Hinge detail. I messed up a bit with the mortise widths, so I cut shims from scrap walnut and sanded them flat. The hinges for this one also open 180deg per giftee's request (they turned out so nice I'm thinking of modifying mine to open flat too)
Putting finish on the inside. I masked the pockets because I'm putting felt in this box (glue doesn't stick well to polyurethane.) I'm using satin polyurethane because it offers a marginally better protective layer than oil finishes, and it keeps enough of the grain texture to avoid looking awfully glassy.
Pickles are various kinds of fermented or refrigerator pickles.
(avoiding botulism in canned pickles is too much effort to bother with)
I cut the radishes into flowers for one of the carrot-radish jars, but the small red radishes were a little too delicate and most of the slices lost their petals in the process.
Still working on box#2! (I borked my leg and picked up a dining-table project, so this is still ongoing)
This means I have the opportunity to take more photos of the steps I missed last time.
After milling out the calipers profile, etch-a-sketch style and seating the tool in the bottom half, I traced out the remainder on the top blank. This one needed a void for the round bezel-dial and not much else.
Here I'm also matching up the ends to mark where the hinges go.
Milling out the bezel void with a ball endmill for extra aesthetics. Hinge and magnet mortises happened afterwards.
I was lazy and decided to lasercut all my inlay pieces. They are 3/32" thick and would've been a pain.
Finished pictures first. I completed the first of my set of calipers boxes (this one was the guinea pig version) and learned a lot about RealPersonWoodworking! The wood is domestic walnut, with cedar inserts on the inside.
This is the Kinematic Coupling Pen! I'm showing the finished photo first, after three coats of walnut oil to bring out the cocobolo sheen. The metal bits are steel, and the pen cartridge is the same as the one in my brass pen.
This completes my 2.750 Kinematic Coupling Lab, and here is my writeup for the class. The following blog post goes more into detail about fabrication and test setup, and is slightly more handwavey about analysis. It otherwise has the same info.
Fabrication of the kinematic coupling started with steel round stock turned to a final outer diameter and a 6mm clearance hole for the pen cartridge. I bandsawed the shaft into halves after turning diameters to ensure they were a matched set, then faced them flat.
Both the socket half and the grooved half used the indexing head on the mill to align features at 120deg angles. The 45deg V- grooves were made with a small endmill followed by a 90deg countersink (start with the endmill for stress-relief!), and the hemisphere sockets were made with a 1/8" ball-endmill.
I then turned some alignment offsets in both halves, but didn't really plan how I was going to cut my parts off the excess stock. Sloppy bandsawing and attempts at salvaging parts left my alignment offsets significantly shorter than I had hoped and my actual kinematic coupling (KC for now on) parts really dinged-up with tool marks.
I cleaned up by first press-fitting a 1/4" dowel into the 6mm hole, then gently filing away the tool marks by hand. This means the parts aren't perfectly circular anymore, but you wouldn't notice unless you spun them at high speeds. Kinematic coupling part of the lab now completed; moving on to the pen itself!
The wood phase of this project took place at the Hobby Shop, because I needed its wood lathe, dust collection system, and expert advice.
Expert advice: Hayami is an awesome guy. I show up with a block of pain-in-the-ass wood, and he showed me everything I needed to complete my pen. He even set up an additional orientation period when I completely forgot to show up to the first one.
Dust collection system: Fun fact, cocobolo dust is super allergenic. You are highly likely to develop skin rashes from prolonged contact, and it's toxic if you breath it in. The wood itself is fine, but the dust is yikesy stuff. Hobby shop gave me a personal respirator mask, and I ran a vacuum at every machine station I touched. I'm pretty happy to say that I haven't developed an allergy to the stuff yet, because I still have plenty of pieces left.
