# A Note on Tolerance

There is one main tolerance (ATOL) that is used globally to control the operations of 3DEC. This tolerance or derivatives of it are used to control:

• minimum distance between gridpoints;
• minimum area (ATOL*ATOL) of sub-contacts;
• tolerance to link gridpoints of joined blocks;
• tolerance to select vertices close to the common plane;
• planarity of block cutting;
• contact detection;
• minimum block edge length; and
• joint plane plot

The default value of ATOL is calculated based on the size of the average model dimensions in x, y and z. The default value calculated by 3DEC is not always appropriate for all models. The validity of the automatically assigned ATOL decreases as the range in block sizes increases.

Some of the side effects of having an ATOL that is too small are:

• creation of very small blocks during block cutting
• thin blocks that collapse during cycling
• zero or negative zone volume during cycling
• very small timesteps and long solve times

Some of the side effects of having an ATOL that is too large are:

• rejected cuts during block cutting;
• nonplanar block faces;
• contacts that form across thin blocks;
• gridpoints that are slaved when they should not be (block join logic); and
• inability to calculate the normals to some block faces.

It is recommended that users define an ATOL (block tolerance) prior to cutting any blocks with block cut commands. We do not recommend changing ATOL after cutting blocks. The value of ATOL should not be greater that 20% of the smallest block edge or zone edge in the model. The value of ATOL can be made smaller that this value to resolve issues such as rejected cuts to blocks when using the block cut command.