Interpretation of system of Fits on engineering drawing

How to interpret Fit mentioned in engineering drawing using the referred fits table

How to interpret Fit dimension and tolerances on the drawing

Lets take this example of a shaft and hole with 50 H7/ p6 which is a preferred Interference fit .

It is a Hole Basis Fit specification indicated by H7

  1. How to find the tolerance ranges
  2. How to find the fundamental , lower and upper deviations

Step 1 :
Refer the chart for preferred fits as shown below

Preferred Fits chart.

In the left side column find the basic size of 50 , for that basic size refer the value of Upper and lower deviations for H7 and p6

Which are shown In table as 30,0 and 51, 32 for Upper and lower deviation respectively . The units are in micrometre.

In terms of millimetre

For H7
Upper deviation = 0.030 mm, Lower deviation = 0 mm

For p6

Upper deviation = 0.051 mm , Lower deviation = 0.032 mm

Step 2 :

Make tolerance diagram as shown . 

The Basic line indicates the nominal dimension of 50 diameter.

The fundamental deviation is 0 .

Step 3 :

Write down the actual tolerances deciphered from the fits specifications.

This is a pure interference fit as the Shaft is always bigger than the hole in extreme case.

Categories: : GD & Tolerancing