I am trying to slow down a chop saw from 3800 rpm to 1800 rpm. I want to replace the cutting blade with a metal cutting blade but the rated speed on these metal blades is a lot less than the cut-off wheels I'm using. Any ideas how I can do this?

Well does it use a gas engine, if it does it should have idle screws and if you turn them outwards it'll slow the speed down, if its electrical there isn't much to do, but use a rated blade. Or a switch (I forget what they are called) to slow it down.

If you have a 110 volt electric motor, there is an electronic motor speed control made for woodworking tools with motors of 15 amps or less. You just plug the unit into the wall outlet, and the tool into the unit. It has a dial for infinite control. I've seen them around but here's a link to Harbor Freight. The unit pictured is typical of the other units Ihave seen though I don't know how reliable it is. But at 10 bucks it may be worth a try.

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=43060

PS: I know I'm late but I was just browsing and saw your question. :D