Little Pond, Big Dive

Dive #1

Cabin Cruiser at 40′, Hathaway’s ponds

With the I.D.I.O.T completed and with waterproofing tested in shallow and fairly deep (80 ft) water, it was time to go take it for a spin in “the deep”. The (purposely) sunken boat in Hathaways ponds seemed like the perfect place to go- there’s stuff to see, and places to swim.

The logistics of a “deep” oxygen rebreather dive were not as simple as one would hope. Since the counterlung is also used for buoyancy, it is not easy to accurately predict the volume needed (without prior experience). It is also hard to descend when you are several lbs positively buoyant. It was decided to descend on a buddies O/C and then breathe the normoxic gas from O/C into the counterlung to provide a reasonable PO2 at depth. This worked more or less perfectly, giving me a rough PO2 in the 1-1.4 range.

Time to follow the string

After completing a swim around the boat, we followed the line across the great murk of the pond to shallower waters (and lower PO2s). However, due to trying to attain neutral buoyancy some gas was exhaled, causing PO2 to increase when the loop was re-inflated. So we made a stop at the “deep” (30ft) platform to take on some more good normoxic air to bring the PO2 back down to oneish. I expected the loop to get a bit rich as I vented air and played with buoyancy on ascent.

2020 a pond Odyssey

From the deep platform we continued along the string to a shallow platform, the mirror, and eventually even found a nice (underwater) chair to sit in.

A motley crew

With the deep testing completed, we headed back to the beach and swapped gear so my buddy could check out the rebreather in shallow water.

Dive #2

Looking very dramatic in <3m of water

Things seemed to be going well until my buddy got a taste of the ol caustic cocktail when he inverted slightly. This caused him to call the dive. Neither of us can figure out where the leak came from since on my dive there was only a little bit of water in the CL after 40 minutes. His flood was at least ~1L of water, which is a significant flood. This underscores the need for backup at all times when diving the rebreather, and this is obviously a reason why people don’t run around breathing off of a single AL6 all the time.

Blowing up my rebreather 😦 + performance notes

Integrated Dive Information Oxygen Transmitter

I’ll start with the exciting part- performance. I used about 800 PSI from my AL6, which is about 1.6 CF of oxygen for a ~45 minute dive. This was supplemented by 2-4 big breaths from O/C, which would be about the same volume. the dive profile was straightforward, straight down to 40′ and then a slow ascent to the surface.

some sketchy math. Basically diver center of gravity will tend to center itself under the center of buoyancy.

I was overweighted with 25lbs with my 7mm hooded vest and 7mm wetsuit with booties. This caused my trim to be basically vertical, and the cl volume needed was essentially the whole counterlung. This is because there is no huge cylinder strapped to my back during the dive, so the belt and counterlung create a huge moment on my body. This means I have to swim to stay in trim, which makes for a bit of a frantic dive. Reducing overweighting would help, but really what I need is to move the weight up to my back. On previous dives with a ~2mm wetsuit I held a rock far out in front of my body, which gave the weight of the rock enough leverage to counter the small amount of weight I was wearing. With 25 lbs, it seems unlikely that I will be able to balance that out without moving the weight.

Blowing up the rebreather was much less extreme than it sounds. I tried to dewater the flooded rebreather by pressurizing the counterlung- what I forgot to do was to open the vent, so I just popped the counterlung. Not ideal! But it should just be a matter of buying another drybag and cutting some holes to replace the counterlung.

It was nice to finally give the I.D.I.O.T a real test and to on a really interesting dive. I hope to return someday soon with my own diluent addition!

Galvanic Sensor and the Science Sr.

The Science Sr. doing what it does best

In the last post I alluded to a larger pressure pot- the Science Sr. This was totally based off of the $50 Cell Checker from the wreckless diver, which is based off of a now defunct post on some other website. I’ll do my best to document what I have made here, since its an awesome tool. I’ll put a standard disclaimer on building one of these- don’t do it, it could blowup, etc, and it could really kill you, and it might hurt the entire time.

