Computer numerical control, or CNC for short, refers to the computer-aided control of machine tools, usually in the field of machining technology, which have to produce parts made of metal, plastic or wood quickly and accurately. These parts can be produced with the help of a milling machine.
This is done on the basis of a program that is usually created in a computer-aided manufacturing (CAM) system. The EIA-274-D standard, also known as G-code, is usually used for this purpose. The M code is also frequently used to call additional functions (e.g. tool change).
CNC emerged in the late 1950s, first for many years with analog computers and much later with digital computers. Like so many technical innovations, numerical control technology arose out of a military necessity. The Korean War (1950-1953) greatly accelerated development, as there was a great need for high-precision parts for aircraft and weapons systems.
In the sixties, American manufacturers such as Kearney & Trecker had an enormous lead in this field thanks to large defense contracts. By the end of the sixties, CNC controls were also being built on a large scale in Europe. It was not until large Japanese suppliers such as Fanuc entered this market that CNC technology became a generally accepted production technique.
New machining techniques
CNC enabled the development of completely new machining techniques such as electrical discharge machining and laser cutting. It would be wrong to say that it is only since NC and CNC that large series of identical parts can be produced. Before the introduction of these controlled lathes, there was the copy lathe, in which a probe passed over a turned template (the workpiece to be copied) and controlled a bit that made the same movements so that identical workpieces could be produced.
Similar methods were also common in milling machines and grinding machines. The major disadvantage of these methods is the large amount of time and resources required to make a new mold when a differently shaped workpiece is to be produced.
New type of control
In 2006, the G and M codes are still widely used, but it is true that a new type of control is entering the market to facilitate milling and turning. This is called dialog control. In this case, the programmer “talks” to the machine in a user-friendly environment. For example, bends are no longer entered as a G-code, but as a “bend” command with a radius, a starting point and a center point.
Of all the G-codes, 4 are used for track control;
- G0 Move to the specified position at rapid traverse;
- G1 Mill in material, travel in a straight line to the specified point;
- G2 Mill an arc with a specified radius, CW to a specified point;
- G3 Cut an arc of specified radius, counterclockwise to specified point.
All shapes can be made with these codes.
Modern machines are equipped with templates linked to dialog programming, where rectangles, circles or complex machining operations can be selected and parameterized through the dialog. The linked macros eventually translate this into G-codes.
An example of such a control is Siemens Shopmill on a Chevalier 1418VMC or a Mazak lathe with a Mazatrol control. Behind this there is still a G-code program. G-Code programs can also be generated directly from CAD programs. Basic knowledge of G-code is no longer required.
Unfortunately, the G-code is not interchangeable between different machine brands due to small differences.
However, there are controllers, such as STYLE 2016 Edition, that allow free contour programming. Minimum values are required for this. With just a few clicks and the input of a minimum number of values, each operation is immediately and continuously visible on the 19″ touch screen. CNC software designed for quick and easy drawing and execution of single parts and small series.
New type of threaded spindle
Characteristic of the CNC machines is that they include a new type of threaded spindle for the movements and for the workpiece or tool. This is a so-called ball screw. This is a thread that is cut with an arc, such as in scaffolding feet for height adjustment. However, this is a more precise version in which ground balls run through these grooves as in a ball bearing. These balls have the same radius as the groove of the thread and are mounted with preload, so theoretically there is no play. Of course there is, but with this type of spindle it is negligible. (Sometimes 0.001 mm and less).
The wear of the ball screw is different from that of an ordinary screw. In the ordinary screw, the clearance increases, but in the ball screw, this clearance may not be present. An automatic lubricator is often used.