When you export to STL format, you may need to make some adjustments in the Export options dependent on the level of smoothness you desire for your model.
Output:
The Units: option selects a unit type from the [Use Design Units], millimeters, centimeters, meters, inches or feet options
The Format: option selects a format type from the Binary or ASCII options, with Binary typically being smaller than ASCII files and calculating faster in third party programs
Faceting Parameters:
STL files will break down any curved faces of your model into Facets so it is easier to compute. The Faceting Parameters in STL Export options will determine how many facets are used to generate said curved geometry.
The Normal Deviation: option toggles whether or not normal deviation is used for faceting parameters which determines how accurately facets are shown in a model which can be advantageous since it does not affect the overall size of a file.
The slider bar for the Normal Deviation option has 10 predefined limits between the values of 0.5 and 40. Enabling the Custom option will make the slider inaccessible but allows any positive value over 0.5 to be entered.
The Surface Deviation: option toggles whether or not surface deviation is used for faceting parameters which determines how accurately facets are shown in a model based on the distance between facets and the surface they represent
The value of Surface Deviation depends on the size of the model which is a fraction of the diagonal length of the bounding box of a face and is independent of the units of measurement of a face. The slider bar for the Surface Deviation option has 10 predefined limits between the values of 0.001 and .05. Enabling the Custom option will make the slider inaccessible and allows any positive value over .0000001 to be entered.
The Maximum Cell Size: option toggles whether or not maximum cell size is used for faceting parameters which determines the edge length of the triangles that make up surfaces of a model and since a facet cannot be larger than a cell, the maximum size of a facet can be determined with this option with larger values creating larger facets and thus a more coarse model and smaller values creating smaller facets, more smooth facets and a larger file size.
The default value of the Maximum Cell Size option is set to 0 with the option disabled. If the value of this option is zero then the maximum edge length is not considered during faceting. The slider bar for the Maximum Cell Size option has 10 predefined limits between the values of 0.05 and 4. Enabling the Custom option will make the slider inaccessible and allows any positive value over .0000001 to be entered.