hemis

[pyrochris732]

Hi,

 

Im thinking of doing some larger shells up to 4 inches. Do you think i should go with paper or plastic? Also, what site other than skylighter would you recommend for good quality?

 

Any pros and cons to each type of hemi? Anything I should stay away from? I think the plastic shells have a sort of built in spolette as well as a fuse ring.

 

Thanks,

 

Chris

[Mumbles]

I can't recommend paper ball shells enough. I feel like they're more forgiving, tunable, and give rounder breaks. Of course, you can make it all work with whatever you choose. I like paper because it feels more traditional, and I don't have to worry about dangerous or environmentally unfriendly fall out. A lot of people go with plastic when they start out, because it seems easier and faster. They also seem to be a little harder to get right. Plastic shells seem to require harder breaks, and more violent bursts in order to get round breaks. Paper usually comes out rounder. Maybe it's just because, you tend to take more time and care when making them.

 

Anyway, I like cannonfuse for their products. They have both paper and plastic materials. Pyrodirect will have both too, but their hemispheres come with 2 pre-punched holes, which I don't like. The paper hemis from cannonfuse are a little lower quality than some others I've used, but the break really comes from pasting not the hemispheres. I'd recommend storing them over a hard ball (I use plastic shells as a base personally). Rubber band maybe a dozen hemis together for a few days, and they'll round out.

[Arthur]

I strongly prefer paper hemis. There is a shrapnel problem with plastic parts that you have to clear up afterwards. With paper shells the debris is only paper which clears up easily or just rots into the soil, so less need to spend hours tidying up after.

 

The techniques for breaking plastic and paper hemis are different so don't try both at the same time. I like to glue a fuse tube into the hemi some days before I want to build the shell. If you try to glue the fuse in then force dry the glued assembly, the hemi usually warps and you can't get a decent looking ball shell.

[jwitt]

I just got my first case of paper products the other day from pyrodirect. 3" hemis (with the holes in one side), gummed tape (lots), and 1/4" ID x 2" long tubes. I'll be putting 1/4" time fuse in the tubes (a nice fit) which stay completely inside the shell as a passfire tube to the center of the burst charge. I'll be adding a little flammable stuff (maybe 4FA powder? or blackmatch pieces) to the empty part of the passfire, which will ignite from the time fuse and spit fire into the center of the burst, which is said to give more reliable ignition and better breaks.

 

I'll share my estimate on how much paper tape I need. Be warned- I have yet to build a shell (first one this week once my rice hulls dry!) I calculated the amount of tape from a very nice shell building tutorial made by 50AE in the members' tutorial section- you might want to check it out.

 

THIS IS ONLY MY ESTIMATE from counting strips on 50AE's work. I hope its fairly accurate, but we'll see- at least it's a starting point.

 

Since I am preparing to make 3" shells, I counted the strips of 30 pound kraft paper you used- about 30 strips per turn.

By my calculations, one shell takes about 37.5 feet of 3/4 inch strips (should be about the same for tape)

This means that a 500 foot roll of gummed tape should be enough to put 10 layers of paper (5 turns of "3 strip method") on 12 shells, with a bit left over.

 

 

 

Since it's been mentioned in this thread- Mumbles or someone else- how should I deal with those two holes in my hemispheres? Just paste over 'em? Will that make weak spots and ruin the break?

[Arthur]

Why not use one or both holes to fit the time fuse?

 

Either you pre-fit a 1/4 x 2" tube and fit QM to the sphere centre, then you fit the time fuse when the ball is complete, OR You glue the time fuse to the hemi first then fit QM or a tiny flash bag on the inside end at the centre of the shell. Larger commercial shells have multiple fuses to reduce the risk of a mis-fire.

[jwitt]

Larger commercial shells have multiple fuses to reduce the risk of a mis-fire.

 

Oh, that makes sense.

