Torsion Spring Bow

This guide will instruct you on how to make a Torsion Spring Bow, like the one above. This is my first Instructable so begging your pardon on any lapses in quality. Also, this is ONLY for the BOW, i am not explaining how to make arrows but there are plenty of Instructables that teach that!

Torsion Spring is a spring that stores kinetic energy as elastic potential energy by coiling, usually in the means of a rubber or metal coil. Torsion Springs are usually used in the Ballista or the Mangonel, the Medieval siege engines, but I thought why not for a Bow?

Step 1: Getting Materials and Tools

Step 2: Measurements and Dimensions

Step 3: Cutting/Sawing For the Pieces

Step 4: Assembling the Handle and filing it to shape

Step 5: Assembling the Torsion Springs

Step 6: Assembling the Bow

Step 7: Stringing the Bow

Step 8: Play with it!

Disclaimer: I AM NOT RESPONSIBLE FOR WHAT YOU DO WITH THE FINAL PRODUCT FROM THIS GUIDE. IT IS MEANT TO BE A TOY AND WHILE USING IT AT ANY TIME YOU SHOULD BE AWARE OF YOUR SURROUNDINGS AND PAY ATTENTION TO THE SAFETY OF THE PEOPLE AROUND YOU. How powerful the bow can be is dependent on how well you put it together. So if you do it well, it can really hurt someone.

MATERIALS:

Plywood (30cm×60cm×12mm) or (0.5×12×24inches)

Plank of Basswood ( 7.75cm×60cm×12mm)

Thick Dowel (Diameter:2cm×Length:1m)

Thin Dowel (Diameter:1.24cm×Length:91.5cm)

Balls of Cotton Twine.

Rivets

*though these are the materials that i used, they aren't set in stone. One thing I've always had problems with, especially living in Singapore , is finding materials (materials can be hard to come by). You can use what materials you can find, it doesn't need to be the same for it to work.

TOOLS:

Fine-tooth Saw

Vernier Caliper

Diamond Files

Pencil

Vices

Files

Saw

Circular Drill Bit

Rivet Gun

*I used hand tools for almost the entire project, exceptions are when i used my Dremel 4000 and the more specific drill bit. The Dremel was just to make my life easier though the drill bit is needed, you can find it a a local, well-stocked, hardware store. Once you have gathered the tools and materials that you will need, you can start building!!

First you need to measure and mark the dimensions of cuts you are going to make. Use a pencil and draw it lightly so that the pencil marks can be easier to erase later. Also make sure to make markings on both sides and be precise. If you do not mark the dimensions clearly and accurately, they will not match up with the other side

You will need to make 4 squares for each Torsion Spring and an extra side for each structure. While the handle, i made on by cutting the shape on plywood for 2 instances and eventually gluing them together for the appropriate thickness. (I know that the picture had 3 sets but I wasn't sure if 2 sets was going to be OK) The straight rectangle on the handle will be used as the grip of your bow, and the triangular ended side is the part of the handle that you will mount the torsion spring on. The parts from the thick dowel you get will be the arms of the bow

Square Dimensions: 7.5cm×7.5cm

3rd Side: 7.5cm×

Thick Dowel: 50cm long, so somewhere near the middle.

Thin Dowel: 4 pieces, 7 cm long each.

The Dimensions for the Handle are in the picture of the individual half, for the other side you can simply mirror the dimensions.

For 4 of the squares that you have cut out, you need to mark out the centre of the 7.5by7.5 square so that you can drill the hole for the torsion spring.

In this Instructable, I'm using the dimensions that i used. I figured them out via the dimensions of my own hand and body so that they were more suited for me, so if you want you can change the dimensions to suit yourself. you can make the length of the grip longer or shorter so if would be more similar to the length of your hand. Or you can change the length of the arm to make the bow longer or shorter.

