# Harpy - an ongoing project



## STO (Mar 2, 2018)

So I have a thing for Bill Hays' designs, the Harpy in particular. HUGE thanks Bill, if you're reading this, for being so generous with your designs. I've made a couple by hand, but I always wanted more. If you've never handled one, it is a fantastic slingshot when it fits your hand, the hook on the end in particular is a great touch, but it really is a pretty size-specific design.

I do a lot of 3D printing, which makes duplicating and scaling things easy, so the natural conclusion seemed to be a 3D printed Harpy. This is actually harder than it sounds, as it is a very complex thing to model.... hence an ongoing project. If I can pull this off, not only will I have a limitless supply of harpys for myself, it'll be much easier to size these custom to fit perfectly, invert them for right and left hand use, and meet the demands of friends and family for slingshots.

So modeling..... here is a quick surface render:








And the first print:









Print here is just for general shape, so you can hold the object, wasn't meant to be structural just fast. And...... failure. It is both the wrong size for my hand, and there isn't enough swell under the index finger-thumb arch area. Try, try, and try again right? There will be more updates....... I'll post in this thread as the project progresses.

*note to mods, if this isn't where this thread belongs, apologies, feel free to move it.


----------



## The Norseman (Mar 5, 2018)

This is a great idea! Sign me up!


----------



## Ordo (Feb 11, 2018)

Great project.


----------



## Toolshed (Aug 4, 2015)

Sounds like you've made something I could use as a mold for casting. Doesn't need to structural at all. Ohhhhhh I so wanted to get into 3d printing....can't afford that gas now....


----------



## Tag (Jun 11, 2014)

Looks great


----------



## Bill Hays (Aug 9, 2010)

On the very first prototype the pinky curl was actually a hole for the pinky... but I changed that to the curl because it only fit certain hands well, and on others it was either to tight or to much spread....

Then after I saw what it looked like after that... the Harpy moniker just kind of stuck


----------



## STO (Mar 2, 2018)

Bill Hays said:


> On the very first prototype the pinky curl was actually a hole for the pinky... but I changed that to the curl because it only fit certain hands well, and on others it was either to tight or to much spread....
> 
> Then after I saw what it looked like after that... the Harpy moniker just kind of stuck


That makes sense, and is really cool. Thank you. :blush:



Toolshed said:


> Sounds like you've made something I could use as a mold for casting. Doesn't need to structural at all. Ohhhhhh I so wanted to get into 3d printing....can't afford that gas now....


So I've actually done resin casting from 3D prints. The trick, as with everything, is surface finish. Once you have your master down though, it works great.



The Norseman said:


> This is a great idea! Sign me up!


I'm so thrilled you like it! :looney: I wasn't sure if this idea would be taken as "oh, just another copycat, and using a 3D printer so the results are PLASTIC." Almost everything flying around in this section is made by hand from wood, composite, or alu.

So this has been brought up twice now, so let me turn around and ask: what is the etiquette on this form surrounding the use of other people's designs. I assume any intellectual property in the templates section is free for personal use, but now we're expanding a bit beyond that. Is it just considered in the public domain, free to use for all purposes? Not looking to step on anyone's toes here.

Latest update on ze projekt:








Profile obviously stayed the same, but once I had an initial size I could take some measurements of the slingshot and my hand to correlate scale to knuckle width. Thus harpy #2 came out just the right size for my hand.









I think it still needs a little more support around the swell in the middle of the hand. That'll be next rev. I guess. It is certainly shootable now as it is, despite being mostly hollow and only really set up as a demo. A couple other little printing quirks to work on as well. I did, just for giggles, try leaning the forks against my workbench and pressing as hard as I could. No failure, not even close, which is a good start.









Don't know if the image res will allow people to see it, but I'm loving these contour lines. Usually with 3D printing they're a real gremlin, you're looking to hide them because there is some sort of collective dislike of 3D printing or something. I never suffered from that, and I think these actually look amazing and add a cool grip. Still not sure about finishing, but that'll come later.









At the moment I have a new Harpy with full reinforcements and the increased palm swell going on the printer. Whether or not that ends up being the final rev. I'm thinking I'll clamp it in the vice and come up with some sort of system for stressing it until the forks fail while measuring the force. I shoot warbows in my free time, both Asiatic and English, but I've never seen a slingshot anywhere close to those sorts of draw weights. At the same time, my hanging scale only goes up to 110 pounds/50 kilos, which probably won't be enough. If I want a safety factor of 2, kinda the bare minimum, that means I'd max out at 55 pounds. Is that enough though? A safety factor of 3 would limit me to just 36 pounds. I guess we'll see if I break it, but I may need a bigger scale......


