Arrival

After two days of traveling by bus, plane, and train, we've arrived safely at KNUST in Kumasi. We spent last night in Accra at a beachfront resort, the Afia Beach Hotel, where I discovered that my stomach doesn't agree with Ghanaian food -- or perhaps it's just food poisoning from Delta. One way or another, hopefully the queasiness will go away by tomorrow.

We're all very eager to get to work, and I believe we'll visit the machine shop Monday. In the meanwhile, I've unpacked and settled in. We strolled the open-air markets this afternoon, attempting to source e-waste type materials for further development on the project, and have a few leads to follow up next week.

Pre-Departure

We've been cleared to leave for Kumasi, Ghana tomorrow, following a regrettable but necessary delay beyond the field expedition's control. As such, we'll only be on the ground for about 18 days, instead of the anticipated ~28. While this will make the project quite challenging, I'm looking forward to the opportunity. I've used the last week to continue stateside production of components, and work out a few small details like mounting locations and hardware.

The project's online presence has spread -- while I'm still using the Google Drive repository for my personal development files, David has installed and hosted a wikipedia site for the project, as well as updating his personal page to reflect the project's current direction.

I'm slightly conflicted about the amount of manufacturing done here in the states. I'm keenly aware that, while the methods and tools I've used are comparable to those available overseas, there's no replacement for on-site development. I'm certain, however, that without prior manufacture, I'd be unable to deploy the devices during my brief time in Ghana.

From an engineering side, development has been frozen with Version 0.33. The manufacturing prototype exhibited excellent vending characteristics, with <2% jam rate and <4% double-vend rate. I'm attributing this primarily to improvements to the inventory chute, which also greatly simplified manufacture of the (previously "W" shaped) faceplate. Pictures of the updated design can be found on the wikipedia as linked.

Further updates from Kumasi as soon as I'm on the ground!

Assembly of 0.32, testing

I spent last evening assembling the latest revision of 0.32, the current production prototype. Below are some images from throughout the assembly process:

Cutting new interface plates from a sheet of steel

Cutting the first version of the new interface plate.

One of the major changes was the move of the interface plate from the inventory chute, to the axle/bracket assembly. Along with the front plate, which has been modified to a curved profile, these components help make the axle assembly far sturdier.

Completed "Magic Box," note the 2nd revision of the interface plate, using countersunk screws

 

 

Countersinks.

This technique requires using a thick (~2mm) steel (preferably) or aluminum sheet for the interface plate -- something rigid enough to hold up under the weight of the inventory, as well as smooth and flat enough to vend smoothly. Below, you can see the thickness difference between the thin casing sheet, and the interface plate's sheet steel.

Note the radius of PN008, the front plate. The interface plate, PN003, is resting edge-on in the vending tray lip.

I've also chamfered the corners of PN008, hoping to reduce the occurrences of condoms catching on the plate during vend. This hasn't been too helpful, as testing has shown. I'm optimistic that improvements could be found by tucking the plate under the interface plate -- a kind of tabbed arrangement, that would eliminate any and all edges from the process. I'd hoped at one point to convert PN003&PN008 to a single part, reducing the number of cuts to two cutouts for cam clearance, but requiring a sharp and accurate bend at the leading edge (where the two parts currently meet). Using the thick steel for the interface plate, however, prevents this -- so I've got to find a way to eliminate the corners between the parts which are causing the problem.

This revision also includes the new cam design, which I'm quite pleased with. I'm going to attempt to produce more of the cams over the next few days, and make sure this design will work for production in Ghana.

Upon assembly, I've collected about 100 data points of vending, 50 per chute. Below are the results:


I've also eliminated the gap designed into the inventory chute: by moving the interface plate to the "magic box" and doing this, the inventory chute is now symmetric -- with the exception of the interface bar, the bottom of the "W" shaped section on the face:

Inventory chute, old design. New CAD model to come soon. Note the W, however.

 I've experimented with flipping the inventory chute over, to determine if the interface bar is necessary for reliability. Below you can see the results.

Compare this with the results after reorienting the inventory chute to utilize the interface bar:

There's an appreciable number of double vends and a few non-vends. Those were all caused by a condom becoming trapped under PN008 during vend, which then necessitated a manual clearing of the jam after another failed vend -- thus a single jam shows as two error points. I'm optimistic that improving the PN008 design can eliminate this issue.

Hinge & Latch Design, Ver. 0.32

I've reconsidered the hinge mechanism, as well as some mechanical improvements to the interface plate and mounting system down there. Also set up P/Ns and hole patterns for security fasteners where necessary, and redesigned the axle mount blocks again. I'm too tired to describe in depth, but most modifications are clearly evolutionary and self-explanatory.

Full Version 0.32

Case hidden

Case assembly, fastener details seen below

These are standard tamper-proof screws, there's a variety of head designs available

The interface plate has been removed from the inventory chutes...

...and now forms the top surface of the newly-christened Magic Box.

