Accuracy of CNC machine tools

Machine accuracy is often the basic criterion for choosing a machine tool. Everyone would like to have the most accurate machine possible. Often, however, the accuracy of the machine is misunderstood and this sometimes leads to confusion. Accuracy of the machine is a general concept and to detail it, the following terms should be used:
straightness of feeds, perpendicularity of the axis, pitch errors, backlash, spindle squareness, positioning resolution, drives resolution, interpolator resolution, positioning repeatability, stiffness.

– straightness of feeds is a parameter that determines the maximum deviation of the tool path from the straight line at a given distance of a given axis

– axis perpendicularity is a parameter that defines the maximum deviation of the perpendicular track of a given axis in relation to the reference axis over a specified distance

– pitch error is the deviation in the value of the ball screw nut displacement from the theoretical displacement resulting from the nominal pitch of the screw

– reverse play is the distance at which when changing the direction movement, the axis starts to move

– spindle perpendicularity is a parameter that determines the error of perpendicularity of the spindle in relation to the xy plane

– positioning resolution is the product of the drive resolution and the screw pitch (the smallest value by which a given axis can move due to the drive capabilities

– interpolator resolution is the minimum displacement that can be given to the drives by the position adjuster (interpolator)

– positioning repeatability is the maximum deviation of the absolute position of the tool during repeated approach to a selected point from different directions

– stiffness is a parameter that determines the value by which the machine will go back by applying the set force at the least favorable axle position.

As you can see, the total tool positioning error is the sum of all these errors. Of course, it may happen that certain errors in given circumstances will disappear, but there is nothing to count on. Additionally, the matter is complicated by the phenomenon of thermal expansion. For steel, it is about 0.01mm / m for each degree Celsius, so when the temperature jumps from 10 to 30 degrees, the screw will expand by 0.2mm / m!

It is not a problem if you machine steel because it has a similar expansion as the feed screw, but if you want to machine aluminum which has a thermal expansion about three times greater than steel, serious problems with tolerance, especially with long parts, start to appear.

Most of these factors affect the so-called static error, i.e. measured at a given point with the machine stopped. There is also a dynamic error, which becomes apparent only during operation and is related to the imperfection of the interpolator and drives. Servo drives work in the so-called closed-loop position control (position feedback). The controller in the servo drive constantly tries to control the motor so that the position error (difference between the set and actual position) is as low as possible.
The servo drive cannot react to a position error immediately, it needs as much time as the period of the position regulator.

Most servo drives have a positioner frequency of only 400 Hz. If the deviation increases immediately after measuring the position, the servo drive will not know about it for the next 2.5 ms, and at a speed of 0.5 m / s, the machine will travel 1.25 mm during this time! This error is many times greater than the sum of all the static errors of the machine geometry.

Unfortunately, machine manufacturers often do not even mention the dynamic parameters of their machines because they usually have nothing to boast about. Bearing in mind this problem, our company has been perfecting digital servo drives made in the “Direct Position Controll” technology for many years, the frequency of the position regulator reaches 20,000Hz, which is a value 50 times higher than in most servo drives. Thanks to this technology, we have significantly reduced the dynamic error of our machines, which allowed for the use of higher speeds and acceleration closely related to the efficiency of machine tools manufactured in our company.

It should also be mentioned that the quality of positioning, i.e. interpolation, is equally important. The interpolator is part of the control system that is responsible for providing servo drives with information at what speed and to what position each axis should reach. The most important task of the interpolator is to synchronize the movements of all axes in such a way that the shape drawn by the tool in space is consistent with the operator’s program. It is a very complex process that requires very fast processors to achieve satisfactory resolution.

For many years, our company has been developing the Dynamic Vector Analysis technology developed by us in shaping the interpolation speed profile. The effect of its operation is, in extreme cases, even a twenty-fold reduction in processing time, especially for details with complex shapes.