Plane Geometry Question

[paulrockliffe]

It's been bugging me for a while, so I'm just going to ask:

Why does a hand plane have a flat sole, but a power plane or a jointer have the sole in two parts with the depth of cut being controlled by the off-set between the two surfaces?

With a hand plane the blade sits lower than all of the surface, so when you're making a cut you start with the front of the plane resting on the wood and following the wood surface, but as you get towards the end, you run out of wood to rest the plane on and support then comes from the rear of the plane. But the rear of the plane is resting at a level that has been reduced by the plane blade's cut, so tilts the plane body and raises the cut. I think.

Anyway, I'm obviously missing something and would like to know what it is.

Thanks

 

[woodbrains]

Hello,

You are right in theory, but in practice, it is not quite like that. With the thin shavings made by a plane, a human cannot push a plane that can make shavings much thicker than 6-8 thou at full width, the wood is compressible enough for the sole to be pulled flat onto the surface. It is precisely this pressure exerted that makes a shaving curl up without too much breakage in front of it, and the finer the mouth, the closer the pressure is applied, which is the principle behind fine mouths causing less tearout. Of course at the beginning and end of the stroke, where not all of the sole is contacting the wood,there is a slight variation in the thickness of the shavings, which is why there is a tendancy for the work to end up slightly convex. A little bit if stop planing will correct this, or being able to exert a bit more down force towards the centre of the cut. Everyone compensates for this, usually without even thinking about it, with a bit if experience. Someone did invention an adjustable front sole on a handplane, but it didn't work. The pressure thing I just described is essential for planes to work well.

Mike.

 

[paulrockliffe]

Thanks, that makes a lot of sense.

 

[Mr T]

Hello,

You are right in theory, but in practice, it is not quite like that. With the thin shavings made by a plane, a human cannot push a plane that can make shavings much thicker than 6-8 thou at full width, the wood is compressible enough for the sole to be pulled flat onto the surface. It is precisely this pressure exerted that makes a shaving curl up without too much breakage in front of it, and the finer the mouth, the closer the pressure is applied, which is the principle behind fine mouths causing less tearout. Of course at the beginning and end of the stroke, where not all of the sole is contacting the wood,there is a slight variation in the thickness of the shavings, which is why there is a tendancy for the work to end up slightly convex. A little bit if stop planing will correct this, or being able to exert a bit more down force towards the centre of the cut. Everyone compensates for this, usually without even thinking about it, with a bit if experience. Someone did invention an adjustable front sole on a handplane, but it didn't work. The pressure thing I just described is essential for planes to work well.

Mike.

Good explanation Mike, although I think there may also be some flexing of the plane sole involved as well. I wonder if compression also explains something that I have guessed about for years. You can plane a shaving thicker to one side by biasing the plane to that side. The classical explanation for this is that it comes from a camber in the blade, but I am sure this can be achieved even without a camber and have asssumed it is due to differential pressure caused by the bias.

Chris

 

[Sgian Dubh]

I wonder if compression also explains something that I have guessed about for years. You can plane a shaving thicker to one side by biasing the plane to that side. Chris

Here's another oddity Chris. I've long reckoned (a couple of decades at least) that I can bend the sole of a long plane (my no 7 for example) to facilitate prepping an edge for a plain edge joint. In other words start and end the cut as normal, but in the middle of a long edge, forcefully try to twist each hand holding the plane up and towards each other as you go to effect a fractionally heavier cut in the middle of the length. I have no real proof it works, just that it seems to, so on quite rare occasions I give it a go. Slainte.

 

[woodbrains]

Hello,

You are right in theory, but in practice, it is not quite like that. With the thin shavings made by a plane, a human cannot push a plane that can make shavings much thicker than 6-8 thou at full width, the wood is compressible enough for the sole to be pulled flat onto the surface. It is precisely this pressure exerted that makes a shaving curl up without too much breakage in front of it, and the finer the mouth, the closer the pressure is applied, which is the principle behind fine mouths causing less tearout. Of course at the beginning and end of the stroke, where not all of the sole is contacting the wood,there is a slight variation in the thickness of the shavings, which is why there is a tendancy for the work to end up slightly convex. A little bit if stop planing will correct this, or being able to exert a bit more down force towards the centre of the cut. Everyone compensates for this, usually without even thinking about it, with a bit if experience. Someone did invention an adjustable front sole on a handplane, but it didn't work. The pressure thing I just described is essential for planes to work well.

Mike.

Good explanation Mike, although I think there may also be some flexing of the plane sole involved as well. I wonder if compression also explains something that I have guessed about for years. You can plane a shaving thicker to one side by biasing the plane to that side. The classical explanation for this is that it comes from a camber in the blade, but I am sure this can be achieved even without a camber and have asssumed it is due to differential pressure caused by the bias.

Chris

Hello,

Yes, there certainly will be some sole flexing, and I was going to mention this, but decided to leave it it for clarity. Like everything in woodworking, there are always other factors that play parts to the outcome. Another reason I left that particular point out, is that it would be plane specific, in other words, a massively heavy cast old infill might have no appreciable flex compared to a lighter Bailey with no webbing. A woodie sole might have compression in its sole as well as the work surface!

You are right about being able to plane a thicker shaving on one side by varying the pressure. I know this because I do not camber my planes! (Except my smoother, which has a very slight camber to eliminate track marks) It is absolutely possible to correct an out of square edge by exerting more pressure towards the 'high' side. It is also why you can compensate for the natural tendancy of the work becoming convex through successive planing strokes, by exerting more pressure towards the centre of the board. I tend to do this more than stop planing, though in practice, I achieve flatness, or slight concavity with a combination of the two.

Mike.