will be concentrated on metal cutting machine tools, using various styles of end mills as the primary tool for contouring. Since an end mill can be used in many ways, first look will be at the various types of available milling machines.

Types of Milling Machines

Milling machines can divided into three categories:

  • By the number of axes – two, three or more
  • By the orientation of axes – vertical or horizontal
  • By the presence or absence of a tool changer

Milling machines where the motion of a spindle is up and down, are categorized as vertical machines. Milling machines where the spindle motion is in and out, are categorized as horizontal machines – see Figure 2-1 and 2-2.

Figure 2-1 – Schematic representation of a CNC vertical machining center Figure 2-2 – Schematic representation of a CNC horizontal machining center

These simplified definitions do not reflect reality of the current state of art in machine tool design. Machine tool industry is constantly changing. New and more powerful machines are designed and produced by many manufacturers in several countries, with more features and flexibility.
The majority of modern machines designed for milling are capable of doing a multitude of machining tasks, not only the traditional milling. These machines are also capable of many other metal removing operations, mainly drilling, reaming, boring, tapping, profiling, thread cutting and many others. They may be equipped with a multi-tool magazine (also known as a carousel), a fully automatic tool changer (abbreviated as ATC) and a pallet changer (abbreviated as APC), a powerful computerized control unit (abbreviated as CNC), and so on. Some machine models may have additional features, such as adaptive control, robot interface, automatic loading and unloading, probing system, high speed machining features, and other marvels of modern technology. The question is – can machine tools of these capabilities be classified as simple CNC milling machines? In two words – certainly not.Milling machines that have at least some of the advanced features built-in (usually many features), are known as a separate category of machines – they are called CNC Machining Centers. This term is strictly CNC related – a manual machining center is a description that does not exist.

Machine Axes

Milling machines and machining centers have at least three axes – X, Y and Z. These machines become even more flexible if they have a fourth axis, usually an indexing or a rotary axis (A-axis for vertical models or B-axis for horizontal models). Even higher level of flexibility can be found on machines with five or more axes. A simple machine with five axes may be a boring mill that has three major axes, plus a rotary axis (usually B-axis) and an axis parallel to the Z-axis (usually W-axis). However, true complex and flexible five-axis profiling milling machine is the type used in aircraft industry, where a multi-axis, simultaneous cutting motion is necessary to machine complex shapes and reach cavities and various angles.
At times, the expression two and a half axis machine or a three and a half axis machine is used. These terms refer to those types of machines, where simultaneous cutting motion of all axes has certain limitations. For example, a four-axis vertical machine has X, Y and Z- axis as primary axes, plus an indexing table, designated as an A-axis. The indexing table is used for positioning, but it cannot rotate simultaneously with the motion of primary axes. That type of a machine is often called a ‘three and a half axis’ machine. By contrast, a more complex but similar machine that is equipped with a fully rotating table, is designed as a true four-axis machine. Rotary table can move simultaneously with the cutting motion of the primary axes. This is a good example of a true ‘four axis’ machine tool.
Each machining center is described by its specifications as provided by the machine tool manufacturer. Manufacturers list many specifications as a quick method of comparison between one machine and another. It is not unusual to find a slightly biased information in the descriptive brochure – after all, it is a sales tool.
In the area of milling systems, three most common machine tools are available:

  • CNC Vertical Machining Center ( VMC )
  • CNC Horizontal Machining Center ( HMC )
  • CNC Horizontal Boring Mill

Programming methods do not vary too much for either type, except for special accessories and options. Some of the major differences will be orientation of machine axes, additional axis for indexing or full rotary motion, and the type of work suitable for individual models. Description of the most common type of a machining center – Vertical Machining Center (VMC) – presents a fairly accurate sample of describing other machines of the above group.

