i'll assume that you have a grotty cable or you have not lined up the cog with the head of the sprocket bolt properly and have split it when tightening the cover back up ,
if the cable does not rotate freely (you should be able to turn it with your fingers easily) it will quickly strip the little bush on the cable side

well the cable was turning freely with my hand, and after i bought another plastic one from rick, the speedo was working good for a few months, and then it started playing up. and then eventually gave up. so i checked it. and the plastic part was broken (crack in the middle and snapped in half), and now my cable wont turn freely by hand, its very stiff. i assume the cable broke and too much resistance on the plastic part snapped it in half.

This seems a bit extreme.

It's like what I said before, you need to find and treat the cause of the problem not just treat the symptoms.

Your approach:

The cog has broken twice, lets make the cog stronger.

The correct approach:

The cog has broken twice, why is this?

Answer:

The cog is plastic so is the weakest part of the link. It will only snap / break IF there is too much force on it.

This can be caused by the cog being fitted incorrectly, the speedo cable being stiff (the cog has to work to hard to drive it) or the speedo itself being stiff, putting force on the cable.

With a good strip, clean and lube the cog should last as long as the bike (providing you make sure everythings cleaned and lubed with regular maintanance). Often the cable is stiff, so replace it.

Answer to original question

The have been made before BUT they are not needed if your speedo system is in proper correct working order.

I've never had one break

Perhaps it's the Scottish weather to blame