Compund bow for kids (8-12)
Draw weight 17-33lbs
Draw length 21-23 inches
Limbs from PVC pipe
The bow is made as follows:
Carefully study the parasolid or STL assembly file to familiarise yourself with the assembly.
Riser:
The riser is milled from 15mm Plexiglass (Acrylic/Perspex) I have nested the parts in the file called 'Milling layout no islands 2.STL'
The layout has been made to fit inside a 300x300 square in order to fit onto the bed of my Roland MDX40 so it should fit most 3 Axis milling machines.
Once machined, the limb bolt and limb pocket mounting holes can be carefully drilled by hand. Make sure the M8 Nyloc nut fits into the milled slot at each and of the riser and that the 8.5mm hole drilled for the M8 bolt lines up with the nut and the hole in the limb pocket. I used the finished limb pocket as a template to drill the holes. Once all the drilling has been done glue the riser sections together with CA glue. There are holes on each joining face for 3mm aligning dowels. I used 3mm dia carbon rod cut into 4 12mm pegs but you can use wooden dowels if you like since the are not structural at all. Sand the riser and paint if desired.
There are holes for a standard bow sight and a hole for a Gerber bolt to mount a standard bow rest. One thing I noticed in the prototype is that the side of the arrow shelf is not wide enough for my rest and if I was to do this again I would perhaps glue in an 8mm spacer between the riser sections just to make the shelf a little wider. (ie. Further from the centre of the bow)
Cams:
The Cams were 3D printed from ABS/Polycarbonate on a uPrint. If you don't have access to a high end printer then just mill them from polycarbonate and then hand cut the cable grooves on a router table or 4 axis cnc mill.
I have used the parallel cam system found on G5 Prime bows.I did this to try and keep the cams from leaning in the PVC pipe. It works pretty well but the limb pockets must be mounted dead straight!
Fasteners:
The cams are held together with M3x12 Socket head cap screws.
The limbs bolts are M8 x 40 SHCS and the limb pockets are M4 Countersunk screws.
The cam shafts are M4 x 50 Screws and I have used 4mm ID x 11mm OD x 4mm Wide bearings on either side of the limbs.
Limbs:
The limbs are made from schedule 40 PVC (1.25 inch) which is heat bent using a heat gun.
Nick from Backyard Bowyer demonstrates the technique here. https://www.youtube.com/watch?v=AKjcrEsYykE&list=PLD816DB6470FEE164
The process for the compound bow limbs is much simpler because you are just making two tapered pipes, not a whole bow, but the technique is the same.(I used two planks and LEGO blocks as spacers instead of making the jig like Nick does).
Once flattened cut the slot for the cams. Make sure to leave large radii in the corners to reduce stresses.
Refer to the parasolid files as a shape guide and you should be fine.
Stringing:
I used 2.5mm nylon rope for the string and cables and 1.5mm nylon string for the cam strings. I have included two diagrams showing the string lengths and their positions.
(DO NOT use the string position in the Parasolid file as a reference for stringing I just put them there for the render.)
To string the bow you need to compress the limbs. You could take it to a shop once you have made all your strings and put it in a proper bow press but this bow is pretty weak so this is not necessary unless you want to. I just tied a length of rope around the limbs and pulled it tight to pull the limbs together. This gave enough compression to string the bow and then I removed the rope carefully once done.
BE CAREFUL!! There is a lot of stored energy in a compressed bow!
Enjoy
DISCLAIMER:
This bow is intended as a target bow with low draw weight FOR KIDS, if you choose to scale to bow to adult proportions you do so at your own risk!
Do not aim this bow at anything living!!
Children should only use this bow under adult supervision!!
Be safe and responsible at all times, don't come crying to me when things go wrong.
