Tolerances: Need to Know to Reduce Assembly Challenges
Tolerance defines the acceptable range of variation allowed on a dimension. It helps to define criticality in the part and ensure assembly.
Ignoring tolerance will add to challenges in assembly and increase cost significantly. Here are our ideas if you need to reduce these challenges.
IT Tolerance
IT grade(s) describe an internationally accepted code system that categorizes the linear tolerances into 12 categories. This allows Product Owners and Data Scientists to handle tolerances with a single number. The system is defined in ISO 286 and frequently used.
The system considers both the nominal size as well as the tolerance width to determine a proxy for the “manufacturing complexity”
IT grades more than IT18
The standard ISO 286 defines the system of tolerances, deviations, and fits only for basic sizes up to 3150 mm. However, IT grades can be extrapolated in the following way.
From IT6 to IT18, the standard tolerances are multiplied by the factor 10 at each fifth step. This rule applies to all standard tolerances and may be used to extrapolate values for IT grades not given in Table 1. For example, the nominal size range 180 mm up to and including 250 mm, the value of IT20 is:
IT20 = IT15 × 10 = 1.85 mm × 10 = 18.5 mm
Fits and Tolerance
The relation between two mating parts due to the difference between their sizes before the assembly, is defined as fits.
There are three major types:
Clearance fit
Interference fit
Transition fit
IT grades with standardized prefix are used to define the tolerance limits that can be used to define the Fits.
IT grades do not specify how the tolerance limits are distributed around the nominal value, IT grades with an alternate prefix are used for this purpose. For example, the prefix ‘js’ is used in place of ‘IT’ to specify the symmetrical distribution, so a dimension 12 js5 is equivalent to 12±0.004 (where 12 IT7 is 0.008).
Standardized prefixes include the letters A, B, C, CD, D, E, EF, F, G, H, J, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC (for holes), and the lower-case equivalents (for shafts). All of these letters represent some kind of distribution around the nominal value. H and h are easiest to explain as the tolerance lies entirely on one side of the nominal size.
For example, 12 H7 means a hole ranging from 12.000-12.018mm and 12 h7 means a shaft ranging from 11.982-12.000mm.
General Tolerance
Tolerances are generally controlled by ISO 2768 standard. ISO 2768 tolerancing is based on the size of the feature. Small feature sizes have closer tolerances and large features feature sizes have larger tolerances.
There are four classes of size tolerances: fine(f), medium(m), coarse(c) and very coarse(v). For example, a company that manufactures precision parts and equipments might select the medium(m) for general metric tolerances. This is given by ISO 2768-m, the tolerances for various dimensions will be given by the general metric tolerance table.
The ISO 2768 only applies to the following drawings with the subsequent features. It is used when these functions do not have custom tolerance indications individually:
o Linear dimensions (external sizes, internal sizes, diameters, distances, chamfer heights, radii)
o Angular dimensions
o Linear and angular dimensions produced by machining assembled parts.
International Standard ISO 2768:1989 was prepared by Technical Committee ISO/TC3, Limits, and fits, it comes in two parts, namely ISO 2768-1 and ISO 2768-2.
o Part 1 – General Tolerances for linear and angular dimensions.
o Part 2 – Geometrical tolerances for features.
The following tables are used to define tolerances for linear and angular dimensions:
The following tables are used to define Geometrical Tolerances for features:
Deviation Tolerance
Deviation is plus-minus dimensioning. It uses a bilateral or unilateral tolerance format, depending on the application. Plus-minus dimension values are placed using the plus-minus symbol (±). For example, 10.5±0.2 or 0.250±0.005.
Bilateral Tolerance
A bilateral tolerance is allowed to vary in two directions from the specified dimension. For example, 10.5(+0.2/-0.1) is an unequal bilateral tolerance and the dimension 10.5±0.2 is called an equal bilateral tolerance.
Unilateral Tolerance
A unilateral tolerance varies in only one direction from the specified dimension. For example, 10.5 +0.2/-0 or 10.5 +0/-0.1.
Limit Tolerance
Limit tolerance is an alternative method of showing and calculating tolerance. With limit dimensioning, the extreme values of the tolerance are given in the dimension. The limits are upper limit and the lower limit.
The upper limit is the largest the feature can be within the given tolerance of the dimension. The lower limit is the smallest the feature can be within the given tolerance of the dimension.
For example: 10.5±0.2
Upper Limit: 10.5+0.2=10.7
Lower Limit: 10.5-0.2=10.3
Single Limit Tolerance
Various features such as chamfers, fillets, rounds, hole depths, and thread lengths, can be dimensioned with single limits. The abbreviation for minimum (MIN) or maximum (MAX) follows the dimension value to specify a single limit application. The unspecified limit is 0 when MAX is used or reaches infinity when MIN is specified. For example, R6MIN means the minimum radius should be 6mm or RMAX6 means the maximum radius can be 6mm.
Reference Dimension
Reference dimensions are used to provide information or visualization only. Reference dimensions are often used as additional information to accumulation of other dimensions or to show a dimension that is defined elsewhere with tolerance. No tolerance is defined explicitly for a reference dimension and no inspection is necessary.
Reference dimensions are defined by using parenthesis around the dimension or using the term “REF” or “Ref.” behind the dimension. For example, (10.5) or 10.5 REF or 10.5 Ref.
Theoretically Exact Dimension
Theoretically Exact Dimensions are used to prevent accumulation of tolerance. Chain dimensioning using Tolerance-based Dimensions can cause accumulation of tolerance as the tolerances of all chain dimensions would add up.
Theoretically Exact Dimensions (TED) or Theoretically Exact Measures (TEM), are also called Basic Dimensions. TED are given from a datum to a feature of interest. TED are defined as a numerical value to describe the theoretically exact size, profile, or location of a feature. Variations allowed to these dimensions are based on feature controls, notes, or tolerance of other dimensions. No tolerance is specified explicitly to TED.
TED are denoted by enclosing the dimension in a rectangle.
For example, here the tolerances of dimension 25, 10 and 15 will add up to define position of the ø10 hole, resulting in a higher tolerance band. This can be avoided using theoretically exact dimensions.
Approximate Dimension
Approximate dimensions are used when the tolerances are not very important. They are indicated by using the term “APPROX.” before or after the dimensional value. They are often indicated using “ca.” or “~”. There is no supervision or measurement of approximate dimensions. For example, 10.5 APPROX. could be any value close to 10.5mm.