The production of the legs was a special challenge, since they represent a majority of the "visible" statics. While there are many opportunities in the body and feet to cover basic wood or metal constructions, the legs offer only a few opportunities. I didn't want visible junction points such as screwed or riveted metal sheet, which must cover the basic construction.
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I manufactured the master model from MDF and polystyrene. |
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The polystyrene sheets were roughened on the back with sandpaper and glued with power glue (Pattex) onto the MDF board. |
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After the completion of the master model I manufactured a silicon mould. |
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Production of the plaster supporting mould. This should be performed with a partner. |
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The master model, the silicon mould and the supporting mould (here however, the GRP "reserve supporting mould" is shown). |
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This huge voluminous part can only be poured in separate layers, because otherwise the thermal activity would destroy the part as well as the silicon mould when hardening (overheating!!!). I increased the strength by introducing 4 layers of reinforcing fabrics, here seen as the red fabric. |
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Basic structure from fibre board and MDF. |
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Fairing with polystyrene. |
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Protruding polystyrene edges can be smoothed accurately with a file or sandpaper. |
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For those people, which want to copy THAT: Omit the grey "structures", the middle part of the Ankle cylinder, as well as the part above and below and make separate silicon moulds. The illustrated part is extremely difficult to pour and to release. Also the production of the casting mould for the silicon is very complicated. |
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When looking at the silicon mould it becomes clear, how difficult building this casting mould was. |
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Since this part contains extremely deep undercuts, I worked here with a two-piece silicon and a three-part GRP supporting mould. |
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In order to test its strength, I carried out a "break test" at the weakest point. I manufactured a partial casting of the Ankle and at the lower end 2 layers of reinforcing fabrics were included. I used a pair of scales in order to determine the exact breaking point. My 60 kg was not sufficient to break the part. The scales showing 68 kg at this time. The test set up weighed 8 kg. When the weight was increased by around 2 kg, with the scales showing 70 kg, the part broke. In my opinion that is strong enough. I nevertheless inserted 6 layers in all future legs. This is, thus, three times stronger than the “break-test” and means that a break is practically impossible here by "normal use" of a R2-D2. |
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Creating
curved parts succeeds best, if sandpaper is used as a template nearly on
the same radius, like the radius which would be reached. |
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The polystyrene sheets used had a strength of 3 mm. |
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Without the appropriate machines it is much simpler to manufacture only the first and last layer in the "horseshoe form" and the remaining parts separately. |
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Indispensable assistants |
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Large slots and recesses in parts should not be filled completely with silicon, so that the part can be removed more easily. You need some space to loosen the silicon mould from the part. |
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The "bottom" of the openings were only lightly fastened during the silicon mould production in the master model. Therefore it was possible to manufacture two mirror silicon moulds with only one master model, one for the right and one for the left side. |
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The first "unification" of the individual parts. Leg and Ankle consist of PU and the foot from GRP. Booster Cover and Shoulder on this picture are master models since the parts had not been poured at the time the picture was taken. |
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