Miniature Dome Stove fittings

This post describes some details on the fittings that I used to make my Miniature Dome Stove and other similar related stoves.

Background

A keen follower Steve who has great blogs on theultralighthiker has suggested that I provide more details on aspects of my Miniature Dome Tent Stove fittings that I have overlooked in my post.

I probably have skipped over these fittings as I have just gone to my ‘junk box’ of ‘old-friend experimental stove parts’ to plug together my next experimental stove for testing. Most of the special fittings are custom made by me from my stocks of somewhat rare stainless steel and titanium foils. “Once I have met the challenge of creating them I probably just ‘take them for granted’.”

My related posts are already too long to add this detail, according to my SEO. Consequently, I thought that a supplementary post would be the best way to deal with this. It can be used in conjunction with past and (no-doubt) future posts that use such fittings. I also can add other omissions that will surely arise

Note: This post is incomplete, but I will revise it and add the missing photos, videos as they become available and other topics as they arise as readers ask pertinent questions.

Mess and chaos can be a virtue

As an ‘inventor’ during my previous research career, I developed an uncanny knack of being able to envision what my invention might do, while managers could not. Luckily, I persisted and they eventually rejoiced in the fruits of success and backed me.

In my experience, staying somewhat blinkered to the ‘proper’ or ‘orthodox’ way of doing something is important for good innovation. If not, ‘rat-racking’ will be likely to limit lateral thinking. I remember a patent attorney looking me in the eyes and saying to me “Tim, each human is unique and if you have independently dreamed up this invention it will have a patentable element.”

I have found that many fellow inventors are somewhat messy (I was, notoriously so) and I think that this is because the chaos of being immersed in technical objects, rare materials and failed test tools/apparatus and chemicals is an integral part of inventing. I think it may be genetic as my late father would empty out several boxes of hardware junk (nuts, bolts bits of WW-2 aircraft etc) onto drop sheets to search through to stimulate a solution to a childhood weekend project that had reached a ‘dead-end’.

In my youth, I remember a family friend (much my senior) who was an innovative industrial chemist. He described his mind as being ‘bowerbird like’ and would tenaciously hang onto useful ideas for the future. I think I have a little of that ‘bowerbird’ in me and my collection of creative junk may be another expression of this.

Back to stove fittings

Over the years, I have posted on a range of developmental miniature backpacking wood-burning stoves. These have culminated in what I think is the ultimate ultralight backpacking three-in-one wood stick powered Miniature Dome Stove. “You may detect some so-called ‘inventors pride’ here.” It is a compact hot little tent heater/cooker, a fast outdoor cooking stove and also a backup alcohol stove. It all fits essentially inside a custom cooking pot with a total weight of ~650g. Two ultralight larger pots (less than 100g each) can be added compactly to the kit.

The lightweight, convenient and compact backpackable properties of the stove depend upon these fittings.

“That’s the end of the stove promotional spiel. Yes, I am aware of the boldness of my claim of an ultimate design, but it does not preclude some tinkering improvements wrought from field use and the experience of others.”

Virtues of round holes and customised connection tubes

During the stove experiments, I have come to the conclusion that round or roundish fittings are the best ones for ‘us backyard engineers’ (or in my case bathroom). Circular holes are relatively easy to cut, form and standardize. When they are cut on compound curves there will be circumferential variations from the true cutter shape.

Corresponding connectors and tubes can be custom made to consistently fit within these imperfect but standardized roundish holes.

Virtues of curving flimsy ultralight foils

The curving of apparently flimsy metal foil (thickness of photocopy paper) greatly increases its stiffness and strength. Locking the overlap of the ring, tube or cone with spot welds adds to these attributes.

I have already posted on rolling smooth curves and cones with simple hand tools. The current post will just focus on some exemplar curved fittings. However, in my experience, the techniques are good for most round stove fittings (rings, pipes, cones, bands, hoops, connectors, sleeves, compression rings, stove bodies and roll up flue pipes.

