At 13 my grandson, Gijs, expressed great interest in everything to do with technology. I figured that it would be nice to make a  ‘Newton Cradle’ for him. It is a simple and well-known device which can demonstrate the law of ‘conservation of momentum’ based on Newton's third law: action = reaction.
Gijs may put this little device on his desk and occasionally play with it . Perhaps he can also take it to school when Newton's laws are discussed in the physics lesson. It is certainly not an unique model, but Gijs can at least say that his grandfather made it for him.

Mounting the steel balls to the supports
It is of great importance that the steel balls hang exactly on one and the same horizontal axis. Therefore, the balls will clash together pretty central which largely determines a beautiful movement behavior of those balls against each other. This can be achieved in the following manner:

1. 1.Drill the 1mm holes in the hollow rivets according to the drawing on page 3.
   2. Glue the two rivets nipples to the balls with Loctite 603; preferably cure for about 1 hour on 50 to 60 degrees Celsius in a oven.
   3. Thread the yarn ropes through the 1 mm holes in the rivets with an excess length of about 20 cm on both sides of the rivets.
   4. Dispense one drop of a 2-component epoxy adhesive (Bison) in the rivets to fix the ropes in it.
   5. Attach a rectangular metal strip with about 8mm height on the wooden base with double-sided adhesive tape, such that one side of it has a distance of 36 mm away from the side of the wooden base; = centerline base (45mm) minus the radius of the balls (9mm).
   6. Thread the wires on both sides through the 1mm holes in the horizontal rods of the support and clamping them more or less stuck with a paper clip on the outside of the horizontal rods.
   7. Slide the ropes through the paper clips on each side until the ball just touches the rectangular metal strip while the bullet also just touches a flat metal strip that is pushed against the side of the bonded strip. In that way all steel balls will exactly be in-line which is a precondition for a good bouncing behavior of the balls.
   8. First glue the ropes to the inside of the horizontal supports with a single drop of acrylic super glue and wait untill the glue has hardened.
   9. Remove the paper clips and cut off the ropes on that side. Glue the ropes also on that side with acrylic superglue. Remove the metal strip on the wooden base.
   

Drawing plan

For those who also would like to make this little device I made a 4-page CAD plan; click here to make a request for that.