Machining Precision in the CNC Machining Industry

We are exposed to processing every day, and we often mention precision in this process. But is your accuracy correct? Or do you mean “rigorous”? Next, let’s follow “Shenzhen China” to learn those things about precision!

The so-called machining precision refers to the actual size, shape, position of the machined part surface and the ideal geometric parameters required by the drawing! For ideal geometric parameters, the accuracy of dimension refers to the average size; the accuracy of shape refers to absolute circle, cylinder, plane, cone and straight line; the accuracy of mutual position refers to absolute parallelism, verticality, coaxiality, symmetry, etc.

1、Dimension accuracy
The so-called dimensional accuracy refers to the correspondence between the actual size of the part after processing and the tolerance zone center of the part size.

2、Shape accuracy
The so-called shape accuracy refers to the degree of the actual geometry shape of the machined part surface and the ideal geometric shape.

3、Position accuracy
The so-called position accuracy refers to the actual position accuracy difference between the relevant surfaces of parts after processing.

4、Interrelation
When designing machine parts and specifying the machining precision of parts, it is usually necessary to control the shape error within the position tolerance, and the position error should be less than the dimension tolerance. This means that the shape accuracy of precision parts or important surfaces of parts needs to be higher than the position accuracy requirements, and the position accuracy requirements need to be higher than the dimensional accuracy requirements.

The so-called machining error refers to the deviation between the actual geometric parameters and the ideal geometric parameters. Among them, the size of machining error reflects the level of machining accuracy, the larger the error, the lower the machining accuracy; the smaller the error, the higher the machining accuracy.

Brief introduction of machining precision
“Machining accuracy” is a term, which is mainly used to produce products. Both machining accuracy and machining error are used to evaluate the geometric parameters of machined surfaces. Among them, the machining accuracy needs to be measured by the tolerance grade. The smaller the grade value is, the higher the accuracy is; while the machining error is expressed by numerical value, and the larger the value, the greater the error; that is, the higher the machining accuracy, the smaller the machining error, and the lower the machining precision, the higher the machining error.

There are 20 tolerance levels, from IT01, IT0, IT1, IT2, IT3 to IT18. Among these grades, IT01 indicates that the machining accuracy of the part is the highest, and IT18 indicates that the machining accuracy of the part is the lowest, and generally IT7 and IT8 are the intermediate grades of machining accuracy.

It should be noted that no matter what processing method is used, the actual parameters obtained will not be absolutely accurate. From the function of parts, if the machining error is within the tolerance range required by the part drawing, then the machining precision is considered to be guaranteed.

Difference between accuracy and precision:
1、Accuracy
When the accuracy is small, the average value of the measured data deviates less from the true value. But in the case of scattered data, that is, the size of accidental error is not clear.

2、Precision
Precision refers to the repeatability and consistency of the results obtained by repeated determination with the same spare sample. However, there may be high precision, but the accuracy is not accurate. For example, if the length of 1 mm is used for measurement, three results will be obtained, which are 1.051 mm, 1.053 and 1.052. Although their precision is high, the values are not accurate. Thus, accuracy refers to the accuracy of the measurement results, while precision refers to the repeatability and reproducibility of the measurement results. Among them, precision is the precondition of accuracy.

Methods to improve machining accuracy
Adjustment of process system
1）Adjustment of trial cutting method
Through a series of processes of “trial cutting – measuring size – adjusting cutter’s cutting amount – cutting – re cutting”, the process can be repeated until the required size is reached. However, the production efficiency of this method is low, so it is mainly used for single piece and small batch production.

2）Adjustment method
The adjustment method obtains the required size by pre adjusting the relative positions of the machine tool, fixture, workpiece and tool. However, due to its high productivity, this method is mainly used for mass production.

Reduce the error of machine tool
1）In order to improve the manufacturing accuracy of the spindle components, the rotation accuracy of the bearing should be improved
①High precision rolling bearings are selected;
②high precision multi oil wedge dynamic pressure bearings are used；
③high precision hydrostatic bearings are used

The accuracy of accessories with bearings should be improved
①It can improve the machining accuracy of box support hole and spindle journal;
②It can improve the machining accuracy of the surface matching with the bearing;
③By measuring and adjusting the radial runout range of the corresponding parts, the error can be compensated or offset.

2）Proper preloading of rolling bearings is required
①Clearance can be eliminated;
②Increase of bearing stiffness；
③Homogenize the rolling element error and reduce the transmission error of transmission chain
1）Less transmission parts, short transmission chain and high transmission precision;
2）Through the use of speed reduction transmission, this can ensure the important principle of transmission accuracy. And the closer to the end of the transmission pair, the smaller the transmission ratio;
3）The accuracy of the end piece needs to be higher than that of other transmission parts.

Reduce tool wear
It is necessary to regrind the tool before the tool size wear reaches the stage of sharp wear.