Easy 3 Inch round shell hemispheres

[TheEskimo]

Hiya everyone!!

Here's a simple shell hemisphere construction tut (for all those pyros with not a lot of money). It can be adapted for any size shell (I think), but I am going to demonstrate a 3 inch shell.

What you need:

Plastic wrap

Tennis Ball

Wood glue

Newspaper

Water

Oven

Sharpie marker

Razor

OK, here goes:

1.Wrap the ball in the plastic wrap. Pull the excess into a strand, cut with scissors, and tape the cut end of plastic wrap to the ball, forming a smooth layer of plastic wrap over the ball.

2.Mix wood glue and water until you get a liquid like thin cream (about a 3:1 mix of water to glue).

3.Cut many newspaper into strands about 3/4 inch wide by 2.5 inches long. Dunk these all into your mixture of water and glue. (About a page or two of newspaper should suffice).

4.Now paste the plastic-wrapped ball as if pasting a completed shell, amking sure all parts of the ball get a consistent layer of newspaper.

5.Continue this until you have 4-6 layers of newspaper.

6.Turn on the oven to 175 degrees Farenheit.

7.Let the shell sit in the open air for atleast half an hour. Then put the shell in the oven. Turn off the oven, but check the oven every half hour to check that it hasn't cooled beyoned 100 degrees. This should take 1-2 hours. If you don't want to watch the oven, place the shells in a food dehydrator for several hours. If you're busy making stars or lift or burst or rockets, or all four!!, place the shells in a dry place for 12 to 24 hours.

8.Find a bucket, pail or drinking glass that wouldn't mind getting Sharpie on it. Make sure the bucket, pail, glass, etc. has an ID of about 2.8 inches. Take your dried shell, and shove it into the glass, until it looks like the rim is on the equator of the shell. Take your sharpie, and draw a line around the shell, using the rim as a guide. Remove the shell, and cut along the line with the razor. Now you have cut your shell, pull each side, and it should pop right off.

Hooray! That took 10-15 mins total work, not including prep time. If you make lots of them, you can have tons of shells for literally pennies.

P.S. When you're assembling working aerial shells, you can read the comics if you made your hemispheres with the right page of the newspaper

NOTE!! After drying for about 10 minutes, take the shell, and compress it in your hands, as if forming clay into a ball shape. This will keep lumps from appearing, and will ensure a smooth, sturdy ball.

Edited August 3, 2009 by TheEskimo

[scarbelly]

I made a set of hemis today using approximately this method. We'll see how it looks tomorrow when it's all dry. Thanks for the tut. My first set of homemade hemis failed awfully, so hopefully this will work better. I used a styrofoam ball rather than a tennis ball. Edited August 5, 2009 by scarbelly

[Swede]

Interesting, and I bet the wood glue will give it decent strength compared to a cheaper glue. Have you tried this with heavier paper?

[TheEskimo]

Nah, I haven't tried it with harder paper. I've been experimenting with shells made out of a layer of masking tape, two layers of reinforced gummed tape, and a layer of masking tape. The take about as long the paper mache ones to assemble, but they can be used right away.

[NightHawkInLight]

I have achieved excellent breaks from 3" hemi's made of newspaper and wood glue in a similar way. It is certainly a viable method. Good job on the tut.

[scarbelly]

So mine are dry now, and they are nice and hard. The North pole of my hemispheres ended up a little bit thicker than the rest. Thank you for the tutorial, I can't wait to go test it out.

[Weasel]

I made a hemi, and decided to stick it in the toaster oven at 150 degrees fahrenheit to dry it out quicker. Bad idea. I burnt my balls. It was also hell getting the hemis off, the plastic wrap melted to the ball. From now on i will just let them air dry.

 

It's not like I need 3" hemis anyway, I just made them for the hell of it. 2.5 inch shells are as big as i'm going next year. The year after that though, I will be moving up to 3" shells .

[scarbelly]

To make a 3 inch shell, does it make sense to do this around a 2.5" styrofoam ball? I figure the final outer diameter will be around 2-5/8 to 2-11/16. Will this be tight enough in a 3" (3.05") mortar?

[TheEskimo]

Sounds fine...I looked up commercial hemis....they all have a dia. of 2 5/8 inches....you should be fine...you'll just have to be careful pasting to get it to fit in the mortar.

