Ball valves are great for projects, but I'm always a little floored by the price.
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Like $15 bucks for 5 cents worth of plastic? Really? And my idea calls for... about 30 of them so...
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NOTE: This is currently work-in-progress and will almost certainly leak like crazy. See below more details.

UPDATE 01/16/2019:
1) Can now generate individual parts in "best" printer orientation
2) Made the math for seals work better
3) Can now select resolution

INSTRUCTIONS:
1) print the ball and 2 seals with max resolution and sand to mate
2) other parts can be less precise
3) Insert both seals, then the ball and handle. Glue housing insert in place last of all.
NOTE: a better strategy might be to buy some cheap seals and design the valve around them!
NOTE: the main chamber and the ball will probably require supports
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TODO:
1) Add fit tolerances
2) Pipe attachment (see http://www.thingiverse.com/thing:436659)
3) O-ring for handle shaft. Right now it will likely leak! (see http://www.thingiverse.com/thing:225603)
4) Handle attachment to ball
5) Add servo alternative (again, see http://www.thingiverse.com/thing:436659)