Basic Principles

CNC (computer numerical control) is the automated control of machining tools (drills, boring tools, lathes) and 3D printers by means of a computer.
A CNC machine processes pieces of materials to meet specifications by following a coded programmed instruction and without a manual operator.
The CNC process involves the removal of material from a solid block or a pre-formed part using various cutting tools, it is a subtractive process.
- The geometry of the part wanted is defined by a CAD model, a machinist uses CAM software to prepare the cutting paths with tool selections needed to achieve the part.
- The cutting paths are then output as Gcode which instructs the machine how it will cut, supplying it with the orientation of the machine head, the part and speed of movement.
- The machine is then set up and the part is jigged (secured) in position.
- The machine then executes the Gcode to shape the part.

These are the various advantaged and Disadvantages:

CNC Machining

There are two categories of CNC machines, 3-Axis and Multi-Axis
3-Axis machines move the cutting tool relative to the part along the x,y,z axis.
Multi-Axis machines add rotation to one or more axis allowing the parts to be cut from various angles.

The following picture illustrates the two:

CNC Mills are quite common and can be used for many geometries
The workpiece is held in place using a jig or vice and then the mill head moves in 3 axes to remove material using rotary tools or drills.
Set-up costs are low since they’re easy to operate compared to other CNC processes
Since they’re quite small, there is a limited range in movement which means that you can’t create some features

This process involves the use of specialized rotational cutting tools to remove material. This is often done in multiple stages, earlier stages using high speeds with a lower accuracy where later stages using higher accuracies to remove small amounts of material left over
The parts are then de-burred and there are also post-processes like surface finishes and paint applied to the part

These processes often require various cutting tools for different cuts and features
1-3 are Flat, Bull, and Ball heads. These are the most common cutting tools for making slots, grooves, cavities and vertical walls. They are selected by the required form of the bottom cavity
4 is a simple drill, they are selected for standard holes
5 is a slot cutter, since their heads are larger than their shafts, it can create undercuts that remove material from the sides of vertical walls
6 is a tap, this is used for threaded holes
7 is a face mill cutter, it removes material from flat surfaces

This process is done using a lathe. These machines can produce parts at higher rates than milling, and turn lower cost, these machines are useful for large numbers of parts
The work-piece is held on a spindle and rotated at high speeds. The cutter is normally a blade and slowly moves towards the part to describe its profile
Lathes cannot produce many features since the machine can only produce “rotationally symmetrical” or “Revolved” parts. For these features, the part is usually transferred to a mill

This process is quite similar to the milling process, the difference is that parts in a lathe will be rotated when being cut instead of milled
When additional features are needed, the part will be transferred to a mill before the post-processes are applied

These machines were developed to reduce the time needed to machine a part
Between operations, the tool/bed can rotate so you can work on the piece from various angles
The ability to reorient the work-piece automatically allows for more complex parts which a conventional 3-axis CNC mill cannot

These machines are hybrids of lathes and mills
These machines are ideally suited for parts that need to have rotational symmetry along with other features. They come at a much lower cost than other 5-axis machines
The part can either be rotated or precisely positioned
Both tools used in Lathes and mills are used to remove material