Why this could be a bad idea

The Science Sr. is based around an air filter canister. These are rated to 125 PSI…with water. they are 100% not for use with air as far as I can tell. Air, unlike water, is fairly compressible. That means if the amount of energy stored in pressurized air is huge compared to a hydraulic system at the same pressure. Filling this to 125 PSI might be fine, or it could explode due to some component being exposed to chemicals or because it has been cycled too many times or because it has been cycled too rapidly. Plastic will fatigue over time, and that will reduce the margin between exploding and not exploding.

Just like the wreckless diver, I have added an over pressure relief valve (OPV) to prevent going over ~45 PSI or so. This is not a guarantee that it will not go over 45 PSI- I have it hooked up to a scuba reg with an IP of ~120 PSI that can provide something like 100 SCFM (cubic feet/min of gas at some standard temp and pressure). That is fast enough to drain a typical AL80 scuba tank from 3000 PSI to 0 in under a minute. There is no guarantee that the OPV can keep up with this flow rate, so I am VERY careful operating the valve.

Also, in some kind of blue-moon case where my first stage reg goes haywire, its designed to fail open. This would be bad news because this will shoot tank pressure right out of all the low pressure hoses. In a dive situation this is great, because you have ~30s to breathe off a free flowing reg. In a pressure test situation this is bad, because up to 3000 psi (more for some tanks) will be shooting out of every reg and inflator hose, including the one stuck to the pressure tester, which will almost certainly explode it.

This was mitigated somewhat by testing with a mostly empty (500 PSI) tank, and by very careful control of the valve. For cell checking applications, it may be a good idea to fill this with water partially to reduce the amount of volume that is full of air, reducing the potential energy. Anyway, on to how to build it.

Science Sr. Construction

Science Sr. Assembly. Note notch/hole at 6 oclock on the blue part.

Here it is, in all its glory. On the top is basically a X manifold that houses all the important stuff. At 12 oclock is a NPT to BC hose adapter going into a ball valve. At 3 oclock is an 1/4-MPT to 1/8-FPT adapter and a 1/8 NPT pressure sensor. These are available on ebay and seem to work just fine. It seems like they are for some kind of automotive application based on the connectors, and they are available in a variety of ranges. At 6 oclock is a 1/4-MPT – 14-MPT adapter. This attaches the female pipe thread cross fitting to the female pipe thread of the canister head. At 9 oclock is a 1/4-MPT 45 PSI overpressure valve.

Close up of epoxy job

On the other end is a 1/4″ MPT- 1/8″ FPT adapter. The threads on the filter are one-time-use soft plastic NPTs, so I essentially “replaced” them with a metal 1/8″ npt. Screwed into this is a 1/8 MPT to hose barb adapter with a bunch of wires epoxied to it, creating an airtight pass through. I like the 60 second epoxies that have narrow mixing nozzles for this, because they cure FAST and the nozzle fits into the fitting well. However, they are expensive and usually only come with two nozzles, which is wasteful if you only use a few ml per fitting. Silicone seems to work ok for this, but its probably best to apply with some kind of syringe. While none of these fittings have shot out the epoxy/silicone plug yet, it is probably best to get as much plug in as possible, especially on the inside of the plug. This should create a step in the plug to make it much stronger.

It is important to install this on the side labeled IN, since there is a large pass through there, and the wires will be much easier to pull into the canister body (see the assembly picture above).

Here is a shopping list:

*I had these on hand but this mcmaster part should be equivalent

Testing and Results

After some initial testing, it seemed like I would need to return to the teflon tape as a membrane to get good sensor response. I suspect this will let electrolyte evaporate out over time (based on previous experience), but it does give a very satisfactory response time, and over short periods the sensor is serviceable enough. It was necessary to make the sensor up in the electrolyte to eliminate bubbles. This meant pouring the eletrolyte into a dish and then wearing gloves to assemble the sensor “underwater”. Excess KOH was washed off.

As a note- for this exact size of sensor, a ~510 ohm resistor seemed “right”. More discussion of that below.

