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Designing a slingshot frame

Posted by ZDP-189, in Technical 17 September 2010 · 9,549 views


Designing a slingshot is easy. In this blog post I will show you my method and then talk about stimulating creativity.


Design Process

The first step is to decide on the slingshot's dynamics.  This determines the way that it shoots.  It is the most fundamental part of the design process.

  • The orientation - the way that the slingshot is held (upright or on its side)
  • The biomechanics - the way that the slingshot will work with the body to resist torque and tension, absorb shock and release the slingshot staying on target
  • The ergonomics - the fit of the slingshot in the hand with reference to the orientation and biomechanics
  • The balance - weight and balance
To simplify things I hold up my hand and imagine myself holding a slingshot.  I line up on an imaginary target with my elbow, arm, etc. all in perfect alignment and position.  I note orientation and the position of my fingers, knuckles, palm, the heel of my palm in this position and the path they take as I follow through.  The other important thing to consider is where the fork tips are and the pouch anchor point.

Once these positions and their path is fixed, I just need to fill in the remaining space with a slingshot. That's  not some Zen-like statement about finding the statue of Buddha in the  tree and carving away all that is not Buddha, it literally means to plan  the position of the fingers and the fork tip which together determine  the dynamics and then connect the dots with an elegant looking frame.

The foregoing is simple and sequential.  The great challenge is to fill in  around these positions to not only fill the hand comfortably, but to  balance the slingshot, and make something pleasing to the eye.  That may come in an instant flash of inspiration, or just through constantly obsessing about it in the shower, on the daily commute, or basically any time you should be working. :)


Case Study

In the case of the T1, I was looking for a small, flat board cut to be a  standard frame for testing new bandsets and to make with less labour so  I could trade with other members.  I also wanted to showcase the use of  plastics for boardcuts.  Up to that time, the use of plastics or  composites was rare and most slingshots were made of wood and/or steel.  The first choice of thin acrylic was less than totally robust - I was making the point that even supposedly weak plastics could be used if the design was complementary.

The shape was based around two concentric circles for the fork and then a fat handle with a cut out for three fingers.  The wide forks and web combined with high fingers reduce the likelihood of fork failure.

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I like to document my work and this forms a body of reference material  that I can turn to later.  Here is some design study work I did when coming up with the T1 Model:

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The T1 design is copyright© and this image and blog post does not
constitute permission to copy the design.


The shape of the T1 was partly based on my prior experience designing The  Shootist.  The Shootist used low forks and a high finger grip to reduce  wrist torque and a narrow raised ridge for the lower three fingers to  grip onto.  If you look at the T1 Ergonomic design study above, you can  see this replicated in one side of the handle loop.  On the whole, the  ergonomics is more or less the same.

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I like to design my slingshot frames around a theme.  In the case of the original T1, the Candy Apple, I wanted it to look like pooled candy or burgundy wine, so the design had a lot of flow to it.  I wanted The Shootist to look like a target shooter's carved stock.

My Desert Ironwood and Steel fork was to introduce myself as a hobbyist knifemaker and incorporates many techniques and design elements common to custom knives.

Desert Ironwood Forged Steel Slingshot

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Scallops was intended to showcase the design and creation process for a cast slingshot, and I wanted to come up with a design reminiscent of the classics from the days when commercial cast metal slingshots ruled.  I gave it the same kind of flow, with a blocky shape that nevertheless typified the master pattern maker's art.  With tips from Pete at Hogancastings, a true mater pattern maker, I came up with this:

Scallops

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The Scallops design is copyright© and this image and blog post does not constitute permission to copy the design.

It does incorporate several design cues from other maker's slingshots.  As I wrote in my original post:

There's  a lot to acknowledge here, as I endeavour to stand on the shoulders of  giants.  I started with Performance Catapults' grip geometry.  I widened  the fork a little and was influenced by A+ Catapults' PS seties, though  my curves will be scalloped like the recent BMW car bodies.  The  overall shape and cuts differ from the PS series too.  The lanyard hole  borrows from Dankung's and Smitty's bent wire slingshots.  The V-plunge  is my own.  It sounds like a hodge-podge of stolen ideas, but it comes  together in a unique way.


What makes it special is the way it comes together.  I adopted a common theme, a sort of Art Deco flow and line and the complementary scallops of a German car.