Because of the compressional flexibility of such fabricated rolled tubes, the circumference and the implied diameter can not be directly measured sensitively with callipers (or other tools that I know of, a pen and strip of paper are not accurate enough). However, the ‘implied diameter’ can be adjusted to within about 0.02mm or less tolerance at the time of welding by using suitable standard rings of stainless steel foil or standardised holes. “Now that is not bad for a bathroom engineer.”

My fine tricky fabrications are made possible by my homemade welding tools (in my bathroom) that are not available to others.

“I have learned how to weld in places and spaces where no conventional/self-respecting welder can or would go.”

Micro spot-welder

This funny ugly tool is key to nearly all my stove stuff so I have put it first. It was the first serious one that I made.

I think many fellow innovators would understand this: How I put off trying it for a while for fear of it failing to work after all the effort put in.

An ode to an innovators tensions between success and failure:

My plans easily hold my wondrous dreams of success,
Fear of failure by testing,……. I delay,…… I confess,
Reality tested, after time invested becomes,
A sensation or just a lesson with a little more mess.

The welder is made of wood (my carpenter fathers medium) a big garden gate hinge (scrap), copper bars (purchased tut-tut), MIG welding tips (made of beryllium copper alloy) and redundant arc welder transformer.

Thrown into the mix is a rope, levers, weights and a pulley system, that hangs from the ceiling (from yachting experience). It provides the required consistent closing force that is needed to form consistently good spot welds.

The mains power to the primary winding of the arc welder transformer is passed through a solid-state relay that is pulsed on for a short duration with a low voltage DC control signal from a 555 timer circuit. The pulse time can be adjusted between ~0.05 and ~3 seconds.

This welder is my baby and I have watched it grow in its capacity to flexibly weld in places that at first seemed impossible. I have a gaggle of alternative electrodes that can be quickly interchanged to weld in the most unusual places (some can be seen hanging in the bathroom in the top right-hand corner of the photo below).

It lives in my bathroom because that is where I first set up the experiment and I don’t dare shift it. Maybe being in this frequently visited location makes me a more passionate inventor?

“Together, we have made possible the making of many apparently impossible and ridiculously light backpacking component. So it probably deserves its prominent place in my bathroom!”

The weaknesses and virtues of a solo welds

The solo weak weld. A spot weld forms a small area, usually circular, where the two contacting faces of the metal melt and fuse together forming what is called a ‘nugget’. A single micro spot weld has a vulnerability to rotational forces around the centre of the nugget.

I exploit this vulnerability to make an easily broken nugget with sub-optimal welding dwell times during welding. This will result in a weld that can be broken without making a hole in either sheet. This means that for example a formed tube can be held together with a weak welded nugget and can be tested (gauged or calibrated) for a perfect fit with another component.

If a better fit is needed, the joint can be marked with a fine scribe, the nugget can easily be broken and welded again after adjusting for a better fit and anther test. When the fit is correct the weld can be supplemented with full strength welds.

This little Instagram video can explain this better than my words can.

The solo strong weld. Even a strong nugget is vulnerable to breaking by the rotational forces. In this case, the breaking will be more difficult and will probably result in a small ‘nugget sized hole’ being ripped from one side or the other.

Ganged strong welds. Because of the above vulnerability of solo spot welds, I never settle for less than two well-spaced welds. It means that the destructive rotation action will not be able to destroy the natural strength. Three or four welds in a two-dimensional array are even better for preventing this type of weld fatigue.

This little Instagram video shows strength is developed by ganged spot welds.

The tiny welded ring, with two strong spot welds, finally broke when I put my weight on it, but it was the metal band that broke, not the ganged welds.

Just for fun I did a 10mm wide foil band with six ganged welds and this easily supported my body weight (~65kg).

An ode to the intrinsic weakness of single spot weld:

A lone spot weld nugget is intrinsically piss-weak,
Without good company so-as-to-speak,
Two welds make rotational stability and ends fragility,
And the strength found was what I did seek.