Step Response

Yellow = sensor, Green = Pressure. 500mV = 1ATM

Here is an example of the step response to pressure. With the ball valve, its a little nerve wracking to throw it fully open, and I wasnt keen on testing the opv, so the pressure step here actually has a rise time of ~250 ms. The rise time of the sensor is about the same- It does not seem like this step is fast enough to elicit any kind of delay in the sensor. The pressure here goes from 1 ATM to 1.5 ATM, and the sensor rises from ~220 mV to 325 mV, which is what we would expect.

The next test was to wrap the sensor up in a bag with the inflator hose, and to shoot O2 straight into it. This should give a value for 100% O2 at 1 ATA.

Yellow = sensor

I wasnt sure how much O2 to squeeze in there or where it was really saturating, so there is a little bump in the middle as I jostled the sensor. But after spewing out a good amount of gas, it looks like the maximum value is 500mV. This is an interesting result and shows that the sensor is actually no good up to 100% O2. If the sensor were linear, the resulting voltage should be 5x atmospheric what it is in normal “air” which is ~20% O2. That would be 1V, which it does not achieve. I believe this means the cell is current limited.

yellow = sensor

Here is the other end of that test- me tearing the bag to introduce normal air back in. From the initial jostle at 10s, it looks like it takes about ~1 min for the O2 to go back to normal levels. This is not representative of sensor performance because the gas actually has to get agitated for the O2 to go anywhere. In other words, it does not purely measure sensor response, but also incorporates the gas diffusing/getting blown away.


A good sensor is a linear sensor

This is a plot of pressure (Y) vs sensor value (X). To produce this, the chamber was cycled several times from 1 ATA (.5V) to 2 ATA (1V). Cursors represent the starting point, and where the sensor “should go” at 1 ATA. Doubling pressure should double sensor value from 190 mV t 380 mV, and that is almost exactly what the line shows. Doubling the pressure represents going from a PO2 of .2 to .4. The voltage is below the 500 mV that the sensor saturates at, so it comes out beautifully linear.

Since the sensor was cycled several times, we can detect some other interesting sensor characteristics, namely a small amount of hysteresis and non-linearity. The cursor generally tracks on the right side on the way “up” in PO2/pressure, an tracks on the left as PO2 drops. This means a sensor will read slightly differently after it has been exposed to higher pressure- it has “memory”. The maximum value of this is almost 50mV!

There is also straight up non-linearity, as the shape of the curve is not straight. However, this seems to be fairly small.

A single very slow run

Here is another hysteresis example, with the rise and fall done as slowly as possible. This should give an idea of the minimum hysteresis.

Slow and fast traces

Here is an example of a slow and a fast pressurization and depressurization. I couldn’t color the two differently, but it is easy to see that the rightmost trace (which was a pressure drop) looks quite different- it drops to 0 pressure then moves tot the left to drop the voltage.

Current Limit/Resistor Size

This is the setup I have, only the amplifier is an oscilliscope

I mentioned above that I used a 510 ohm resistor as a current shunt, and that the cell is current limiting. O2 cells, while read out in mV, are actually current-producing devices. Typically this is measured by putting a shunt resistor across the cell, and measuring the voltage. With low resistances, this produces a small voltage drop which can be difficult to measure. Higher resistances produce higher voltages- but you can only go so high! This is because the cell can only tolerate so much voltage across the cell*, and can only generate so much current. Therefore it is a balancing act to find an appropriate resistance to use- too small and readings get jumpy, too big and the cell wont change value at all, since it will be current limited.

*I am admittedly fuzzy on this but you probably do not want to get close to the open circuit voltage of the cell, which is 1.2v



Building a galvanic O2 sensor is possible and actually fairly easy. I suspect even the polarigraphic sensor would have been fine if capped in electrolyte, and if it did not leak. The sensors can obviously be tuned to give good responses, as this is exactly the same way that commercial O2 sensors work. However, building a really good sensor that could be used for diving, where the sensor is monitoring life support, requires a lot of sensor characterization which is way beyond what I want to do. There’s a lot more to an O2 cell than just getting it to spit out a voltage proportional to the O2 concentration- for example:

  • Temperature compensation
  • shelf life/storage condition determination
  • repeatable assembly/manufacturing processes

Any one of these could take a month (or longer) to do, and they would require a lot of units. I will stick to commercial O2 cells for now (and in the future), but now that I have a cell checker/pressure pot it should be interesting to compare a “real” cell to home made!

