I was cutting a load of 25mm MDF a while ago with a 6mm cutter like the one in the first post. About £3-£4 I think on eBay. I broke one about every hour or so until I swapped to an 8mm cutter and had no problems. The 6mm bits have that tapered section and this is exactly where they broke. The 8mm bits don't have that and the main body is straight with the cutting bits sticking out welded to the sides. Without that stress raiser that seems to make them more durable and with the thicker body they probably vibrate less. Worked for me . . .