The micro spot-welder is limited to welding thin steel, stainless steel and titanium foil up to about 0.3mm thick. (Aluminium foil from a ‘soft drink can’ could be added to the list, but it is so tricky that it will have to be in another post, as will other perverse metal combinations).

Consequently, I needed a more powerful welder to weld thicker sheet metal that I use for small but critical load-bearing stove components and multiple laminates of foil.

Here is a little Instagram video of an ultralight telescopic tube that is spot welded together with my micro spot welder from stainless steel that is only 0.03mm thick.

Bigger spot welder

For thicker sheet metal welding, the Tecna mobile welder shown below is my next step up. It is too powerful at its lowest setting to weld thin foil to thin foil. Fortunately, it can weld thin foil to thicker metal at the lowest settings. “So both welders make a good team.”

Exploiting the welded seam overlap

At first sight, the small seam overlap in my welded tubes and cones just look like a pesky necessity.

For example, they ‘catch’ in a rotary sense when used as pipe holders on a roll up flue pipes. If placed with the ‘raw edge’ edges facing one another on the flue pipe seam. “This is why I keep them all facing the correct way by sliding them off the flue pipe and directly on to the storage cylinder as in the photo below.

“My ‘bower bird brain’ said Tim you could use these pesky flaps to your advantage by making them bigger when making stove components.”

Snail-tail-rolley. My most obvious use of the larger overlap flap is in the roll up flue pipe storage tube. I call it a snail-tail-rolley . The photos below best describe the multiple functions. It makes the rollup safe, efficient, clean and causes less ‘crinkle damage’ to the foil.

Interleaving seam lock. If the overlaps are designed intelligently they can be enlarged and welded in a way that leaves the seam flap open as either an ‘internal’ or ‘external one.

Essentially, tubular fittings that connect one inside the other can be counter-rotated to interleave their flaps with one another to lock the two fittings together. Unlocking is as simple as reversing the rotation.

Compression rings with retaining overlap flap. My roll up flue pipe has a compression ring within the bottom of the pipe. This stops the pipe collapsing where it is inserted into the flue pipe port. Such simple rings can easily fall out and be lost. “I know this from years of experience.”

Recently, I solved this problem by leaving a very long overlap tail on the outside of the welded ring or tube. The tail (~1/3 of the circumference) of the ring can be interleaved with the overlap of the flue pipe and protrude outside of the pipe. Then it can be clinched in place by lowering a tight-fitting flue pipe retaining ring. The fitting also has the effect of expanding the pipe circumference so the external retaining ring also can not fall off. “The tail is so long that it protrudes out beyond the finish of the flue pipe seam as a reminder that it is in there.”

[Photo of compression ring protruding from and locked into the bottom of a roll up flue pipe]

[a video of inserting and locking of a compression ring]

Overlap flap used as retaining tabs. Another use of the seam overlap, with heavier gauge metal, is to use a tiny portion of it to form a retaining tab. This is done by making a small cut at right angles to the seam line, before welding. The small portion of the ‘overlap’ can then be folded out to make a small tab after welding. If not obvious, such tabs are desirable to keep fittings that reach 350+C in their proper place while a stove is operating in a small tent.

Overlap flap used as a holder for retaining tabs. When a fitting is made entirely from very light foil an alternative option for a strong retaining tab is to insert a thicker ‘L’ shaped sheet metal tab into the seam and spot welded it in between the light foil seam overlaps.

[Photo of heavy tab welded into light foil fitting]

When I first started to use the retaining tab I thought that provision of a ‘keyway slot’ would be the only way to allow the fitting to be inserted and then rotated to lock it in position. This is true for parallel tubes. The final resting position of the tab should be as close as possible to 12 O’clock if the fitting is bearing a strong gravitational loading. However, if you read on about conical fittings with tabs you will find that the keyway may not be necessary.

Welded foil elbows

I make ultralight elbows from foil by first making a tube. Then I cut the tube, on an angle, into two portions. I cut small foldable tabs on one cut face and bend them to fit inside the other portion of the tube. Then with the pieces held together with strips of packing tape, I progressively weld all the tabs.

