I am having a problem with my Jointer.  The problem I am having is that the work piece is high in the centre.  I think the problem started after I changed the Knives.  I have a 6″ jointer with a total bed length of 46″.  Currently the work I am machining is 55″ long.  When I change the knives I set them very carefully using a dial caliper and set the height within .001 of the height of the outfeed table.  I checked the infeed and outfeed table this morning and the infeed table might be low by only .002 at the very end.  I remember reading of this type of problem before but cannot recall what the solution was.  As I recall there was a specific reason why a work piece would be bowed or tapered.  Any suggestions would be helpful.