CNC drilling and milling machine, short for Computer Numerical Control drilling and milling machine, is an automated drilling and milling machine equipped with a program control system. This control system can logically process programs specified by control codes or other symbolic instructions, decode them, and represent them in coded numbers. Nanjing No.4 Drilling and Milling Machine Co., Ltd. inputs these instructions into the CNC device through an information carrier. After computation and processing, the CNC device sends out various control signals to control the actions of the drilling and milling machine, automatically processing parts according to the shape and size required by the drawings.
The positioning accuracy of a CNC drilling and milling machine refers to the positional accuracy that each coordinate axis of the machine can achieve under the control of the numerical control device. The positioning accuracy of a CNC drilling and milling machine can also be understood as the motion accuracy of the machine. Ordinary drilling and milling machines are manually fed, and their positioning accuracy is mainly determined by reading errors. However, the movement of a CNC drilling and milling machine is achieved through digital program instructions, so its positioning accuracy is determined by the numerical control system and mechanical transmission errors. The movement of each moving part of the drilling and milling machine is controlled by the numerical control device, and the accuracy that each moving part can achieve under program instruction control directly reflects the accuracy that the machined parts can achieve. Therefore, positioning accuracy is a very important aspect of inspection.
1. Linear motion positioning accuracy detection
The positioning accuracy of linear motion is generally tested under the no-load conditions of drilling and milling machines and workbenches. According to national standards and the International Organization for Standardization (ISO standards), the detection of CNC drilling and milling machines should be based on laser measurement. In the absence of a laser interferometer, standard scales can also be used for general users, accompanied by an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument must be 1 to 2 levels higher than that of the measured object.
In order to reflect all errors in multiple positioning, the ISO standard stipulates that for each positioning point, the average value and the positioning point dispersion band formed by the -3 dispersion band of the five measurement data shall be calculated.
2. Detection of repeatability accuracy in linear motion
The instrument used for detection is the same as that used for detecting positioning accuracy. The general detection method involves measuring at any three positions near the midpoint and both ends of each coordinate stroke. Each position is positioned quickly, and the positioning is repeated seven times under the same conditions. The stop position values are measured, and the maximum difference in readings is calculated. Half of the maximum difference among the three positions, with a positive or negative sign attached, is taken as the repeated positioning accuracy of that coordinate. It is the most basic indicator reflecting the stability of the axis movement accuracy.
3. Detection of return accuracy to the origin of linear motion
The accuracy of returning to the origin is essentially the repeated positioning accuracy of a special point on the coordinate axis, so its detection method is completely identical to that of repeated positioning accuracy.
4. Reverse error detection of linear motion
The backlash error in linear motion, also known as loss of momentum, comprehensively reflects errors such as the backlash zone of the driving components (such as servo motors, servo hydraulic motors, and stepper motors) in the feed transmission chain of the coordinate axis, as well as the backlash and elastic deformation of various mechanical motion transmission pairs. The greater the error, the lower the positioning accuracy and repeatability.
The method for detecting reverse error involves moving a certain distance in the forward or reverse direction within the range of the measured coordinate axis, using the stop position as a reference. Then, a certain movement command value is given in the same direction to move a certain distance, and subsequently, the same distance is moved in the opposite direction. The difference between the stop position and the reference position is measured. Multiple measurements (typically 7 times) are conducted at three positions near the midpoint and both ends of the range, and the average value at each position is calculated. The maximum value among the obtained average values is taken as the reverse error value.
5. Positioning accuracy inspection of the rotary table
The measuring tools include standard turntables, angular polyhedra, circular gratings, and collimators (straightness instruments), which can be selected according to specific situations. The measuring method involves rotating the worktable in a positive (or negative) direction by an angle, stopping, locking, and positioning. This position is taken as the reference, and then the worktable is rapidly rotated in the same direction, with locking and positioning every 30° for measurement. The maximum difference between the actual rotation angle and the theoretical value (instruction value) at each positioning position is the indexing error. For a CNC rotary table, each 30° should be taken as a target position, and for each target position, rapid positioning is performed seven times from both positive and negative directions. The difference between the actual reached position and the target position is the positional deviation. Then, the average positional deviation and standard deviation are calculated according to the method specified in GB10931-89 "Evaluation Method for Position Accuracy of Numerically Controlled Drilling and Milling Machines". The difference between the maximum value of all average positional deviations and standard deviations and the sum of the minimum values of all average positional deviations and standard deviations is the positioning accuracy error of the CNC rotary table.
Considering the practical requirements for dry-type transformers, key measurements are generally taken at several right-angle bisectors such as 0°, 90°, 180°, and 270°. The accuracy of these points is required to be one level higher than that of other angular positions.
6. Repeatability indexing accuracy testing of rotary table
The measurement method involves selecting three positions randomly within one rotation of the rotary table and repeating the positioning three times, conducting the test in both forward and reverse directions. The maximum difference between all readings and the theoretical values at the corresponding positions is the indexing accuracy. For a CNC rotary table, one measurement point is taken every 30 degrees as the target position. For each target position, five rapid positioning tests are conducted in both forward and reverse directions, measuring the difference between the actual reached position and the target position, i.e., the positional deviation. Then, the standard deviation is calculated according to the method specified in GB10931-89. The maximum value of the standard deviation at each measurement point is six times the repeatability of the CNC rotary table.
7. Precision detection of the rotary table's return to the origin
The measurement method involves performing an origin reset from seven arbitrary positions, measuring their stopping positions, and taking the maximum difference in readings as the accuracy of the origin reset.
It should be noted that the current detection of positioning accuracy is measured under rapid and precise positioning conditions. For certain CNC drilling and milling machines with poor feed system performance, different positioning accuracy values are obtained when using different feed speeds. Additionally, the measurement results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. Currently, most CNC drilling and milling machines adopt semi-closed-loop systems, and the position detection components are mostly installed on the drive motor. It is not surprising to produce an error of 0.01~0.02mm within a 1m stroke. This is an error caused by thermal expansion, and some drilling and milling machines adopt the method of pre-stretching (pre-tightening) to reduce the impact.
The repeated positioning accuracy of each coordinate axis is the most basic accuracy indicator reflecting that axis. It reflects the stability of the motion accuracy of that axis, and it is unimaginable that a drilling and milling machine with poor accuracy can be stably used in production. Currently, due to the increasing functionality of CNC systems, systematic errors such as pitch accumulation errors and backlash errors in the motion accuracy of each coordinate axis can be compensated by the system, except for random errors, which cannot be compensated. The repeated positioning accuracy reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by CNC system compensation. When it is found to exceed the tolerance, only fine adjustment and correction of the feed transmission chain can be made. Therefore, if it is allowed to choose a drilling and milling machine, it is better to select one with high repeated positioning accuracy.