Connecting cones

Making reproducible close-fitting joints between round holes and matching handcrafted tubes is important for making my ultralight stoves. This is not easy when they are made of thin metal foil. If made with a suitably tight fit they are impossible to fit into their respective holes. If they fit easily they leak easily.

However, a suitable homemade intermediary cone can make the fitting tight, easy and tolerant of minor dimensional and shaping errors. “These are so good that, in my early tinkering with stoves, I even made these cones that were held together with rivets made from fine flathead nails!”

An ode to the fit of a cone:

Fit a rigid crude circular hole around a compliant cone,
Their intimate fit stands alone,
If you stuff it in tight, the seal will be right,
No gaps, no leaks or reason to moan

Fit a crude tube inside the same cone,
The tube will find its way home,
Shove it right, the fit will be tight,
You will barely hear a moan.

Make a round hole that is smooth but rude,
And a tube that is equally crude,
By stuffing them tight they will unite,
Via a cone, so no; flame, smoke or gas will exude.

Examples of the utility of sheet metal connecting cones

Using retaining tabs without a keyway

Above I mentioned the use of a keyway slot to allow the entry of the retaining tab through a round hole. At first, it seemed to be essential. Now, I have found that with the use of conical connectors they can have a critical taper that allows entry of the end of the cone, with a tab, without the need for a keyway slot. If the initial entry fit is too tight it can be eased a little by trimming the length of the tab if required,

[Video of keyless fitting of a cone with a retaining tab]

Is it an elbow or cone or an elbcone?

I have recently made elbow ‘fittings’ that can be made with a suitable conical taper so that it does not even need the ‘cone’ as a separate interface. What’s more, such a tapered fitting can also transition from a male fit on one end to a female fit on the other. This can be very functional with a fitting such as my Miniature Dome Stove flue pipe elbow. This ensures that any flue pipe creosote does not trickle down and leak out and burn to make smoke in the tent.

The wider female arm can easily be fitted with another cone that allows easy removal/replacement of the flue pipe. It can also be designed to stop the flue pipe slipping in too far.

This allows the user to prime the flue pipe draft with a burning waxie fire starter and then quickly replace the flue pipe. “This gets a fast stove startup under freezing conditions when you are cold and the fuel sticks are damp or frozen.”

Lastly, easy removability of the flue pipe makes it simple to quickly ‘kill’ the power of the stove to prepare it for packing up. “No draft, little combustion and not much heat.”

[Video of sight and sound of flue pipe priming with a flaming waxie]

Easy entry cone

Here is a little Instagram video of connecting the elbow assembly and setting the slope angle.

Here is the last ode for a cone:

A truncated cone is one hermaphrodite,
Unlike snail, male and female in sight,
With mounting corrected and polarity respected,
The pleasurable union will stay creosote tight.

Flame guide

My Pudding Bowl Stove and Miniature Dome Stove and other related miniature stoves require the use of a flame guide that prevents the flame from taking a ‘short-cut’ to the flue pipe. This helps to distribute more heat to the fire dome and less heat to the flue pipe.

The flame guide has 1,000 C charcoal on one side and 700C flame and gas on the other. It has no substantial attachment to less hot stove parts to use as a heat sink to moderate its temperature.

To add to this challenging environment there is also a high oxygen content for the initial part of the burn zone near the very hot charcoal. This means that the stainless steel Or titanium) foil, that the guide is made from, will oxidize and break down after about 50h of operation.

Consequently, I designed this component to be cheap, light and removable (without tools). It also means that replacement guides could be sent in flat form by cheap letter post. Then they could be bent into shape by the user when required.

It also means that virtually any thin ferrous sheet metal (eg bean can) could be cut with a knife and bent be used to make a substitute flame guide if ‘need must’.

The photo below may help to explain a little more about the way the flame guide works and why theflame guide gets so hot.

Lastly, for this stove design, the removability of the flame guide was necessary for the fire dome to be converted to a blower stove and also to facilitate easy packing of the stove components within the fire dome during backpacking.

