IT Grades as Standardized Tolerance

Ever wish you could deal with the several toleration methods in a single standardized way? Here’s how… 

Tolerance is defined as the allowed deviation from the nominal value of a dimension, also is one of the elements of technical drawings. Tolerances can apply to many different units. For example, deviation tolerance, limit tolerance, general tolerance, etc. These are significant elements for production, but several data analytics problems do not need this granularity to be solved. In fact, for problems to be resolved, it requires a simple indication of how difficult it is to reach tolerance.  

Fortunately, there is a simplified indicator exists: ISO 286 species the International Tolerance (IT) Grades. The species are internationally accepted tolerance codes for linear dimensions that range from IT01 to IT18. Low numbers remark the tight tolerances (I.e., difficult tolerances), and large numbers indicate wide tolerances. 

By dealing with so many parameters, it is possible to make life easier for engineers who want to walk the rope without the wrong steps. The right step for the engineers is Werk24’s API which automatically calculates the IT grade for all finite tolerances that it extracts. The example of converting tolerances into definite IT grade is given below:  

Why the Tolerance Width Is Not Enough 

Of course, Werk24 also generates the tolerance width. Still, for most applications IT grades are more meaningful. The reason is simple: a tight tolerance on a large measure is more difficult to achieve than the same tolerance on a smaller measure which are represented as on the images.

Which Tolerance Methods Have No IT Grades

IT Grades require the tolerance width to be finite. Tolerance widths that only have an upper (e.g., max. 4) or lower (e.g., min. 10.0) limit, or approximate measures (e.g., ~40) are thus not given an IT grade. The same is true for measures that have no tolerance because they are marked as “Theoretically Exact” or reference measures.

Relationships with Fits

IT grades should not feel completely unfamiliar; most of us will have seen them already in the context of fits. Fits are defined in two parts: (I) the location – described by 1-2 characters and (ii) the width – described by the IT grade.

Additional information about the location does not normally affect pricing or feasibility checks. It simply adds an offset to the nominal size. In considering the following example of fits, it is converted by using the translation table initially and then centered. For the chosen nominal size 12, the IT grade IT7 corresponds to a tolerance width of +/- 0.009. This width could be maintained for all locations of the fit.

Conversion and Extrapolation

Converting the tolerance widths into IT grades is not a rocket science. The following image shows how Werk24’s technology understands all these different ways of tolerancing and provides a single output.

The ISO 286 defines the international tolerances grades for nominal sizes up to 3.15 meters, which can be seen in the illustrated table. Let’s start with choosing IT12, one of the IT Grade from tolerance bands to demonstrate how it works. Supposing that the nominal size is over 20.0 mm, the size ranges the values between 18 and 30 mm on the table. The convergence of defined column and line is equal 0.21 mm where IT12 allows deviation of 0.21 mm on the nominal dimension. The standard table shows IT Grades from IT01(most precise) to IT18(least precise); these grades can come through by using a simple formula, thus even more variations become available if needed.