BY ADAM GRACE
The first APU part to be made was the gun. It was required for filming in the USA shortly after the props manufacture workshop was set up, and before final APU designs has been completed. A team of modelmakers and moulders quickly manufactured patterns from Sergei’s drawings which were moulded in silicone rubber and cast in fibreglass and urethane resin. Additional structural components were machined from aluminium and steel. From the moulds two lightweight fiberglass guns were built for the APU.
The next item to be made was a 1:5 scale model of the left side of the APU. The APU was essentially symmetrical about the vertical axis and so only one side was required. This enabled Martin to fine tune design details taking into consideration the engineering requirements that the full size APU would need. The model was completed at a later date to be used as an aid in planning and setting up shots.
The APU had a steel “skeleton” covered by fibreglass and ABS shells with a high density urethane foam bonding the shells to the frame. Steel was chosen for the internal frame, offering the benefits of ease of manufacture and maintenance, with structural strength. Joints typically involved a 25mm steel shaft linking 60 – 80mm steel bosses with a nylon spacer. Bolts on the outside faces could be tensioned to lock off the joint if required. The waist used a tapered spherical bearing to allow maximum ease of rotation.
As the frame was being manufactured, the various shells and components were made. There were two main processes used. Patterns of parts were either moulded and reproduced in fibreglass or used as vac-form patterns to produce ABS plastic shells. The patterns were generally made from jelluton: a close-grained lightweight timber, MDF: a composite timber sheet, and “chemiwood’: a dense urethane resin compound. Allowances were made in the patterns to include “draft angles’: a slight taper on vertical faces to enable the withdrawal of the finished mould or vac-form shell. The shells were attached to the frame by locating the joint bosses in the shell, sealing the seams and pouring in a two-part urethane foam. The parts were supported by timber panels to stop distortion until the foam had set.
Parts such as the links in the toes and shoulders, and the carriage trusses were produced in cast aluminium. This process required a pattern to be made a small percentage larger to allow for the shrinkage that occurs when casting aluminium components. Using cast aluminium parts meant that an inner steel frame encased in a shell would not be necessary. The pattern would still need to be made but manufacturing time for the finished component would be reduced. Moulds of finished patterns were made from fibreglass, or if too complicated or small, silicone rubber was used. Small parts were cast in urethane resins.
The method of frame manufacture, pattern making, moulding, reproducing and fixing to the metal frame was determined by the requirements of the particular component and its function on the APU. Virtually all of the components were constructed so that they could support the weight of an actor climbing around and over the APU. Every item required the foreman and engineering foreman to discuss and confirm the structural needs, the pattern and moulding requirements, how the shells were to be produced, and how the finished parts would assemble. Filming requirements and VFX and art department requests were co-ordinated with Art Director Jules Cook. The details were then passed onto the personnel involved in the engineering, pattern making, moulding, and finishing of all the APU components.