Insulated clip-on hole cover

The 20mm hole in the fire dome is primarily there to be the port for the blower when converted to a blower stove. Even if the stove is not to be used in blower mode the fire dome will still be provided with this hole as it makes an excellent ignition port for re-starting the wood gas flame with a waxie.

The little hole cover shown below with an insulated curved handle can be safely fitted and removed from the stove while it is running at high temperature. It particularly improves stove combustion performance when the stove is configured with the largest diameter fuel port.

Hole cutting

I find that any rotary cutting of titanium or stainless steel sheet metal is terribly harsh on these cutting tools, even if the sheet is paper-thin foil. Consequently, I avoid drilling, hole sawing etc. The exception with this the drilling of necessary pilot holes for punching.

Little holes. For necessary small holes, I use a sacrificial step drill for the purpose. I run the drill at the lowest possible speed and liberally apply my DIY cutting fluid and push through with a lot of drill pressure.

“My theory is that the thick swarth that comes off this way can carry more heat away from the cutting point than a thin broken up drilling chips and thereby keep the drill tip cooler. Who knows, but it works and I am still using the original step drill that was demoted to this demeaning task.”

Even then, before drilling, I punch a big penetrating hole on my centre mark with a long narrow DIY prick punch made from a concrete nail, so that it is easy on the blades of the first step of the drill. These punches are very cheap and can have very long points (unlike commercial punches) that make the job easy.

If I require fine holes, I never use my precious drills. Instead, I punch a pilot hole with the above prick punch and use a long tapered hardened steel awl to stretch the hole to the required size. Then I dress off the bur on the back of the hole with an aluminium oxide stone in a Dremel tool. This abrasive just eats hard titanium and stainless steel.

“Incidentally, if you get similar looking pieces of stainless steel and titanium mixed up, they can easily be separated by the colour of the grinding sparks. The titanium sparks are beautiful vivid white, while those of stainless steel look a little yellowish.”

[Video of titanium and stainless steel sparks]

“I also accidentally discovered that I could make little holes by electro explosion but that will have to keep for another post.”

Hole punching

Big holes. Earlier in this post, I extolled the virtues of round holes for stoves and fitting. My prefered method of making them to a tight specification is to use a punch and dies. These work best with my thicker sheet metal in stove bodies and fire domes.

I have two punch and die types, one that uses a screw and wrench to force the punch down into the close-fitting cup of the die. The other preferable one uses a hydraulic ram to effortlessly provide the punching force. It also can easily fit into places where the screw punch will not go.

[Photo of screw punch]

[Photo of hydraulic punch]

Cutting holes in thin foils with a punch and die. The thin titanium and stainless steel foils that I use in my stoves are not naturally compatible with the above punches and luckily my current stove designs have moved away from needing many of them.

The punches tend to draw the metal down into the die and distort the sheet, then rip it, rather than making a clean cut. Both the above punches can be made to work by putting thicker backing sheet metal behind the foil and cutting through both. The ends of a bean can, with the rim left in place, make ideal rigid backing plates for this task.

“Even the leftover lid with a precise hole can be used as a tool for setting and precisely gauging tubes and cones to fit appropriately before welding.”

Cutting holes in thin foils with a craft knife. Even thin foils are cruel to drills. However, they cut easily even if tediously with a simple craft knife blade.

“It is hard to believe, but the blades just seem to keep on cutting without getting noticeably blunt. They do infinitely better than my poor twist drills”

[Photo of custom made craft blade holder]

I cut to the inside of the marked shape. Then I use a DIY grinding cone to dress the hole to the marked line with a reciprocating action. I use a piece of cleanly cut off pipe as the anvil to support the sheet during this dressing. “It may sound tedious or impossible, but it is surprisingly quick and satisfying to make a hole this way. The grinding can be done with the cone in a drill press, but it quickly wears out the aluminium oxide abrasive cloth. Grinding stone cones and cylinders work also, but they throw burrs at the edge of the hole that is nasty and must be dressed off. More to come.

Tim