I have had a 6" Delta Variable-Speed tabletop jointer for several
years, and have not had much luck getting straight edges out of it. I
tried checking the tables for co-planarity, but my dial indicator was
giving me readings all over the place, even on the same table.
Shimming the table tops didn't seem feasible when I had nothing to use
for a reference.
Yesterday, I bolted the jointer down to a piece of 3/4" MDF and bolted
the MDF to the table on my knee mill. I shimmed the MDF to make sure
the knife drum was parallel to the mill table and took light cuts with
a flycutter to true up both tables. They were both warped, cupped,
and bowed. There didn't appear to have been a flat surface anywhere
on either table.
After bolting the jointer back on it's stand and re-setting the knife
blades, it works much, much better.