Rebreather i4 “Nudibranch”

Holding onto the rock of great buoyancy, in the Atlantic

After feeling a bit like a snail in a shell in my last rebreather, I decided I wanted to make something a lot smaller and more ergonomic. Hence the next generation of the rebreather being called the Nudibranch- Latin for “naked lung”. A suitable name for a design with a scrubber-in-lung and with the counterlung unenclosed in a shell.

Improvements over i3

I knew there were some features I wanted to keep from the previous revision, and some things that needed to be improved. The materials for example, all proved to be robust and appropriately resistant to chemicals. the manual add valve (MAV) functioned well, and the counterlungs were of a comfortable volume. Scrubber duration was adequate as well. However, there were some issues with i3:

  • Bulky design
  • Difficult to reach gas controls
  • MAV hard to locate
  • Low gas volume
  • Assembly took too long

With a focus on simplifying the assembly, i4 looks and feels more like a simple rebreather rather than some kind of science project.

Streamlining/shell-less design

photo of a medi nixe from, an excellent resource

The main inspiration for the new design was the Drager Model 138 rebreather. This model was famously used by Hans Hass in the Red Sea and later went on to be a “sport rebreather” diving system used for leisure, re-branded as the barrakuda or medi nixie. It turns out that it got a lot of things right.

One of the interesting aspects of the model 138 is that it is mostly cloth. When working at the scale of the human body, the cost of materials and the size of parts can add up quick. The last rebreather was encased in a large aluminum shell bolted to an 80/20 frame. This frame covered the gas cylinder, as well as two 8L couterlungs. Since the scale of these parts is cm, the small offsets to accommodate things like the counterung bulkheads or the gas addition fitting took a lot of room. Wrapping that all in aluminum made the shell quite bulky. comparing the two units above, you can see that i4 is much smaller.

The breathing hose routing is a lot cleaner, since the hoses run over the shoulder instead of straight outwards. This means I need less hose length. In the future I hope to convert to 1.5″ diameter hoses that are ~6 in shorter, for lower work of breathing.

Gas Volume/Valve Access

The Model 138 also has front mounted gas, which means all the valves and controls are easily accessible to the diver. They also are not mounted on a whip/hose like on the i3. With the i3 it was possible to “loose” the MAV since it was not mounted to anything.

Swapping bottles/shutdown- Photo taken with a nikonos on 400 speed film

The i3 valve was accessible, but it was extremely challenging to reach and required flexibility. On the i4, the valve is in the front and stationary, and only requires one intermediate hose compared to the two for the i3.

I knew from my previous design that I wanted a lot more gas so that I could stay underwater longer. I still haven’t figured out the best way to reliably get oxygen fills, so I stayed with the hardware store welding bottles, but I doubled up on them. More can be read about that here.

Ultimately, the only real issues I had with this design were MAV related. The gas regulators seem to restrict the flow far more than the old regulator, which makes the counterlung fill very slowly. It also seems like they cut off at a higher pressure, leaving a significant amount of gas left over in the tank. This makes buoyancy and breathing comfort a challenge, and limits range. However, twice as much gas is twice as good, and dives up to 45 minutes have been conducted without complete gas exhaustion.

Counterlung material selection and design

This is really the complicated part, and its particularly fraught since it is where all of the rebreather magic happens. The bag must be, relative to the demands of the dive (duration, pressure, volume of bag) completely watertight and made of a waterproof material. Some leak rate is permissible, but it has to be low. In order to capitalize on pre-existing materials and advanced technology, I decided to use a drybag. The other options are to sew a bag and seal it, heat seal a bag, or sew and completely impregnate a bag with silicone.