----------



## Hobbit With A Slingshot (Mar 14, 2017)

The contour lines do look really cool there. Be interesting to see what you could do with them and some different colored filament.


----------



## The Norseman (Mar 5, 2018)

The contour lines give it character, and probably keep it from sliding around in your hand. I would leave them there, except for on the fork tips. Depending on how abrasive they are, they could shorten your band life.


----------



## STO (Mar 2, 2018)

FAILURE!... sort of anyway. The 50 kilo scale got maxed out immediately and the slingshot wasn't even close to yielding. I guess I need a more powerful load cell.  I backed it off a whisker so it'd display a force, rather than FULL, but I'll have to do this again. It was definitely a jenky enough rig, one of those safety squints engage, two condoms, mother on speed-dial sort of situations. I probably would have clenched more if the load cell hadn't topped out so quickly.

















So the basic setup:
At one end we have the slingshot, clamped in the vice, and with 550 cord looped on the forks to apply load just as the bands would. The orientation in the vice is such that all the various loads are going to the correct places.









From there the paracord is attached to the load cell in the middle....









Which in turn is attached to a come along that can ratchet up a couple tonnes.









The whole disaster looks like this, including my very very messy bench:








But clearly 50 kilos wasn't enough to break it, or even stress it really, so I've got a crane scale on order. Should be here in the next couple days.

On the up side, at least the palm swell is right now. 










The Norseman said:


> The contour lines give it character, and probably keep it from sliding around in your hand. I would leave them there, except for on the fork tips. Depending on how abrasive they are, they could shorten your band life.


They're not really abrasive, although I had similar thoughts. I have a plan for smoothing out the fork tips so it'll hopefully be a non-issue.



Hobbit With A Slingshot said:


> The contour lines do look really cool there. Be interesting to see what you could do with them and some different colored filament.


I just prototype in that white stuff because it is cheap and doesn't foul the printer with dark colors that'll stain other prints. You can go from white to any color, but if you just printed a black or dark blue it'll be a bit of cleaning before you can print a light color. Fear not, I'm working on colors and exotic materials shortly.


----------



## The Norseman (Mar 5, 2018)

How come you need to test it further? It seems to me that 110 LBs is a sufficient test. I know that even if I could pull back 110 LBs my wrist couldn't support that kind of strain. WAIT! I know now! STO is a code name for Bruce Banner! He wants a slingshot that he can use while he's the Hulk!


----------



## STO (Mar 2, 2018)

The Norseman said:


> How come you need to test it further? It seems to me that 110 LBs is a sufficient test. I know that even if I could pull back 110 LBs my wrist couldn't support that kind of strain. WAIT! I know now! STO is a code name for Bruce Banner! He wants a slingshot that he can use while he's the Hulk!


Hahahahaha. So the serious answer to your question is what in engineering is called "safety factor." So, for example, a bridge which is rated for 100 tonnes and has a safety factor of one is expected to fail at loads greater than 100 tonnes. That is all well and good with car traffic and such, but what happens when the bridge is a few years old, there is a wind storm, and the teamsters union is in town? You might rightly be nervous driving on (or under) said bridge. This is why basically nothing is actually built to a safety factor of 1.

Similarly with this slingshot, in my size, right now brand new on a good day the slingshot took 50 kilos once, or rather a couple times. But scale the slingshot for smaller hands, deliver a few fork hits, etc..... see where I'm going with that? This is why I want to test this to either failure or a safety factor of 5 (five times intended maximum load), whichever pucker factor kicks in first.  I should add that, while I've never done it with a slingshot, I can draw a 110 pound bow and have vague plans involving slingshot power/velocity records so.......


----------



## The Norseman (Mar 5, 2018)

OOOOHHHH. Ok I see! Still it would be cool to be the Hulk!


----------



## STO (Mar 2, 2018)

The latest: Got a 300 kilogram load cell. That certainly should be enough to break a slingshot right?

Here we are zeroed out: 








Same setup as before with the paracord on the forks and the come-along to tension it:
















That is 141.5 kilograms, which is 312 pounds-force. The pucker factor is real here, if this fails there will be violence!









And at that force, as you might imagine, the slingshot has a hint of flex to it.

















