Always cut to length first. Makes it easier - in fact planing long lengths
flat might be impossible if there is too much distortion.
On the other hand to be realistic, a bowed piece might well be OK, once it's built in and constrained by whatever you a doing, so perfect flatness not always essential, if the surface is OK.
But long or short - machinist skills count and hand and eye are important. It's not all done by machinery!
The main thing is to keep pressure in the right place.
If you plane the
convex face of a bent board, feeding it through with pressure on the front and moving pressure back as you go, then the face will stay convex as you follow the curve.
So a convex face you'd pass over the planer with pressure in the
middle as far as possible. This would also take care of a twisted board as the centre where you start planing would be in the midpoint of the twist so average out the cut.
With the
concave side down, pressure in the middle might bend the board down so it'd spring back and stay concave. So you try to keep pressure at the
ends instead. If also twisted, then you need to make sure you are getting the twist out equally at both ends, so you keep pressure on the corners and sides which would be in contact with the bed , opposite one another- i.e. the high-points when you turn it over.
Whatever you do you need to keep checking what's happening by squinting down the board length, and adjusting your technique accordingly
Does that make sense?
cheers
Jacob
