Handplane Performance Tuning #2: Sole Flatness

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Blog entry by OSU55 posted 01-15-2014 05:04 PM 6166 reads 4 times favorited 7 comments Add to Favorites Watch
« Part 1: Sharpening Blades / Irons Part 2 of Handplane Performance Tuning series Part 3: Chip Breakers & Cap Irons »

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.

7 comments so far

View Paul M's profile

Paul M

119 posts in 2755 days

#1 posted 02-27-2015 02:52 AM

I am beginning woodworker, with a new interest in hand planes. (Note that this scares me after seeing the hand plane fanatics on this site !!!) I have seen on several sites that I should have the iron and cap lever installed when flattening the sole. This is apparently to ensure that the lever does not distort the plane casting. But can this really happen? A hand tight spring lever, centered in the frog body, somehow warping a heavy iron casting (with 1” high sidewalls)? I just don’t see it. Is this just myth? I found it much easier to start cleaning the sole without the frog and handles installed. Thanks

-- Paul M

View OSU55's profile


3034 posts in 3441 days

#2 posted 02-27-2015 12:47 PM

Well, it’s not myth, it is fact, but it is a matter of degree. Typically the sole flattening process is as I describe above, using human power to do the flattening. Why not get the sole flat as possible in the shape it will be used to cut wood (frog and blade in place)? I have experimented some with it, and we are only talking 0.001” or 2, which is really only relevant for smoothing and maybe jointing, but since it takes no more time or effort, why not clamp things up and get the sole dead flat?

What method are you using that makes it easier to not have everything installed? I surface ground a #7 using a very accurate oscillating bed tooling grinder. The plane bed was stripped since there wasn’t a good way to fixture otherwise.. Due to the flexing of the long plane bed it was still out by a couple of thousandths, and when I mounted everything, it twisted the bed differently and created high spots in different areas.

View Paul M's profile

Paul M

119 posts in 2755 days

#3 posted 02-27-2015 10:40 PM

Thanks for reply. I am flattening on wet/dry sandpaper on a marble base. I started with only the bare casting at 80 grit (there was alot of pitting, is an old plane that I am using to learn with). Once I got up to 220 grit I re-assembled the plane as you described, and finished removing any scratch marks. I just find it easier to apply consistent downward pressure on the bare casting, that is all.

I don’t yet have the tools to measure the few thousandths that the plane sole could warp when preparing. I am just wondering from a mechanical point of view how that casting can warp with only the spring pressure from the cap lever? Note that I could see this on a small block plane, just not on a large bench plane.

Anyways, thanks much for your inputs. By the way, I was able to finally use the plane today, and spent 30 minutes shaving down some 2×4s. I hope I am not hooked.

-- Paul M

View OSU55's profile


3034 posts in 3441 days

#4 posted 02-28-2015 12:34 AM

With everything in place, if you marked up the sole with ink and went back to your 220gr, you maight see that the surface did change some. Not an issue unless down the road you have trouble getting thin smoother shavings or jointing an edge very straight.

Just like any of the other bad habits, just keep doin’ it long enough…................

View Paul M's profile

Paul M

119 posts in 2755 days

#5 posted 02-28-2015 04:33 AM

don’t get me wrong, I will follow the best practice. Just still intrigued by the phenomenon. Need to see it tested.

Thanks again.

-- Paul M

View Don W's profile

Don W

20380 posts in 4020 days

#6 posted 02-28-2015 02:45 PM

there are differences of opinions about the importance of having the plane together when flattening. Hell, there are even differences of opinions as to whether the sole even needs to be flattened. The bottom line is how the plane works.

As Osu55 stated, not an issue unless it doesn’t work as expected.

-- - Collecting is an investment in the past, and the future.

View benchbuilder's profile


285 posts in 3902 days

#7 posted 04-29-2015 02:07 PM

If its not broke dont fix it!!

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