Why Sole Flatness?
Convex (bulging out) and concave (hollowed out) soles will cause uneven cut depths and skipping and chattering. For a convex shape, the plane rocks front to back and/or side to side. A concave shape will cause heavier cuts at the start and end of a surface, and possibly no cut in the middle. Different amounts of downward hand pressure can affect each stroke causing more confusion. Even with a very flat sole varying downward pressure will affect the cut. Reduce the variables as much as possible. A smoother should be FLAT – ideally within 0.0005”; a jointer within 0.002-0.003”; a jack within ~0.005”. The flatter the better. Not the entire surface but the areas hilited in the picture:
You can see the faint remnants of magic marker used to measure progress.
How To Get Sole Flatness
Check the sole with a straight edge with a light behind it. I use the ruler from a carpenter’s square, but anything perfectly straight works – the thinner the better. Also, mark the sole with a magic marker and stroke the plane on sandpaper on a flat surface without much downforce. I use plate glass glued to masonite fiber board that is 22” long x 9” wide – 2 sheets of sandpaper just fit. Tables for saws and jointers work, I just don’t like having to remove the sandpaper and clean up the table to use the machine. I leave paper attached to the glass for various lapping activities. A light spray of aerosol adhesive at each end of the sandpaper holds it to the glass. I have found a resin coated type sandpaper is best – similar to sanding belts and just as effective.
The primary concern is flatness, not smoothness or surface finish. If you want to polish the sole to a mirror shine, it won’t hurt anything, but I don’t find it helps. After planing a few boards the sole gets scratched up anyway. I start with 50-60 grit paper, and maybe work up to 120. Anything beyond that looks better, but doesn’t help friction after the sole is waxed. I use furniture paste wax with no silicone. During use I use a crayon, paraffin, or a candle to wax the sole. Some use feeler gauges to check flatness. I find that if the magic marker is getting removed fairly evenly by the sandpaper (see pic above), a 0.0005” feeler gauge won’t fit.
It is important to clamp, or wedge, the blade in place just as it will be in use, but retracted from contact (~0.020” or so) with the sandpaper. All handles should be in place and tight. The points of contact with the plane body do create stress and cause the sole to move. I find holding the tote to push and pull, and using my other hand to apply downward pressure in different spots, depending on where the high spots are, works best. I will apply significant downward pressure initially if quite a bit needs to come off, and let up on the pressure as the sole starts to flatten out. Check the sole often with the straight edge so you know you are addressing the correct area. I’ll finish the flattening (and all smoothing/polishing steps) applying light downward pressure, just like planing. I use a shop vac with a brush tip to vacuum the iron dust and loose abrasive particles every few minutes. I have had to spend several hours to get a flat sole, but that was because I didn’t use aggressive enough grit to start. I will usually mark the sole and make a few passes at 120, then decide how aggressive of a grit will be needed.
The edges of the plane sole need to be rounded or tapered a bit (front, back, each side) so the sole doesn’t hit a sharp edge in the wood and stop (such as the misalignment of board edges in a panel glue up) and the sides can glide over sharp edges when skewing the plane. If the sole has fairly sharp corners, I’ll use a file to break the edge and round it over to about 30°. Starting with 120 paper, I hold the plane at different angles over to about 30° as I stroke it on the paper, and sand the ends by hand. It is not a lot of material removal.
I have used large granite inspection tables at work to flatten soles, and find they do not work any better than the glass/masonite setup. The glass does need to be supported on a fairly flat surface for the entire length, such as a workbench. It will bend if not supported. The 22” length will handle up to a #5 length. Longer planes like a #7 or #8 will need a longer surface. You do not want each end extending off the sandpaper more than an inch or so as you stroke it through, as it will cause a concave surface.