Hand Tools

Re: Questions on sole flattening

Wiley Horne
Bill asked,

1. So, how flat is essential for peak, or acceptable, performance?

2. Related, if a novice planer is using an unacceptably unflat plane what would they experience that would tell them they need to flatten it?


Q1. Leading edge of mouth must sit down hard on the work. You don’t want it in the air, even in the low-thousandths.

Q2. If mouth is not on the work—is in the air—you get erratic chip-pickup, depending on blade projection. Plane is in the shaving, then out. Not getting a full-length shaving.

For example, at a standard fine shaving of 1/1000”, you get pickup starting at the near edge, because there the mouth is on the work. But as the plane gets fully on the board, the heel is now lifting the plane and creating an air-gap under the front-of-mouth—the blade lifts out of the cut. It may reenter when plane hits a high spot, but then out again

At a heavier shaving, say 5-6 thousandths, the effect is less pronounced unless the gap is a bit larger. In a heavy cut, if the sole is concave, the plane will deflect down—particularly on a longer plane. The cut first pulls the plane down into the work, but then it will spring back out leaving a torn shaving.

The cure for these issues, is to get the front of mouth onto the work.

Diagnostic 1: Is the sole convex or concave? Test with straight-edge to get the overall picture. Big engineer’s square or on a long plane a milled-edge level with light behind.

Diagnostic 2: I’ve never had a convex sole—if it’s convex, I think you’d need to drawfile it flat or slightly concave, then proceed with lapping. But the typical case is a more or less concave sole, in both directions. Test: run the plane over 150-220g sandpaper on a flat surface—like a cheap Grizzly granite plate, or equal. Look at the sole—where are the scratches? Is the front of mouth showing scratches all the way across? If not, you want to (1) surface grind at machine shop, or (2) use engineer’s scraper with blue on granite, or (3) some form of lapping—until front of mouth is showing even scratches all across.


Caveat: the above will get the plane working. A more advanced approach would use winding sticks to test for twist. I’ve never had a twisted plane, but surely they exist.

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