Author: Isabelle Roy
Publication: Cadalyst.com
Date: October 13, 2011
URL: http://www. cadalyst.com /hardware/3d-scanners/temple-team-captures-3d-data-craft-traditional-hindu-sculpture-14119
Using laser scanning to create a CAD model
of a sacred bull makes a mammoth custom manufacturing project more manageable.
Vectra FORM Engineering and Solutions is a
Creaform Handyscan 3D distributor based in Coimbatore, in the southern part
of India. In 2008, the company came across the opportunity to help create
a nandi - a sacred bull sculpture - for the Shiva temple of a well-known spiritual
foundation. In Hindu mythology, a nandi is a bull that carries the god Shiva
and serves as the gatekeeper of Shiva and Parvati. Traditionally, every Shiva
temple has a nandi facing the main shrine, and worshippers pray to the nandi
first before attending to Lord Shiva.
The Sadhguru, or chief priest of the temple,
was keen on the idea of having a sacred bull different from those in existence
across India, and wanted it to last more than 500 years. A stone or rock nandi
was the first option considered, but the spiritual foundation had previously
been disappointed when attempting to build three stone sculptures of Lord
Shiva. Those figures were hand-sculpted from drawings and pictures, without
the aid of any 3D scanning or CAD technology. The sculptures did not fulfill
the original design intent, and furthermore, working in stone consumed a great
deal of time and effort.
Because of this experience, the foundation
was seeking a more reliable alternative. At this point, the temple project
leader visited VectraFORM, and selected the company's services after viewing
a demonstration of the Handyscan 3D technology.
True to Life
To obtain the 3D shape of the sacred bull,
the team considered scanning live bulls. After weighing the inevitable difficulties
of dealing with live animals, however, they settled on taking pictures of
bulls of various ages and studying their characteristics. The best details
from various photographs were combined to create an ideal specimen.
Within a month, the temple sculpting crew
created a plaster of paris model from the pictures, striving to imitate the
position when a bull begins to stand up. This position, with one foreleg extended,
indicates that the bull is "recognizing the master before him."
The model was built at 1:6 scale, and measured approximately 2' x 3' x 4'.
Once the concept was approved, the project
moved to its next phase: 3D scanning of the model. The plaster model was scanned
using a Handyscan 3D scanner. The CAD model obtained - once enlarged by six
times to its actual size - was used to calculate information such as the weight
of the bull when built with various wall thicknesses.
Although the team edited the 3D scan data
from the plaster model with Geomagic and Rapidform - adjusting the hump size,
head position, etc. - the results were not satisfactory. The temple priest
and the core team preferred to work with a model they could touch, rather
than look at the CAD model on a computer screen. VectraFORM considered using
rapid prototyping or CNC machining to create a new model that incorporated
the CAD changes, but the core team opted instead to make a wax model that
could be corrected by hand.
A Second Model, a Second Scanning Session
Next, the team made a wax replica of the
plaster model, and modified it by warming select areas to make adjustments.
The final wax model was then scanned using a Handyscan 3D handheld, self-positioning
laser scanner. Once the optimal laser power and aperture time settings were
determined for this particular surface, the scanning process required about
five hours of work, plus sixteen hours of post-processing to assemble the
various scans.
The team used Creaform VxScan to export the
STL file in binary format, and imported the model into Rapidform 2006 and
Geomagic 9.0 for processing and assembly. Geomagic and Rhinoceros were used
to convert the data into STL format, and later into full-scale 2D drawings.
After the data was imported into Rhino, it was divided into parts. The individual
parts were cut into 100-mm layers for designing the 16-mm rebar frame of the
final sculpture (except the head, horns, and ears). Then 2D drawings were
printed from Rhino using 8'-wide vinyl printers to actual scale (1:1).
Since the physical model's underside wasn't
scanned, the CAD model was filled using the post-processing software to make
it watertight. Then it became possible to calculate some critical data, such
as the bull's surface area.
Final Fabrication
The initial plan was to cast the bull's entire
body, but the team faced obstacles such as a limited number of technical foundries
within the region, as well as a limited budget for the project. In addition,
the chief priest preferred to make the metal sculpture by manual labor instead
of with automated tools. Ultimately, the team decided to have the head cast,
and the body manufactured using mild sheet steel to ensure good quality and
maximum lifetime for the sacred bull. The 2D drawings and design prepared
with 3D scanning enabled the fabrication team to adhere to the original intent
as expressed in the wax model.
To simplify manufacturing, the sections were
taken on a single plane (xy, yz, or xz) that were prepared using manual methods.
The skeleton was completed in parts, and was composed of 16-mm rebar rods
mounted on a 50-mm L angle on the outer edge, and in the inner areas to reinforce
the frame. Before draping the 8-mm mild steel sheeting onto the skeleton,
the team put all the parts together to check whether they fit - which they
did.
However, the most important piece was still
missing. Manufactured in a location 500 km from the temple site, the head
was CNC-milled from polystyrene foam, then invest-cast so that the features
perfectly matched the CAD model. The head was cast in seven pieces, which
were welded and assembled on-site. Supporting bars were used to help compensate
for the deformation or warping of the castings, and a 100-mm buffer zone had
been planned in anticipation of the head and torso assembly. This way, it
would still be possible to join the two even if slight dimension variations
existed between the manufactured skeleton and the cast head.
The sacred bull was inaugurated before 250,000
people by the Sadhguru of the temple in February 2009. The temple team met
their goal of honoring their deities with a realistic, long-lasting statue,
while attracting more worshippers.