Capturing the exact value at which something breaks is quite difficult. You have to be staring at just the right moment, and nobody will ever fully believe you because you'll never have an image that "proves" it...... unless you get unbelievably lucky:









Yeah, was taking a picture at the very moment if failed, and captured the scale flying to the side as it showed the final value. 156.0 kilograms, or 344 pounds force. if you assume a safety factor of 5, that is to say the slingshot's "rated" safe operating load is 1/5th the maximum load, that is 69 pounds or 31 kilos. Should be adequate for safety right? :rofl:

Aftermath: 
































It took fifteen minutes to find the part of the slingshot which broke. It clearly had bounced off several things before coming to rest. There are a number of interesting things to note here in the failure. The most obvious is that it didn't fail in the thin arm section where I expected it to. This is probably for two reasons. The first is, the way I print, I do a fixed wall thickness followed by an internal supporting structure. This gives strength and light weight, but it also means that the surfaces which actually bear most of the load are the same thickness in the thin sections as the main grip. Thus the increasing lever-arm length as you get down into the slingshot body increased the force and it failed further down. On this test mule, I went with a quicker-to-print internal supporting structure. Changing that supporting structure to honeycomb, or even cubic honeycomb would likely add another 8-20% greater strength to the system. Of course so would printing it in 20% carbon fiber filled polycarbonate. I think this is enough though.

So I've now sufficiently proven to myself that the design is strong and safe, time to make it pretty! I'll update with pics when I've got some shmexy designs printed out and finished. I have some real ideas for this......


----------



## The Norseman (Mar 5, 2018)

Wow! That is unbelievably strong for a slingshot! Have you tested it against sudden loads aka forkhits?


----------



## Ordo (Feb 11, 2018)

Quite an interesting journey. Thanks!


----------



## STO (Mar 2, 2018)

Ordo said:


> Quite an interesting journey. Thanks!


You're very welcome.  It isn't over yet, we're just getting to the fun part!



The Norseman said:


> Wow! That is unbelievably strong for a slingshot! Have you tested it against sudden loads aka forkhits?


Thank you. The short version is yes, I tossed the broken frame on the ground outside and pounded it with a few steel balls at high velocity. Most skimmed off, some left visible impacts. Really depended on the angle of impact and a few other factors. I wish I'd have photographed it, but I didn't.

That said, I'm really not very concerned about fork hits for a few reasons. First and foremost is that I basically never have fork hits, so while I'm concerned about breaking a fork under draw force and having it come back and make me a lot less pretty, the chances that I'll actually encounter this are pretty slim. Secondly, the mechanical term for impact resistance is "toughness." Toughness numbers will vary wildly depending on ambient temperature in this polymer (ABS gets brittle in the cold, and I tested it in a freaking snowbank), and generally vary wildly across the spectrum of polymers I can 3D print. I'm not going to go out and test all of them, but I have four polymers in mind for these slingshots and they all have a higher strength than the ABS I used to print this one. And finally my hand is right there, so if I started losing control of the steel balls frequently, I think I'd be inclined to design a slingshot to protect my hand..... or just quit.


----------



## MakoPat (Mar 12, 2018)

Sto, friend, that was an amazing read. You engineers make every thing more fun. 
Thanks for sharing.


----------



## STO (Mar 2, 2018)

I'll probably do more, but in a quest to move from simply functional to pretty it is time to add color and some design!

I did this bad boy for much smaller hands in translucent orange and filled it with honeycomb. For some reason though, while it is quite orange, my camera thinks it is UNBELIEVABLY orange.

















Next up is metallic gold. It is a metallic fleck material that looks very cool. Very visible is where I smoothed over the forks, although band wear doesn't seem to really be an issue even if I don't. The party piece of this bad boy is the pattern on the flat side. It has that life in it like figured wood or stirred honey. Hard to capture on film, but stunning in person.

















And last of this round is this slingshot I printed in bronze composite. It is a strange material, expensive and difficult to print, as it is something like 90% bronze by mass. The results are something that looks like investment cast bronze. It is a bit of a mind **** when you handle it, because your brain can't quite label it as anything. It has weight and cold feel kinda half way between metal and something else. I put some effort into weathering and finishing this one to make it look kinda like some sort of ancient alien artifact, something other-worldly. It is easily my favorite so far, definitely going to be doing more slingshots in this material.


----------



## Ordo (Feb 11, 2018)

Unbelievable work. Congratulations.


----------



## The Norseman (Mar 5, 2018)

NOOOOO WAAAAAY!!!!! That bronze composite is unbelievably..... I don't know! That is the coolest thing I have EVER SEEN! Incredibly great work!


----------



## Brook (Mar 4, 2018)

This has been really interesting to follow and excellent work. Cheers


----------

