Understanding CNC Milling – A Beginners Guide
What is CNC Milling?
CNC (Computer Numerical Control) milling, like other subtractive manufacturing forms, works by removing material to create a final design. The CNC Milling process involves multi-point cutting tools being controlled by a computerised programme which reads and translates machine paths. These machine paths originate from CAD/CAM software (Computer Aided Design/Manufacture) programmes which are used to create a computerised design of the product.
In this article, we're going to delve a little deeper into the set-up and functioning of CNC mills, taking a look behind the scenes at their various functions and uses.
Programming the Milling Machine
The CNC machining process works by the machine reading its instructions from a design that was inputted on a CAD programme. These part designs can be 2D or 3D. Once completed the designs are converted into a machine-readable format. CAM (Computer Aided Manufacturing) software then exports this to a CNC machine programme which then acts as the instructions to direct each and every move that the machine will have to make in order to replicate the CAD design with the chosen material.
Before the manufacturing can commence, the machine needs to be set up with the right CNC machine tools for the job. The operator will also place the workpiece on the work table and fix it in place with a vice or custom jigs and fixtures. These steps are important to ensure the machine can carry out its job without the material moving out of place. When the necessary preparations are complete, the operator can launch a programme using the machine interface, and the manufacturing process can begin.
Milling in Action
When the manufacturing begins, the tools can spin at thousands of RPM and, as such, can cut into even the strongest materials with ease. During the manufacturing process, the tool and workpiece will be moved in relation to each other in order to access different parts of the design. The angles and direction depend on the specific design requirements of the piece but the principle of feeding the workpiece to the rotary tool remains the same.
Types of Milling
Plain milling (otherwise known as slab milling or surface milling) involves introducing a cutting tool whose rotary axis is parallel to the surface of the material at hand - hence its reference as 'surface' milling. It is used to create a flat surface on a part.
In face milling, the rotary axis is perpendicular to the surface of the material. The tools for face milling tend to have more teeth than plain milling. The teeth on the edge of the cutting tool are used for cutting whilst the teeth in contact with the flat material are used for finishing.
Angular milling (or angle milling) involves, as the name suggests, the rotary axis of the tool being at an angle to the workpiece surface, rather than perpendicular or parallel, these machines cover the angles in between.
Form milling is used for parts without flat surfaces. These machines can be used for materials with curves and contours using specialised tools depending on the type of curve.
As a rule, superficial surface cuts will be carried out by larger cutters, whereas a more narrow cutter can be used for finer details. Initially, larger amounts of material will be removed with a coarse-toothed tool, then a faster, more fine-toothed cutter is used for more detailed parts of the design and for finishing.
CNC Mill Anatomy
To fully understand the working of a CNC Milling machine, we've compiled a brief list of the main parts that a CNC mill consists of:
- Worktable - This is where the material is secured in order to be worked on.
- Saddle - This is just below the worktable to support it and is adjustable in order to fix the worktable in relation to the spindle axis.
- Knee - Below the saddle, this supports the worktable and saddle and can be moved up and down depending on what height is needed.
- Column - This is the fixed base of the machine which can also house things like coolant or oil for the machine.
- Spindle - This component is driven by a motor within the column by which it is supported. This is what controls the machine tool.
- Machine tool - This component is sometimes referred to as a mill cutter. This is the equipment which touches and removes parts from the workpiece and will vary in type depending on the job at hand, e.g. cutter, drill, bore etc.
- Arbour - The arbour essentially connects the machine tool to the spindle.
CNC mills can work on a variety of materials such as plastics, metals, wood and even glass. Milling machines can be programmed to function at a variety of angles and depths, as well as having the ability to work on large, heavy workpieces making them a popular and versatile machine.
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