The man from BOC
I phoned up BOC Tradequip at Crawley and the chap (Chris) on the other end was very helpful. Their set-up sounded like it was well suited to industry, but what about a domestic user who wanted just one bottle for maybe a year ?
No problems. I needed to hire the bottle itself, which involved an annual rental, then buy the actual gas Ė which (compared to the small disposable bottles) worked out amazingly cheap per litre! I also needed to buy a proper regulator and an adaptor to convert to the MIG unit. So, if I could get all my welding done in less than a year (Iíd hope so), then the cost of the gas, bottle, regulator would be around £150.
BOC could also supply 2kg aluminium reels of 4043 alloy wire, which is far more economical than the mini-reels. These 2kg reels are the same diameter as the 5kg reels of steel wire, but Aluminium is less dense than steel so the reels are lighter. I was convinced this was the way forward, so I went off to Crawley BOC to pick up a 5m3 Argon bottle, a regulator, two reels of wire, and an adapter to go between the regulator and the small diameter MIG welder gas pipe. Now I was ready for some serious welding!
The final countdown
A great tip I read in the "Metal Fabricatorís Handbook" (available from Amazon) is Never use a project to practice your welding skill. Practice on scrap first.
So I ordered 5 metres of 25mm x 10 gauge tubing from Outlooks metal suppliers, just to practise on. This bit kind of hurt. I was buying good metal, just to do practise welds, and it would then end up as scrap. But then again, the tubing wasnít hugely expensive, and I did need the practise, especially as this tubing was exactly the same alloy and the same thickness as the hovercraft frame and I didnít want to turn the hovercraft itself into scrap!
The real welding begins
The practise tubing was a good move and having reached a point where I was happy with my level of welding (thatís happy not ecstatic) I decided the time has come. First I degreased the frame and the support strips with white spirit to remove the cutting oil (from the M6 and M3 thread tapping) which seemed to have got everywhere. Then I welded the deck support strip to the outer frame and the deck support onto the inner frame. Having previously spent some considerable time screwing the support strip to the frame I could see that this was time well spent. The outer and inner deck frames were welded up with the deck panels in place to minimise warping, so it didnít need any additional clamping, I could just concentrate on the welding - which didn't take too long..
I started off running 6 inch weld beads, and not concentrating in one area for too long to try and prevent heat build up, which might lead to the metal warping. This worked OK but the start of the weld is always a little poor, so I ended up doing 10 inch runs, which meant fewer starts were required.
Feeling Hot !!!
Those July weekends saw me doing a lot of welding in the garage. This point was the hottest part of the Summer and I was wearing my overalls, but little else underneath. The sweat was running down my face and either collecting inside the welding mask, or dripping onto the hot metal and fizzing. It made me think; why was I cutting freezing cold sheet metal back in January, and now Iím welding aluminium with 3kW of heat just a few inches from my face?
Having finished the space frame deck, I turned the hull upside down so as to fit the T-section triangles. The triangles were fabricated one by one on the bench and then welded into place. They started off difficult. Much of the triangle fabrication needed butt welding and so I had to be very careful not to overheat the weld area, otherwise, it would burn through, and I'd be left with a hole. This was a particular problem towards the end of each weld bead, as the heat built up excessively. But once I had discovered the technique of using a steel backing piece things got much easier. A backing piece helps dissipate the heat, and if things do get too hot it supports the weld pool, preventing a hole.
I went to the Claydon House race in Buckinghamshire. Claydon is a great event. Itís set in a very picturesque estate, and is very well organised by the Chiltern hovercraft club. I saw Tony Broad again, and met the boss of Connick Tree Care, who lives just a couple of miles from me it seems.
I cut out the rear corners of the hull and bent them to shape Ė these are a quarter of a cone each. I thought I might need to use a mangle roller to bend the sheet, but bending it on the bench is much more successful than I had thought. I think this is because the curvature that I am after is relatively slight.
Jonathan Curtis aluminium hovercraft
I had an email from a chap called Jonathan in Devon who was just finishing off his own aluminium hovercraft. Mine and his could hardly be more different. Jon's is made from folded panels, with an internal frame structure, using the principles of aircraft design, and fabrication, i.e. internal formers and stringers etc. Mine is based on a space frame and detachable panels. Jon has used 3000 blind pop rivets, so he says he now has a very strong grip! Mine uses a welded frame and hundreds of M6 secrews. Jon is using a Rotax 503, whereas I am using an ex-motorbike engine.
I am particularly impressed by the metal hull (first photo above) and by the size of his workshop - it's at his works. Thanks to Jon for the photos.
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