housekeeping, and add beams

.gitignore

 **.3dmbak
 **.rhl
-**drawing/
\ No newline at end of file
+**drawing/
+**cam/

README.md

@@ -4,50 +4,60 @@ These are machine design elements that we use at the CBA when we're spinning up
 
 ## Machines Using RCT Gantries
 
-[Little Rascal](https://gitlab.cba.mit.edu/jakeread/littlerascal)  
-[Mother Mother, a generalist](https://gitlab.cba.mit.edu/jakeread/mothermother)  
-[Claystack, a ceramics 3d printer](https://gitlab.cba.mit.edu/jakeread/claystack)  
-[the Madison Park Vocational Machine, for high schools](https://gitlab.cba.mit.edu/jakeread/mpvmachine)  
+[Little Rascal](https://gitlab.cba.mit.edu/jakeread/littlerascal)
+[Mother Mother, a generalist](https://gitlab.cba.mit.edu/jakeread/mothermother)
+[Claystack, a ceramics 3d printer](https://gitlab.cba.mit.edu/jakeread/claystack)
+[the Madison Park Vocational Machine, for high schools](https://gitlab.cba.mit.edu/jakeread/mpvmachine)
 
-# Usage 
+# Usage
 
-This repository should help you figure out how to build your own axis: it contains some explanatory drawings, videos, as well as CAD files and Bills of Materials for various design elements. 
+This repository should help you figure out how to build your own axis: it contains some explanatory drawings, videos, as well as CAD files and Bills of Materials for various design elements.
 
-A typical machine-level workflow is described [for machineweek, at this link](https://gitlab.cba.mit.edu/jakeread/machineweek-2018). 
+A typical machine-level workflow is described [for machineweek, at this link](https://gitlab.cba.mit.edu/jakeread/machineweek-2018).
 
 ![](video/SEQ-fusion-parametric-enc.mp4)
 
-First, find the model you'd like to start with on this page and download it, or clone the whole repo. The ```.f3d``` files are parametric Fusion 360 files. Files here are tuned for material thickness choices - but the exact thickness of your material can be tuned in the model: i.e. the 0.375" HDPE model will also work for ~ 8 - 11mm HDPE if you're working with sensible units in another country. 
+First, find the model you'd like to start with on this page and download it, or clone the whole repo. The ```.f3d``` files are parametric Fusion 360 files. Files here are tuned for material thickness choices - but the exact thickness of your material can be tuned in the model: i.e. the 0.375" HDPE model will also work for ~ 8 - 11mm HDPE if you're working with sensible units in another country.
 
