i use the slitting disks they are great(can even cut curves if your carefull)
BUT
there are good ones and bad ones
the ebourer make from srewfix are very good as are the rodus ones from tool station the norton ones on the other hand are carp (as quick as you touch the steel they disapeer)
they even do a 2mm thick one for the 9" grinder (great for box/tube