 

I would rather have time fuse/passfire coming from the bottom of a hemi than from an angle, and I don't want to use two pieces of time fuse. The two holes are to either side of the "pole" of the hemisphere.

[dagabu]

Larger commercial shells have multiple fuses to reduce the risk of a mis-fire.

 

Chinese shells use two time fuses, the good shells have a spolette. The rest is true, if there is a 1:100 failure in time fuse, thus the chances of both failing are 1:10,000 max.

 

If this is your first, you may wish to consider top fusing rather then bottom fusing if you haven't had the chance to block passfire before. For smaller shells, the block can be hot melt glue but as the size increases, so does the resistance to lift as all things at rest have a tendency to remain at rest.

 

D

[dagabu]

Oh, that makes sense.

 

I would rather have time fuse/passfire coming from the bottom of a hemi than from an angle, and I don't want to use two pieces of time fuse. The two holes are to either side of the "pole" of the hemisphere.

 

Addition: Keep the exposed amount of time fuse very short on the outside of the hemi, the lift can and does tear time fuse away from the hemi causing a flower pot.

 

D

[Arthur]

I always add a fillet of glued string round the fuse to reduce the chance of lifting the fuse into the hemi and having it flower pot. It's worked to 6inches for me. Alternatively put the fuses at the top and bring the leader through the lift and back up to the top and into the fuse buckets. Multi break cylinders are usually top fused!

[dagabu]

I always add a fillet of glued string round the fuse to reduce the chance of lifting the fuse into the hemi and having it flower pot. It's worked to 6inches for me. Alternatively put the fuses at the top and bring the leader through the lift and back up to the top and into the fuse buckets. Multi break cylinders are usually top fused!

 

No need to back track, cut a hole in the leader, place the cross match into the leader and tape it back up. This way you only have one place where the ignition can fail, not two. The only thing worse then having a shell not fire is to have it fire blind.

 

I agree with pasted string but for a first try, stay with simple. Get a few shells in the air and go from their.

 

D

[Ralph]

Chinese shells use two time fuses, the good shells have a spolette. The rest is true, if there is a 1:100 failure in time fuse, thus the chances of both failing are 1:10,000 max.

 

 

were are you pulling these numbers ? (is this place brown and smelly ?) I can tell you now if there was a 1% failure rate single fused shells would probably be illegal many a 1:10 000 rate is considered unacceptable by companies such as liuyang fireworks consider 1:10 000 to be far to frequent and this is why they use dual fuses

 

 

 

[Mumbles]

Ralph, it's called experience. It's something you may have some day.

 

Whether you like it or not, chinese fuse is still chinese quality. It's cheaper to add two fuses, rather than engineer a fuse that has a much more acceptable failure rate. It may also have something to do with less than ideal priming methods. Some brands are better than others of course. The failure rate may not be as low as 1:100, but it wouldn't be unreasonable to hear something on the order of 1:500 for medium quality pyro. Crap fuse from crap companies will use two fuses, while the only shells I've seen in the last 5 years with single fuses have been good quality to begin with. There was a batch of fuse a few years ago at the PGI that had shells dropping left and right. Failure rates significantly over 20% from what it sounded like. It's not an anomalous batch.

[dagabu]

LOL!!!

 

Ralph, yes I pulled them out of a smelly brown place!

 

They were completely anecdotal and not related to fuse in any way, its only math, where the failure rate is multiplied by itself to give you the new failure rate for a double fuse.

 

Mumbles, I have several rolls of the "crap time fuse", there are many that hate it because of all the bounces they got using it. The problem turned out to be the cross match though and had nothing to do with the time fuse. They switched from the thermolite to black match cross match and used it in several of the AllStars shells last year at the PGI convention.

 

Not one failed to light. That being said, I will always use two time fuse or one spolette on shells 4" and larger.

 

D

[Ralph]

Im using Japanese time fuse and Im yet to have it fail me it lights nice and easily and has never gone out

[dagabu]

That's wonderful! I just wont pay what they want for Jap TF, Ill pound spolettes first.

