CNC machine tools have been created primarily in order to increase the efficiency and accuracy of processing by reducing or eliminating physical human work. In times when labour costs are getting more expensive, the natural purpose becomes the production of as many of goods as possible by one worker. Company owners who want to improve their capabilities of producing goods begin to analyze the possibility of buying a CNC machine tool. Frequently, they choose the price of the machine as a basic criterion. They do not realize that CNC machines are divided into professional and unprofessional ones. Professional machines are the machines that allow generating income, that is they can produce a product that can be sold for an amount exceeding the cost of producing it. The production cost consists of the price of the material (together with waste and defective products), the amortisation cost of the machine tool, the cost of electricity, the operator's labour cost, the cost of providing technology and machining programs, the cost of cutting tools. Unprofessional machines are the machines which are not able to generate profits due to their design, the materials of which they are made, the possibilities of the control system and widely understood performance. These machines are usually a group of machines which are often much cheaper than the professional ones. These machines are perfect for fun, e.g. for a modeller who makes model airplanes, ships, etc. as a hobby and where the processing time is not essential.
It is worth emphasizing that usually the times of processing the same workpiece on a professional machine and on an unprofessional one differ by not tens of percent but processing on a professional machine is dozens times faster, which allows to generate a considerable profit. While processing the same workpiece on an unprofessional machine, we may suffer big losses. This is due to the fact that almost all the factors responsible for the production cost of a given workpiece depends on the time of machining and intermediate times, that is on the efficiency of the machine.
So how to distinguish a professional machine from an unprofessional one? The price is not always the determining factor, because some of the companies, especially those with a brand name, produce unprofessional machines which are not cheap at all and the user pays primarily for the known logo on the housing of the machine. There are also machines that are offered as professional ones but they only seem professional, which is to encourage potential customers to make a purchase. To avoid a situation where you have made the purchase and the machine appears not meet your expectations, you should carefully examine the real possibilities of the machine you are interested in.
First of all, the fundamental issue is to visit the company which offers such a machine. It is best to go to the manufacturer, because in addition to the demonstration of the machine, we will be able to evaluate how and on what hardware these machines are made, and what are the conditions for the production. Trading companies usually offer to see the machine in operation at the premises of another customer, which can make requests for the demonstration of all machine capabilities quite embarrassing. Most frequently it causes the presumption that the machine performs the treatment correctly, which does not mean that it really will. During the demonstration at the producer's you can usually be more fussy, because then the both sides do care about the transaction.
When we look at the machine, we should pay attention to following details. A professional machine should be made on the basis of steel construction containing as few connecting elements (bolts, clamps, screws, etc) as possible. It should be a closed spatial structure providing high rigidity of the machine. Aluminium profiles connected through twisted elements are often unstable, prone to deformations during transport and may lose the geometry of the machine because all elements are held by means of friction. The situation in which machines are imported in parts and assembled at the customer's is a total misconception.
It is ideal when the machine is composed of the minimum amount of parts, i.e. the machine frame is monolithic and the gate is not a removable part. Admittedly, it forces the manufacturer to have huge machines which allow machining such large elements in one clamping, but only then the user has the guarantee that he will have a machine with the correct geometry for many years.
All elements which move one another should be free from sliding elements in favour of rolling bearings. This provides many years of work without replacing components that wear out in a natural way.
The bearing of each axis should be on at least two guides and four carriages. Converting rotary drive into linear drive should be done by means of ball screws. In the case of the drive of a mobile gate, the mobile gate must be powered by two ball screws which are synchronized in order to maintain the correct perpendicularity of axes! It is very important because otherwise the gate will have very low torsional rigidity.
Ball screws are precision rolling gears and therefore they must be protected from dust and chips occurring during processing. The ball screws which are exposed to direct contact with contaminants should be obligatorily protected by safety covers.
The machine must weigh sufficiently. If we are able to lift up the machine from its either side by human power, it must be a toy, except for CNC desktop machines. The weights of industrial machines are measured in tons.
As for the drives which are used in such machines, the best solutions are digital servo drives which work in the DPC (Direct Position Control) system and which are characterised by high precision movement in dynamic states. It is very important because the accuracy of machines are usually given in static states, which does not allow to estimate the real machining accuracy.
The machines which are driven by servo motors should reach speeds from 300 mm/s and and higher. Step motors should not be used for professional applications, but this is acceptable in cae of lighter machines provided the use of well-matched engines with drivers and the application of resonances damping system. They should reach a speed of 100-150 mm/s.
A good control system is half the battle. The rate of growth of this market segment makes several-year-old machines of well-known brands not suitable to use because of the outdated control system, although they are still in perfect condition. Therefore, it is very important that the control system would allow subsequent upgrades to make it adjusting to future standards.