As is the current custom, I've uploaded 0.32 to the public Google Drive Folder. This version is pretty much ready for production, barring the cam alignment and functionality issues that I'll fight with for the next month.

Cams

I spent a trying three hours in the Student Machine Shop today, attempting to cut out cams from a section of 3/16" pegboard. This turned out more difficult than I'd have expected, as I'd foolishly selected such a dense material that the laser-cutter was incapable of cutting at speed. I was able to cut about half of the laminate sections for one machine, which was enough to convince me that this wasn't saving any effort.

But hey, at least it looks cool, right?

I've got more than enough cams now, however, to experiment with different profiles over the weekend in a controlled fashion. Tuesday is the next opportunity to use the campus lasercutter, but I'll likely cut new cams tonight using traditional methods. The incorporation of the alignment peg has allowed me to improve the cam mounting system as well -- before and after models seen below.

Old cam mounting method, note the professional screw and cotter pin

New cam mounting method, cutaway view

Both cam halves visible here.

 This new design constrains cam motion better, allows installation and removal of cams without tools or permanent modifications, and reduces each cam location dimension to a single cotter pin mounting point -- it looks more complex, but simplifies the installation and construction of the cams greatly. I'm also very optimistic about a split cam, as opposed to a solid cross-section. I believe that this will increase pressure at contact points, increasing the likelihood of non-slip frictional contact there.

0.32, Oh god it's June 5th already

Weeks stretch into months, as usual. With finals week(s) and a vacation afterwards to return to human, I've finally gotten back to work on the project in earnest this week. I've worked out a number of small issues which Dr. Eglash, David, and I identified last semester -- most on paper and a few still in my head. The last outstanding issue, as always, is these cams. I've realized that the leading edge, the highest point on the cam, is extremely critical, as it has to catch -- not slip-- in order to vend.

Guaranteed not to scale

I think I've been shooting myself in the foot with designs incorporating a fillet on that notch. I've redesigned the cam, scaling it up by 7%, reducing the fillet to 1mm, and incorporating an alignment peg system to allow lamination of variable cam width from on-hand material -- I see no reason this wouldn't work as well with lasercut scraps of underutilized, burning plastic from the Accra scrapyards, but that's another project.

 I'll be testing this new design tomorrow, hopefully all will work out well. Apart from a few little things, like moving the hinge to the side and incorporating riveted box design, the system will be essentially set to manufacture -- the next unit made will be a production model from here.

Version 0.31 update

I've been working on preparing the design for production this summer, as we've now hit 10 weeks until I leave for Ghana. I've re-packaged the CAD model (Dragon 0.31) and uploaded it, but more importantly, the Google Drive Repository now includes PDF blueprints. This effectively marks the release of the design for public development, and I hope to implement more detailed assembly instructions and imagery as the design continues to evolve.

I've been working on assembling a copy of 0.31, using "basement power tools" level equipment. I'm not entirely satisfied with the system, particularly with the supports for the cam assembly. I'll be redesigning those and releasing a new update (0.32 probably) within the next week or two. However, I have assembled a case and functional front access cover, which exceeded my expectations. By using a sheet steel and end cap design, I've been able to reduce the thickness of sheet steel to 28ga  -- about 0.3mm. This has brought cost down by an order of magnitude, and makes cutting and bending components far easier.

While the incorporation of plywood has eased assembly and reduced the complexity of the model, I feel that there's still improvements to be made. I'm interested in replacing the endcaps with sheet metal angle brackets, perhaps riveted. I'm slightly concerned about accessible screws on the outside of the device, and using rivets would also solve that -- while I've designed the case to be impossible to access without access to the rear or the padlock, I'm concerned about encouraging petty vandalism with exposed Phillips Heads. Rivet guns are cheap and readily available in Accra, I'll want to make sure both options are possible. There's also currently unused scraps created from cutting out the case, I'm interested in reducing material requirements by using those.


I've done some initial hunting for sheet steel and other suppliers in Accra, and the results are promising. While I won't know for sure until I get there, it looks like many materials should be easier to find than I've feared.

Here's some pictures of things and such:

Face of inventory chute (PN001), cut from 23/32 plywood

Inventory chute (PN002), also from 23/32 plywood this time

Drilling the mounting holes in the case. Note shameless product placement.

The case was cut by printing the pdf blueprint at 1:1 scale, rubber cementing it over 28ga sheet steel, and using tin snips to cut along the lines. Pretty painless. All blueprints are also fully dimensioned, allowing traditional layout methods as well, if a printer isn't readily accessible.

Here's the case and inventory chute inside. Note the lock mount, and two hinges. These aren't in 0.31's CAD models or blueprints, but I'll incorporate them in 0.32.

Mostly closed. The 40x150mm slot for the customer to recieve the product hasn't been cut yet, since I'm still adjusting the design of the cam mechanism and the location may change slightly.

Case, in open position, and removed inventory chute. The blueprints are still on the outside of the case, since they look cool and I'm too lazy to remove them. It's also much easier to mark on paper than the steel underneath, and protects the steel from scratching during assembly.