Homemade tennis ball machine
The University of Victoria's engineering students made some prototypes to assist mobility challenged individuals. One indoor, one outdoor model. They included their findings on a free PDF they've made available to the public. See link here.
By denise w (Guest Post)
My hubby told my son to make one of these for school project he used 4 cans without the edge on bottom need to be asparagus cans and used duct tap to tape them together leaving the bottom in one can only. Then you put the potato or tennis ball in and hold your hand over top using butane lighter fluid. You needed to poke hole in bottom can. put butane in there. Remove your hand and light a lighter near the small hole now it shoots out.
So radio shack is doing a clearance on their dc small motors why not use the can idea with the small motor and a paddle of some type say an old plastic spatula connecting it to the motor with a pulley. You should be able to make one very cheap. Hope I helped. d
By Jill 
I looked; and they're $149; but that is still ridiculous! I actually had one of these when my dog (who is now 8 and a half) was small, but it was only about forty dollars then! It only lasted a matter of months-hopefully they have improved that.
I had ordered it because I had given myself a wicked case of tennis elbow playing with her! Since that time, we have found several methods that work well for us. The easiest/best for us is to use a "ball flinger", which is essentially a long plastic "arm" with a claw on one end that will grasp a tennis ball, then give you extra leverage to fling it long distances. I prefer the "Launch a Ball" from my daughter prefers the "Chuck It" from Petsmart. The best part of these is that you don't have to touch the slimy ball!
I have also seen devices that are much like slingshots in their action. I once bought a device that looked like a golf club for my brother to use with his dog.
I really do recommend the long plastic flingers, though. I have found them to be worth their weight in gold!
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