Edge jointing cupping re-visited

[JPG]

As for why:

Consider the effect the 'droop' will cause(end result center cupping) .

When the work piece is tight to the infeed table the cutter will cut along a line above and parallel to the infeed table plane.

As the work piece is advanced the new bottom edge will ride on to the front edge of the outfeed table.
Blades set too low will cause the work piece to snag on the front edge of the outfeed table.
Blades set too high will cause the work piece to clear the front edge of the out feed table by a small amount proportional the the amount of droop.

As the work piece is further advanced, the leading end of the work piece will clear the outfeed plane by an increasing amount.


Now as the operator transfers down ward pressure from the infeed table to the outfeed table. the work piece will 'rock' down tight against the ouffeed table plane. As a result the trailing end of the work piece will rise slightly above the infeed table plane.

NOW the cutter is cutting along a line that is parallel to the outfeed table plane AND closer to the trailing bottom end of the trailing end of the workpiece. That results in an edge that is straight along TWO different lines that intersect near the point the pressure was transferred from the infeed table to the outfeed table.

Precise coplaner grinding of the two tables allows precise edge straightening. No guarantees though as other stuff(blade adjustment, operator technique . . .) can throw it off.

All this explains just ONE thing that can go wrong.

[Matanuska]

As for why:

Consider the effect the 'droop' will cause(end result center cupping) .

When the work piece is tight to the infeed table the cutter will cut along a line above and parallel to the infeed table plane.

As the work piece is advanced the new bottom edge will ride on to the front edge of the outfeed table.
Blades set too low will cause the work piece to snag on the front edge of the outfeed table.
Blades set too high will cause the work piece to clear the front edge of the out feed table by a small amount proportional the the amount of droop.

As the work piece is further advanced, the leading end of the work piece will clear the outfeed plane by an increasing amount.

Now as the operator transfers down ward pressure from the infeed table to the outfeed table. the work piece will 'rock' down tight against the ouffeed table plane. As a result the trailing end of the work piece will rise slightly above the infeed table plane.

NOW the cutter is cutting along a line that is parallel to the outfeed table plane AND closer to the trailing bottom end of the trailing end of the workpiece. That results in an edge that is straight along TWO different lines that intersect near the point the pressure was transferred from the infeed table to the outfeed table.

Yes, this is similar to the logic I was following when I thought the outfeed table trailing edge droop was responsible for the cupping and that milling would be required to solve the problem. With enough droop (much more than what my jointer has) the cupping would occur even if the knives were set with no gap above the leading edge of the outfeed table (as I have it set now). The board would pivot on the leading edge of the outfeed table as weight is transferred, causing cupping. As it turned out, the table droop was a red herring and caused only imperceptible rocking and associated cupping. As you pointed out in your original post, the knives being too high was by far the main cause of the rocking and cupping. My outfeed table still droops but with the knives now set equal to the outfeed table leading edge height there is no perceptible cupping. Thanks again for putting me on the right path.

Matanuska