How to test CNC machines?

CNC machine tools were created primarily to increase the efficiency and accuracy of machining by reducing or eliminating human physical work. At a time when human labor costs more and more, it is natural for one worker to produce as many goods as possible. Fast machines are needed for this. When starting to analyze the possibility of purchasing a CNC machine tool, the owners of companies who want to increase their ability to manufacture goods often choose the price of the machine as the basic criterion. They often don’t realize that CNC machine tools can be divided into professional and amateur machines.

Professional machines are machines that allow you to generate profits, i.e. they can produce a product that can be sold for an amount exceeding the cost of its production. The cost of production includes the price of the material with waste and shortages, the cost of depreciation of the machine tool, the cost of electricity, the cost of the person operating the machine, the cost of preparing technologies and machining programs, and the cost of cutting tools.

Amateur machines are machines that, due to their design, materials from which they are made, the capabilities of the control system and the broadly understood efficiency, are not able to generate profits. Such machines most often constitute a group of machines that are often many times cheaper than professional machines. These machines are perfect for fun, for a modeller who makes models of airplanes, ships, etc. as a hobby, where the processing time is not important.

It is worth emphasizing that the most common times of machining the same detail on a professional and an amateur machine differ not by several dozen percent, but by several dozen times! Which in the case of a professional machine allows you to generate a lot of profit, and by processing this detail on an amateur machine, we will incur considerable losses. This is due to the fact that almost all factors responsible for the cost of manufacturing a given detail depend on the machining time and on intermediate times, i.e. on the efficiency of the machine tool.

So how to distinguish a professional from an amateur machine? The price is not always the decisive factor, because some of the companies, especially those with a well-known brand, produce amateur machines that do not cost at all, and the user pays primarily for a well-known logo on the machine housing. There are also machines that sellers offer as professional, but they are professional only for these sellers, which is to encourage a potential customer to buy. In order to prevent a situation in which, after purchasing the machine, it turns out that the machine does not meet our expectations, the actual capabilities of the machine we are interested in should be carefully analyzed.

First of all, the basic thing is to visit a company offering such a machine. It is best if we go to the manufacturer, because in addition to the demonstration of the machine, we will be able to assess how, under what conditions and on what equipment these machines are produced.

Trading companies usually offer to see the machine at a customer’s location, which makes requests to demonstrate the machine’s capabilities while it is running often embarrassing. This usually results in the presumption that this machine correctly performs the machining we are interested in, which does not mean that it will actually be so. During a show at the manufacturer’s, you can usually get a bit more fussy, because then both parties care about the transaction.

If we are already viewing the machine, please pay attention to the following details. A professional machine should be made on the basis of a steel structure containing as few connecting elements as possible (bolts, clamps, screws) etc. It should be a closed spatial structure ensuring high rigidity of the entire machine. Aluminum profiles connected by twisted elements are usually not very stable, prone to deformation and during transport the machine may lose its geometry because all elements hold together by friction. Machines that the seller brings in parts and assembles at the customer’s place is a complete misunderstanding.

It is ideal when the machine is assembled from as few parts as possible, i.e. the machine frame is monolithic and the gate is one part that cannot be dismantled. It is true that this forces the manufacturer to have huge machine tools that allow for processing such large elements in one clamping, but only then the user has a guarantee that he will have a machine with the correct geometry for many years.

All mutually moving elements should be devoid of sliding elements in favor of rolling bearings. This ensures many years of work without the need to replace elements that wear naturally.

Each axle should be mounted on at least two guides and four carriages. Converting the drive from rotary to linear should be done with ball screws. In the case of a moving gate drive, it must be driven by two drives, synchronized in order to maintain the correct perpendicularity of the axis! This is very important as otherwise the door will have very little torsional stiffness.

Ball screws are precision rolling gears and therefore must be protected against dust, chips and dust generated during machining. It is obligatory to use covers protecting ball screws exposed to direct contact with contamination.

The machine must weigh. If we are able to lift the machine by human force from either side, then apart from CNC desktop machines, it is definitely a toy. Industrial machines have weights already counted in tons.

As for the drives used in such machines, the best are digital servo drives operating in the DPC (Direct Position Control) system, which are characterized by high motion accuracy in dynamic states. This is very important because the accuracy of the machine is usually given in static states, which does not allow to assess the actual accuracy of machining.

Machines driven by servo motors should reach speeds of 300mm / s and up. Stepper motors should not be used in professional devices, but it is allowed in lighter machines, provided that well-selected motors with drivers are used and a resonance suppression system is used. They should reach speeds of 100-150 mm / s.