Another aspect concerning the control system is its speed. The speed of the control system of a CNC machine tool is the ability to process a certain number of blocks of a program in a unit of time. The speed of the control system is primarily important in the work in which there are complex shapes consisting of a large number of vectors (and such ones are processed mostly on CNC machines).
In this situation, another parameter of the control system, which is the possibility to analyze more than one block of a program at a time, becomes very important. When analyzing several thousand vectors forward within a second, we are able to adjust the speed in knots between vectors so that in case of small angles between them it is possible to cover them with a speed greater than zero. This way of the interpolator performance is called "Dynamic Analysis of Vectors".
Another aspect of the control system also applies to the interpolator performance. First of all, the PC is not suitable for a direct interpolation of movements for CNC machines. The hardware resources of a PC are not equipped with a precise timer that could be the basis of time for the interpolator. In addition, most operating systems like Windows and Linux are not real-time systems, which means that impulses generated directly by the PC, e.g. to the printer port, may be delayed of an unspecified value. This causes that the movements generated in this way will always have a very poor quality (vibration, oscillations, jerks), which is caused by irregular generating impulses. The solution to this problem is to use a hardware interpolator operating on an entirely different processor. They are usually very fast DSP processors. In this case, the PC is used only as a user interface rather than an interpolator.
To ensure the communication between these two parts of the system in the real time, they must be connected to a very fast data bus. Solutions like serial port, parallel port or USB are not suitable for that. The only possible option is Ethernet, usually on a modified transport layer.
A good control system should also allow for the smooth regulation of the machine feed rate from zero to the preset speed. It should allow generating the toolpath based on drawings in dxf format, etc. automatically, including the correction of the tool diameter, extracting pockets, detecting islands and drilling holes. It is recommended that the system can display all the data concerning the machining with the visualization of the work progress on the screen in the real time.
To find out the possibilities of the machine, it is essential to perform test treatments, whose results will help to answer most of the questions about the reasonableness of the purchase of a given machine.
We should ask for the realization of several geometric figures, that is: a square, a triangle, a circle and an ellipse, of the size of 100mm and the thickness of 5-8mm, with the speed of at least 50mm/s, in materials at least as hard as those which we want to process on this machine.
Firstly, we cut out a square and then we examine its corners in particular. They should be straight and sharp, and should not be rounded. Any undulations should not be visible near the corners. Examining waste material we have to pay attention if the cutter did not move in the corners too far. If we notice the effects mentioned, it means that the machine has rather small rigidity.
We measure the dimension in both directions using an electronic caliper. If the deviation is within 0.03mm in case of milling machines or engraving machines and 0.05mm in case of milling plotters, the result is satisfactory. But the difference between the two dimensions should not exceed 0.02mm and 0.04mm respectively.
Looking against the light between the angle bar and the square, we should not see any clearance. We can also cut out two squares and place them together after turning one of them upside down. They should ideally coincide with each other. If they do not coincide, it means that there is no perpendicularity of axes XY in the machine.
Then we cut out a triangle. Here, apart from corners we draw our attention to sloping walls which require the simultaneous movement of two axes. Now we assess the quality of interpolation. The rougher the surface is, the worse the interpolator and the drives operate.
Now we cut out a circle. While machining the circle we have to pay special attention to operating speed and possible vibrations, jams and other incidents that may cause e.g. significant decreasing the feed rate in comparison to cutting a square. If we notice that the circle is performed more slowly than the square, despite the fact that the preset speed is the same, this means that the system cannot keep up with processing large quantities of vectors, or emulates a circular interpolation with low resolution. This can be recognized on flat surfaces which form this circle and which are visible on the side of the circle. The circle should be round. We measure it using an electronic caliper at various angles and check the dimensions as well as in case of the square.
Now it is the time for an ellipse. Here, most common problems occur with the processing speed in the control system, and therefore during the treatment of an ellipse, we have to pay attention primarily to the speed and smoothness of the movement of the machine.
Then we "plan" the surface with dimensions of approximately 100x100mm vertically, and horizontally nearby, with a 10mm dia milling cutter so that the distance between the successive paths was 9mm. After treatment we check the smoothness of the machined surface. If we can feel "stairs" with your finger on any of the planned surfaces, it means that the spindle of the machine is not perpendicular to the table.
Another important test is to perform machining of the stamp. We mill the stamp in the shape of a cube with dimensions of 40x40x40mm, so that the cutter performed subsequent squares by lowering layers every 1mm down. Then we measure the dimensions using an electronic caliper in both directions near the surface and near the base. If the upper and lower dimensions differ by more than 0.03mm, it means the Z axis is not perpendicular to the axes X and Y.
If the machine passes all these tests successfully, it means that we are dealing with a perfect machine which is definitely worth to buy it.