 In Fusion, you can open this file up and use (from the top menu)
 
 ``` Modify >> Change Paremeters ```
 
-Each of these models should have some parameters starred, these are what you'll want to configure. Go ahead and set axis lengths and material thicknesses according to what you're doing. 
+Each of these models should have some parameters starred, these are what you'll want to configure. Go ahead and set axis lengths and material thicknesses according to what you're doing.
 
 There are also two sets of hole patterns in the last two 'groups' of features on the feature history bar. One puts a 20mm square grid on top of the axis, the other on the bottom. You can leave these in, or customize them, or add whatever mounting-to-the-next-bit design you'd like.
 
 When you're satisfied, you can export the model as a .step file, using the file menu, to prep it for fabrication.
 
-``` File >> Export ``` 
+``` File >> Export ```
 
 Make sure to change 'type' to .step, and check the 'save to my computer' box. Now you're ready to import it into another assembly.
 
-**Alternately,** you can save the changes to a new file and import that model into another Fusion 360 Assembly. 
+**Alternately,** you can save the changes to a new file and import that model into another Fusion 360 Assembly.
+
+# Material Choice
+
+ - bring over notes on material choice
+
+ - acrylic, cool, but req' move to heat-set everwhere: cannot tap cannot heat-ineret (very well)
 
 ## The Blocks
 
-### [N17 Linear and Pinion-Type Belt](n17_linearPinion/)
+### [N17 Linear and Pinion-Type Belt](gantries/n17_linearPinion/)
 ![n17lp](n17_linearPinion/n17-assem.jpg)
 
-### [N17 Linear and Pulley-Type Belt](n17_linearPulley/)
+### [N17 Linear and Pulley-Type Belt](gantries/n17_linearPulley/)
 ![n17pp](n17_linearPulley/n17pulley.jpg)
 
-### [N23 Linear and Pinion-Type Belt](n23_linearPinion/)
+### [N23 Linear and Pinion-Type Belt](gantries/n23_linearPinion/)
 ![n23lp](n23_linearPinion/RCTN23-alu-img.jpg)
 
+## The Beams
+
+### [Torsion Ain't Easy](beams/)
+
 ## Bonus Kit
 
 ### [E Chain with Tape Measure Backbone](tapeChains/)
 
-![tc](tapeChains/tapeChains-2.jpg)
\ No newline at end of file
+![tc](tapeChains/tapeChains-2.jpg)

beams/README.md

+# Beams for Machines
+
+Torsion ain't easy, as they say. In a break from a bonafide software extravaganza, I've been walking through this design for a decent beam from flat stock, to back on RCT Gantry types.
+
+The beam design is straightforward, but I've also been occupied with joinery. A big driver here is the desire to make machines with laser cutters and acrylic, to really lower the threshold for fab-your-own-machine-ing. To that end, joints in acrylic that don't cause cracking has been the order.
+
+Typically, laser cut acrylic members are joined a-la this classic machinescrew-and-captured-nut joint:
+
+~ pic of makerbot type joint ~
+
+These are overdue to crack at the stress concentration around the nut:
+
+~ microscope pic of your nut beam ~
+
+This is largely the fault of Acrylic, which, despite its low cost, relative high stiffness (~ 3GPa Modulus), and ease-of-lasercutting (about 50W machine can chew through 7mm OK), is very brittle.
+
+To handle this, I was interested in developing some 3D Printed load-distributing members. I went through a few designs, settling on something like this:
+
+~ pic of joint ~
+
+There are 'singlet', 'tee' and 'corner' members, each having a heat-set insert in the rear, and some arms to grip the reciprocal laser-cut feature. I'm a minor fan of these so far, but I think the design can still be improved. In general, I'd love more algorithmic generation - a mechanically sound method for generating 3D Printable 'node' joinery based on pin-frame beams is a bigger desire.
+
+Beams made up of these joints feel pretty satisfying, although the subtle 'creak' of the acrylic tells me they're not 100% quite yet. However, serviceable.
+
+## Usage
+
+**Material choice**
+
+Like I mentioned, these are sort of designed with Acrylic in mind. That said, moving to Delrin (Acetal) will up the overall durability for a similar modulus, but also double your cost.
+
+Moving to aluminum will similarely double your cost, and put you in a new fab-bracket (I use a waterjet), but brings your beam from 3GPa material -> 60GPa (also ~ 2.5x density). With aluminum, this stress concentration issue isn't so controlling, and the 3D Printed joint effectively adds a relatively-spongey moment in the structural system, so, with complete time, different joinery should be developed for metals. Alas, it'll do as is.
+
+**Be careful about**
+
+- thicknesses: the 3D Printed joints are *much* happier going together if the arms are dialed in to pinch on to laser cut members, so if you're going to assemble lots, test a pair first
+- beams on pulley'd axis: there's not room (at the moment) to accomodate for the pulley's return path. when I spin these through CAD->CAM, I add that accomodation manually (in Rhino). this is much easier with wider beams, as is planned for the [little rascal](https://gitlab.cba.mit.edu/jakeread/littlerascal) machine.
+
+**Doing it**
+
+Apart from that, download the .f3d model (should be in this repo, when the update comes), set your parameters, and las / print.
+
+My workflow is like this:
+
+ - make parameters for material thicknesses in beam, and gantry model
+ - export both .step
+ - (if a pulley) bring both into rhino, modify ears to suit, and add pulley allowance to beam webs
+ - las / wj / 3dp
+
+## Todo
+
+### Completion
+
+when acrylic arrives, laser and test assemble, photograph, document.
+there's a temptation to update the carriage(s) so that beam-face machine screws can rest non-flush on the surface. this makes assembly much easier
+for littleRascal, complete the bupdate:
+ - 110mm wide beam, having belt accommodations
+ - beam ear accommodations: motor and idler
+ - beam support, and support accommodations
+ - the same for cable accommodations
+
+### Documentation
+
+nothing is here yet - add
+ - f3d and step for beams
+ - f3d and step for bmhws (beamhardwares)