R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Table 6.2 Process steps and associated tolerances
Case studies 241
Process step Source of error Tolerance
Impression taking Human / skill level No value
Casting study model Human / skill level No value
Optical scanning of study model Scanner +/−0.050 mm
Creating Polygon computer model Software setting +/−0.050 mm
from point cloud data
Import into CAD software Software 0.000 mm
Design in CAD software Software setting +/−0.001 mm
Export of CAD data in STL fi le format Software setting +/−0.010 mm
Physical manufacture using SLM TM RP machine +/−0.100 mm
Removal of RP pattern from machine, Human / skill level No value
cleaning and support removal
Surface preparation and polishing Human / skill level No value
Total +/-0.211 mm
and might range from zero to complete failure, discussion is not included
here. However, as this study aims to investigate the implications of adopting
CAD/CAM/RP technologies in dental technology, it is appropriate to
attempt to illustrate their potential contribution to error in the fi nal RPD
framework. The tolerances used in this table indicate typical or nominal
fi gures, which are quoted by manufacturers or set as parameters in
software.
From the processes stated in Table 6.2, it is reasonable to expect a tolerance
of approximately 0.2 mm for these parts. It should be noted that
cumulative negative and positive tolerances from the various steps might
also partially cancel each other, resulting in a lower overall tolerance. The
contribution of each individual step would be diffi cult to demonstrate
without a statistically signifi cant number of cases. The closeness of the fi t
and effective clasping observed when fi tting the frameworks to the patient
cast, as shown in Figs 6.90 and 6.91, suggest that SLM TM RPD frameworks
are in fact within this tolerance.
Error analysis
RPD frameworks are by defi nition one-off custom-made appliances specifi -
cally designed and made to fi t a single individual patient. In addition, the
anatomically fi tting nature of RPD frameworks means that they are complex
in form and do not provide convenient datum or reference surfaces. This
makes it diffi cult to achieve an investigation that provides detailed quantitative
analysis of error. Therefore, it is not practical to perform the type of
repeated statistical analysis that would be commonly encountered in series
production or mass manufacture. Instead, it is normal dental practice to