Originality

With so many excellent designs being shown every day, it is a major challenge to design a slingshot which is  totally fresh and has never before been seen, yet has an elegant simplicity so  that it is obvious in retrospect and an instant classic.  As I have shown above, the trick is to find a new aspect, either to imagine a new theme in which the design can come together, or to come up with a totally new geometry like the T1.  Jörg Sprave and Bill Hays are two designers that come to mind that have mastered that and their portfolios are rich and exciting because of that.

If you are a new maker, the temptation to copy all the great designs that you admire can be irresistible.  I myself learned knife making that way, buy working from kits, duplicating knives I couldn't afford to buy and from learning under direct instruction by master smiths.  When I started making slingshots, I copied Geko's 'Favorit' shooter as he encourages people to do so, and I duplicated A+ Slingshots' PS-1 under paid license.  However, the danger in duplicating others' work is that one never goes through all the design step and becomes a slave to precedent. This is especially so if the majority of the work is farmed out.  Unless you go through whole design and making process, you don't really develop as a maker.

Another problem with copying designs is often you miss the details that really make the design and come up with an imperfect clone.  Here, I copied Perry's (A+ Slingshots) PS-1 design.  I wasn't trying to be cheap; I happily paid him the price of an original slingshot to be allowed to make and show it, but I just couldn't wait for the post.  At the same time, i placed an order for a real PS-1.  However, I used super-duper Jade G10 and figured I'd done a really good job of creating an exact clone.  When the real deal showed up, I was astounded.  I had gotten it completely wrong.  You'll have to look really carefully at these two photos to spot the differences, but if you saw them in person, held them in your hand and shot them, it would be completely obvious.

The Clone Trooper

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A+ Slingshots Maple PS-1 #071

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For a start, the PS-1 is much smaller than my copy, and I have very small hands too.  Secondly, I had rounded over, but I completely missed the way Perry cuts his gorgeous and deep symmetrical scallops.  You can see them below:

A+ Slingshots Bamboo PS-1 #035

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Thirdly, he makes his slingshots out of very light woods like maple and bamboo.  The way he cuts his hole feels different in the hand to the mine turned out.  The bands are completely different it look and performance.  Altogether, my slingshot proved to be a very different thing to the one I had copied.  They were like chalk and cheese.  If I had not bought the original slingshot, I would have learned very little about Perry's design.  My re-make was one of the closest clones to Perry's that I have seen too.




:) Excellent !It's a Bible !Thank you for your work.Xavier
Will it work?

It's common to see newbies on the forum asking if a certain design will work. Honestly, many designs will work to a degree even if they lack the potential to be tournament winners. Also, it's hard to know for sure without extensive trial and error. The idea is to do all your trial and error gradually building up to progressively more onerous levels.

Step 1: Conceptualisation

After some experience, you develop a knack for the initial conceptual design, and get the basics right, like short narrow forks with low moment arm and the centre of thrust going through the palm, then the wrist and between the radius and ulna and with the elbow at the right angle.

Step 2: Visualisation

Draw your design out on the computer or on paper. One can visualise whether a design will work or will not work even before making it. Imagine a point between the fork tips and the slingshot resting on this point on a notional short spike. Now imagine doing one handed press-ups holding the handle. If it looks like the slingshot will be hard to hold, will twist or pivot, then fix it or give up on it. You also want to visualise how the slingshot will release. You want a clean release and to avoid an unpredictable twisting flick.

Step: 3 Initial Hand Fitting

The next step is to make it in paper or cardboard and cut it out to see if it fits the hand. More often these days, I'll make a Formica, aluminium, or acrylic scribing template and actually do some adjustment on this before moving on.

Step 4: Full Prototype

If it makes logical sense and fits the hand, I go ahead and make a full prototype and try to do an actual one handed pressup resting on the fork tips.

Step 5: Stress Testing

If that works, I'll soft clamp it in a vice and put 40lbs of pressure on each fork tip in turn and inspect for damage.

Step 6: Test Shooting

If that works, I spend an evening shooting it with light, medium and then Hunter Bands.

Step 7: External Feedback

Only then would I send them out to friends for field testing and feedback before declaring the design done and dusted. If it only fits you it's not a product. Also, sometimes it's easy to get attached to a design or its features and you need a dispassionate person to tell you it needs changing. It's hard to get accurate and detailed advice unless that person has actually shot with it.

...

This is not the only way to do it. Most people will blend all these steps into one and 'wing it', starting with a piece of wood and whittle or carve at it till it feels right. I sometimes do that, but that process doesn't lend itself to symmetry, line and form very well and it can be hard to reproduce a design time and again, or to improve on previous models in an evolutionary manner, because you have no templates or drawings to work from.
Today I have been thinking about sculpted handles and finger grooves.

