To make what you are describing, first grind down the tip of the bolt you are going to use so it is completely flat and goes up into the threads. This ensures the threads are at the tip of what is left and that there is no taper there. This will allow you to get to the bottom of the hole (if this is a through hole, you don't really need to grind the tip and might work better with the tip on it). Next, when grinding the flutes, you'll want several on there. Your flutes should be wide enough so you have equal amounts of flute to what's left of the bolt threads. Four would probably be good, but three (or even two) would work if the bolt is too thin for more flutes. Make sure these go up the bolt quite a ways. You want to make sure there is plenty of meat left at the center of the bolt so as to support itself. Last thing you want is for it to crank around itself and get stuck in the hole. The flutes are there to pick up any ash/trash, not to cut new threads. When you run it down the hole, it will be slightly precarious, so you have to be careful you don't cross thread it.
While a tap would work for this operation, you really run the risk of cutting new threads with it if you don't do it right. A bolt cut with flutes should be quite a bit softer, but be hard enough to straighten out threads and remove ash/trash without too much of a hassle. If you've not used a tap before, I believe using this would most likely be a better (read: safer) solution.