But let's talk about the wood lathe. I was pretty happy to finish the actual kinematic coupling part early on, because then I had an excuse to spend lots of time (2 weeks) learning woodturning. My 2x4 scrap of cocobolo got bandsawed to two rough rectangular pen-blanks, then I found the rough centers and drilled a 1/4" through-hole.
The 1/4" through-hole served two purposes: one, I needed a through-hole for the pen cartridge; two, the Hobby Shop pen-turning kit consists of a 1/4" bolt and a thumb-nut that clamps the workpiece to the lathe.
The woodturning process has three parts. First, the pen blank is made cylindrical and roughly even using a scallop-shaped rough gouge. Then, a hemisphere-shaped gouge turns the proper outer diameter and shapes contours. Finally, a parting tool forms tongues for alignment with the pen tips and cuts my parts off the original stock.
After the wooden parts were done, it was back to MITERS for the steel pen-tips. The tapered front was made using similar methods as the brass pen, except this time I didn't bother dealing with cutting threads in steel. Drilling tiny through holes was bad enough (I broke 1 drill bit and 1 centerdrill in the process, but 3rd try is the charm!)
Once all the components were complete, they were glued together with 30min epoxy and brought to 2.750 show and tell the next day! Following that, I sanded all the components' edges flush and cleaned off all remaining epoxy residue. Officially finished!
Of course, there's still science to be done. I evaluated the kinematic-coupling part of the pen with two tests: a repeatability test and a stiffness measurement. In both cases, I wanted to better simulate a freely-waving pen solely held in place with magnets, which meant I had to avoid standard laser pointers (since I'd be doubling the weight of my pen!) My test setup consisted of a disassembled laser diode taped to the top of the pen in an attempt to add as little weight as possible.
An aside about this laser diode - it's a really fantastic 400nm blue experimental laser diode
Bayley let me borrow so that I can eventually make a shiny knurled case for it for science,
where the normal application is for something like ridiculously-specific X-ray equipment.
But more importantly, the laser diode emits a very beautiful blue hue.
Given my contact surfaces of steel and neodymium alloy, I expected both high stiffness and high repeatability, so my test setups required magnifiying small errors over long distances - Abbe Error!
image credit: Matt Rosario
Basically Abbe error (sine error) takes advantage of magnifying angular error over distance to make observing tiny errors easier. In my case, that meant clamping my pen in the mill and observing laser pointer errors 24.5 feet away (giving me a magnification factor of 840:1)
I got an average error of 8μm, which was the expected order of magnitude for a steel-steel manually-machined object. I could have probably cut down on error if I had invested more effort than just taping the laser pointer on, or if I had been more careful machining the ball sockets in the kinematic coupling (one is noticeably shallower than the others)
Setting up laser diode for repeatability test
Laser pointer is blue dot in center of blue circle
For measuring stiffness, I used the same laser diode and attached it mostly parallel to the bisector of the magnet-triangle, and pointed it at a wall 15' away (528 magnification factor). Then I pushed on it perpendicularly with a force sensor (approx. 1N) and measured laser error on the wall.
I got an average deflection of 3.5μm after 5 trials, which resulted in a measured stiffness of 0.30 N/μm. This was twice the stiffness predicted from the kinematic coupling spreadsheet (0.14 N/μm), but is pretty close. Discrepancies could come from many sources, ranging from assumptions in the spreadsheet to again user error with the laser pointer, so it's unclear how closely my pen follows the theoretical model.
Finally, I've been doing some user testing over the past week. It writes! Unfortunately, due to the size of the kinematic coupling it has the ergonomics of an expo marker. The size of the pen threw me off initially but I got used to it eventually.
Additionally, the magnets used in this pen are not so strong that they perfectly resist my hand, so the back of the pen wobbles a bit if I write too quickly or squeeze the joint. This means my hand is located further down the pen than it normally would be when writing with an expo marker or a different pen.
