You’ll often come across two camps – one who thinks no plane needs to be flattened and one who believes all should be or that such flatness is only available in premium planes. Within the flattening group are the hands off folks and machinists, and the DIYers.
If you ask a machinist (one is active touting plane truing as of this post), they will tell you that nobody can flatten a plane sole by hand and “i’ll run a dial indicator across it and prove it”. This is the view of a machinist who perhaps hasn’t run into many old school machinists who did the finest work by hand. But they will have a case talking about seeing planes that were poorly flattened or made worse.
First, if you aren’t going to flatten a plane or don’t have confidence that you can – stop reading and decide how you’re going to proceed. You may want to buy premium planes or have a machinist do the work for you. The rest of this short bit is for everyone else.
If you want to flatten plane soles, you need a way to check them, at least if you’re going to make wise use of your time. We are talking only about steel and cast iron planes. You have to have something truly flat, and having feelers isn’t a bad idea as chasing the condition of no light anywhere under a starrett straight edge is some form of insanity. You’ll find that after a .0012″ or .0015″ feeler won’t come close to fitting in a gap, light will still creep through. My preference is to try to land at no light but to have no important area of any size come close to allowing a .0015″ feeler get through.
I don’t know of anything better than a 24″ starrett straight edge for this, but they are not cheap. If you can find a discard from a machine shop who is replacing one, but check that the one they’re letting go is good against a newer reference, that’s great.
Flattening a small plane on a lap only can take as little as 5 minutes, and i’ve spent up to about two hours on the worst of stanley 8s. If you try to lap a plane only and avoid the spot removal i mentioned, you’ll struggle to do much of anything on some planes in several hours.
Lastly, a machinist may check the whole surface. if you have 1/8th of an inch at the edge of a plane or the last quarter inch of the ends rounded over and it would take a lot of removal to deal with that when the rest of the sole is flat – forget about it and stop there.
Check anything that is being used on the whole sole with the same straight edge – for example, if you’re using a glass lap, the lap when it’s clean should also not allow a feeler. The surface under the lap needs to be flat and clean – even a single grit of coarse abrasive will have you fighting the lap to do something the straight edge likes.
You can use a large marker or marking fluid to check progress. When does progress end? when you can take two flat planes and plane the same flat surface and have neither show a tendency to take the ends off of a board before cutting full length. Why is this test so appropriate? If you’re working by hand doing more than following a planer, you will actually be doing this – there’s no great sense in planing a surface flat with a try plane or jointer and then fighting the smoother to take a cut. If your planes meet this standard without any work, maybe you don’t need to do anything. if they don’t, it’s fixable.
The typical condition that I have seen in later low-wear planes and some earlier (if you can find them without wear) is a toe and a heel low. This must be an artifact of manufacturing – if you check the plane with a straight edge, the toe and heel will touch the straight edge and light will pass through in the middle. This is the opposite of what we’d prefer – flat or a little bit in the other direction.
If you need to remove metal from the center of a plane instead, finding a way to do it (small block, coarse paper, or a somewhat flexible mill file and draw filing) without reaching the edges of the plane is a good idea, then lap at the end. Notice how much is removed when you do each different method so you don’t create a problem that’s difficult to counter (e.g., you’ll regret it if you hollow 8 thousandths at a time out of the center of a long plane).
The longer the plane, the more sensible this spot work becomes instead of just lapping. If you find yourself lapping a short plane only and not getting closer to straight, stop and figure out why.
Of Making by Hand with Wood and Steel 