1/250 scale Zeppelin beta builder(s) wanted...

[Rubenandres77]

Originally Posted by Kugelfang

...
Is there any rule of thumb when designing internal parts to allow for paper thicknesses? Like... circular formers should have a diameter equivalent to 98% of the diameter of the 'outside' parts? ...

You may wish to have a look at the first post in this thread:
Making paper cylinders with a ledge & full inner layer
Although it is applied to a different challenge, I think the explanation
on how to compensate for paper thickness may be useful for your case.

[thorst]

You need to account for the paper thickness in mm, not in %. That means that you need to set the scale for your model before designing the actual parts. In general, I use 0.1mm thickness per 100g/m² (that means 0.2mm for 200g/m² and so on). In most cases, the outline of the parts you want to print is the shape of the part in the neutral axis (see here: https://en.wikipedia.org/wiki/Neutral_axis). So assuming you design a cylinder with a diameter of 100mm with the formula

part_length=2*r*Pi

and don't account for the paper thickness, the outer diameter of the cylinder will be 100mm + paper_thickness , the inner diameter will be 100mm-paper_thickness. A former should thus have the diameter 100mm-paper thickness.

On the other side, you may want to have a cylinder with the outer diameter of 100mm. Then you need to take it into account before calculating the length: D=2*r=100mm=2*r_neutral_axis+2*paper_thickness. The length of the part needs to be 2*r_neutral_axis*Pi, and the diameter of the former is 100mm-2*paper_thickness.

When modeling with 3D-software, you can just trace the part directly in the neutral axis and add half of the paper thickness in each direction. This way, you can account for the paper thickness when designing formers of irregular shapes.

[Kugelfang]

Thanks for the links and explanations. I'm using 65# cover stock (so a thickness of ~ 0.198mm). Since my model is actually polygonal in cross section rather than circular, are there any calculations to determine tolerances for the corners of the polygon? Do creases affect the Neutral axis? Or is this usually not a factor?

The above questions are for future reference.

In this case I think an empirical process is going to work better for me. While I could revise the original 3d model to a high level of tolerance to account for the paper thickness, I can't really export only the revised parts to the SVG format for the paper model. I'd have to export the entire model to ensure that all the parts are printed at a consistent (though not necessarily accurate) scale. With Wings3d, that is an art, not a science--scale consistency is the BIGGEST challenge in this work flow. It also would set me back all the way to the beginning of layout and graphics in Inkscape.

Instead, it's probably a more effective to make minute reductions of the formers in Inkscape, print and assemble to determine when I've reached a good fit.

Oh, well, it is a learning project, after all.

--jeff

[thorst]

If you want to use your current Inkscape files, I would try to offset the outlines of the formers by ~0.1mm to the inside for cylindrical sections, and to about 0.3mm for the front formers. At the formers in between, just use values in between. That's just an estimation, you'll see if it works later.

In general, I found the method of adding half of the thickness in each direction to work well also for folded parts. Trying to draw even more exactly quickly leads to cases where the design accuracy is less important than printing- and building accuracy. So just try if it fits together after using the method above, and then tweak it slightly if it doesn't work.

Thorsten

[Kugelfang]

I got lucky. Looks like the first size reduction I made will work. I'll be able to tell for sure once I assemble the rear portions. But for now... back on track.

(Also uploaded the revised version to the link in the first page of this thread.)

--jeff

[missileer]

Jeff,

Thanks for the update and discussion of the sizing problem. I found that I had to trim the connecting rings and make some spacer cuts in f.04 to make it fit. I thought that I had really lost my cutting skills and was going to beg off on the beta build. I'll download the update and try again.

John

[Kugelfang]

Hi John,

Never assume I know what I'm doing! If there's a problem in building, more than likely it's because I messed up the design.

I appreciate you taking on the build.

--jeff

[Don Boose]

This and your other dirigible build thread are exceptionally informative. Great Imagineering, engineering, math, and rollation.

Don

[Tino]

Originally Posted by CharlieC

Not true - the UK's official measurement system is metric but there are special dispensations such as the road signs which still use miles. There was a recent article in our local Oz papers about what Brexit will mean for measurements - there are the Poms who want to go back to the Imperial system when England leaves the EU.

Regards,

Charlie

Really they want to do that? They should also forbid the EU to use the english language!


I would like to build this model but with the available time that I have and the Akitsushima very far from finished I cant for now.

[missileer]

Jeff,

Now the glue strips are slightly too short and the formers slightly too small in diameter. Circumference wise, the former f.04 is approximately 1/4 inch too small. I am using 65# (or 176 g/m) paper.

John