About the machining accuracy of machine tools

We often encounter accuracy problems when processing parts. I have been engaged in mechanical design for more than 5 years, and I have encountered many problems related to accuracy. In most cases, they don't ask for a thorough explanation. The project is just over and done with, without spending energy and time on sorting out and summarizing it.

Some processing plants often say that the precision of our machine tools is very high, reaching 1 micron, or reaching IT3 level. The next question is, what is the difference between the accuracy of the machine tool and the accuracy of the parts we want to process? If there is no difference, can the accuracy of the machine tool reach 1 micron? Then the accuracy of the parts processed by this machine tool can reach 1 micron?

Now I will discuss this concept of accuracy with questions. It's just a discussion. The content published is just a summary of my experience in the work process and a summary of my understanding of the processing industry. I may draw wrong conclusions. I hope everyone will criticize and correct me.

Let’s first give an example to explain the problem:

The factory will choose different processing methods according to the drawing requirements. Take drilling as an example. The processing accuracy of ordinary drilling machines is IT10, which means drilling machine processing. The highest accuracy is IT10, which is the highest theoretical processing accuracy of this drilling machine, which means the highest dimensional accuracy of the parts processed by this drilling machine. The processing accuracy is not only related to the processing accuracy of the machine tool itself, but also related to the operator's level, the use of processing tools, the use of fixtures, the use environment, and the processing technology of the parts. For example, when processing a 100x100mm part, there is no need to have the accuracy of each dimension be 0.001mm. In this case, even if there is no problem with the processing accuracy of the machine tool, the entire processing technology will be very different, and the processing cost will be much higher. For some We can give high precision to the key matching dimensions during the design process. There is no need to give high precision to the appearance surface and non-fitting dimensions. Just use a larger knife, a larger cutting amount, and a faster feed speed. It can be processed quickly. For dimensions with high precision requirements, a small knife can be used to reduce the amount of cutting. Multiple cutting, multiple measurements, and multiple tool calibrations can be performed to ensure processing accuracy.

An example of analogy: a vernier caliper. The accuracy of an ordinary vernier caliper is 0.02mm. This can be seen. Each scale is 0.02mm. We can also say that its measurement accuracy is 0.02mm, rigorous. He said that his highest measurement accuracy is 0.02mm. Because the same ruler may have an accuracy of 0.03mm when used by different people. For example, incorrect holding methods, incorrect reading methods, unscientific measurement methods, etc. may lead to different measurement data.

In addition, let’s deepen the concepts of precision and tolerance here

For example, we are talking about 60H7, the size of this hole, its diameter is 60mm, the tolerance zone code is H, and the tolerance level is IT7,

Pay attention to the three conditions for the size above, nominal size, tolerance zone code, and tolerance grade. Only when these three conditions are obtained, can we determine the position of the tolerance zone. In the past, I always saw this statement in the technical requirements of some engineering drawings: If the tolerance is not noted, it will be processed according to IT10 level. In fact, this statement is wrong. How can I process it according to IT10? You did not give the tolerance a code. I don't know if this is an old standard, but it is wrong to label it this way now. The country has given the unindicated tolerance standard GB/T1804-2000.

The following table is taken from the Machinery Manual. The header clearly states the tolerance levels that can be achieved by various processing methods.

Note: The selection of processing accuracy and processing methods is in the Tolerances and Fits chapter of Volume 1 of the Mechanical Design Manual.