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 Post subject: Spoonapult v1.0 the wreckening
PostPosted: Sat Mar 14, 2009 6:29 pm 
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While not a unabashed success, the spoonapult succeeded in throwing the cherry. We discovered several important things:

Pine veneer is not structural. As we tightened the rope it bent the handle of the spoon 90 degrees. It bent the metal support too. The bowl looks excellent and held up beautifully. We straightened it out, tried again with less tension and it worked. Not amazing, but a solid 20' throw with non-optimal trajectory.

The rope spring is excellent, but wooden windlases aren't strong enough.

The frame and elevation method rocks.

Wood glue over a layer of papermache makes a nice surface for foam sculpture.

The frame cherries and hands are ready for v1.1

Pine veneer and bondo is a great if smelly method to build non engineered things.

St. Paul constabulary in the park don't seem to be interested in small catapults but paused to look.

Notes for v1.1:
Deceleration and forces at the spring are damaging to throw arm.
Cherries need to be a little lighter.
Square steel tube for the spoon arm.
Foldability is essential for car transport. Maybe mortise and tenon if not folding.
Metal sleeves over the windlases should prevent the compression of the dowels.
Release mechanism works well, and is adaptable to rope release.


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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 8:31 am 
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metis wrote:
The frame and elevation method rocks.

*whew*
With a title like "the wreckening", I thought there might've been more crunch issues.

So. Spoon. 1/8" veneer strips, sawn from a seasoned pine board, then slapped together with epoxy & Gorilla Glue over soft steel... isn't strong enough to withstand the awesome forces of the rope spring. You know what I'm thinking... it might be time to start playing with fiberglassing techniques.

Obviously impact force is significant, even with the really nice leather-and-foam pad on the top of the frame, otherwise I'd suggest carbon fiber immediately; as it is, fiberglass is strong & cheap, and MUCH easier to conform to tight curves and odd angles. Or, if there's appropriate sheets to be found for not much ca$h, Kevlar. (Maybe at that fabric outlet Michael mentioned. I want to check that place out anyway.)

I'm envisioning fiberglassing the Spoon a bit like higher-precision, higher-tech papier-mache. As far as the layering, it's likely we'd be able to alternate between spiral-wrapped strips and straight longitudinal strips on the handle. The bowl probably wouldn't need anything more exotic than widely-used boat-repair techniques, but with some good overlap from the handle strips for strength, at the tight bend.

Is Spoon v1.0 too damaged to be sanded & used as the base for 'glassing over? Can we progress to Spoon v1.1 or do we truly need to shelve the original & start work on Spoon v2.0?

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 10:32 am 
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It would be pretty easy to wrap the existing spoon with a couple (or few) layers of fiberglass, could also use basalt fabric which is supposed to be a bit stiffer and is cheaper. My understanding is kevlar isn't much fun to work with, difficult to wet and much rougher than glass, basalt or carbon. It's also a pain to cut.

I'm starting research on vacuum bagging for my rocketry stuff which makes glassing irregular surfaces much easier.

This a good and cheap source for supplies, I could also probably volunteer some glass and epoxy for the project.

http://www.uscomposites.com/

These guys sell basalt, although it looks like they only sell it in sleeves or as carbon/basalt hybrid fabric. A sleeve (basalt or otherwise) might not be a bad idea for the the handle.

http://www.sollercomposites.com/

I'm not entirely sure just wrapping with fiberglass will provide the strength needed, perhaps the high stress areas could be taken down to the metal and build back up with pure composites.

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 10:42 am 
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Thanks for the leads, I've never even heard of basalt fabric. It's possible we'd only need to reinforce the handle anyway. Not sure how practical it'd be to target just the high-stress areas of the Spoon at this point.

I've heard that Kevlar is a pain to work with, too. Maybe if there's such thing as Kevlar ribbon, or tape, then that could be the spiral wrap. (One criss, one cross; other layers could be normal 'glassing fabric.)

I'd hate to strip all the wood & bondo off the Spoon:
    1) it'd be a serious pain
    2) it's already shaped
    3) it does have some strength as is

Wrapping with fiberglass (basalt, etc.) would add strength, though, and quite a bit of it without also adding too much weight. (The rope-spring and windlass can handle the weight just fine, as it turns out; but the lighter the launching arm, the better the throw.)

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 10:55 am 
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Theo wrote:
Thanks for the leads, I've never even heard of basalt fabric. It's possible we'd only need to reinforce the handle anyway.