I also experienced the fun thing where the tip of my pen cartridge is also steel and will therefore adhere to magnets, so I have to take care when removing the ink cartridges. The pen cartridge-tip gets press-fit in the pen, and the first time I tried pulling it out I accidentally separated the ink reservoir from the steel tip. Ink got everywhere! So, in order to remove ink cartridges I have to tap the pen-tip with the steel back of the pen.
A happy side-effect of the pen-tip getting press-fit within the pen is that the pen-tip stays put under normal writing conditions, even though there is space in the back of the pen. That means I don't need to make a cap for this pen, and can instead tap the writing-tip inside the pen-body if I want to store it in my bag.
Woooooaaah, new stuff.
Our first lab in 2.750: Medical Device Design was a refresher in mechanical engineering fundamentals - designing, calculating, and validating measurements for a kinematic coupling.
Kinematic couplings are really cool - a simple, deterministic way to prevent things from wobbling or be otherwise imprecise in all 6 DOF. One of the professors for the course is really into kinematic couplings, and so of course we the students needed to share in the joy by making some ourselves!
I have to build more gift-pens anyway, so I thought it would be cool to magnetically connect the two halves via a kinematic coupling using buckyball-magnet-toys.
We'll see how my design performs in real life... in ~3 weeks :P
Okay, maybe not that legitimate. But closer on the spectrum.
I started by waterjetting flat patterns from 0.5" 7075 MITERS stock,
originally from JACD and Battlebots 2015!
(they stuffed the plate in the back corner, for reasons I'll get to later)
Why five?
Two are holiday presents, but the quality of a carabiner is dictated by how it performs under load. I will analyze the performance of these carabiners by doing experiments with the three sacrificial biners, and will also use them to figure out how these things get put together.
Below is one of the nice carabiners, currently getting engraved on the newly renovated MITERS CNC mill.
Why 7075? Compared with normal 6061 aluminum, 7075 has significantly more tensile strength while keeping similar density and shear strength. This is important in a carabiner, where the strength/weight ratio means everything. Below is a comparison of the two materials, taken from Makeitfrom.com
selected graphs from MakeItFrom.com
However, 7075 only exhibits these qualities at T6 temper. The plate in the back corner was T0 (un-heat treated) and was therefore fairly useless -- it was so soft you could dent it with your teeth.
My process for taking 7075-T0 to T6 went as follows:
- Heat to 900°F for 2 hours
- Immediately quench in water until room-temperature
- Heat at 250°F for 18 hours
Proper heat treatment of alloys is a somewhat precise operation, one I couldn't achieve during this project. For starters, the kiln and kitchen oven I used for heat treatment had a large degree of error. Second, proper T6 requires parts to be strain-hardened while in the kiln but mine were not. Experiments later will show how this imperfect method compares to the datasheet.
I was aware that heat treatment would distort my parts, but I didn't expect the bubbly surface or the iridescent oxide layers that formed.
I also didn't expect cracks to emerge. Half of the parts had cracks consistently 1/3 thickness from the surface, so I attribute this probably to some defect in the original plate. I attempted to fix this by removing more material from the smaller side when facing the parts (ending up with final thicknesses of 0.375".)
Let's talk a little bit about carabiner design.
I chose an offset-D shape (the most common) for my carabiner patterns. This blog has a good explanation of the different kinds of carabiner shapes. The key parameter for an offset-D is a basket (the bigger curve) around 3x as wide as the narrow curve. This shift transfers forces away from the weak gate and concentrates load on the stronger spine (the straight part.) In addition, the curvature radius on the two sides of the D should be identical and large enough to accept the rope you plan to use with it. All other dimensions were mostly eyeballing.
Another major choice was type of closure mechanism.
simply circus.com
There are two main types: keynose (left in the diagram) and hooknose (right). Keynosed carabiners have a slight advantage by not as easily catching on things, and not failing as catastrophically if they do. Black Diamond has a good article about hook-nosed biners catching on things. So with that in mind, I decided to try my hand at making keynoses for my biners.
