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Vacuum Cannon Tips

After sending out a few cannons, I decided to send out a “Tips” sheet in the cannon packaging to help people get started. I’m reproducing it here for those who didn’t get it:

Vacuum Cannon Tips

  • The pipe suppliers are used to providing for large scale filtration systems. They care not about little nicks and scratches on the pipes. I sand off the more obvious burrs (so the pipe might look a bit battered) but they will have no effect on the performance of your cannon.
  • Please don’t underestimate how loud it is close up; it makes your ears ring for some time afterwards if you fire it with no protection. The shooter DEFINITELY needs ear defence of some sort because they won’t have their hands free to cover their ears.
  • Flanges (pictured) do make it easier to prepare and fire. There are many ways to obtain some. You can use cardboard, old mini-whiteboards, 3D printing… Thomas found that his tech department were very happy to be involved, knocked out laser cut flanges within a day and enjoyed the test firing process!
  • The tapered holes for the valve make a very good seal, but if in doubt PTFE plumbing tape will make the seal even better. Thomas has not needed it apart from the early prototype.

Firing

Please warn your colleagues! You don’t want an evacuation because of a suspected shooter on the premises. This has happened. “Duds” are possible, but can be mostly avoided with the following approach:

  1. Don’t forget to roll the ping-pong ball all the way down the tube. This is easier before you add the tape because of the ball’s tight fit.
  2. It is worth taking care with the taping of each end. The rear just needs to be secure. The front needs to be secure but not too secure! Take care on the front, cut a square piece of tape and carefully fit it to the front of the tube. Try to fold back and stick as little as possible. You want the least folding and sticking that will support the vacuum. Trial and error will help you work out the optimum. We believe the tape blows off, so you want that to happen before the ball arrives.
    (If you have flanges, this step is much easier, just cut a square and make sure it is securely pressed all round the circle of the tube and gently stuck to the flange.)
  3. Let the pump run for a while. You know very quickly if you have a bad seal because you will hear cracks from the tape and then the frustrating hiss of air being sucked in. If it is working nicely it gets to a pretty good vacuum very quickly but if you wait 10 seconds or so it continues to increase.
  4. Pierce the rear tape. Scissors are fine. It fails catastrophically and very very loudly. It is worth checking there is no tape hiding in the tube before loading the next ball.
  5. Please share your experiences on twitter @physicstp or Facebook.
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Making a Vacuum Cannon

The Tube

The tube does not need to be super strong (the tape will fail long before the tube), but it does need to be close in diameter to a table-tennis ball (40mm). The right tube is known as Imperial 1½ inch PVC, common in the USA but not so much in the UK. I collected it from Koi Logic for £5.05 a metre since delivery is quite expensive and it is not too far away. CLEAPSS suggest the plastic pipe shop where it is about the same cost but you get a longer length (so more expensive and a lot of waste).

1m of tube is fine, it is easy to transport and store. You can get tremendous speeds from a 1m tube. CLEAPSS recommend that you do not go longer than 1.5m. I chose 1m because above that, the delivery charges get silly.

The valve and its Hole

You need a way to connect the vacuum pump to the tube. Patrick Kaplo sent me a link to the hose elbow he used in the USA and CLEAPSS suggests a metal schrader valve that you just self tap (force in to the PVC) and glue in to the pipe. Their instructions here are a bit vague on the size of drill bit you need. A very unscientific poll of physics teacher friends suggested that they all had rubber hoses on their vacuum pumps so the elbow is much more sensible.

Being a bit of a perfectionist I thought that tapping it in to a tapered thread would allow for cheaper components, a better seal and for replacing components if needed. I found some cheap nylon fittings at Wreking Pneumatics, all they needed was a correctly threaded hole.

For pneumatics I learned you really should used tapered thread (BSPT) holes to get a good seal with no need for tape, glue or sealant. Tracy tools are helpful on the phone and sell reasonably priced tap, dies and drills. I wasn’t sure what would work best so bought a plug tap and the correct size drill (8.4mm for the ⅛” plug tap) for both ¼” and ⅛” tapered thread holes. The prototype has ¼” BSPT but I learned that the schrader valves suggested by CLEAPSS and potential vacuum gauges all use ⅛” BSPT so the ¼” tap I bought was redundant. (I actually bought another but it took so long to come in the post via amazon I gave up on it; Tracy Tools delivered within 24 hours of ordering.) Tapping PVC is a doddle compared to steel.

The Tape

*just* an overlap

The tape was the cheapest “three inch” (78mm wide) tape I could find on eBay, just bought in bulk. The prototype used “two inch” and worked, but barely covered the width of the tube (see photo).

The Balls

The balls are practise balls from eBay, bought in bulk. “Real” balls are much stiffer and about twice the mass. I bought 300. Softer balls expand a little in the tube I think, so possibly get more push, and being lighter can accelerate faster? This is worth investigating perhaps?

No Flanges 😥

Flanges make a great stand for the cannon and provide a good surface for the tape to adhere too. The best I could find was £8 a flange, but that would almost double the cost of the cannon. Robin and I tested the cannon with no flanges with very good results. My tech department whipped two up for free with no problem and you can make basic ones out of cardboard or 3D print them. There are many options. Our tube’s diameter was 48mm (Officially 48.26mm according to the Standard). This is a good opportunity for the school to personalise their cannon!

Adding a Pressure Gauge

I found pressure gauges on eBay for £7 including postage. They come with a ⅛” BSPT (taper) male and screw straight in to another ⅛” tapped hole. Just search for:
1/8″ BSPT pressure gauge.

If you make one, do let us know how you get on:

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Selling a Vacuum Cannon

We are selling Vacuum Cannons in the shop. I am setting it up with some trepidation: if nobody buys one I will be several tens of pounds out of pocket. If everyone buys one I will be unable to make them fast enough to keep up with demand. Of course, the most likely thing is somewhere in-between. Being part-time I can respond quite fast, and I can limit the number I have to make and dispatch by setting the stock levels in our shop software. And through the piss-poor planning you have come to expect, we are releasing them a day before I go away for a week over half-term. Oops.

The pay-off is that more people will get to play demonstrate with this fantastic toy piece of equipment. I had never heard of it, nor had Robin, but CLEAPSS are all over it so it must be pretty well known. When I tested the prototype in the kitchen I didn’t really think through the consequences of a ball travelling at several hundred miles an hour vs my kitchen wall. There was ball debris everywhere; I found one shard 5m away on a high window sill two weeks later.

I have set up stock of 2 in the shop but with unlimited back orders. Once I know what the demand is (or indeed if there is any!) I will start making and shipping them in batches. I intend to keep doing them at £20 until I lose the will to go through all the guff of making, packing and dispatching them. Then I will either give up or raise the price. So buy now to avoid disappointment.

Anyone with access to a vacuum pump can of course make their own. If that is your plan, you could use the CLEAPSS guidance I mentioned above. I also made my own “How To” notes which includes links to where I purchased things.

If you make one, please let us know how it goes.

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