What is CNC Machining?
CNC machining is a manufacturing process performed using a pre-programmed computer software, which controls and dictates the order of operations and movements of the machining tool during the manufacturing process. In this production process, layer by layer of the raw material is substracted until the final part is obtained.
This production method is suitable for a wide range of of raw materials such as metals, plastics, wood, glass or composites. It is used as the preferred manufacturing method in a wide variety of industries.
By being a computer-controlled manufacturing method, CNC machining enables the production of parts in an accurate, inexpensive and efficient manner suitable for production in small and medium quantities.
However, although machining has advantages over other manufacturing methods, the level of complexity that can be achieved in design and the economic efficiency in the production of complicated parts is limited.
Overview of the CNC machining process
CNC machining is a manufacturing method that turns raw material (metals, plastics, wood, etc.) into a product or part that is pre-designed with the help of computer software.
Today modern processing machines are capable of performing a wide range of processes and capabilities. Yet the basic principles of all these processes are the same, and they include the following steps:
-Designing a 3D model of the part that we want to produce
-Converting the 3D model to a command file to the machine
-Preparing the machine for the production process
-Execution of the operation and production plan of the machine with the help of a computer software
-Design and planning of the computerized model of the part in machining
Any machining process begins with the creation of a 3D model of the part in a CAD software (such as SOLIDWORKS AUTOCAD etc.). These programs allow engineers and designers to design and plan the part or product according to the required dimensions and geometry.
On CNC machined parts there are a number of limitations due to the capabilities and way of machining, for example, in most milling machines, the milling used by the machine is round resulting in the restriction that sharp corners can not be milled within the machined material, but only round corners.
In addition, the raw material properties and capabilities of the machine cause design limitations like requirement for minimum thickness of the part and inability to produce geometries and complicated interior spaces.
Once the 3D model is ready, the next step is to turn the 3D model into a command file and instructions that the machine can execute.
Common machining processes
CNC machining is a manufacturing process that is suitable for a wide range of applications and industries such as military, aerospace, agricultural, automotive and more. This is because of the ability to produce many types of products and parts.
The production process includes a number of chemical, mechanical, electrical and thermal processes performed in a controlled manner by a computer.
The most common CNC machining processes are: drilling, milling and turning.
The drilling process is a machining process that uses a drill or a number of drills that rotate at high speed in order to create a cylindrical hole (drill) in the raw material or the worker.
When the drilling is performed in a computerized manner, the machine will usually perform the drill when the drill bit enters the raw material surface vertically. This, in order to obtain a vertical straight drill with a diameter, equal to the size of the drill diameter with which the drilling was performed.
Drills can be made at an angle but to do this it is necessary to make use of lapping machines, and different configurations of processing machines so that they can be suitable for this.
With the help of a drilling process, additional machining operations can be performed, such as drilling a funnel-shaped hole at the top (countersink), drilling a drill with a round socket at the end (counterboring).
Milling is a CNC machining process in which the processing machine removes layers from the raw material that the machine works on. There are several different types of milling, the type of milling is chosen according to several parameters such as the type of milling to be performed, the raw material and more.
Common operations that can be performed using milling are:
Surface milling, which is performed on a flat surface of the raw material at a low depth and thus allows to create a uniform surface quality or to create depressions and ditches with shallow depth.
Reed milling, into the raw material, with the help of this action it is possible to perform internal and deep milling in the raw material.
The engraving process is performed by removing layers from the raw material with a sharp knife which comes into contact with the CNC machined part while it rotates at high speed and is performed with a lathe.
Unlike the drilling process, in the engraving process the raw material is the one that rotates quickly and not the machining tool.
The engraving process will usually be performed in a way that the machining tool moves directly along the surface of the machined part and removes material from the circumference. This happens until the desired diameter is obtained, thus allowing cylindrical parts with internal features such as grooves, threads, conical drills, large diameter drills and surface machining.
Equipment and components of CNC machines
As we have seen, there are a number of machining processes that can be performed.
Because each such machining process is unique in the way it is performed, there are differences between machining tools and machines in each of them, which require a different skill and method of work.
