CNC Knowledge: Calculations of dimensions and fit tolerances

Tolerance levels are divided into IT01, IT0, IT1 – IT18. Accuracy levels decrease sequentially and tolerance values ​​become larger and larger.

1. Dimensional tolerance

1) Hole tolerance

TD = |Dmax-Dmin| = |ES-EI|

2) Shaft tolerance

Td = |dmax-dmin| = |it’s an egg|

2. Fit tolerance

1) Amount of the difference

Xmax = Dmax- dmin = ES- ei

Xmin = Dmin- dmax = EI-es

2) Amount of interference

Ymax = Dmin- dmax = EI-es

Ymin = Dmax- dmin = ES- ei

3) Transition amount

Xmax = Dmax- dmin = ES- ei

Ymax = Dmin- dmax = EI-es

4) Fit tolerance

clearance adjustment

Tf = |Xmax- Xmin|= TD+Td

tight fit

Tf = |Ymax- Ymin|= TD+Td

Transition adjustment

Tf = |Ymax- Xmax|= TD+Td

3. Calculation examples

1) Which of the following sizes Ф10-0.0220, Ф250-0.044-0.015, Ф70+0.075+0.105 has the highest level of accuracy? The lowest level of precision?

Solution: Check the GB/T1800.3 standard and get the highest accuracy level Ф250-0.044-0.015 (IT6) and the lowest accuracy level Ф10-0.0220 (IT8).

2) Mounting parts, hole Ф200+0.013, maximum clearance Xmax=0.011, fit tolerance Tf=0.022. Q: Axis deviation? Deviation below axis?

Solution: ∵TD = 0.013, Tf = TD+Td

∴Td = Tf-TD = 0.022-0.013 = 0.009

∵Xmax = ES- ei

∴Difference under axis ei = ES-Xmax= + 0.013-0.011 = +0.002,

Deviation on the axis es = ei+Td = +0.002+0.009 = +0.011.