Tips: As you can see the dimensions in the pictures, the cuts between pieces that u want to use should have a gap between them marked out clearly. This is actually for precision of the final pieces you will get, as the saw will force a gap in the wood. If you just saw the wood where you marked out without a gap, the actual pieces that you get will be a little shorter that you actually wanted. I will describe further in the next step.

This part is actually fairly simple, you just need to cut the pieces out. Though, this entire process might be the most taxing part of the entire process.

The saw (blue-handle) is hard to make stay on the right alignment for the cut, so i use a smaller fine-toothed saw to make the first cut before using the bigger saw.

During Sawing, use some clamps so hold down your wood or it will be damn near impossible to do. The clamps can however leave marks, so best to sandwich the piece you want between junk wood or even some spare cloth.

As you can see from the picture of the wood that is clamped down, as you saw the wood, the wood will not be in the exact shape or dimension that you want. So, you'd want to file or whittle the wood down so that it matches the dimensions that you have marked on both sides. If not, the edges of the pieces will be lopsided.

To drill the big hole for the torsion spring, like how you made a small cut to use the bigger saw better, nail a small hole in the wood as a guide hole for the drill. Again make sure to use a piece of wood under it that you do not want cause the drill will drill into it. The Hole i drilled had a diameter of 5cm, by using the specific drill bit that I had.

After sawing the pieces out and filing them into shape, you'd have the 2 pieces ready to be joined for making the handle.

Glue the 2 pieces of the handle together. If in the process of squeezing them together, any glue flows out of the crack, wipe the residue away. If it drys, it will cause a lump which is a pain when you are filing the handle into shape. So put it in the right alignment with each other, clamp it together and let it set.

The handle as it is is rectangular with relatively sharp edges to it, so once it dries you can start with the filing of the edge to make a smooth agronomic handle. Clamp down the handle and file the horizontal edge at an angle. Vary the angle while you file to form a curve, like in the picture of the curve with the respective tangents. Try holding the handle now and then to check how comfortable the curve is, it should be easy to hold and match the curve in the meaty part of the palm near your thumb and the curve of your fingers flexing around it.

At the same time, file all the edges of the handle a little, so there are no sharp edges or loose splinters.

Firstly, you need to glue the sides together. Sandwich the pieces between the pieces with holes in them, as you can see from the picture. Ensure that all the sides of the torsion springs are perpendicular to each other, if not it will not be stable or strong. If not it will be like applying pressure on opposite sides of a rhombus, the supporting sides will cave in. You can ensure that it stays straight up and level by filing the shorter sides connecting to each other are exactly 90 degrees to the long sides.

When applying the glue, press the sides together till the pieces are firmly in contact with each other. Sometimes the glue while applying pressure, have a tendency to act like a lubricant and cause the pieces to slide out of the position you want it in so be sure to take note of the shifting.

Secondly you will need to glue the additional supporting side onto the current structure that you have, this side is going to be the front facing side and will be connected to handle. Again it must be perpendicular the the other sides of the structure.

Once the structure has dried, you can weave the cotton twine to form the business part of the torsion spring. I'd suggest referring to the pictures for a good representation of the process of weaving the torsion spring and what it should look like at the end.

The length of the twine needed to tread the torsion spring is very long. A good approximation is 6-8 times the length of your arms held out from end to end, as i wrapped the torsion spring twice for each. Whatever length you use, it must be enough to wrap around the dowels from end to end at least once.

Tie down 1 end of the string to one of the 7cm dowel pieces. I'd suggest that to ensure that the string does not shift while you tighten the torsion spring or while weaving twine, file a small pit in the dowel around 2.5 cm away from the ends of the dowel. Tie the string in the pit tightly and the string should not shift from that pit, again at the other end of the dowels once you have finished weaving the string around them

As shown in the pictures, with the dowels placed at the holes of each side of the torsion spring structure, pull the twine out along the same side of the dowels as it goes it. Go around the dowel and pull the twine pass and through the other side of the dowels. Pull the twine tight against the dowels after each pass and hold the string tight as you tread the twine again. Repeat the process as the string wrap goes down the length of the dowel and reaches the other pit that you filed. At this point, you can stop, tie off the end and cut off the excess, or you can weave the string around again to from a second layer of the string wrap. This would increase the torsion spring strength. To do so, overlap the sting over the previous layer of strings and go around for another round from end to end.

