Based on a given size of a CNC component, what tolerance can CNC milling or turning process can achieve? The International Tolerance Grade chart specifies standardized tolerances for designing mechanical components. The IT Grade number indicates the general applications or suitable processes of these grades.
International Tolerance Grade, also known as IT Grade, identifies what tolerances a specific process can produce for a given dimension defined in ISO 286. It establishes the magnitude of the tolerance zone or allowed amount of part size variation for internal and external dimensions. There are totally 18 grades of tolerance, the smaller the International Tolerance Grade number, the smaller the allowable tolerance zone, the higher the precision level.
The IT Grade is used to indicate how precise an industrial process is. In designing, the engineer will determine a key dimension and relative tolerance for a particular feature. The tolerance for a particular IT grade can be calculated through the formula as below. The calculation formula can help engineers to know what IT Grade is necessary for producing a part with certain specifications. If the manufacturing process can’t reach the grade, you should consider another method.
T: tolerance in micrometers [μm]
D: geometric mean dimension in millimeters [mm]
ITG: IT Grade, a positive integer
How precise are common CNC machining processes? Different IT grade numbers represent different precision and suitable varying processing techniques.
IT Grade 1-4: very precise, usually used for Gage making and precision work, precision measuring tools
IT Grade 5-16: suitable for cutting operations like turning, milling, boring, grinding and sawing.
IT Grade 12-16: suitable for manufacturing operations like pressing, rolling & other forming operations.
Fundamental positions are expressed by Tolerance position letters. Capital letters are used for internal dimensions (hole), such as H and lower case letters are used for external dimensions (shaft), like h. But letters “I, L, O, W, Q” and “i,l, o, w, q” won’t be used.
For example, in the expression of 35H8, 35 refers to the basic size, H refers to fundamental deviation, 8 refers to IT Grade, and H8 refers to tolerance grade.