[Aussie Mike]

I don't reckon a 1/4" cutter will ever cut a 1/4" slot. The flutes of the end mill will tend to pull into the workpiece in one direection of traverse and ride up over in the other direction. Also the cutter will move in the slot just with the flex in the tool itself regardless of how rigid the mill is.

I use a cutter smaller than the slot and cut it in several passes and cutting the 2 sides of the slot seperately. For the most accurate slotting in a mill I reckon you need to use a slotting blade.

You can still get accurate results with the lighter machines, it just takes longer because you have to take smaller cuts and more of them.

Cheers