R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

4.18 Close up view showing facets.
4.19 Facet deviation.
Working with medical scan data 53
Distance at midpoint
Facet edge
True curve
which is the inside and outside surface). Usually, small problems with an
STL fi le can be corrected with specialist software or the RP machine
preparation software. STL fi les can be generated from practically all threedimensional
CAD systems. Solid modelling CAD systems rarely have problems
creating STL fi les, but surface modellers can pose problems if the
surfaces are not all properly stitched and trimmed.
When exporting an STL fi le from a CAD system, the user will normally
specify a resolution or quality parameter to the STL fi le. This is normally
done by specifying a maximum deviation. The deviation will be the perpendicular
distance between a facet and the original CAD data where the facet
forms a chord at a curved surface, as shown in Fig. 4.19. In essence, a
smaller deviation will give a more accurate representation of the CAD
model, but this will result in an STL fi le with a greater number of smaller
facets. The fi le size depends only upon the number of facets and so a
smaller deviation will give rise to a larger fi le. The effect is illustrated in
Fig. 4.20.