Step 1: Materials
- 720 6mm stainless steel ball bearing (I recommend getting plenty more in-case some roll away)
- Two 5x420x720mm and one 2x420x720mm sheets of clear acrylic
- Five 500mm lengths of 6mm stainless steel screw thread (and about 30 6mm bolts).
- About 100 18mm long 5mm thick bolts and nuts (I recommend checking whether the bolts you use leave enough clearance for the bearings to roll free. I ended up using countersunk bolts and drilling a countersink into the bolt holes where need by).
- Strong fishing line
- A couple of meters of elastic resistive exercise tube
Step 2: Design
Central to the design is a method to laser cut bearings. Each bearing contains 3 rings of stainless steel ball-bearings. There is a central ring of 10 ball bearings and an upper and lower ring of 15 ball bearings each. There are a total of 18 of these bearing units in this arm functioning as both pivots and control-line guides. They're not nearly as efficient as ready-made sealed bearings. The advantages are that you can design and make them yourself and modify them to be a hybrid of control-line guide and structural element.
Behind the shoulder joint is a long extent for the elastic part of the control-lines. This is the area that could be modified in future to include any manner of motors able to automatically pull the 6 control-lines.
Step 3: Cutting
Handy Hint: You might find you're missing or have broken a component and need to cut a replacement. If you change the color of all the lines that have been already cut, then you can place new shapes within the sheets you've already cut. I save a new copy of the cutting PDFs each time I cut an additional component.
Step 4: Assembly
Handy Hint: When mistakes happen I found a big magnet really useful for rescuing misplaced ball-bearings.
Bolt the components together as your build. I found it useful to temporarily hold the vertical stacks of bearings in place in nuts on the scaffolding of vertical screw threads. This involves a lot of spinning nuts up and down the screw threads as each bearing is constructed.
Handy Hint: I found a handheld power-tool with a polishing attachment really useful for quickly spinning nuts up and down the vertical screw threads. I used a Dremel (eg: amazon.co.uk/Dremel-Rotary-Tool-Series-Accessories/dp/B000KJRQN0 ) on its lowest power setting.
When all the components are bolted together in place cut away the screw thread scaffolding supporting the two arm joints (I used the Dremel again, Caution: the threaded bolts get very hot when cutting, don't get burnt, and if you smell acrylic starting to melt then stop and let it cool down).
Step 5: Rigging Control-lines and Muscles
Each control-line loops once around each bearing it passes. The loops prevent each line from leaving its guide no matter how the arm bends. The lines alternate between looping clockwise and anticlockwise. This criss-crossing seems to help isolate the effect of each muscle to one joint. Without the criss-cross arrangement the whole arm tends to bend one way or the other and is less able to pull joints in opposite directions for an s-shaped reach.
The rear of each control line is tied to a length of elastic. Knot the lines at a length where the elastic cannot go slack when the arm is bent, and so that the muscles are all the same length when holding the arm pointing straight ahead.
There you have it. A lovely looking bendy arm. The next iteration will refine the design and extend it to include motors of some sort. Let me know if you build the arm and let's put our heads together to build it better.