Flowering Elbow

The cone of the cyclone is arguably the hardest part to make, especially if we are making it from clear plastic, rather than sheet metal. The bill of materials for all the parts of the cyclone (and for that matter the whole DS) can be found on the ‘Resources & References’ page at the end.

After I ruled out oven forming (hehe that was a funny experiment), I moved on to cold-bending, which is quite possible with polycarbonate. Polycarbonate, also known as Lexan, is the tough stuff they make bullet proof glass, machine tool safety guards and goggles from. It has glass-like transparency, is super strong and has quite good cold shaping characteristics: the rule of thumb seems to be that you can bend a circle of radius 100 x the sheet thickness. Thus, if we have a sheet of polycarb that is 0.75mm thick the smallest tube we could make by wrapping it up, will be one of radius 75mm.

So for the DS we are going to bend this rule a bit, because we want the diameter of the bottom of the cone to be close to 70mm. The very bottom of the cone (the ‘dust chute’), which becomes more cylindrical than conical, is more like 51mm. But we can use the lower half of a standard polycarb half pint glass to form this ‘dust chute’.

For the cone, begin with a nice big sheet of 0.75mm polycarbonate (wants to be about 780mm * 780mm). This thickness is ideal as it wraps up tightly enough to form our cone and is thin enough to be cut with tin-snips or very good scissors. You might think that this is quite thin for such a rugged application, but we can wrap it, so that it overlaps several times, making the finished thickness more like 2.25mm, which, with the mounting supports and the one-piece dust chute is more than enough. Hover over the photos for more…

Each 71 degrees of the flat arc represents one full loop of our cone when the material is wrapped up. With the first experimental cone I made, I measured and marked out for one loop with a small overlap (thinking that I could then make another separate cone to go inside). Don’t do it like this – you want the cone to simply overlap on itself a few times without the faff of joining several individual cones and stacking them inside each other. So to do this we need to mark out a very big arch – more than a semicircle’s worth.

When it comes to glue it in place, I used clear hot melt glue. It is best to roll up the shape you want first. Then once it is in the correct position, hold it there with a little masking tape. Now for the inside of the cone, you can peel back a little flap, and run the glue gun’s nozzle inside the seam, making a nice long continuous bead of glue. Let the flap close and press the parts together. Remember you don’t want any roughness on the inside of the cone where the air spins, so anything that splurges out or accidental blobs needs to be cleared away (use a sharp knife).

For the outside seam,  you can glue it in stages – undo a bit of masking tape near the top, pull back the polycarb making a flap and inject some glue under it, hold together till set, then move on to another bit, etc. When you are done and the cone shape is secure, you can run one continuous bead along the outside seam, to ensure air tightness. Of course all this would be easier if you can borrow an extra pair of hands for a few seconds to hold the cone for you while you glue it.

This is the cutting diagram and measurements to make the cyclone’s cone (click for full size image).

Glue problems: ‘polyplastic’, a solvent based plastic ‘weld’, seems to instantly fracture the thin polycarbonate when trying to glue anything that is under any tension. My suspicion is that it is making the polycarbonate brittle and because it is ‘cold-bent’ it cracks :( Instead, for all joints I found standard hot melt glue to work really well.