Howto print a hollow half-sphere/helmet

Hello,

for an helmet-like object I require a hollow half-sphere.
Any ideas how to print such a thing because of the
restrictions on overhang?

My best idea at the moment is to segment it. Have
the outher side of each segment round and the inner
flat (because the inner side sits on the printing bed).
Then combine them with screws and either rips or small
helper-objects that go between 2 such segments.
This would essentially create an outher sphere with
an inner dodecahedron-shaped hole.

Any better idea?
I guess supports are still way too experimental to be used?

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Trying the challenge?

Support is not that experimental, its just not the best solution for support, we would need another material for a good solution. Give it a try, you will get a lot of strings so its a good advice to stay near to remove some o that during printing.

Does the sphere need to be perfect?
You could do something that is a dome on the outside, and close to a dome on the inside.
The popular whistle shows that overhangs can be possible in some conditions. Filling up the dome fulfills (at least one of) these conditions.

It depends on your extruder and the size of the dome if this is going to work, and it depends on your application if this is acceptable.

Se attachment for the idea.

M.qrius Wrote:
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> Does the sphere need to be perfect?

As I said. "helmet". A human head has
to fit in it.

> You could do something that is a dome on the
> outside, and close to a dome on the inside.
> The popular whistle shows that overhangs can be
> possible in some conditions. Filling up the dome
> fulfills (at least one of) these conditions.

What condition does it fulfill?

> Se attachment for the idea.

The overhang in your version is much worse
(90° to the vertical axis everywhere). that object
would be completely unprintable.
That would be something like 200-300g of material
being printed in mid-air. It would simply fall down.

I guess as supports do work I'll do some sphere-sections
of about 20-45° size with a way to connect them using
screws or some kind of snap-in.

Marcus

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No the shape M.qrius proposed could be printed without support. We can span gaps as long as it is supported at each end. The first layer or two may sag a little in the middle but after that it will be normal. See [hydraraptor.blogspot.com]

However it might not fit a normal head and something big enough for a human head would take tens of hours to print.

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[www.hydraraptor.blogspot.com]

MarcusWolschon Wrote:
-------------------------------------------------------
> M.qrius Wrote:
> --------------------------------------------------
> -----
> > Does the sphere need to be perfect?
>
> As I said. "helmet". A human head has
> to fit in it.

Right, that's going to be hard


> What condition does it fulfill?
>

Take for example the whistle. Many people have printed it, even though it has a totally horizontal overhang. this is possible, because the molten plastic is a bit springey, so if you start a line on top of something else, and end it on top of something else, it can clear a pretty big gap inbetween.
You have to be careful though, that no lines are started in mid air. This will cause problems.
That is the condition.
The design I posted satisfies this condition, while your original one does not: the inside boundary of one of the higher circle layers will be printed in mid air.
My change basically removes the inside boundaries.


> > See attachment for the idea.
>
> The overhang in your version is much worse
> (90° to the vertical axis everywhere). that
> object
> would be completely unprintable.
So has the whistle.

> That would be something like 200-300g of material
> being printed in mid-air. It would simply fall
> down.
You may be right about that. The overhang would be very big, if you have a dome the size of a head.



Of course, you could do the reprap logo design as a helmet (attachment)

For a more sensible idea, consider this (attachment)
You'd get a bit of a flat top on your helmet, but not much. Also, the inside is a dome.

Heh, M... I'd totally buy a helmet that made me look like a Conehead.

Still, could you just half it lengthwise and use a heat gun to fuse the two together?

Edited 1 time(s). Last edit at 10/29/2009 01:01PM by Lethosos.

Heh, perhaps Marcus will be selling them soon?


You can't halve it lengthwise, that doesn't solve the problem.

I think the minimum number of parts would be 3:
halve the dome horizontally at the 45 degree angle, then halve the top half lengthwise, and build those on their sides...

M.qrius Wrote:
-------------------------------------------------------
> For a more sensible idea, consider this
> (attachment)
> You'd get a bit of a flat top on your helmet, but
> not much. Also, the inside is a dome.

That's actually a great idea!
If I constructed the support-structure you added
as a kind of honeycomb that can be broken/cut off easily
that would work out.

Why didn't I think about flipping the sphere before. It's so obvious.

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M.qrius Wrote:
-------------------------------------------------------
> Heh, perhaps Marcus will be selling them soon?
>
>
> You can't halve it lengthwise, that doesn't solve
> the problem.
>
> I think the minimum number of parts would be 3:
> halve the dome horizontally at the 45 degree
> angle, then halve the top half lengthwise, and
> build those on their sides...

Yes, that would work.
Leave some slots at the end where a screws
with washers can fit and it can be held together.

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You could build it in pieces that would fit together like a geodesic dome. Probably wouldn't want to use it as a crash helmet.

R.J.Bowman Wrote:
> You could build it in pieces that would fit
> together like a geodesic dome. Probably wouldn't
> want to use it as a crash helmet.

And how exactly do you propose to combine the pieces?

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Hi Marcus,

... look at the images: here i stacked some sheets and milled them afterwards and here i presented another method for fabbing hollow objects ...

I don't have a CNC milling-machine yet nor do I plan to get one
in the near future.

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Why not just print the support (dome-shaped, maybe with some edge supports around it), brush it with mold release so the new plastic doesn't stick to it, and then print the actual part on top of it? Pop it off, print the next part, rinse, repeat. Then you use an ABS glue to stick the parts together.