The Sole of an Old Machine

Instead of writing about the flatness of plane soles, perhaps I should talk about something less controversial, such as religion or politics.

When purchasing a vintage plane, the flatness of the sole can be critical when making a purchasing decision. So I’m going to man-up here and talk about how I approach this potential problem.

The soles of vintage handplanes can be warped for a variety of reasons. Perhaps they were poorly manufactured. Perhaps they weren’t properly stress relieved and the casting moved over time. Perhaps they were dropped or abused in service.

Whatever the cause, an out-of-true sole can affect the way a plane cuts, especially when trying to take thin shavings. When I purchase a vintage handplane that I am going to use for smoothing or jointing, I’ll check its sole with a straightedge and feeler gauges.

Some people will instead skip the feeler gauges and just look for light between the straightedge and the sole. I think this can be misleading (the gaps look alarmingly large) and it doesn’t tell you how much of a convexity or concavity you have. That’s where the feeler gauge comes in.

I assemble the plane completely, apply a working tension to the lever cap and retract the iron into the body of the tool. Then I set the plane upside down in a vise. I don’t clamp the vise jaws down , I merely position the jaws to support the tool’s sidewalls.

Then I place the straightedge on the sole of the tool and start with the feeler gauge that is .0015″ thick and see if I can slip it under the straightedge at any point. Then I move the straightedge and try again. If I can slip the gauge under the straightedge I’ll switch to a thicker leaf, such as .002″ and try that. I keep moving up in thickness until I cannot slip the gauge under the straightedge.

Do enough testing and you’ll get a fairly good topographical map of your plane’s sole. But how do you read the map? Your sole doesn’t have to be dead flat to work great. The plane’s sole has several critical areas that need to be coplanar. Check out the photo. All the red Xs are the critical areas. However, you don’t really want the sole to be convex in the areas behind or in front of the mouth. Big hills are bad news.

Then it’s time to make a call. For a plane for rough work, almost anything is acceptable because its shaving is so thick. For a jointer plane or a smoothing plane, I start to get concerned when the sole is out in a critical area by more than .002″. I have found that errors such as that can cause problems.

So what do you if the sole is out of whack? If it’s a jointer plane, I’ll usually pass on the tool and keep looking. I have had difficulty in the past flattening jointer plane soles. Plus there is no worldwide vintage plane shortage. If it’s a smoothing plane I might still buy it. I can true those easily.

There are lots of methods to true soles that are published on the Internet, and I’ll cover my methods in a future post.

– Christopher Schwarz