A Guide for CNC Milling Machines

In manufacturing, the milling machine is one of the most captivating tools for those who are enthusiastic about machining and manufacturing. A milling machine is a machine that cuts, roughs, and even drills different raw materials; typically metal. The milling machine achieves this by removing the workpiece chips through the combined high-speed rotation of the milling cutter and the maneuvering of the axes, either of the mechanism or the element. A milling cutter is a tool used within a milling machine. The cutter has at least one or more cutting edges that remove metal chips when the tool is circulating. There are a variety of milling cutters for the different processes on a given part. This article will review the components of a milling machine and the different types of milling approaches.

The Anatomy of a Milling Machine 

Taking a look at the anatomy of a milling machine will help explain its great functionality. Because parts and accessories vary, depending on the type of machine, we will highlight the most common and important ones. 

Table: This part of the machine is where the machinist will place the workpiece to mill. On the table, the raw material will be fixed, using different systems. The most notable system is to grasp the workpiece with a vice or clamps, t-nuts, and screws, as the workpiece has to be secure and in the correct position before the milling process begins. 

Cranks: The role of cranks is to give movement to the cartesian axes. There are three axes: “X”, “Y” and “Z”. A machinist can move the part secured on the bed and the spindle that has the milling cutter in various directions with the crank. Milling machines can have more than three axes depending on the number of required movements to machine the part, or if a fourth rotation axis is added. 

Head: This component of a milling machine is designed to contain the device that securely holds the cutting element. In some scenarios, it may have a power button and accessories such as coolant hoses and clamps. It also has the tool axis or spindle, the circulating element on which the milling cutter is placed. 

Motor: The motor is an essential component of a milling machine. It can be viewed as the heart of the machine. Depending on the model and intended utilization of its power, torques and revolutions vary. There are motors with changeable speed control, which is achieved electronically, and fixed-speed motors that control the speed and power with a system of gears or pulleys. 

Gearbox: Because different materials cut at different speed rates every blend of cutter material and processed material calls for a tool speed that can offer the best achievable finish and ensure that tool life is not impacted. The important gearbox takes the power output from the engine and changes it at different adjustable speeds, or in different directions. 

Types of Milling Machines

There are different types of milling machines that have distinct functions, characteristics, and sizes. These descriptions are based on the spindle orientation, which refers to the direction of the cutting tool. With traditional milling machines, this is only one direction. Below are descriptions of milling machines according to the characteristics we think are most significant. 

• Horizontal milling machine – The spindle axis is horizontal, the cutters are firmly fixed on a horizontal shaft referred to as an arbour. These machines are the first of the milling machine family. They have a horizontal spindle, a bridge that holds up the arbour in a very rigid position, and a coordinate table that slides up and down on the vertical axis. 
• Vertical Milling Machine – With this type of machine, the position on the spindle is vertical, perpendicular to the coordinate table. If it is possible to tilt the spindle head to perform certain procedures at an angle a swivel head can be incorporated. Vertical milling machines are more known today and are the standard when it comes to CNC centers. With older machines, the part typically slides up and down (similar as on horizontal machines), however for the majority of modern milling machines, the head is the one that slides up and down to machine the part. 
• Universal Milling Machine – Are milling machines that have a combination of horizontal and vertical milling functions and have the possibility of also using the tool horizontally and vertically. 

The Number of Axes

Axes distinguish the types of maneuverability the milling machine will have. The greater the number of axes, the better the machining possibilities. Below we name a couple of the axes available on milling machines: 

• 3-axis – These are the critical orthogonal directions of a machine with such characteristics. They portray the X, Y, Z axes, to machine vertically, horizontally and front to back.. 
• 4-axis – This is an additional axis added to the 3-axis that allows the possibility of rotating the workpiece to be machined on the table. The 4 side faces can be milled or a tool can also be placed to repair 4 parts rather than one in each cycle. 
• 5-Axis – Also added to the axes of 3-axis machines for more complex part fabrication. Read more on 5-axis machining here. 
• 6-Axis – These types of machines are generally not too common because of the expensive price-tag associated with them. While they can be up to 75% faster than 5-axis machines, the necessity of such capabilities does not often justify the expense. 6-axis machines have a stationary worktable and all the freedom of maneuvering is left for the cutting head. 6-axis machines can move in 3 directions and rotate around each one.

A 9-axis machine is also available, however, we will hold that for another blog discussion.

Structure 

Standard milling machines are designed in a C-shape for the machine elements to be distributed. The most key feature of the milling machine makeup is rigidity, to resist the milling forces without contortion, and to maintain the specified tolerances of the workpiece. 

Classic vertical or horizontal milling machines, known as Knee Mills, generally have an X and Y coordinate table that slides up and down the Z-axis on firm support known as a console. The majority of today’s milling and vertical machining stations usually have stationary X and Y axes and it is the head that slides up and down on independent guides, not the workpiece. Both share a similar C-structure. 

The structures that provide the strength of a milling machine are generally created from cast iron to support very rigid and heavy parts that hold the shape of the milling machine and consume vibrations. Additionally, there are welded steel frames, which are typically less rigid.

Movable Gantry or Bridge:  With this component of the milling machine, the table and the workpiece do not move. However, the structure which holds the head moves. Generally, they are very large and used on complex, flat milling pieces. 

Column Milling Machine:  In this approach, the spindle is located in a vertical column. It slides up and down and the milling piece comes out and enters through an arm. This design is used when the parts are large and heavy. It is easier for the machine to maneuver around the workpiece. 

Beyond Milling Fundamentals

Milling machines are generally used in machine shops for manufacturing prototypes, single, and large parts. This article addressed the overview as a guide for CNC  milling machines. There are many aspects and approaches to milling we will update in future posts. Check our blog on The Process of CNC Milling

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