Wednesday, August 17, 2022

What is a CNC machine?

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CNC (Computer Numerical Control) machining makes use of machines controlled via a set of instructions sent by an activator. The code used, or G-code, is in the form of a list of coordinates and any machine controlled through this code, whether it is a lathe, a milling machine or even a plasma cutting machine, can be considered as a CNC machine.

In this article we will focus on some types of China CNC machining, such as lathes, milling machines and others that combine features of the previous two. These machines can move along certain axes: the X, Y and Z axes following the classic Cartesian vectors but also, for more advanced machines, A, B and C, representing possible axes of rotation around the work piece. Typically, a CNC machine can operate on up to 5 axes. The most common machines are listed below:

CNC Lathe:  A lathe works by turning a work piece fixed on a chuck. A tool is caused to move along 2 axes in order to make cylindrical cuts of material. Thanks to CNC technology, a lathe is able to create curved surfaces that would be difficult or impossible to machine using a conventional manual lathe. The cutting tool itself is usually inert, but there are machines that can operate with rotary tooling.

CNC Mill – A CNC mill is typically used to create flat parts, but some machines with more degrees of freedom can create more complex shaped parts. The material to be machined is held still while the spindle drives the tool in rotation. The latter then moves on 3 axes in order to cut the material. There are variations where the material moves while the spindle is held still.

CNC Drill – These machines are similar to milling machines but are specifically designed to cut along only one axis. For example: a drill can only move along the Z axis to drill through material and cannot cut along the X and Y axes.

CNC Grinder – These machines drive a grinding disc which they bring into contact with the material in order to perform high quality finishes on the surfaces of the workpiece. Designed to remove only a small amount of material from hardened metals, they are mainly used for finishing operations.

Subtractive Manufacturing

CNC machining makes it possible to create parts using the subtractive manufacturing process: to put it simply, material is gradually removed from a billet until it obtains the desired shape. This can be achieved through the previously mentioned methods (turning, milling, grinding or drilling). In contrast, the additive manufacturing process consists of creating the part from scratch and adding material. 3D printing is an example.

Tools

The cutting work is carried out using specific tools. Tools are typically mounted on a tool holder and inserted into the spindle as required. In machining, it is practically impossible to complete a work using a single tool and it is quite normal to have to combine several tools of different shapes to achieve this. The most common tools are listed below.

Milling tools

End Mill – The end mill is the most common type of milling cutter and is typically capable of cutting in 3 different directions. There are all kinds: flat, trimming, roughing, concave or conical to name a few. They are characterized by their number of teeth, their helix angle as well as their material of manufacture and their coating.

Face Mill The face mill is designed to cut through large surfaces, also known as “face milling”. Its cutting edges are normally located at the ends of the tool and its teeth are made of carbide inserts.

Thread milling cutter a thread milling cutter is designed to make threads. Its mode of operation is to rotate around a rod in a helical pattern to form the desired thread.

Slot Cutter – This type of cutter is used to make T-slots along a work piece. Due to its particular geometry, the tool can only cut from an open face of the part in question.

Turning tools

External Cut Turning – As the name suggests, this type of tooling is designed to cut to the outside diameter of a part. It can be either a one-piece tool, molded to the appropriate shape, or carbide inserts.

External Cut Grooving and Threading: This type of tooling normally has a tapered shape to allow them to reach the internal parts of a part. For example, they can be used to ream or thread the inside of a previously drilled hole.

Separation: A separation tool is used as the last step to separate the part from its original block once it is completed.

Drilling: These tools are used to drill holes along the longitudinal axis of the part. The resulting holes still need to be properly reamed to meet the required tolerances.

Tools and materials

The different types of tools can be classified into subcategories according to their material of manufacture. The most common materials are listed below:

High Carbon Steel: These tools are some of the cheapest on the market but have a relatively short lifespan. In addition, they lose their hardness around 200°C.

High Speed ​​Steel or High Speed ​​Steel (AR or ARS) – These tools are used more frequently than high carbon ones because they have a longer tool life. In addition, they retain their hardness up to 600°C, which allows them to be operated at higher speeds.

Cemented Carbide: Cemented carbide tools enjoy a higher hardness than ARS tools. However, they are more fragile and tend to break if mishandled. They can operate at temperatures up to 900°C.

Ceramic: Ceramic tools boast exceptional hardness and are typically used to cut very hard materials at very high speeds. The two commonly used variants are silicon nitride and aluminum oxide.

Cubic Boron Nitride: Tools in this category are ideal for cutting through hardened steels or high performance alloys (super alloys). They benefit from an excellent thermal resistance, coupled with a strong abrasive power.

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