What is the design goal of the new foot?
Good question. 
The short answer…
The design goal for this project is to implement a foot that can engage with better contact on the terrain it encounters.
I wish to study the way in which the foot will react when engaged.
Force sensors will be the next stage!
As the foot will have more contact with the ground I’m thinking it will allow for better control when walking incline.
And also the tibia will only have one single point of contact to the foot, so it wont compromise its performance.
I hope this has answered your question?
What’s the ball and shaft diameter, and the shaft length?
Steel parts? Something easily found?
Alan KM6VV
all i know is:
its rust proof
max mounting weight is 3KG
base diameter 65mm
length (fully extended) 100mm
thread diameter 6mm
you can get them from local security stores, ebay for £1.99 or Google: MOUNTING BRACKET FOR CCTV CAMERA
not yet got my order for the parts, but as im waiting i have done a short vid showing xans great work from his phoenix v2.0.
youtube.com/watch?v=mgktlqBxff8
shortly after this vid was taken i took it back inside and re-calibrated the INIT pos as it wasnt perfect. apart from that im now just waiting for parts.
ok well parts arrived…
but i sent them back… 
the item described was not what they sent. apparently they dont stock that part anymore and the company they get them from upgraded it to the one they sent me, without telling me it was different.
anyway im still searching…
anyone know where i can get a Low fiction Ball joint like this?
http://i531.photobucket.com/albums/dd355/innerbreed/Capturevhutj.jpg
p.s iv already asked matt!
Any bigger picture of that ball joint? Maybe we can work up a drawing?
Alan KM6VV
The foot is attached to the tibia via an 8mm ball (A) which locates into the upper foot socket (B).
This upper foot socket is then attached to the lower foot plate © via two ball joints to form a hinge between (B) & ©.
There is a PCB (D) mounted to the top of (B) which has a micro switch (E) soldered onto it.
There is also a retaining plate (F) screwed to the top of (B) to hold (A) in place.
At the front of (B), there is a M3 grub screw which pre-tensions the micro switch and stops the hinge between (B) & © from opening too far.
The spring between (F) and the tibia base re-aligns the foot when it is not in contact with the ground.
Supplied by Matt Denton.
ISO view
front view
Sounds like a patent application!
Matt has definitely put some thought into this foot. It reminds me of a digitizer probe for CNC. Here the idea is to be compliant in three dimensions, and to open a switch at the slightest contact. Uses switch contacts orientated in a triangle. I might explore that idea more.
What’s the approximate width/length of this foot? Does Matt sell them?
Thanks for the PIX!
Alan KM6VV
it does doesn’t it. all the information given is here so if i find out ill let you know. they are currently not for sale, but i think he is happy for the design to be issued. switch contacts orientated in a triangle is a very clever idea i thought.
a very smart way to make sure there is contact on every step.
i think this design would be quite easy to replicate.
of course not wanting to copy it but the triangle idea should differently be explored.
Looks like it would work great in the dirt, mud and grime of the real world… 
been looking at these.
HD Ball Link w/Lock 4-40(Ea) - Bulk Pack Of 6
http://www.wheelspinmodels.co.uk/products/ripmax/f-sln562_1.jpg
That’s an interesting part. And a good fit of a spring. The spring is often the most difficult part to “design in” on a mechanical project. I hate springs!
Alan KM6VV
Agree, you pretty much have to custom design the spring to work with your custom design, or, design your part around the spring. 
What we need is a 74LS series for springs!
Alan KM6VV
There are standard stock springs that are designed using a specific wire type, gage, OD and thickness (number of turns). There are charts showing the force provided by each style of spring. I spent the weekend working on an SES tubing ground contact leg switch. I think I have it! I will start a new thread with drawings illustrating what it looks like. 8)