Thursday, September 13, 2007

Redesigned hip structure with partial assembly steps

It's been only two or so months of service, and sad to say, the hip structure I designed started to show structural wear. Originally a ball joint approximation with hinge structures, it wasn't user friendly in design, and required a lot of proper manipulation to pose properly. This was due to the lack of proper ball joint design.

There are a few issues with ball joints made of paper/wood derivatives:

1) Making a uniform sphere. The best thing you can get is an approximation of a sphere by rolling a length of paper around a shaft, with some trapezoidal cross sections. For this exercise, I needed small spheres, otherwise, the housing would exceed the desired dimensions. The older unit used a 20mm diameter circular plate to flesh out the hinge area. I decided to use an 8mm diameter sphere, created by wrapping an 18cm long x 8cm tall strip of 110lb cardstock trimmed to taper down from 8mm to 2mm wide around a wooden bamboo skewer for structural support.

Two spheres were made, and placed on the ends of a 32mm long bamboo skewer. The spheres are force fit and removable, for later disassembly and maintenance.

2) Making the housing. The housing needs to be strong enough to support the outward force of the sphere, restrain the sphere in all directions except for rotation, allow for movement, while keeping enough friction to allow for posing the leg without need of external supports. It also must not occupy too much volume. For this, I'll need a strong material. The first housing consists of two 15mm x 87mm Macetail Hystrodon card sections rolled around an 8mm diameter cylinder as a guide. These strips were pre-sanded to allow for maximum adhesion, rolled, and form an outer diameter of 12mm. The bottom housing was done in the same manner with an Abjure. One 4mm wide by 7mm long cut was made down one housing, while the other was cut to fit around the other, making a crude T-Section pipe.

These are the end results of the housing manufacture process.

The design ultimately ends up with an 8.3mm sphere fit inside a 8mm inner diameter tube. The housing will bulge outwards a bit due to the force fit, but this will be corrected by adding the rest of the assembly, namely the end caps.

The next element to build is the lower leg attachment assembly.
This piece was rolled around a 36mm long bamboo skewer to form a stepped rod. this was inserted into the housing bottom, and the rest of the Macetail Hystrodon and Abjure card scraps (about 30mm x 87mm) were rolled along the thinner end to form the pin housing.

The next step is to thicken the legs. Random pink cardstock was used to flesh out the legs as needed, and the 110lb white cardstock was used to cover the upper part of the knee assembly.

The red bag like objects are made of rolled cardstock tubes glued around a straight 110lb cardstock cylinder over the thigh assembly. The top part has the end caps in place, made of a Malicious Advice. It is composed of a 5 walled box with a slit cut through the center.
This piece has a 14x4mm slit in the middle to allow for 90 degree movement, and was placed on the end of the housing. I've trimmed the box to fit better in the space allotted, so the actual dimensions needed are unknown. After the end cap was placed, it was glued and secured with layers of cardstock to hold it in place. Then the process of fleshing out the leg was freehanded with simple primitives covered by a tube of 110lb cardstock. The end result was sanded and painted.
The legs actually look better since it's smoother, shows less mechanical elements (the screw access holes for tightening), and hides nearly all of the elements that make it work.Here is a finished shot of the legs in action. These hopefully will last longer than the original legs.

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