Sealing the end of the bag

Since I managed to find an appropriately sized drybag, I ended up just buying one. Testing of several bags revealed that the stock roll-top seals of dry bags are inadequate. Specifically, the way that most of them close is by clipping the ends together and rolling them shut. While this is fine for splashes or even putting it on top of water, submerging the bag will squeeze the air out of the wrinkles created by bending the rolled up seal.

Without bending the seal, it is possible to get good enough contact between the two sides of the bag that it will form an airtight seal. However, the particular bag I chose had stiff mounting points for the buckles- these interfered with collapsing the seal in that area, and provided a leak path. Absolutely fine for a drybag, but not good enough for a counterlung.

By removing the stiffener and making some aluminum clamps, I was able to seal the bag well enough. I flipped the bag inside out so the smooth urethane coating was on the inside of the bag, instead of the textured nylon surface. Additionally, it is important that the bag material was very thin so the seam in the material does not disrupt the seal.

This is really a dream for scrubber access and drying since the whole lung can be rinsed and turned inside out to dry.

Making holes in the bag

I knew there would need to be at least 3 penetrations into the bag- inhale, exhale, and gas add, in addition to the large hole in the bottom. all of these of course, are leak paths!

The inhale and exhale penetrations are sealed with these (put link here) handy PVC bulkhead fittings, which conveniently fit a piece of 3/4″ pipe stub on one end to attach the breathing hose. Instead of the stock gasket, I used an oring and a retaining groove. The gaskets that were provided may have worked, but I wanted to be sure that I could get good contact along the whole circumference. The provided gaskets had a rectangular cross section and were hard- requiring a lot of force to seal. The replacement orings are soft, have a reduced cross section, and a much finer surface finish.

As you can see in this diagram, some water is actually allowed into the bag via the threads on the bulkhead fitting, but it is stopped by the oring and the glue on the bulkhead.

The other hole is a standard dump valve/BCD inflation port. These were “successfully” used on the i1 and i2 rebreathers- successful in that they did not leak, less successful in terms of actual diving.

The modified component is the small white disk under the spring

The last (and yet tested) item is the drysuit exhaust valve/ overpressure relief valve. Once installed properly, this should help avoid over pressurizing the lungs on ascent. Its stock cracking pressure is adjustable between 3-10″ H2O, but by modifying the spring spacer I bumped it up to 8-16″ H2O. This is probably the leakiest of all the valves because it requires a large hole- a larger hole is harder to seal because the seal is longer. Usually these are installed with a sort of large rubber washer with a C-cross section. The material the seal is attaching to goes in the open part of the C, and the washer is glued in. Since I don’t have that washer, I haven’t installed it yet and I have been avoiding overpressure via the “oro-pharyngial” valve.

Leak Testing

Intensive leak testing (more here) allowed me to validate the design before committing to a whole build. That was important, because a lot of things did not work initially. For example, I couldn’t apply enough torque to the brass thumbscrews to seal the bag, so I had to switch to a nylock nut. Another example is that the original gasket for the bulkhead seals was determined to be inadequate.


harness hardpoints highlighted in orange

Just like with i3, the last thing I was really prepared to deal with was attaching this to my body. Unlike the i3, I knew that this challenge was coming. I considered something like a vest a la model 138, but that would require buying a few yards of fabric and knowing what to do with it, and getting a sewing machine. Instead, borrowed some inspiration from an oxydiver design (link) and backplates and designed in slots for a hogarthian harness. In the end, I actually think this is better since it is now compatible with all kinds of 2″ webbing accessories.

Divers view of the gas

Of course, there is no backplate, and the gas is mounted on the front of the diver. After playing around with a bucket of sliders and clips I came up with a simple loop around the two canisters, with a 2″ long loop at one end to put the waist strap through.


Another gem from the Nikonos

This rebreather is actually very fun to dive. I would actually consider diving this over OC on shallow dives. However, no project is ever perfect. The three things I will likely seek to improve in the inevitable i5 are:

  • Terrestrial O2 regs rust and fail. O2 is expensive in small disposable bottles, and having a million of them around is silly. There is also no SPG. Normal scuba cylinders and regs are far superior.
  • No place to put an inflation cylinder for drysuit. Even in 20′ of water I think I would prefer a drysuit over wet- especially if I were to dive in the winter.
  • People say YERGONNADIE if you dont know your PPO2. There is some merit to this claim, but I feel it is not likely if you respect the depth limitations of an o2 rebreather.