We know that the drilling operation uses drills. To this end, the drills with which the machine performs the drilling are designed in such a way that at high rotational speeds the tip of the drill acts like a knife, thus removing the chips from the raw material and producing the drill.
There are several types of drills, each drill has the most suitable use for it.
Examples of common types of drills available on the market are center drills (allows high precision to create a drill at a shallow depth or an initial center drill that can later be extended to a desired size drill), taps – a type of drill that allows tap production as the drill allows the screw to be “caught” inside the drill and reamers (allows to increase existing drills in the part).
Most often when the drilling process is performed by a CNC machine, the machine uses unique drills that are specifically designed for the machine so that it can perform the drilling operation. However, drills can also be created with the help of mills and lathes.
Types of milling
As mentioned earlier, in the milling process the machine uses a certain milling in order to mill layers from the raw material until the final model is created. Each mill has a specific use for which it is intended, some are known for use and milling into the material vertically and some for milling the material horizontally.
Common types of these millings are finger milling, round milling, crushing milling, slot milling, etc.
It is common to distinguish between two types of milling commonly used in the industry:
-A computerized milling machine controlled by a computer which controls and dictates the movement of the milling machines in a vertical or horizontal configuration.
-A conventional mill that is controlled by a human operator who dictates its action and movements.
Basic mills are able to move along 3 main axis but there are also more sophisticated machines that are able to move along additional axis.
The engraving operation uses a lathe and a cutting knife to remove layers of raw material from the worker which rotates at high speed. The design shape of the cutting knife varies according to the type of processing we would like to perform on the raw material.
Examples of such knives are a length engraving knife, a slitting knife, a threaded knife, an internal tap knife, an internal engraving knife and more.
The engraving process also uses two common types of lathes:
-A computer-controlled lathe that performs the engraving and control operation of the knife independently.
-A manual lathe in which the movement of the knife is performed manually by a human.
Considerations in selecting the raw materials for processing
CNC machining is suitable for a wide range of engineering materials such as:
-Metals (aluminum, brass, stainless steel (stainless steel), steel, etc.)
-Plastic (nylon, Teflon (PTFE), acetyl (POM), etc.
The choice of the optimal raw material from which the part is to be made depends largely on the use or application of the final product.
Many types of raw materials can be processed, as long as the selected material is able to withstand the production process, that is, has suitable mechanical properties such as hardness, yield point to stress, tensile strength, thermal and chemical resistance.
Using these mechanical properties it is possible to define the optimal turning speed, progress and approach at which the processing machine will work.
The turning speed is the parameter that determines the speed at which the machining tool removes the raw material along the machining area and it is measured in units of meters per minute.
Progression is the motion of the machining tool relative to the machined raw material, its function is to ensure a uniform design of the surface (lowering the chip along the entire machined surface), the advancing motion maintains the continuity of the machining process by changing the relative position of the cutting edge. Progress is measured in units of mm per round.
The depth of cut defines the depth of machining to be performed by the machining tool, meaning how deep the cutting tool will go into the processed raw material, it is measured in units of mm.
In many cases when a CNC machined part is produced, they will first be coarsely machined into the raw material until the desired general shape of the final part is obtained.
Considerations in designing the size of the processed part
CNC machines can be found in many industries, from automotive to aerospace, to construction and agricultural industries.
But despite great versatility and the ability to customize the process, when wanting to produce very large or particularly heavy parts, there are a number of difficulties that we need to take into account:
-Use of special and very complex equipment which is suitable for moving and cutting and processing the raw material during the process.
-Increasing the factors that affect the accuracy of production.
-A very large amount of heat which is emitted during the spinning process.
-Higher chance of deformations in the material.
Additional production methods
Although machining demonstrates many capabilities and advantages over other manufacturing methods, it is possible that this manufacturing method is not suitable for the part we want to produce, but another method would be a more suitable alternative in terms of price and speed.
There are currently several alternatives on the market that can be used, for example:
– 3D printing
– Laser cutting
-Electrical discharge machining (EDM)
-Electrochemical machining (ECM)
-Laser beam machining
– Water jet cutting