It's a throwback to my knifemaking, but I am biased against finger grooves and overly sculpted handles. In knifemaking, a curvaceous but less sculpted design is usually called for because you want to be able to shift your grip about. A smooth handle with some palm swell is safe and secure in your hand even when wet but you to grip it in various positions to carve, whittle, lever cut, choke up, apply penetrative force, scrap and skin.

Clearly this does not apply to slingshots. You grip it one way and it stay there throughout the draw and shot.

However, I make my slingshots for other people and these people vary in hand size and proportion. I used to ask for detailed hand size information and scaled my designs to fit, but friends may shoot them or the slingshot may be given or sold later so it is better to make a universal design.

My early slingshot "The Shootist" failed in this respect. It was a beautifully carved slingshot frame sculpted to fit my hand perfectly and boy, did it look impressive! The problem was it was nearly impossible to ensure it fitted the person for which I might make it unless they were stood there in my shop as I made it.

Likewise, I was recently asked to review Hogancastings' Target Master. It's a beautifully sculpted cast slingshot with palm swell and finger grooves. Upon receiving it I was taken aback. It was made for somebody with much bigger hands than mine and I could barely grip the fork arms in a pinch grip. The palm swell was in the wrong place for me and the finger grooves were not where my fingers should be. It was a disaster. I never even put bands on it. Having agreed to do a review, I actually made a full-on 80% scale reproduction in my shop for Pete Hogan to recast for me. The smaller slingshot fittes perfectly and everything fell into place. It felt superb and will make an excellent target shooter. At the same time, a person whose hands would fit the original larger model would not be able to hold my mini version.

There is an imposrant lesson to be learned here: Overly sculpted slingshots become hand-size (and possibly individual) specific.

If you want to make a universal slingshot, gentle curves are the way to go. Avoid deep finger grooves, pronounced projections, or single finger holes.

If we look at competition bows and target pistols and rifles, we see the same thing. Some target guns do force the hand or body into a certain position, but these are generally adjustable, with spacers and sliding blocks.

If you have to err, external shapes should be made to fit a small hand. A big hand can grip tighter. Internal shapes should fit a large or gloved hand.

I am a big Fan of Bill Hays. All his slingshots feature pronounced finger grooves; it's his signature style. Yet, I have yet to hear of somebody complaining that they bought one and can't hold it properly. I've arranged a trade for one and I'll see if I can hold it. Let me speculate for now. I noticed on his video that the slingshot looks surprisingly small. His finger grooves are large and more widely spaced than designs that would bunch the fingers together. I notice that in more recent designs the finger grooves are becomming less pronounced too.
Back to guns, one of the most universal grips is the 1911-A1 a combat and civilian classic. Just about anyone can get a good grip on it. It has a simple grip that reduces in size as it gets closer to the trigger.

The same is true for the Dankung Cougar. It has a simple, conical grip and the pinkie hole is a big, long slot. My only criticism of the Cougar is it's very big, being designed for large Non-asian hands. In a small hand, it is unwieldy.

The Cougar was made in response to Westerners complaining that they couldn't fit their fingers in the pinkie hole of Chinese sized Dankung bent rod frames. That design flaw could have simply be remedied by a handle shape that accomodated larger hands and still provided lower finger support to counteract draw torque.

The Pickle Fork is a very small design with all-ouside curves and it fits just about anyone and shoots very accurately. Slingshots do not have to be big.
Here's a clever and informative post from Bill, primarily regarding target-shooting starships:

It is correct that there is no perfect slingshot, but there are slingshots that will give you advantages in certain circumstances. A well balanced and designed slingshot with a stabilizer will shoot somewhat more accurately than a light one without a stabilizer, just as the case with a target bow. A plain fork like a wire frame or board cut or tree fork has some advantage when wing shooting or at any moving target. Any wrist braced slingshot will let most people shoot heavier bands and shoot longer at a time. Extended forks let you shoot faster if locking out on your cheek. A narrow fork has a little advantage when flip shooting. There are a lot of factors to consider when choosing a slingshot for your main shooter. If you stick to one all the time you will be a better shot, so you need to decide what kind of a shooter you will be, a target shooter, a plinker, a hunter, an exhibition shooter or maybe a combination. Also Bands enter into the equation different bands for different styles and different people such as small lady, verses a large muscular man. I shoot light bands when wing shooting with marbles so I can see the shot better. -- Tex

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