In the precision parts processing, the error due to the size and shape as well as the processing position error caused is called, in order to ensure the quality of precision parts machining, we will require error which must be controlled within the range, this range is tolerance. In general, the higher the level of tolerance, the processing of the better parts accuracy, the same processing difficulty will be higher. According to the theory, since the error can not be eliminated, so the tolerance value is not zero, and is an absolute value. Then the higher the level of tolerance is not better?
　　In the process of the design process, the designer in the design of manufacturing tolerances, it shall be selected to ensure reasonable use of the processing conditions, the cost at the time of selection should be sturdy precision parts machining, of course, the higher the level of tolerance well, this product accuracy is high, but this process also requires the use of processing technology and equipment must meet the relevant level of the job such harsh conditions, processing costs will be a lot of money.
　　So tolerance class how to make a choice it? The question you have to look at the demand side of the process, if only just consider the economic, then select the tolerance value, we will choose to ensure the precision parts processing products can be used normally under the premise that a lower level of tolerance values for some of the more sophisticated development of products, in the case of sufficient funds, we recommend choosing a slightly higher tolerance values ​​that level. You know, absolutely impossible to eliminate tolerance, so to protect the product is absolutely no error that is not possible, we can only be human error controlled within a reasonable range in order to ensure the normal use of the product.