I'd hate to strip all the wood & bondo off the Spoon:
    1) it'd be a serious pain
    2) it's already shaped
    3) it does have some strength as is

Wrapping with fiberglass (basalt, etc.) would add strength, though, and quite a bit of it without also adding too much weight. (The rope-spring and windlass can handle the weight just fine, as it turns out; but the lighter the launching arm, the better the throw.)


There were two failure points that I saw, one was the obvious one at the tip of the handle where it stuck into the rope, that's already down to the metal at the very tip, might want to take it back a bit and build it back up. The other place was at the bend of the spoon where it was delaminating, perhaps tightly wrapping that area with a few layers of fabric would help to hold it together. Wrapping the whole thing with a layer of glass (or a tight sock) would help to prevent delamination.

So if you went that way the majority of the weight would be near the center of rotation so shouldn't have a huge impact.

The trick with fiberglass to keep the weight down is to minimize the amount of epoxy used, which is where vacuum bagging comes in, you wrap the object in a porous (but not absorbent) sheet and that is backed by absorbent material that sucks up the excess epoxy.

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 10:58 am 
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Wups, I was editing my response as you were responding... anything to add? :|

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 11:10 am 
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Soller Composites is fascinating, btw. Man, I wish I was rich.

UPDATE: Some of that 2" basalt sleeving sounds like it might be just the thing for reinforcing the handle, as long as the reinforcement could also be extended out into the bowl of the spoon, somehow. $30 for 15' isn't peanuts (and that's just the cloth!) but it ought to do the trick.

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Last edited by Theo on Sun Mar 15, 2009 12:03 pm, edited 1 time in total.
Basalt sleeving!


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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 1:22 pm 
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fiber layups are really *not* ideal for this application.

they're very strong along the axis of the plane of layup, but remarkably weak against it, and *not* keen on impact forces. think about taking a hammer to a boat hull straight on.

while they'd probably do ok while under tension, the impact of hitting the release bar could very well shatter them, dependant on what hardening agent was used, and getting a *good* layup with good resins is not a simple task. one bubble in there and it spews glass shards everywhere. also the compression around the hollow core of the shaft wouldn't be happy at the spring. if we can bend 1/8 steel at less than a 1/4" radius, it's not going to go well.

if we did a wood veneer a bit more engineered than hacked laminate core and sheathed it with fiber that might work, but would be tricky to layup well around the steep curve of the neck of the spoon.

the gorilla glue on wood held up fine where it wasn't snapped in half however i think a high quality laminating glue with proper clamps is really necessary to ensure even adhesion, but we couldnt' have gotten the lay up done in as few sessions as we needed to make our time line. the bowl is a good example of this method as stable where it'd not expierencing really harsh forces.

the snap at the spring i wasn't expecting but makes sense. there is a delamination at the neck on the inside, which appears to be an impact related stretch issue. the lamination didnt' get in tight to the metal there, so as the head smacked into the stop and the arm tried to keep going, it popped those two layers.

nick and i were more or less in agreement that thin walled steel square stock is the way to do this *strong* and we could repeat the bowl methodology on that w/o significant changes.

i still think that a wood layup is a viable way to go, but that it really needs to be done more accurately.

so: do we want to build a new spoon? do we want to build v2.0 and leave 1.0? (n.b. i'm not suggesting anything nearly as agressive a schedule, but keeping *doing* something maybe once a month)


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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 2:36 pm 
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I think fiberglass is more durable than you think. Boats have a layer of gel-coat on top of the glass that is very brittle and crack prone. High power rockets routinely have recovery failure and fall from great heights and either bounce, tent peg or core sample and the airframe often survives. Some people do use kevlar in certain areas to improve long term durability.

Also we're talking about using fiberglass on a supporting structure.

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 Post subject: Re: Spoonapult v1.0 the wreckening
PostPosted: Sun Mar 15, 2009 4:24 pm 
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all of those examples are inline with axis of layup and not dealing with crossing impacts. think of a fiber golf club: it transfers a lot of energy through some spring and momentum to another object via the head but it you hit it against a tree it shatters. we need to hit a tree.

now i really want to do some layup stuff too, but i dont think this is the appropriate project. im just not seeing a lot of successful fiber/resin throw arms for impact release engines. now, the trebu-foshay would be ideal to have a foam frame laid up around a laid up core as the strains are all with the form. add in a foam outer shell to match the building shape and a few coats over that....

the spoon handle will have to be redone. the neck is damaged and the spring end is gone. the bowl is very salvageable though. we may have to notch out the inner side to fit new handle attachment but its primarily sound. it may be easier to repair the current spoon and use it as a prop/publicity item than an actual item.

anyone have a metal bandsaw?


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