There was some mention of this in a related thread on flattening plane soles, where someone mentioned that a plane will flatten wood only until toe, cutting edge, and heel are just touching. In short, producing a shallow curve through these three points. This is sometimes shown as a diagram in text books. However, I believe this is wrong.
Why? well consider the process of taking stop shavings*, also often quoted as a good way to ensure a flat surface. This makes the surface very slightly hollow (2 or 3 thou or so) to allow subsequent use of a square with no possibility of it rocking.
You keep taking stop shavings until the plane does not cut - nothing coming out of the mouth. It is at this point you have produced the slight hollow as defined in paragraph one above. You have planed a hollow of a shaving's thickness over the length of the plane (or 2 shavings thickenss over twice the length etc.)
Then you take full passes (ie. over the full length of the timber) until you get a continous, full length shaving. At this point the surface should be still slightly hollow, but less hollow than as described in paragraph one above.
The effect of a convex sole will be to increase the depth of the hollow produced by this process by roughly the amount of the convexity. A hollow, concave sole reqires the iron to be set further out before it cuts full shaving, and the above process may produce a convex surface, which you don't want.
At this point we'd better consider the bending of the sole under planing action. My Record No.8 will deflect** between 1 and 2 thou under firm downward planing pressure, and my Clifton 6 about 1 thou. In practice a well set plane requires little pressure to maintain a cut, and a firm downward load is only applied as the toe leaves the end of the board. However, the possibility of sole flexing suggests a slight hollow in the length may be less of a problem in a try plane.
From this you can see roughly how flat a plane's sole needs to be. Your best smoother, probably as flat as you can*** if you like a planed (no sanding) finish, and your try plane not more than a thou or so hollow.
It looks to me as if the old addage of para. 1 is only half the story; maybe the rest (if my thoughts are correct!) got lost in the rapid decline in handwork as working wood became industrialised.
* Stop shavings: start and finish planing about 1 cm from the edge of the work to introduce a hollow over the length. Repeat this till plane stops cutting. Now take full length shavings until you get a continuous ribbon from edge to edge; then stop.
** plane resting on mouth and heel, measured by clock gauge about half way in between, with load on the tote.
*** wet/dry or lapping film on cheap granite surface plate.
Why? well consider the process of taking stop shavings*, also often quoted as a good way to ensure a flat surface. This makes the surface very slightly hollow (2 or 3 thou or so) to allow subsequent use of a square with no possibility of it rocking.
You keep taking stop shavings until the plane does not cut - nothing coming out of the mouth. It is at this point you have produced the slight hollow as defined in paragraph one above. You have planed a hollow of a shaving's thickness over the length of the plane (or 2 shavings thickenss over twice the length etc.)
Then you take full passes (ie. over the full length of the timber) until you get a continous, full length shaving. At this point the surface should be still slightly hollow, but less hollow than as described in paragraph one above.
The effect of a convex sole will be to increase the depth of the hollow produced by this process by roughly the amount of the convexity. A hollow, concave sole reqires the iron to be set further out before it cuts full shaving, and the above process may produce a convex surface, which you don't want.
At this point we'd better consider the bending of the sole under planing action. My Record No.8 will deflect** between 1 and 2 thou under firm downward planing pressure, and my Clifton 6 about 1 thou. In practice a well set plane requires little pressure to maintain a cut, and a firm downward load is only applied as the toe leaves the end of the board. However, the possibility of sole flexing suggests a slight hollow in the length may be less of a problem in a try plane.
From this you can see roughly how flat a plane's sole needs to be. Your best smoother, probably as flat as you can*** if you like a planed (no sanding) finish, and your try plane not more than a thou or so hollow.
It looks to me as if the old addage of para. 1 is only half the story; maybe the rest (if my thoughts are correct!) got lost in the rapid decline in handwork as working wood became industrialised.
* Stop shavings: start and finish planing about 1 cm from the edge of the work to introduce a hollow over the length. Repeat this till plane stops cutting. Now take full length shavings until you get a continuous ribbon from edge to edge; then stop.
** plane resting on mouth and heel, measured by clock gauge about half way in between, with load on the tote.
*** wet/dry or lapping film on cheap granite surface plate.
Whether it's flat to within a gnats dooberie is purely academic - Rob