Once you have finish weaving, tie of the end tightly and cut of any excess string. It should look something like the 3rd last picture in the image set. Remember to make 2 of these!

You can skip tightening the spring till after you have assemble the whole bow but since it is part of the torsion spring, I'll explain how to do that bit here.

Insert the 50cm long thick wooden dowel in to the gap between the strings in a vertical position to act as the bow arms. Began twisting the short dowels of the torsion spring in the same direction and amount. The strings will apply pressure on the bow arms and it will lean towards one side, eventually looking like in the first picture of this step's image set. This step is quite hard and will require some wrestling on your part to press and wiggle the dowel to increase the tension. If you really can't move the dowels any further into position, you can try securing the torsion spring, then use a mallet to hit it into place.

Once it is finished you can try moving the bow arm and it should be quite hard to push it against the tension. Take note that as you push the bow arm from the resting position, the strings may stretch or twist and make the spring less compressed. You can compensate for the reduction by just twisting the dowels more after the movement.

Once you have the 2 torsion springs and the handle, you can start to assemble the whole bow. You can glue them together, with the handle at the centre of the torsion spring. The handle should be 2.5cm from the ends of the surface that it is being joined to on each side.

While letting it set, you should clamp the pieces together to prevent the handle from shifting and balance it with the handle of the bow vertical. Do not balance the bow on its side as gravity can cause the handle to shift out of position as well. I actually used a set of my drawers, open at different lengths to balance the pieces while letting the glue set but i doubt you can do the same thing. If you can't find a way to let it be vertical while drying, i suggest first using a glue that dries faster and hold it in place before using the stronger glue.

Even the strongest glues may have a problem holding the torsion spring and the handle together considering the forces the torsion spring can come up with. So we'll need to reinforce the joint with a rivet on each side.

To use a rivet, drill a hole near the end of the handle that is the diameter of the rivet, the rivet must be longer than the length of the hole, insert the rivet into the end of the hole and then use the rivet gun. You'll need to do this for each of the ends of the handle connected to the torsion springs.

You can refer to the link below for a better explanation of rivet use.

http://www.diydoctor.org.uk/projects/Poprivets.htm

Yay! This is the last step before your bow is complete!

You'll need to tie a bowstring to the arms of the bow with a bowyers knot. To do so, first notch the bow at the ends with a shallow pit into the bow arm. The knot should stay in this pit and get locked into it once you string the ends.

The actual length of the bowstring that you will need can be gauged by the length of the arms across while the bow has not yet been strung, like in the first picture. So cut out a length a bit longer than that and tie it down at the notch of one bow arm. An excellent explanation of how to tie a bowyer's knot is at the link below.

http://www.bickerstaffebows.co.uk/bowyersknot.php

Once the knot at one end is tied, tie another knot on the other bow arm, this time it will be further down the arm, closer to the handle. Though you'll need this knot to be tight as well, do not tie it too tight as you'll later need to slide the knot up to the notch on the other end.

Now you'll need to actually string the bow. The pictures show clearly how to do this. Place the arm with the knot already in the notch onto another surface (like your floor or in my case, bed), hold the handle with your right and the other bow arm with your left. (First Picture)

Now press down on the handle with your right and pull the bow arm inwards toward the other bow arm with your left. Both bow arms should be shifting and the "curve" of the bow should start to show. During this time, hold the unsecured knot in your left hand and start sliding it upwards towards the notch. (Second Picture)

Still pressing down on the handle and pulling inward with the bow arm, pull the unsecured knot all the way to the end till it is stuck in the notch of the other end of the bow arm.

Congrats! The Bow is complete!!! Have fun with it and once again, Safety First!