A good control system is half the battle. The speed of development of this market segment means that several-year-old machines of well-known brands in excellent technical condition are not suitable for use due to the outdated control system. Therefore, it is very important that the control system allows for future upgrades to adapt it to future standards.

Another aspect of the control system is its speed. The speed of the CNC machine control system is the ability to process a specified number of program blocks per unit time. The speed of the control system is important primarily in works where there are complex shapes consisting of a large number of vectors (and these are most often processed on CNC machines).

In this situation, another parameter of the control system becomes important – the ability to analyze more than one program block at a time. This is important because when analyzing several thousand vectors forward per second, we are able to adjust the velocities in nodes between the vectors so that, in the case of small angles between them, it is possible to travel them at a speed greater than zero. This way of working of the interpolator is called “Dynamic Vector Analysis”.

Another aspect of the control system operation also concerns the operation of the interpolator. First, a PC is not suitable for direct motion interpolation for CNC machines. The PC does not have a precise timer in its hardware resources that could be the time base for the interpolator. In addition, most operating systems such as Windows and Linux are not real-time systems, which means that pulses generated directly by the PC, eg to the printer port, may have delays of an undefined amount. This means that the movements generated in this way will always have a very low quality (vibrations, vibrations, jerks), which is caused by uneven generation of impulses. The solution to this problem is to use a hardware interpolator running on a completely different processor. The most common are very fast DSP processors. In this case, the PC only serves as the user interface and not as an interpolator.

In order for the communication between these two parts of the system to take place in real time, they must be connected by a very fast data bus. Solutions such as serial, parallel or USB ports are not suitable for this. Only Ethernet remains, and most often on the modified transport layer.

A good control system should also enable smooth regulation of the machine advance speed from zero to the set speed. It should also enable automatic generation of the toolpath based on * .dxf drawings, etc., including tool diameter correction, pocket selection, island detection and hole drilling. It is also good if the system can display all processing data on the screen along with the visualization of work progress in real time.

To find out about the possibilities of the machine, first of all, you should perform test machining, the result of which helps to answer most of the questions about the wisdom of purchasing a given machine.

You should ask for a few geometric figures. Namely, a square, triangle, circle and ellipse, 100 mm in size and 5-8 mm thick, with a speed of at least 50 mm / s, in materials at least as hard as those that we want to process on this machine.

First, we cut a square, we look, first of all, at the corners. They should be straight and sharp, the corner should not be rounded, nor should there be visible undulation near the corner. in the waste material, pay attention to whether the cutter in the corners did not go too far. If we see the described effects, it means rather low stiffness of this machine.

The electronic caliper measures the dimensions in both directions. If the deviation is within 0.03mm in the case of milling machines, engraving machines and 0.05mm in the case of milling plotters, it is satisfactory, while the difference between the two dimensions should not exceed 0.02mm and 0.04mm, respectively.

Use a tool square to check the right angle. We should not see the clearances when looking against the light between the angle and the square. We can also cut two squares and put them together by turning one upside down. They should coincide perfectly. If they do not match, the machine is not XY perpendicular.

Then we cut a triangle. Here, apart from the corners, we pay attention to sloping walls that require simultaneous movement of two axes. Here we evaluate the quality of interpolation. The rougher the surface is, the worse the interpolator and drives work.

We now cut a circle. When machining the wheel, we pay special attention to the speed of work and possible vibrations, jamming and other phenomena causing, for example, a significantly reduced feed speed compared to cutting a square. If we see that the circle is running slower than the square, even though the set speed is the same, it means that the system cannot keep up with processing a large number of vectors, or it emulates circular interpolation with low resolution. This can be recognized by the flat surfaces visible on the side of the wheel that make up the wheel. A circle like this circle should be round. We measure them with an electronic caliper at different angles and check the dimensions as in the case of a square.

We are left with an ellipse. Here, the most common problems with the speed of processing in the control system and when processing the ellipse, we pay attention primarily to the speed and smoothness of the machine’s movement.

Then we “plan” a surface with dimensions of about 100x100mm vertically, and next to it horizontally, with a 10mm milling cutter, so that the distance between successive paths is 9mm. After processing, we check the smoothness of the treated surface. If you feel steps under your finger on any of the planned surfaces, it means that the machine is not perpendicular to the table.

Another important test is the punch processing. We mill a cube-shaped punch with dimensions of 40x40x40mm, so that the cutter carries out subsequent squares, lowering in layers every 1mm down. We measure the dimensions with an electronic caliper in both directions right next to the surface and right next to the base. If the upper and lower dimensions differ by more than 0.03mm, it means that the Z axis is not perpendicular to the X and Y axes.

If the machine passes all these tests, it means that we are dealing with an ideal machine and it is definitely worth buying