A lot of these changes or issues have implications- if you have an inflation cylinder, why not make it breathable as backup? If you add oxygen sensing, why not add that breathable cylinder as dil? If you give an engineer a cookie…


Manifolded O2 For Rebreather

MAV-1X and MAV-2X

The more gas you have on a dive, the more time you have underwater, and the larger a safety margin you have- although for an oxygen rebreather in a non-overhead environment, it mostly adds to dive time. I wanted to be able to carry not one, but TWO disposable oxygen bottles on a dive, so I went ahead and built a “manifold” based on the MAV components from my previous rebreather.

Here is the MAV 2x in the exploded state. With all brass components, galvanic corrosion, should be kept to a minimum. I may also try nickel plating them after polishing, for even more corrosion resistance. Polishing reduces surface area, which also helps combat corrosion.

At the bottom are a couple of off the shelf mini-oxygen bottle regulators. These have a male (thread) B-sized oxygen fitting on them, for an oxygen hose. Since I want to stick these into a manifold and not have the bottles interfere, I needed a rigid connection that lets the two sets of threads be independent. These fittings are right in the middle- one end is NPT and one end is a female B size fitting (P/N WES-123 for the curious). The NPT end is affixed to the manifold, and then the nuts are tightened onto the regulators. Since the threads are on the nuts, and not on the stem attached to the NPT, the bottles can be in any orientation.

Original 1X MAV

This is very similar to the original MAV, but it has been simplified by putting the MAV directly on the cylinders, which will be front mounted on the diver. This simplifies the gas routing, and eliminates a connection. Another improvement is that the bore for the cartridge valve is not a through hole- this means that the back does not need to be sealed up, eliminating an extra seal.

The cost for the reduced leak path count is that chip evacuation is more difficult in the long hole (roughly 4.25″ x .25″ drill- 17xD hole) and there is an additional, difficult to machine cover that retains the cartridge valve. If this is not installed, the cartridge valve will shoot out of the MAV under pressure. It is definitely an area to watch out for when the valve is pressurized. However, with such a long bore, there is not a good way to be able to machine the valve bore from the reverse side. Fortunately, the cross sectional area of the valve is quite small, so even at 500 PSI, which is the tank pressure, the cover only needs to resist ~50 lbs, across four M3 screws.


Of course, not every dive requires two oxygen bottles, and with no gas installed the inside of the regulator would be filled with water. So that the future rebreather can be operated with just one cylinder, a 1X MAV was produced. Cant wait to try it out!

3D Printed Arbor with Flexures

Bulkhead parts are in black

I recently had an issue with tightening bulkhead connectors. Since one side is round, its hard to really crank down on the nut that seals the flange! Grabbing the tiny, round flange just does not provide enough torque, no matter what you do. I am using these on a new counterlung design, so my goal was to make some small tools for assembling or breaking down the rebreather, but they needed to be hand-operated. While I did consider drilling holes in the flange and making a pin spanner that would be annoying since that is a sealing surface and because I would need to modify off she shelf parts whenever I got them. So I decided to build a arbor to grip the ID of the part.

Interstate - 1-1/2" Face Diam x 4" OAL Expanding Lathe Arbor ...
Typical expanding arbor, this one is from MSC

Typically an arbor is pretty simple- a wedge is screwed into a slitted block, and the block (in this case a cylinder) expands. This is pretty simple, and is a wonderful one piece construction. However, if you tried to print this it would be unlikely to work because of the huge stress concentrator at the root of the cut, and because you are creating a torque that is pulling right where the weak layer lines of the print would have to be!

Flexures are cool

single-extrusion-width flexure for parallel motion

My solution was to use single-extrusion-width flexures instead. A flexure is more or less a skinny piece of material that is bendy and stretchy…but on purpose. Because the material is thin, the maximum compressive and tension forces on the material stay small, even when the material is bent. Consider a piece of polycarb bent to make a face shield vs a sheet of bulletproof glass. If you bend one, it will spring back to being flat, while the other one will snap in half!

