Merge branch 'patch-1' into 'master'

ussm3

See merge request jakeread/ussm!1

README.md

 # micro Stress vs Strain Machine
 
+## uSSM #3 
+
+![img](images/ussm3_chassis.jpg)
+
+Theres a few new criteria that ussm-3 aims to achieve:
+- Manufactured only using simple shop tools, PLA 3d printers, and a standard bed size (24'x 24') laser cutter. 
+- Pulling force of around 600N
+
+In order to be laser cuttable, acryllic and delrin were the main two materials considered. Delrin was the go to option to avoid shattering.
+
+This design uses mainly the following adapted gantry system: [gantry](https://gitlab.cba.mit.edu/jakeread/rctgantries) along with this beam design: [beam](https://gitlab.cba.mit.edu/jakeread/rctgantries/tree/master/beams) for rigidity.
+
+The main idea is to attach a beam to the linear axis and use it as a carriage to hold fixturing for tensile testing on the top half and compression testing in the bottom half. The gantry system shown above does not supply enough torque, so an adapted version was made shown below.
+
+![u3_carriage](images/motorcarriage_vertical.jpg)
+
+The uSSM #3 design takes advantage of the beam design by attaching 4 different beams to a larger "O-face", with webs in the corners to prevent torsion. The larger face itself is split into a bottom and top in order to make assembly easier, make each piece smaller to fit into laser cutter bed, and to have the possibility of changing top for larger specimens. The beam design also utilizes custom joinery talked about in the [beam](https://gitlab.cba.mit.edu/jakeread/rctgantries/tree/master/beams) repository. This makes the machine take some time to build, but it allows delrin sheets to be used, allowing many fabs with laser cutters to be able to make this machine.
+
+![u3_corner](images/uSSM3_corner.jpg)
+
+### fixturing
+Fixturing is being designed to use mainly 3D printed parts. First up, PLA is being in the jaws to see if it can withstand the 600N load alongside a few other McMaster parts and this [load cell](https://www.amazon.com/Pressure-Force-S-type-Sensor-Cable/dp/B01F6IOWDG/)
+![u3_fixturing]( images/USSM3_fixturing.jpg)
+
+
 ## Drawingboard Return
 
 With \#2 feeling somewhat unloved ('both overdesigned and underdesigned'), I'm back at the basics for \#3. There's a few major selections, and decisions to make: