Handplane Performance #2: The Main Things - Sole Flatness

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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. Really, the best check is to mark the sole with a magic marker and stroke the plane on sandpaper on a flat surface. 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. P120 is usually good enough – no need to go beyond P220. For a really bowed or twisted sole I may go as low as 60 grit paper, but usually 80 grit until flat, then progress through to 120 and 220. I use furniture paste wax after smoothing the sole. 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 donward pressure in different spots, depending on where the high spots are, works best. I will apply significant donward pressure initially if quite a bit needs to come off, and let up on the pressure as the sole starts to flatten out. 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

Handplane Performance #2: The Main Things - Sole Flatness