Introduction: Ruben's Tube Fire Scythe Engineering Project / Prop

About: The name "Ikkalebob" was invented by my cat when she ran across the keyboard. I attempt all manner of projects, from home engineering to prop replicas. Follow me on Instructables and my YouTube chann…

  • UPDATE: Two years after I built this project I decided to revisit it, using a cheap micro Bluetooth speaker to deliver the sound. I also decided to test it indoors, so I have new footage of it working correctly! I'm trying to grow my YouTube channel so that I can make more videos and Instructables, so any subscriptions are appreciated.

A Ruben's tube or simply fire tube is a set-up in which flames will become higher or lower according to the frequency of the sound entering the tube. Essentially, the flames act like an oscilloscope for the sound entering the tube.

In this project I decided to replicate this effect except, in the spirit of Halloween, I mounted it onto an 8-foot scythe. It should go without saying that this project is quite dangerous if you don't play it safe, so keep this in mind if you decide to have a go.

Unfortunately it only took a very small amount of wind to make the effect difficult to notice, but if I had a room with a high enough roof to be able to light the scythe indoors, you should be able to notice the effect. On the first video, notice when the singer says "big canoe" around 0:16.

Step 1: Materials and Tools

Here's yet another example of how buying something locally and getting a price face to face can save you a lot of money. I had originally planned to buy my steel from a hardware shop, which would have cost £40 for only just enough steel, not quite the right thickness and in not great condition. After visiting a local steel supplier, they offered me roughly 5 times the amount of steel, some with a little bit of rust on the surface, but for only £15. I now have more than enough steel sheet to complete this project 3 times over! The moral of the story is to shop around, and don't be afraid to ask if they have any offcuttings.


  • 1x1 Metres 3mm thick steel sheet
  • 1.8 Metres 4cm thick steel tube for handle
  • Around 2 Metres 3cm thick steel tube for flame tube
  • Fixed Bike forks
  • Solder
  • Flux
  • Gas line
  • Brass fittings for gas line
  • Steel Basket
  • Copper Wire
  • Cotton sheet
  • Wooden dowel
  • Screws
  • Small canteen bottle
  • Phone/Mp3 player/iPod speakers
  • Jubilee Clips
  • Headless Bolt with nuts
  • Blowtorch head
  • Epoxy glue


  • Grinder
    • Grinding Disc
    • Cutting Disc
    • Wire Brush Attachment
  • Welding machine (MIG or TIG)
  • Blowtorch or Soldering Iron
  • Files
  • Wire Brush
  • Drill
  • Drill Press

Safety Gear

  • Overalls (optional, but keeps your clothes nice)
  • Leather Apron (again optional but can save you a lot of pain/clothes)
  • Eye protection (absolutely not optional)
  • Welding mask
  • Leather gloves
  • Ear protection
  • Bandana for welding (keeps your hair from melting)
  • Boots (don't weld in crocs - trust me)

Step 2: Handle

The handle wasn't a total success, in fact it took a few attempts. The first try was some heat-bended PVC pipe reinforced with fibreglass, but this turned out too weak for the steel head and inevitably it snapped in half. The only real solution was to make the handle out of steel tubing.

A quick trip to the scrapyard and I found an ideal piece of steel tubing for only £2. It had become quite rusty so I used the wire brush attachment on my grinder to get rid of it. This revealed a peculiar copper coat, which I didn't expect to find under the rust but it actually looked really good. In order to join the blade to the handle I wanted to make sure I had an exceptionally strong joint, since the blade is extremely heavy and even unsharpened it could do some serious damage if it were to fall off. In order to do this, I used the drill press with a metal holesaw to make a large hole all the way through, where the flame tube would go.

The holesaw wasn't properly attatched to the drill press as I later found out, and unfortunately this made it vibrate a lot and the hole came out much bigger than it should have. In order to thicken the flame tube section, I used a slightly larger tube as a collar. Not only did this solve my problem, but it added strength to the structure.

Step 3: Cutting Out the Steel and Edge Grinding

I transferred the paper cut-outs on to my steel sheet and used an angle grinder to cut them out. A lot of inexperienced people (myself once included) seem to think that the more experienced you are the less you end up needing safety gear. The reality is, the inexperienced people forego safety gear and end up hurting themselves, whereas the experienced know it's just generally more comfortable and easier to wear it. I've set my trousers on fire and gotten some short term hearing loss from using an angle grinder irresponsibly, and now I'm more experienced I always wear gloves, goggles, ear protection, leather apron and something over my head so the sparks don't make me bald.

Safety rant over, it's time to get cutting. These were difficult to cut since the steel was quite thick, but its better to cut outside your lines first then grind back to refine the shape. Don't saw with the grinder, instead gently apply pressure and slowly move along your lines. Be aware of where your sparks are going, and try not to do this inside because the sparks embed themselves into your stuff.

I then made a rough edge grind on the bottom section of steel with a grinding wheel (don't grind with a cutting disc if you like your face intact!) and further refined this with a flap wheel. This took around 3 hours from cutting to grinding the edge, however, when I did this on my sword/machete it took about 15 hours. I've become more efficient because my technique has improved, I'm using the right tools for the job and, crucially, I've learned from my mistakes last time around.

A note on grinding technique is that its best to drag towards yourself using the section of the disc furthest from you, rather than pushing away, this will make you less likely to make gouges. I finished by using the flap wheel to eliminate the sharp edge I'd made, because I didn't actually plan to take any souls with this scythe, I only planned to blow minds with it.