[scarbelly]

Are you saying I should paste extra as to make it more tight? Or more conservatively so it doesn't become too tight? About how much thicker do you think pasting will make the shell?

[NightHawkInLight]

Are you saying I should paste extra as to make it more tight? Or more conservatively so it doesn't become too tight? About how much thicker do you think pasting will make the shell?

You'll figure it out. For your first shell just have the mortar there as you are pasting to judge how much more you should do. If it ends up being only a few layers you can fit on then consider making your hemi's smaller next time. On the other hand, if pasting ends up at 1/2" for a 3" shell, make the hemi's larger. Remember you don't want the shell to be quite a perfectly snug fit after pasting because you need room for your quickmatch leader. Also remember that the pasting will shrink some when drying. It's a process you will have to figure out on your own making home made hemi's.

[scarbelly]

Ok thanks. I'll keep that in mind.

[KruseMissile]

I finished my hemi's, at first i didnt think they would come out very well, but they look great. I prolly should have made them thicker but too late now lol. And i cut it off center(by just a tad). take a look(pictures are a little blurry).

Brian

[TheEskimo]

Looks good. Tell me how they work.

[KruseMissile]

i gotta wait a few months

[TheEskimo]

Ahhh...Why?

[Twotails]

I use a hairdryer in between layers, I made a tut on this a wile ago on instructables, havent thought of it sence, but if you were to take two (metal, wood or plastic) blocks, and turn them into molds, wrap moldes with plastic wrap, place your ball into it coverd with the glue/water/paper, then put it into a vice, and slowly make it tighter, untill the ball compacts.

 

or if you used the above molds with metal, or maby wood, you could use wood pulp/paper pulp and glue. theoreticly you'd just have to make a female part to fit in mold. ( and make two sets of molds for each hemi, so you'd need to make four sides)

 

try not to laugh at my sad exuse for a diagramhemi.bmp

Edited August 18, 2009 by Twotails

[KruseMissile]

Ahhh...Why?

 

I have to wait till late september or october to go to Ocotillo Wells(its the only place i can do fireworks). Look up the Witch Creek Fire or the Cedar Creek fire, it happened in my town.

[Mumbles]

Twotails, that is now commercial hemispheres are made. A few flower shaped pieces of paper are given a coat of glue and are offset, and pressed with something like you diagrammed. I recall something about hot molds. There may be a heat activated glue, or it is designed to dry faster or something.

[scarbelly]

I sort of tried something like that with elmers glue, and it didn't work at all. I would have had to leave it in a long time. Until I find a better glue, I like TheEskimo's method. unfortunately I killed my styrofoam ball after just one set of hemis

[scarbelly]

Sorry, I forgot to comment on this after using my aerial shell with these hemis, but it worked great.

[KruseMissile]

Mine worked perfect! Here is the video...

[Ventsi]

I've got a bunch of these made and some more that I'll make tonight.

 

I read that some of you are having trouble with build up on th poles. Here's a simple solution, don't have any poles! After you paste one layer paste the next layer perpendicular to the previous much like plywood is made. This way the hemis are nice and even, and I would think that the plywood like construction would give them more strength.

[firetech]

The Chinese have special cutouts that they use to make hemi's. There's not possibility of build up on the hemi's and they are rock hard. My suggestion is once it starts building up on the poles, stop applying paper there. Pretty obvious eh?

You might be able to find the cutout on google. I'll keep my eyes open. From there all you need to do is size it to your needs and make a form.

[Mumbles]

I have this file, but have never used it. It's at least an idea. Is is probably easiest to put it in a program like photoshop or microsoft publisher so you can print off a template fit to size. Go a little big, and trim the edges to make them fit nicely.

 

A few convenient paper thicknesses

 

4 sheets

 

4 ply poster board - ~1/16"

5 ply poster board - ~3/32"

8 ply poster board - ~1/8"

14pt file folders - ~1/16"

#70 chipboard - ~1/8"

 

5 sheets

 

4 ply poster board - ~3/32"

6 ply poster board - ~1/8"

12pt file folder - ~1/16"

#90 chipboard - ~1/8"

#120/#125 chipboard - ~3/32"

 

6 sheets

 

5 ply poster board - ~1/8"

10pt file folders - ~1/16"

14pt file folders - ~3/32"

#90 chipboard - ~1/8"

#120/#125 chipboard - ~3/32"