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What Factors Determine the Accuracy of CNC Machining?

When measuring the linear movement of the CNC machining, generally we will use the linear detection elements, called direct measurement, the position closed loop control is called full closed loop control, the measurement accuracy mainly depends on the precision of the measuring element, it will not be affected by the transmission accuracy of the machine tools. Due to there is an accurate proportion for the linear displacement of the machine tool and the rotation angle of the driving motor, the moving distance of the workbench can be measured indirectly by driving the detection motor or screw rotation angle. This method is called indirect measurement, the position closed loop control is called half close loop control, the measurement accuracy depends on the precision of the detecting element and the feed chain of the machine tool.


The CNC machining accuracy of closed-loop CNC machining tools is largely determined by the accuracy of position detection device, the CNC machine tools has very strict requirements for the position detection elements, and the resolution is typically less than or equal to 0.001-0.01mm.


Ⅰ. The requirements of the position measuring device for the CNC machining feed servo system


1. It is less affected by temperature and humidity changes, reliable, high precision assurance, strong anti-interference capability.

2. It can meet the requirements of the accuracy, speed and measuring range.

3. Easy to use and maintain, adapt to working environment of machine tools.

4. Low cost

5. Easy to achieve high-speed dynamic measurement and process, easy to achieve automation.


According to the different classification methods ,the position detection device can be divided into different types. According to the output signal in the form of classification, it can be divided into digital type and analog type; according to the type of measurement base points, it can be classified as incremental and absolute; according to the classification of motion forms for the position of the measuring element transformation, it can be divided into rotary type and linear type.


Ⅱ. Diagnosis and elimination of the CNC machining detection device failure


Compared with the numerical control device, the probability of failure of the detecting element will be higher. It often appears as cable damage, component stained, phenomenon of collision deformation. If detection element status of health is questioned, you should first check whether the wire is broken, cell stained or deformation, etc., you can also measure the output to determine the state health of the detecting element, it requests to be extremely skilled with the working principle of the detecting element and the output signal.


The output signal: The connection of the numerical control system position control and the position detection device.


There are two forms of the incremental rotary measuring device or the output signal of linear device: the first form is a sine voltage or current positive rotation signal, thereinto, EXE is pulse shaping interpolator; the second form is TTL level signal.


CNC machining tools in the process of movement, there are three sets of the signals from the scanning unit output: two sets of incremental signal are generated by four photocells, connect two photocells which are 180° different together, their push-pull forms two sets of approximate sine wave of 90° different, amplitude is about 11μA, le1 and le2, one set of reference signals also consists of two 180° different photocells into a push-pull form, the output is a spike signal le0, its effective component of about 5.5μA, this signal is only generated through the reference marks.

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