The cutters in my bench planes all have cambered irons. The jack has the most – a 10” radius curve – followed by the much slighter curves of my jointer and smoothing planes.
The curves do two things: They prevent the corners of the iron from digging into the work and creating “plane tracks,” tiny unsightly rabbets. And the cambered irons also allow me to remove material in certain areas to true up the edge or face of a board. When I have a high spot, I can put the camber over it to reduce it without affecting the surrounding wood.
But there are some woodworkers who work without a camber. They use irons that are sharpened straight across.
While this isn’t the way I work, the methods are helpful to know, even if you use a cambered iron. I use them at times in a jointer plane to make swift corrections to a wonky edge.
Here’s how it works.
First, Forget Tipping & Leaning
It’s tempting to think that you can easily control an edge by slightly tipping the plane or leaning hard on one edge. You can, but it takes practice and is not as predictable as other methods.
When you are trying to make things true, you want a predictable path and a guaranteed outcome. So my advice is to ignore advice to tip the plane (unless you are already an ace at it).
Use the Mouth
The heart and soul of the method of using a straight iron is to use the tiny bit of plane sole that is next to the mouth opening. Does this part of the tool have a particular name? Probably, but I’ve never seen it called out. I call it “the corner of the mouth.”
What’s important about the corner of the mouth is that when you run it over wood, it won’t cut it. That sounds obvious, I know. But it can be a powerful technique.
So let’s take a simple example to show how the corner of the mouth works when it’s at its best – dealing with the edge of a board. Let’s say you are planing the edge of a board and the edge isn’t square to the face of the board – a typical problem. How do you correct the edge so it’s square?
If you have a straight iron in the tool, place the corner of the mouth over the low side of the edge. That way the plane won’t cut anything away from the low side and will cut only on the high side.
Take a couple passes and check your work. When you get close to square (about one shaving away) shift the tool laterally so the entire cutter sees the entire edge and can give you a square and true edge.
Planing faces with a straight iron is dang tricky if you want the face clear of plane tracks. I’ve done it. You can do it. But I have to be honest here. In my opinion it’s far more difficult than sharpening a simple camber on the edge of your smoothing plane.
Here’s the best way to explain the straight-iron method: Let’s say you are mowing the lawn. The wheels of the lawnmower represent the corners of the plane’s mouth. And the blade of the lawnmower represents the blade of the tool.
And for the sake of example, let’s say this grass is stiff. Really stiff. As stiff as wood.
So you push the lawnmower across the grass along your street and make what is essentially a super-shallow rabbet. Now you go to make the second pass on the lawn. You put two of the wheels of the mower on top of the stiff grass. The cutter is now tilted slightly.
If you are taking a shallow cut, then you will leave a surface without harsh corners or plane tracks. The wheels on the already-cut grass will hold the cutter above the grass. The stiff, uncut grass will hold the cutter at an angle so there are no harsh corners.
The trick is to overlap your passes with care so that the corners won’t dig in. And you need to take a shallow cut – less than .002” is enough.
One More Method
There’s a third hybrid method: Clip the corners of the iron but sharpen the middle part straight across.
This method works much like cambering the iron – it tucks the corners up into the mouth of the plane. What I don’t like about this shape of this cutter is that it will tend to scrape the wood at the corners of the mouth. The surface looks OK and will look just fine when you put a film finish on it.
A straight iron works best in a jointer plane. You can straighten an edge easily with a jointer plane with a straight iron. And when you work faces, the plane tracks from the jointer plane are fairly irrelevant.
However, a straight iron in a jack plane will give you trouble if you use the tool in a traditional manner (taking off huge slivers of wood). And a straight iron in a smoothing plane is (I think) a fussy proposition.
— Christopher Schwarz