Selection of Fits

From the above it will be realized that are a very large number of combinations of hole deviation and tolerance with shaft deviation and tolerance. However, a given manufacturing organization will require a number of different types of fit ranging from tight drive fits to light running fits for bearing etc. Such a series of fits may be obtained using one of two ISO standard system:

The Shaft Basis System

For a given nominal size a series of fits is arranged for a given nominal size using a standard shaft and varying the limits on the hole.

The Hole Basis System

For a given nominal size, the limits on the hole are kept constant, and a series of fits are obtained by only varying the limits on the shaft.

The HOLE SYSTEM is commonly used because holes are more difficult to produce to given size and are more difficult to inspect. The H series (lower limit at nominal, 0.00) is typically used and standard tooling (e.g. H7 reamers) and gauges are common for this standard.

Clearance Fits ISO Standard "Hole Basis"

 Transition Fits ISO Standard "Hole Basis"

 Interference Fits ISO Standard "Hole Basis"

 Comparative Roughness Value

Engineering Communication (Dimension Tolerances)

Tolerance

Important to interchangeability and provision for replacement part

It is impossible to make parts to an exact size.

The tolerance, or accuracy required, will depend on the function of the part and the particular feature being dimensioned.

Therefore, the range of permissible size, or tolerance, must be specified for all dimension on a drawing, by the designer/draftsperson.

Nominal Size: is the size used for general identification, not the exact size.

Actual size: is the measured dimension. A shaft of nominal diameter 10mm may be measured to be an actual size of 9.975mm.

General Tolerance: In ISO metric, general tolerance are specified in note, usually in the title block, typically of the form: “General tolerance ±0.25 unless otherwise stated”.

Specific Tolerances

Specific Tolerances Indicate a special situation that cannot be covered by the general tolerance.

Specific Tolerances are placed on the drawing with the dimension and have traditionally been expressed in a number of ways:

Limit are the maximum and minimum sizes permitted by the tolerance.

All of the above methods show that the dimension has:

a Lower Limit = 39.97mm

an Upper Limit = 40.05mm

a Tolerance = .008mm

Manufacturing must ensure that the dimensions are kept within the limits specified. Design must not over specify as tolerances have an exponential affects on cost.

Limits and Fits

1. Clearance Fits:
   The largest permitted shaft diameter is smaller than the diameter of the smallest hole.
   
     
2. Interference Fits:
   The minimum permitted diameter of the shaft is larger than the maximum diameter of the hole.
   
   
   
3. Transition Fits:
   The diameter of the largest allowable hole is grater than that of the smallest shaft, but the smallest hole is smaller than the largest shaft. 
   

ISO Tolerance Designation

The ISO system provides for:

• 21 types of holes (standard tolerances) designated by uppercase letters A, B, C, D, E....etc. And
• 21 types of shafts designated by the lower case letters a, b, c, d, e....etc.

These letters define the position of the tolerance zone relative to the nominal size. To each of these types of hole or shaft are applied 16 grades of tolerance, designated by numbers IT1 to IT16 – the “Fundamental Tolerance”:

ITn = (0.45xD^0.33+0.001 D) Pn

Where D is the mean of the range of diameters and Pn is the progression: 1, 1.6, 2.5, 4.0, 6.0, 10, 16, 25....etc. Which makes each tolerance grade approximately 60% of its predecessor.

Experience has shown that the dimensional accuracy of manufactured parts is approximately proportional to the cube root of the size of the part.

Example:

A hole specified as: ø30H7

The H class of holes has limits of i.e. all tolerances start at the nominal size and go positive by the amount designated by the IT number.

IT7 for diameters ranging 30-50mm:

Tolerance for IT7=(0.45x40^0.3+0.001x40)x16=0.025mm

Written on a drawing as Ø30h7