Fabrication of the nose and gate began with straight slots and pockets as rough approximations, then I refined the angles by filing down the edges until the gate smoothly fell into place.
Following carabiner construction, I had to figure out its spring mechanism.
Now this part was an absolute pain. I thought at this point I was on the home stretch, but ended up spending two additional days being frustrated at bent metal things.
According to the patent drawing above, solid-gate carabiner hinges consist of a compression spring (#40) housed within the gate, actuated by an angled leaf spring (#101) lodged in a notch. The assembly is wedged within the gate when the hinge pin (#36) is in place. You can see the leaf spring in a real carabiner (upper right) peeking from the gate.
Finding a small compression spring was easy; finding a suitable leaf spring was not. After much trial and error, I discovered that constant-torque flat springs didn't work, and actual spring wire (balanced under a set screw) wasn't helpful. I eventually realized that the leaf spring didn't need to be springy at all; it simply needed to have the correct shape. Whacking and grinding small finish nails into shape successfully completed my artisanal hinge assembly.
Testing springiness of rough carabiner
Finally, I rounded off edges with files and passed paracord through to ensure no fraying ropes.
The gift carabiners were then complete!
Woo! Now for science.
The three remaining carabiners also had their load-bearing edges rounded off and received identical leaf-spring notches, though I did not insert springs into the gates. These got pull-tested until failure on an Instron tensile machine. The experimental setup, shown below, was conducted with a 20kn load cell at constant displacement (~2mm/min)
It turns out I made a stupid mistake when designing my gates -- the gate's arms that grab onto the keynose were made too shallow and bent outwards at just 5kn, allowing the keynose to slip out.
Past 5kn, all my carabiners behaved as if the gates never existed, and failed at the standard 6-7kn for an open-gate 7075 biner.
On the bright side, having my carabiners fail at 6-7kn means that my amateur heat treatment actually worked! Woah.
carabiner slowly peeling apart
all three carabiners broke their gates first (left) and a closeup of the damage (right)
The 2002 MIT experiment cycled commercial carabiners on a tensile machine for several iterations and monitored their load-displacement curve. They have an interesting change in slope when the gate catches against the nose, then another change in slope when the carabiner itself starts deforming.
My load-displacement curve is more disappointing, but follows the same pattern. There is a slight change in slope around the 1.5kn mark where the gate engages, then the slope levels out at plastic deformation. There are also distinct short spikes wherever the gate slips, followed by a final plummet at failure.
The third carabiner started out with a small crack near the hinge pin (I messed up while hammering the rivet) and therefore has a really weird graph. Oops.
Trial 1 (a&b):
The machine suddenly cut the experiment short, so the second attempt was with a likely compromised biner
Trial 2:
First downspike is the gate slipping; second is failure
Trial 3:
This gate failed early, probably due to prior cracks near the shear pin
Carabiners post-experiment from left to right
(I couldn't find one of the pieces after the biner exploded)
Notes for the future if I make any more of these:
The keynose slot can afford to be shallower, so that the gate can be made thicker
I should have made the gate pattern to be thicker than the carabiner pattern in the first place, instead of making them the same size.
My derpy heat treatment method wasn't terrible!
It still probably isn't quite on par with commercial 7075-T6
Normally carabiners peel apart at failure, but mine exploded -- I probably didn't anneal it for long enough (250deg oven) and the metal was probably on the brittle side.
The main fracture planes were, for the most part, right at the upper basket as expected.
Making a keynose was not as difficult as I was originally led to believe.
I should engrave things post-heat treatment, not before - removing the deformed bits while keeping the engravings intact was a pain.
I can expect the two gifted carabiners to hold up to 5kn loads and not fail until 6, making them more resilient than the waterbottle/backpack carabiners at the top of the post. They are definitely nowhere near the strength of true climbing carabiners!
((Update 2/24/2017))
Found a cool video of the manufacturing process for real carabiners!