Vertical Machining Centers

CNC Vertical Machining Center ( VMC )

Vertical machining centers are mainly used for flat type of work, such as plates, where the majority of machining is done on only one face of the part in a single setup.
A vertical CNC machining center can also be used with an optional fourth axis, usually a rotary head mounted on the main table. Rotary head can be mounted either vertically or horizontally, depending on the desired results and the model type. This fourth axis can be used either for indexing or a full rotary motion, depending on the design purchased. In combination with a tailstock (usually supplied), the fourth axis in vertical configuration can be used for machining long parts that need support at both ends.
The majority of vertical machining centers most operators work with are those with an empty table and three-axes configuration.
From the programming perspective, there are at least two items worth mentioning:

  • Programming always takes place from the viewpoint of the spindle, not the operator’s. That means the view is as if looking straight down, at ninety degrees towards the machine table for development of the tool path motion. Programmers always view the top of part !
  • Various markers located somewhere on the machine show positive and negative motion of the machine axes. For programming, these markers should be ignored! These indicate operating directions, not programming directions. As a matter of fact, typically the programming directions are exactly opposite of the markers on the machine tool.

Horizontal Machining Centers

CNC Horizontal Machining Center ( HMC )

Horizontal CNC Machining Centers are also categorized as multi-tool and versatile machines, and are used for cubical parts, where the majority of machining has to be done on more than one face in a single setup.
There are many applications in this area. Common examples are large parts, such as pump housings, gear cases, manifolds, engine blocks and so on. Horizontal machining centers always include a special indexing table and are often equipped with a pallet changer and other features.
Because of their flexibility and complexity, CNC horizontal machining centers are priced significantly higher than vertical CNC machining centers.
From programming point of view, there are several unique differences, mainly relating to the Automatic Tool Changer, the indexing table, and – in some cases – to the additional accessories, for example, pallet changer. All differences are relatively minor.Writing a program for horizontal machining centers is no different than writing a program for vertical machining centers.

Horizontal Boring Mill

Horizontal boring mill is just another CNC machine. It closely resembles a CNC horizontal machining center, but it does have its own differences. Generally, a horizontal boring mill is defined by the lack of some common features, such as the Automatic Tool Changer. As the name of the machine suggests, its primary purpose is boring operations, mainly lengthy bores. For that reason, the spindle reach is extended by a specially designed quill. Another common feature is an axis parallel to the Z-axis, called the W-axis. Although this is, in effect, the fifth axis designation (X, Y, Z, B,W), a horizontal boring mill cannot be called a true five axis machine. Both the Z-axis (quill) and the W-axis (table) work in opposite directions – towards each other – so they can be used for large parts and most of hard-to-reach areas. It also means, that during drilling, the machine table moves against an extended quill. Quill is a physical part of the spindle. It is in the spindle where the cutting tool rotates – but the in-and-out motions are done by the table. Think of the alternate method offered on horizontal boring mills – if the quill were to be very long, it would lose its strength and rigidity. The better way was to split the traditional single Z-axis movement into two – the quill extension along Z-axis will move only part of the way towards the table and the table itself – the new W-axis – will move another part of the way towards the spindle. They both meet in the area of the part that could be machined using all other machine tool resources.
Horizontal boring mill may be called a 3-1/2 axis CNC machine, but certainly not a 5-axis CNC machine, even if the number of programmed axes is five. Programming procedures for CNC boring mills are very similar to the horizontal and vertical CNC machining centers.

Typical Specifications

On the preceding page is a comprehensive chart showing typical specifications of a CNC Vertical Machining Center and a CNC Horizontal Machining Center. These specifications are side by side in two columns, strictly for convenience, not for any comparison purposes. These are two different machine types and comparison is not possible for all features. In order to compare individual machine tools within a certain category,machine tool specifications provided by machine manufacturer often serve as the basis for comparison. These specifications are contained in a list of verifiable data, mainly technical in nature, that describes the individual machine by its main features. Machine tool buyers frequently compare many brochures of several different machines as part of the pre-purchase process. Managers and process planners compare individual machines in the machine shop and assign any available workload to the most suitable machine.
A fair and accurate comparison can be made between two vertical machining centers or between two horizontal machining centers, but cannot be done fairly to compare between any two different machine types.
In a typical machine specification chart, additional data may be listed, not included in the earlier chart, depending on the exact features. In this handbook, the focus is on only those specifications that are of interest to a CNC programmer and, to a large extent, a CNC operator.