The question of how flat a sole of a hand plane need to be comes up all the time. Here is my opinion on the subject.
The following would hold true for a smoother and a jointer (which is also used for flattening)
I scrub, jack, or try plane doesn’t need to be flatten at all. (assuming it has been twisted in a fire or seen similar blunders)
I picked this plane. I would say it’s a little worse that the typical, no where’s near the worst I’ve seen, but a bit worse than average.Tt is a type 19, and it’s the reason a lot of folks don’t care for the planes made after type 16. Its this kind of lack of quality often seen. However once it’s rectified, the planes all perform the same.
I plane doesn’t need to be flat the whole distance of the sole. You need a flat spot the width of the plane right in front and right behind the mouth (this is the most important spots). A decent size flat spot in front (the toe) and the back (the heal) of the plane is also required.
I flatten either on a piece of granite or my cast iron table saw top with sanding belts that have been cut. I usually use used belts, although new ones will obviously be faster.
I’ve marked the spots I’m referring to. should there be a hollow in between these marks, it wouldn’t kill your performance.
(All notes are for pictures below them)
As I start flatten on this, we can see a hollow where the arrow is. This will have a dramatic affect on performance of this plane. We will want to get that out
Note how this hollow follows through to the mouth. This is important to note. If the same hollow was where the arrow is, I wouldn’t need to worry about it. That’s not saying it will hurt to get it out, it just won’t have an affect on performance.
Not the hollow in the back is now gone. We are making progress.
Progress, and we’re getting close to it not making a big difference. Its starting to show we’re hitting behind the mouth, so at this point the plane will start to perform well.
Note the circle is now away from the mouth just a little bit. This plane will work. I just want to go a little more for security.
Now we’ve made this plane a worker, but where the arrows point bothers me just a little. I would say it probably wouldn’t matter, but I’m going to keep going.
Its getting smaller.
There still a small spot. If I decided to stop right now, I’d never see the difference in performance of this plane.
Ok, So my laziness is starting to overcome my perfectionism. I’m going to stop here, knowing this plane is as flat as it will ever need to be, and the only thing continuing will do is possibly make it look a little better. The shaving will not get thinner and the wood will not get smoother.
I typically stop at 180 grit. Once in a while I’ll go to 22 if the paper is new enough, but not very often. Keep it waxed and it will be all that’s ever needed.