 

D

[Mumbles]

Exactly my point. Japanese take pride in their shells and products. Chinese go with what ever is most economical. Quantity over quality.

[Ralph]

That's wonderful! I just wont pay what they want for Jap TF, Ill pound spolettes first.

 

D

 

You make it sound like a last resort pressing spoletes isn't to bad once you get in the rhythm of it and you can get nice accurate timings I do it often (use them about half the time)

[dagabu]

Considering that the vast majority of the people here do not have a lot of cash and even less experience, spolettes just become another variable that could be better controlled by using a premade fuse for a first shell.

 

Jap time fuse is also difficult to find, I know a master shell builder that shoots his salamis at the all stars show that was looking for Jap time fuse this past winter and came up empty for a whole month till Lloyd dug some out for him.

 

And no, spolettes are not easy nor are they accurate when made by someone that has not done the exhaustive back work on homemade powder or uses the incorrect commercial powder. How many have access to commercial meal?

 

How about back drilling? Would a beginner know anything about back drilling or for that matter, how thick to roll the tube?

 

Lets get through a basic shell first and then make him do a four break Italian with brickwork stars and inserts with bottom shot next.

[jwitt]

So Chris- decided to go with paper yet?

 

Wow...now I'm thinking two time fuses wouldn't be so bad. With all the work that goes into a shell, it would be a shame to bounce it. Not to mention in the dark, hand firing a bunch of shells, just waiting for a dud to cook off 10 feet away. Makes me want to wear my steel pot instead of hardhat (kidding, mostly).

 

But, it seems kind of inconvenient to have two time fuses plus passfire tubes coming into the shell at angles, the way the two fuse holes are. I guess I'll figure that out quickly tonight when I start work on my tester shell.

 

 

 

Edit:

 

I've read that many folks make "time fuse" out of tape-wrapped visco instead of using 1/4" powder core time fuse. If the cheap chinese time fuse is so bad, I'm guessing that visco "time fuse" is even more failure prone?

Edited May 4, 2010 by jwitt

[pyrochris732]

yea, i guess ill go with paper. But im wondering why i cant find reinforced kraft tape anywhere. Everyone seems to only have the non-reinforced stuff. I have two rolls of this now, will it be sufficient to use over the reinforced stuff?

 

Thanks

[dagabu]

Reinforced kraft HERE but you want to use non-reinforced kraft for shells.

[jwitt]

Chris- a tip for you re: paper hemis: Mumbles' suggestion to rubber band hemis together to round 'em out is a good one. My second shell with actual round hemis looks a lot nicer, and I can only assume those good looks translate into (slightly) better performance.

 

I admit that it's obvious that a suggestion from someone who knows what they're doing will be a good one, but having tried it and seeing how well it works, I strongly recommend doing it.

Edited May 10, 2010 by jwitt

[pyrochris732]

thanks jwitt. So I should wrap them around a ball and then rubberband them? What ball for a 3" shell? Or do you mean just hold the hemis together wrapped in rubber bands?

[TheEskimo]

Dagabu--What's your reasoning for not using reinforced gummed tape? I use it all the time, and it seems to work just fine. Apart from it not being totally biodegradable, I see no issue for it. And why order it online? Just go down to the packing department at your local Staples/office supply store--that's where I get my rolls.

[dagabu]

Yup, you can get it at the local store too, a bit more expensive per roll but its your money.

 

Several reasons why reinforced kraft isn't used regularly for shells.

 

-Not biodegradable (like you said)

-More expensive

-Does not offer a better break

-Harder to get to lie down flat over spiking

-The fibers are not taut on spiked shells so they dont offer more confinement, just added cost

 

Then again, I only use plastic strapping tape on my plastic ball shells so to each his own but simple kraft packing tape will work as well as the reinforced stuff