Step 4: Flame Tube

I had hoped to find some maths on the topic of flame tubes, but unfortunately I had no such luck. As such, I didn't really know if what I had would work, in fact I expected that with all the holes from both rows, the gas wouldn't be flowing fast enough for it to light properly. As I found out in an early test, I was quite wrong.

I took an old rusty barbell and used the wire brush attachment on my grinder to get all the rust off. I then drilled two rows of holes using a drill press. My holes were around 3mm, and If I were to change them I would make them smaller because this makes the fire come out a bit more aggressively than I had planned for, but the upside is that bigger flames look more impressive and less like candles. Another important point is that I drilled the holes up to one end of the barbell but left lots of space at the other end where I could attach it to the handle and also figure out the sound system.

At this point I taped over one end and taped a blow torch to the other to test it out. I shouldn't need to tell you that this isn't very safe. In any case, it proved that the tube was fit for purpose.

I then ground off the end of the barbell at an angle so that this end would follow the contours of the top of the blade, and in general would end up more aesthetically pleasing (Its easy to understand what I mean if you look at the final pictures). I cut out an ellipse in 1.5mm sheet steel (thicker would perhaps have been better) and MIG welded it over the end, making an excessively large weld to ensure that no gas would be escaping from the end. I haven't been welding very long so I can't really give any tips, but I would strongly recommend watching some videos for beginners to welding.

Taking my very ugly weld which I fortunately forgot to photograph, I ground it down to make it significantly neater. I also ground along the top and bottom edges of the barbell (along its length) where I was to weld the top and bottom sections of the scythe. This is important, particularly if you're welding something chrome plated because the welding machine will not be able to make a circuit properly if there is an obstruction like rust, dirt or a chrome coating.

I fitted the steel collar I talked about in the "Handle" step, it was extremely tight fitting so I used a blowtorch to heat the collar and therefore expand it.

Step 5: Welding It All Together

Welding it a whole lot harder than it looks, in fact its a skill that takes years to master. Part of the reason I wanted to do this project was to get some more experience with welding, and despite having welded a few things before I still consider my welds very ugly. A great deal can be learned from the internet, although at the end of the day practice is what makes you a good welder.

The first problem you're likely to encounter is holding everything in place while you make your first tacks. I strongly recommend getting a magnetic fixture to hold everything in place, but if not you can use clamps. Bare in mind though that it can get really hot so don't use a plastic clamp.

I also welded some bike forks to the bottom to add height and make a neat little cage for the gas canister to sit in. Using some thinner tubing left over from the barbell, I welded both the forks and the rest of the handle to this tubing (this is easy to see by looking at the pictures) Doing it this way makes the joint stronger than if I had just welded the forks to the handle, because it means that if the welding is weak or flexible, the forks' movement is restricted by the interior tube. At this point the scythe became very difficult to photograph.

As a final note, a wise man learns from his own mistakes, a wiser man learns from other men's mistakes. Learn from my mistake and avoid massive burns by wearing gloves that don't have holes in them, and by not touching steel immediately after it's been welded.

Step 6: Electronics and Fuel System

Extreme caution must be taken while fitting the gas line, since the project involves open flames. If you're using this instructable as a template for a similar project, its important that you consider, as I have, where on the structure it will be hottest. From this you need to position melt-able parts as far away as possible from the hot areas.

I drilled holes at the top and bottom where the gas line would protrude from the handle, and threaded it through. It's important that you file all the edges of your holes to make sure there are no sharp areas where the tube could get punctured. Using some push-fit fittings, I attached the lower end to the gas canister and the upper end to the flame tube by soldering the fitting to the tube.

I found some old Sony Ericsson speakers and phone which were just the right size to go over the flame tube. Using a small canteen bottle, I mounted the speakers to the rest of the scythe, using a lot of epoxy to ensure all the parts were air-tight. One of the speakers was mounted on the outside, so that people would actually be able to hear the what the flames were dancing to. I secured the phone to the handle with a cable tie but this fell off. Ideally I would have liked this to be a wireless system but I wanted to have it done for Halloween!

The blowtorch fit nicely into the gap I had made for it, but in order to make it extra secure I bolted through the bottom of the forks and tightened it up to bend the forks inwards as well as putting in a massive jubilee clip for extra security.

Step 7: Fiery Basket and Lighting

A double Ruben's tube wasn't enough for me, so I needed to add an extra load of fire. I found an old steel basket, with what appeared to be glass beads on it. As I quickly found out, these were not glass, and I now have various degrees of burns standing testament to this fact. In any case, I mounted the basket on using a piece of dowel which fit very tightly in the very top of the scythe and had to be hammered in. I then screwed the basket into the dowel.

I used a cotton as wick for the basket's flame, but the best option would be some kevlar wick like what you would get for circus equipment. I simply screwed up the cotton into a tight ball and held it all together with some copper wire, since I knew this couldn't burn up or melt. I soaked this in paraffin and put it inside the basket.

Now was simply a case of turning it all on and lighting it! The end result was quite spectacular, although the ruben's tube effect was fairly weak outside in the wind.

Step 8: Lessons Learned

  1. Safety can't be stressed enough with these kind of projects. I have around 6 burns from this project, one from touching welds with my bare hands (my glove split) and the other 5 from the shower of molten plastic from the melting plastic beads on the basket.
  2. Get your settings right on your welding machine, I have a lot of splattery welds which could have been avoided if I had the feed speed and heat set correctly.
  3. Welding is hard. It looks so easy but its probably the hardest workshop skill I've ever tried to learn.
  4. Consider the properties of your materials before you use them. The original handle was a big waste of time when I should have immediately identified that PVC tubing was too flexible for thick steel.
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