How not to make a fuselage

[Hobbes]

I've been experimenting with scratchbuilding a fuselage. My idea was to drill a hole through some wood, and use that as a template or mould: fit some plastic sheet inside and glue it up.

The first result isn't what I'd hoped:

The mould consists of 4 sheets of MDF. The hole is as near perfect as I can make it (using a column drill).
I fit two sheets of plastic, one inside the other, then applied pressure to get them to conform to the inside of the mould.
As you can see, the plastic isn't quite round: the ends don't meet neatly.
Not visible in the photo is that the sheets have crumpled a little: the plastic has been overstressed in places, rather than having a nice even radius. All in all, the plastic was too thick for this. The hole is 19 mm across.
This was the thinnest plastic I have (0.3 mm), so there's not much scope for sanding down the imperfections.

I could try using paper instead (less stress, so it'll probably give a better-shaped cylinder), but then there's the question of how to stabilise/strengthen the paper after shaping it. Maybe a few layers of lacquer will do the trick.

I suspect Geoff's technique, starting with a smaller shape and then adding styrene strips, will give better results.

[kitnut617]

Did you heat the plastic before you applied pressure? 

In the steel fabrication industry, when we want to make a cylinder like this, we have to have some run-off tabs along the edges that get joined. This is because of what you have just found out, the last little bit doesn't bend so you need the extra so the the bend continues to the exact point where it's supposed to join. We then cut off the run-off tab at the desired point, usually leaving as near a perfect diameter as you could possibly get.

My other suggestion would be to make two of these and then just use the sections from each one that have formed good to make one complete section.

Have you thought of using that expanding draught stopping foam as a core? That might force the shapes into a round shape while it's glueing.

[John Howling Mouse]

Was your actual goal to end up with a simple, thin-walled plastic tube?

[Hobbes]

JHM: Yes, I wanted to create a simple cylindrical tube. Wall thickness wasn't important.

Kitnut: I didn't heat the plastic. I found that when you heat plastic, there's a very narrow temperature range where the plastic is malleable. Above it, the plastic will warp, buckle and char; below it, it's just as stiff as at room temperature. I haven't found a way to reliably arrive at this temperature.
The correct temperature is also above 100 C, too hot to handle.
There's no way I could heat the plastic, then roll it up and insert it into the mould.

[sequoiaranger]

>I didn't heat the plastic. I found that when you heat plastic, there's a very narrow temperature range where the plastic is malleable. Above it, the plastic will warp, buckle and char; below it, it's just as stiff as at room temperature. I haven't found a way to reliably arrive at this temperature.  The correct temperature is also above 100 C, too hot to handle.
There's no way I could heat the plastic, then roll it up and insert it into the mould.<

Roll it up into the mold first, then insert a hot hair-curling iron from the inside. Watch it carefully for that moment when the plastic warms up enough to "sag" into place.

If you just want a circular tube, you can probably find something suitable in a commercial plastics shop ready-made. Your wooden-block and heat method will probably be better for other-than-round fuselages. Of course you would have to have the woodworking skills to make the mold shape the way you wanted, but it might work with the inside heat I mentioned.

[Hobbes]

Up to 12 mm, you can get any size you like, above that the shops I've found only stock a few standard diameters. You're limited to electric pipe (15 or 22 mm), or drainpipe (25, 32?, 40?mm).

[ysi_maniac]

Hi Harro,

Do you want to make a cylinder?

Did you ever had built a cardboard model? like this, for example http://www.mundoradiocontrol.net/index.php?main_page=product_info&cPath=95_105&products_id=848

Use styrene sheet 0.25 mm or 0.3 mm as if it were thick paper. This way, you will be able to produce many different shapes, cylinders, cones, trunks, etc...

[John Howling Mouse]

JHM: Yes, I wanted to create a simple cylindrical tube. Wall thickness wasn't important.

Could you simply use a short length of 3/4" wood doweling with the center drilled out to reduce weight (or would it still be too heavy for your needs, i.e. if you have a tricycle gear project and this cylinder is aft of the centre of gravity?). 

[Hobbes]

I didn't go with wood because I wanted to avoid the endless PSR that entails. Also, if the dowel is much thicker than the diameter you want to end up with, you have to be very careful to maintain the cylindrical shape while sanding.

I've never built a cardboard model, so I've no idea what sort of construction is used in those.

[jcf]

Can you find a length of conduit or pipe with an approximately 19mm inside diametre measure?

Roll your plastic (let it overlap itself don't try to get a butt joint) and insert it into the conduit, plunge the whole assembly into very hot water(less than boiling, try @ 85 C) for a couple of seconds and then into cold water. After the plastic is set remove it from the conduit and trim the overlap.

The other way is to wrap it around a pipe or dowel with a diameter just less than your desired cylinder, the diameter depends on the
wall thickness of your finished piece, again let the sheet overlap. Secure the plastic at several points with cable ties and do the hot water dip
followed by a trim.

I've used both methods, the former giving somewhat better results.

HTH

Jon

[cthulhu77]

I am suprised you can't find a ready made product in that size. Glass suppliers here stock every size imaginable, from stretched sprue thin, to 6' (72") diamater plastic tubing, in a variety of colours.

[famvburg]

    www.plastruct.com

I've been experimenting with scratchbuilding a fuselage. My idea was to drill a hole through some wood, and use that as a template or mould: fit some plastic sheet inside and glue it up.

The first result isn't what I'd hoped:

The mould consists of 4 sheets of MDF. The hole is as near perfect as I can make it (using a column drill).
I fit two sheets of plastic, one inside the other, then applied pressure to get them to conform to the inside of the mould.
As you can see, the plastic isn't quite round: the ends don't meet neatly.
Not visible in the photo is that the sheets have crumpled a little: the plastic has been overstressed in places, rather than having a nice even radius. All in all, the plastic was too thick for this. The hole is 19 mm across.
This was the thinnest plastic I have (0.3 mm), so there's not much scope for sanding down the imperfections.

I could try using paper instead (less stress, so it'll probably give a better-shaped cylinder), but then there's the question of how to stabilise/strengthen the paper after shaping it. Maybe a few layers of lacquer will do the trick.

I suspect Geoff's technique, starting with a smaller shape and then adding styrene strips, will give better results.

[Gary]

I am in awe of guys like you who take projects like this on in the first place.

[Hobbes]

The Plastruct site is a good tip. I've checked several local stores and websites, but none of them carry the diameter I need, so I'll order a bundle of tubing directly.

Gary: don't think too much of it. The experiment cost me next to nothing (€ 5 for a drill bit, the wood was scrap) and took no more than two hours, at the end of which I'd learned some valuable lessons. It's sort of thinking with my hands, trying to see what works and what doesn't.

[jcf]

The Plastruct site is a good tip. I've checked several local stores and websites, but none of them carry the diameter I need, so I'll order a bundle of tubing directly.

Gary: don't think too much of it. The experiment cost me next to nothing (€ 5 for a drill bit, the wood was scrap) and took no more than two hours, at the end of which I'd learned some valuable lessons. It's sort of thinking with my hands, trying to see what works and what doesn't.

Be aware that most of the Plastruct architectural/piping modelling tubing is butyrate not styrene, especially over 3/8", however they do have some ABS and acrylic tubing, I'd recommend either the ABS or the acrylic. Gluing any of them requires the correct cement.

Jon