Mechanisms: Couplings | Hackaday
photo from hackaday

Another important note about flexures is that they can be very rigid in some directions, but flexible in others. That is how flexure motor couplings work- they are rigid in rotation, but flexible in other directions, as you can see. And since they (should) operate in the fully elastic region of the materials stress, they should also operate more or less forever, with no maintenance. This is highly desirable compared to multi-part assemblies.

However, flexures tend to be long compared to the amount of motion they can produce. A good rule of thumb is the length of the flexure (including total length of a zig zag) will be 10-20x the length you can expect it to move.

I had already done some experimentation with single-extrusion width flexures for a small parallel motion flexure, so I knew they worked. I had not tried to make a zig-zag of them, but I knew in theory the slicer would slice them and that they would be extremely flexible.

Photo of flexure

Here you can see the folds of the zig-zag flexure. To increase the amount of friction, I glued a piece of 1/32″ EPDM to the outside with superglue. Superglue seems to have ok bonding properties with superglue, as long as it is glued to something rigid. I could probably peel the EPDM off of the mandrel if I wanted, and in previous tests the EPDM has failed, not the superglue (CA glue). One thing to note is that some of the zig-zags are actually stuck together by stringing on the 3d printer- this is really annoying, and this design would benefit from better retraction settings, or more space between lines.

Expander wedges shown in cross section

Here you can see how it works- two cones are pulled together, which acts on a conical surface in the arbor core. This pushes the wedges outward, and into the ID of the part. This is achieved with a heat-set M3 insert on one side, and a long m3 bolt that goes through the whole assembly. While it would be possible to have just one wedge, I thought that two would be better since it would prevent twisting the flexures and create an expanding cylinder shape instead of an expanding cone shape.

Another critical feature seen here are the fins on the cones that go into matching slots in the flexure. These prevent the nut from spinning freely, and it also allows transmission of torque from the handwheel to the arbor, once the wedges are locked in place rotationally.

The orange handles let you really crank down!

Here it is in use. the large diameter of the bottom wedge is for gripping- it is about the same size as the grip for the nut, and by using these two the nut can be tightened about as tightly as I could want.

Notes for successful flexure printing:

-make the width of the flexure the same width as your nozzle/whatever width your printer thinks it is printing

-flexure bending axis will be paralell to z axis for this technique. I could imagine a way to make a flexure on the build plate but that’s different than this technique

-try to convince your slicer to prioritize outside “skin” layers, including the flexure.

-Explicitly tell the slicer not to put the z-seam on the flexure. While flexures are bendy, the z seam in theory/in practice is thinner and weaker, and putting it on a flexure is a good way to snap it. A few seams here and there are ok

-Attempt some kind of strain relief where the flexure goes into the part-put a biiiig radius on the flexure before that point. since we are relying on a single width of filament to bond to the rest of the shell here, adding a radius here probably wont help since even the smallest printable radius is much much stiffer than a single wall thickness.

What I would do differently/what went wrong

The flexure I made is pretty aggressive in terms of width between folds. I could probably go down to fewer folds since the wedges dont need to move very far. This would give me more clearance between lines, which would help with the stringing issue Or I could even go down to just two wedges, which would give me a lot of room for flexures and could even give me some extra surface area.

Additionally, the angle of the wedges is a little narrower than some previous prints. This makes them tend to get stuck, as less of the restoring force of the flexures is pushing them up and back out of the cone of the flexure. Its only about a difference of 8 degrees included angle, but it is significant. This makes it a little fiddly to get the flexure out of the hole once it has been set. this could be improved by reducing friction between the printed surfaces, but it seems simpler to make it work right off the printer. It would be distracting if you were servicing something using the tool.

I will also probably change to some kind of hand-operable and fully captured screw. as it is, it requires an additional screw (allen key) to actuate the clamp. While that is ok, it would be very convenient to have a clamp that did not require a small, easily corroded tool, and it would be nice if the screw couldn’t fall out.