Plane irons may benefit from rounding across their breadth, but rounding on the bevel edge or flat face means they are dull. Bench plane irons work with their bevels down toward the wood. To see the problem with a rounded bevel, put your index finger on the table and push it forward as though it were a plane iron. Unless your fingernail is much longer than mine, its edge will not make contact. This is exactly what happens with an ill-ground or dull plane iron—it rubs and nubs but never cuts.
Before you can sharpen the iron, you have to take it out of the plane. Iron-bodied planes usually have a cam-lever that locks the iron in place. Pull the lever and the iron comes free. On a wooden plane, strike the fore end of the top of the body sharply with a wooden mallet as you support the plane with your free hand. Often a wooden plane will have an inset "start" or "strike button" at this point to save the body of the plane from wear. On small planes the strike button may be on the rear, and that is where you should tap to release the iron.
I say "iron," but you probably found two irons, back to back, held together by a screw. The lower one is the plane iron, the upper one the cap iron or chip breaker. This upper iron is an innovation from the late eighteenth century to help the plane leave a smoother surface. When a shaving is severed by the edge, it slides up the face of the iron and can build enough leverage to lift a shaving ahead of the iron. This lifting can cause the wood to split a tiny bit deeper into the surface than the iron can cut. The chip breaker prevents this by sharply turning the shaving and breaking it before it can do any harm.
The cutting edge of the plane iron and the breaking wedge of the cap iron work as a team, but we need to separate them for the moment. Loosen the screw holding them together and remove the cap iron, turning it sideways and carefully avoiding touching and harming the cutting edge. This care to protect the edge is even more critical after sharpening. One slip and you're back to the stone.
The shape and bevel of the plane iron come from the grindstone. For a plane iron you plan to hone with a secondary bevel, grind to 25 degrees. Grind at 30 degrees if you are going to hone the whole bevel. You can always tell a 30-degree angle by the length of the bevel. When the bevel is twice as long as the thickness of the iron at the end of the bevel, you've got 30 degrees.
The flat side of the iron is where the steel is. In older plane irons you can see the color change or the weld where the steel layer ends. The flat side of the iron must remain flat right up to the very edge. It should never need grinding — just a dead flat polish on the whetstone.
Check the iron with a square before and as you grind to help you judge the degree and evenness of the widthwise curvature. It's a very slight arc on a jack plane, just 1/16 inch at most. Smooth, trying, and jointer planes just get the very corners rounded off. Just bear down a little more on the ends of the iron as you sweep across the grindstone, and, later, across the whetstone.
The cap iron needs no attention unless you find shavings catching under it. It must bed tight against the flat of the plane iron with a smooth face ready to give a quick turn to the shaving.
Carefully reassemble the irons with the big screw. A jack plane may have the breaker as far back as 1/16 inch; a smooth or jointer should have a setback of down to 1/64 inch. The smoother and finer the cut, the closer the breaker iron should be to the edge. Difficult wood needs the breaker set as close as possible to the edge. In nicer stuff, you can back off the breaker to make the cut easier. I have a very old and beautiful wooden single-ironed jointer plane that is a joy to use. It seems to glide down the edges of boards because there's no chip-breaking work adding resistance.
Before the iron goes back in an iron-bodied plane, look at the sloping ramp where the iron rests. This is called the frog, and if there are two screws holding it to the body of the plane, it is an adjustable frog. If you loosen these screws, you can use the screw behind the frog to adjust the mouth, the opening in front of the iron. For fine shavings, narrow the mouth. For coarse work, widen the mouth so the fat shavings can pass through. You'll need to tighten the frog screws and lock the iron in place before you can check the mouth opening. Work by trial and error, taking great care not to touch the freshly sharpened iron against the metal body of the plane. All-wooden planes don't come with adjustable frogs, but they as well as iron-bodied ones benefit from narrow mouths. That's how planes eliminate the wedge effect, ensuring that the bottom of the plane holds down the wood until the last possible instant before the edge of the iron shears it off.
When you try the plane, you may hear and see chatter. Chattering occurs when the iron repeatedly bends back and then springs forward as the plane progresses along the wood. The iron is either too far extended or is not well
supported. Very thin irons or long sharpening bevels are also prone to chatter. Either reduce the amount of protrusion of the iron or correct its bedding on the frog.
Setting a metal plane is as easy as turning a knob. Hold the plane upside down and sight along the bottom. Thumb the depth-adjusting nut around until the iron shows its head. Push the lateral adjustment lever left and right to even the iron. Now turn the depth nut back to take the iron out of sight. Turn it back again until enough iron shows to take a paper-thin shaving. If you go too far, back up and come out again. To take up play in the mechanism and make the iron stay where you want it, always make the final adjustment in the direction that moves the iron outward.
For a wooden plane, it's just tap, tap with the mallet or hammer. Place the iron back in the plane, bevel down, and push the wedge into its seat. Turn the plane upside down and sight along the sole to judge the squareness and depth of the iron. Set the iron a tiny fraction of an inch shallower than you want and lightly tap the wedge home. The iron will usually travel a bit deeper with the wedge. If the iron appears to be too deep, a sharp tap on the heel end of the body will raise it. This loosens the wedge as well, so it will want another tap. If the iron is too shallow, tap either on the front end grain of the body or on the top end of the iron itself. The wedge may again be loosened by tapping on the iron, so see that it is well secured before you test the plane. You can make all the adjustments on a wooden plane with a mallet, but a hammer on the top of the iron works faster and finer.
In the long run, finer and faster go together in planing. As Bergeron advised in 1796: "Often an amateur tries to advance his work by giving too much iron, but in little time his plane is choked. He is obliged to withdraw the shavings with an iron point; thereby he destroys the plane's mouth, nicks the iron, and loses much time."
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