Geoff Ames replies: Given that your infeed and outfeed tables are parallel, the problem with jointing usually results from one of three factors involving setup and technique:
Assuming you are flattening a board, run the board concave side down. The same is true for edge jointing. And for safety’s sake, always use a push stick when flattening a workpiece.
Jon Siegel replies: Problems with jointer adjustments are almost entirely of two types: a) the outfeed table is too high (work rides up), or b) the outfeed table is too low (work snipes at the end). Neither of these defects will cause the work to turn out convex or concave. The only way that can happen is if the tables are not parallel. If this is the case you can’t fix it (sell the jointer).
So the question is wrongly conceived. For example, a good question would be: “I am having a problem with my jointer. When I joint long boards, it seems to take off less and less as I go, so by the time I get near the end, it isn’t cutting anything. What’s wrong?” Answer: The outfeed table too high.
Joe Barry replies: You need a proven straight edge – preferably a machined straight edge that you use only for machine set-up. First check each table from end to end and corner to corner to ensure flatness. If each individual table is not flat, it is time to contact the manufacturer or a machine shop to either replace or flatten the table.
Rotate the cutter head by hand with the power disconnected so the knives are out of the way. Raise the infeed table level with the outfeed table. Use the straight edge again to check that both beds are co-planar. They should be in line with the straight edge along their length and corner to corner.
If the beds are out of plane, you need to use the table gibs to adjust and maybe even add shims to adjust the plane of the table(s). Then drop the infeed table and adjust the high point of the knives to be equal to the outfeed table’s plane.