2014-05-14 (W) Tennis Ball Launcher

Over the last few days I've had plans to get back on track with other projects such as the ASETNIOP keyboard and the podcast with Joe Robertson.  However these are both things which are done from the comfort of my own apartment and that was not where I felt like being so I spent some time at hardware stores and the hack space.

Enough background.

Hardware stores were visited to find parts for the Tennis Ball Launcher.  Some hardware, such as 1 ½” PVC pipe, 5/16-18 nuts, 5/16-18 lock nuts, washers, and aluminum bar stock was already owned.  More 5/16-18 x 2” bolts were purchase, 1 1/4” metal pulleys, 48” bungee cords, and a flat corner brace were purchased for the project.

Bar stock:        $5.00
PVC 1 ½”:       $3.00
Miscellaneous hardware:         $5.00
Corner brace:   $1.19
5/16-18 bolts: $1.89   (pack of 10)
Pulley 1 ¼”:     $5.90   (2 @ $2.95)
Bungee:           $5.98             (2 @ $2.99)


The available PVC was cut into two pieces each 26” long.  They were cut on a radial arm saw.
 Clamped PVC pipes

Freshly cut PVC

5/16” holes were drilled all the way through the PVC 1” from one end.  This hole will hold the rear pulley.
Marked 1 inch from the end

Set up on drill press

Freshly drilled 5/16" hole

Pulley in PVC

The corner brace was drilled to 5/16” in the corner where it can act as a hinge.  The wiggle room for the hinge was calculated by shifting the corner brace around the hinge, sketching the placement and measuring the greatest distance from the hinge point.

5/16" hole drilled into corner brace

Moving the corner brace around to determine the necessary size of the hole

The corner brace, as shown on the drawings, will serve to push the tensed bungee off the catch, similar to a crossbow. 

The first attempt at the hinge-catch portion resulted in an unusable block of wood.  It is important to use a large enough bit when drilling down through the hinge-catch for the corner brace and even more important to keep that hole aligned with the hinge hole drilled for a bolt.

A new hinge-catch was cut to length on a radial arm saw and shaped with a band saw.
Hinge-catch portion

The piece had a groove cut down the middle using a radial arm saw raised up so the piece was not split.
Cutting a groove with a radial arm saw

The bolt hinge position was marked by holding the corner brace in place.  The 90° will need to be rounded on a grinder since it sticks out.  The hole was drilled to 5/16” like the corner brace.
Position of hinge hole

Drilled hinge hole

Above and in line with the bolt hinge hole a 1 1/16” hole was drilled with a Forstner bit.
Lines indicating where to drill the hole

1 1/16" hole drilled in catch-hinge

A 5/16” bolt was inserted into the hole and the corner brace should work well to push the elastic off the catch when moved.  This movement will likely be initiated by the user pulling a bicycle brake handle mounted as the trigger.
Movement of corner brace

A phone discussion with my teammate also explored the idea a spring-loaded model which would have a very simple/elegant design.

To do:

  • Grind corner brace 
  • Revise design to use pulleys I place of bearings 
  • Build 
  • Test 
  • Revise 
  • Document

 Journal page 1

 Journal page 2

 Journal Page 3