Once I had the major components for the spaceship laid out I designed up a truss in LibreCAD.
I needed some way to hold all the truss tubes in position while they get silver soldered (silver brazed). I considered small welding magnets but found they had a plastic part that would more than likely melt under the sustained heat of the propane torch I was going to use. In the end I decided a steel jig was required and I needed a milling machine to make it. Lucky for me I currently work in a machine shop and was allowed some time on the mill to knock something up.
Below you can see the result which is a slab of mild steel with 6mm wide slots 2mm deep machined into it. Then later at home I used a hole saw to cut away the area of the jig where the torch flame would need to heat the matimng surfaces of the tube. In the CAD drawing above you can see a small cross section where the 8mm tube locates in the 6mm slot and only touches on the edges, hopefully minimising heat transfer from the tube to the jig.
A clamping plate was made in the same way out of thinner material and without the slots. The slotted plate had four holes drilled and tapped for some clamping cap screws.
The tubes were cut to length in the mini lathe using a live centre as a length gauge. My high speed steel parting tool is 1mm thick and is mounted upside down so the lathe runs in reverse to part off. I used a little water soluble oil coolant squirted on from a spray bottle to ease the cutting.
The next problem to tackle was a mini tube notcher to "fishmouth" all the mating tubes some at 90 degrees and the others at 59 degrees. I figured I could use an 8mm end mill in the chuck of my mini lathe and make a tool post mounted jig to hold the tubes in position while the notch is cut. The jig had an adjustable end stop, with an 8mm diameter spigot which would align the previously fish-mouthed end of a tube with the cutting of the other end. I made up two of these end stops, one for the straight cuts and one for the angled cuts. The main body of the jig is a piece of 25mm square brass with a chunk of bronze bolted on that gets clamped in the tool post. The 8mm through hole was done by mounting the blank brass holder in the tool post and drilling through with the drill mounted in the chuck so the centre height would be correct for eventually cutting the fish-mouths.
Once the tube was locked into the jig by nipping up the cap screw, the carbide cutter was run up to around 2800 rpm and the carriage was then wound left towards the cutter .The mini lathe is not the most rigid of machines so I could only cut 1mm at a time so it took about 4 passes to cut to full depth.
The depth of the cut was adjusted with the cross slide and by setting the dial to zero at first touch. Each full rotation of the cross slide handwheel and the dial is 1mm depth and generally the full depth of cut required was 4mm. To make sure I didn't lose where I was with the depth and to make all the tubes the same I marked a line in white fine paint pen on the cross slide base.
The angled cuts where done in the same way using the same straight end stop for the first end swapping over to the angled end stop for the other end. The desired angle of 31 degrees was achieved by rotating the compound slide and locking it in position.
It took a couple of days to cut to length and fish-mouth all the tubes for the sides of the truss.
The silver soldering process could then begin by fitting the tubes into the jig and clamping it down. Liberal amounts of silver brazing flux was applied to the joints and just as the tubes were reaching cherry red the silver solder was applied making sure the capillary action drew it along the joint. Then the clamping cap screws could be removed and the jig moved along to the next position locating on the previously brazed last vertical tube, repeating the process along the full length of the truss. Once I started I realised it would be better to have an even number of bays in the final truss so added some to the long tubes with a small insert of mild steel rod inserted to hold the extra bits in position.
Once the two sides were made I then had to join them together with cross members. The cross members needed an extra notch cut at 90 degrees to the main fish-mouth as they had to join at a junction of an existing upright tube. This notch also had to be slightly angled as the truss is trapezoid in cross section, angled in for the bottom cross member and out at the top. See the CAD drawing for what I mean. The picture below shows one of the bottom cross members receiving that secondary notch at 90 degrees to the main one.
I was worried that I would need another jig to hold the sides at the correct angle but found that I could clamp the sides together with four cross members temporarily in position using a protractor to set the correct angle. The clamps held sufficiently to solder a couple of cross members at each end and then the whole thing was locked into position. All I had to do was then snap the remaining cross member into position and solder them up.
As I was only using propane as a heating source I had to use the lowest melting point silver solder. Unfortunately that is the one that has the greatest silver content and is consequently rather expensive. I used 5 lengths of 1.6mm rod which when I purchased it some years ago was around $100 AUS dollars. It is now even more expensive than that. Conversely I also did the whole lot with a 1.25 KG gas bottle using a medium sized burner.
The hearth I used was just 4 lightweight autoclaved aerated concrete blocks known here in Australia as Hebel. I have found them very good for this job as they are very good insulators.
Before I started I was concerned that the truss may end up twisted once finished as I had no way of keeping the whole length aligned as I soldered. In the end It came out reasonably well with a slight banana curve in the vertical direction which probably wont be noticed when the model is complete.
A couple of steel plates were soldered to the ends for attaching the modules. M4 button head cap screws screw into an 8mm block of PVC glued to the top of the modules to secure them in place. I also added a couple of strips of steel either side of the centre cross member of the truss to attach a 15mm water pipe flange which will be part of the model support structure.
The next task which was to clean off all the flux and scale using water steel wool and assorted wire brushes. I then primed it with a zinc auto primer as it started showing signs of rust.
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The picture above shows the model support made from water pipe fittings. There will be a bottom mount and a side mount in case I ever manage to build a motion control camera and want to film the underside of the model unobstructed. There will be a central styrene box built around the mounting which is still to come.
All in all it took a solid six days to complete this truss and it is incredibly strong. I can easily stand on it without any deflection what so ever.
Thanks for looking.
More soon...
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