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Newsletter Article: How to True a Board's Edge

Squaring the edge of a board to a flat (“reference”) face is a simple but critical skill for all woodworkers.  It should be one of the first skills a woodworker learns.  Rarely do you get a piece of lumber with square edges and fal reference faces, ready to use. 



The theme of this monthly newsletter is, “How to six-square a board.”  A board has six planes or sides (2 x faces, 2 x edges, and 2 x ends).  Before starting most woodworking projects, you must make your lumber flat, with parallel faces, edges square to the faces, and ends square to the faces and edges.  The process of achieving this is called “six-squaring” or “dimensioning” a piece of lumber.  The first step in dimensioning is to flatten one face of the board.  This becomes your "reference" face. Squaring one edge to this reference face is traditionally the second step in six-squaring a piece of lumber. 




Squaring a board’s edge to its reference face can be done with a power Jointer or a hand plane.  I think the hand plane is more pleasurable to work with than a power jointer, it's faster than a power jointer (if I am not doing production work), it's more precise than a power joint, and it's certainly cheaper than a power jointer.  This article is all about how to square a board’s edge to the reference face using a hand plane.  If you are a hybrid woodworker (use both power and hand tools) a good technique is to square an edge on the power jointer and then use a hand plane to perfect the edge.  Use a hand plane after the power jointer and you will be surprised how rough and imprecise a power jointer leaves an edge.


Ok so let’s define our starting point.  I will assume you already have established at least one flat face on your piece of lumber, this will be our “reference” face.  Meaning, we will reference all our other edges and faces of the board to this face in order to make everything either square or parallel to this “reference face”.  If you do not know how to get a flat face, see Rob’s video on “six squaring a board” and Jake’s video on “How to use winding sticks” in this newsletter.   


To square a board’s edge, you need four tools: a hand plane, a square, a straight edge, and a workbench.  The general rule of thumb is the longer the sole of the hand plane is the easier it is the square and flatten an edge.  This is the operation that a “jointer” hand plane is made for.  A jointer is typically between 22” and 24” long.  Using the Stanley numbering system, a jointer plane is called a #7 or a #8. 



A jack plane (#5 through #6) will easily square medium length boards, say less than 36 inches long.  Yes, it is possible to square an edge with a shorter smoothing plane (#3 through #4-1/2) but it will take more time and be more difficult to achieve the same level of precision as what you can achieve with a jack or jointer plane.  You will also need a 6- or 4-inch square, combination or fixed.  Ensure it’s a square, square.  Don’t use a square you bought at the big box store for $7.00; its not accurate enough.  If you are going to be a hand tool woodworker, you need an accurate square.  I recommend and use a 6-inch, PEC combination square



You also need an 18- to -36-inch-long straight-edge.  Like the square it must be accurate.  If you are using a jointer plane, you can use the sole of the plane for the straight edge.  You can purchase a straight edge, or make a wooden one on your own (it's fairly easy). 



If you have The bottom of winding sticks, they can be used as a straight edge.  If you already have a flat surface (e.g. the top of your workbench, table saw, or power jointer) you can use its flat surface as a straight edge – I will explain how to do this later.  Finally, you need a workbench with a face vice to hold your board on edge while you plane it.


Your goal is to achieve the following two conditions:  1) an edge that is exactly 90 degrees, measured along its entire length, to the reference face and 2) an edge that is perfectly flat along its entire length.  So how do we test to see if our edge meets these two conditions?


First, let’s test for squareness to the reference face.  Using a square, place the square’s reference edge against the board’s reference face so that the square’s blade is positioned over the board’s edge that you are testing.  Keeping the square’s reference edge tight against the board’s reference face, slowly lower the square’s blade down to the board’s edge until the edge of the blade is resting on the board’s edge.  Sighting down the boards edge as you point it toward a light source, look where the square’s blade meets the board’s edge. 



You should not be able to detect ANY light between the blade and the board, across the width of the board’s edge. 



 Do this test along the board’s edge about every six to eight inches.  If every test shows no light, the edge is square to the board’s reference face, but the edge may not be flat along its length.      


 Now let’s test the board’s edge for flatness.  There are several methods to test for flatness, but first sight down the edge with your eye and see if the edge appears flat or if it is obviously convex or concave.  If it appears convex or concave to the eye then plane it until it appears flat, but don’t lose your squareness to the reference face.   Once the edge appears flat to the naked eye, test it for flatness.  You can hold the edge of your jointer plane against the edge of the board and look for any light passing between the edge of the plane and the edge of the board (you want no light). 




 You can also do the same test using a straightedge. 




My favorite flatness test is to place the board’s edge down on a flat reference surface (workbench top, table saw, or power jointer table) and see how it reacts to movement.   Using your forefinger and thumb, lightly grasp one of the ends of the board and very gently push the end of the board forward then back by about 6 inches, while observing the other end of the board.  If the board is flat, or very slightly convex (which is OK), then the board will pivot on the opposite end from where you are holding the board (this is what we want). 



If it pivots somewhere other than the end, there is a hump on the edge at that pivot point.  I also like to “poke” one end of the board with my finger and see if the board will spin, indicating a hump near the middle of the board’s edge (a common problem).        


Now that you know how to test a board’s edge for square and flat, let’s talk about how to get there.  If you have a rough-cut board with a jagged edge, you must first plane the rough edge to get it generally flat and square so you can do a squareness test with your square.  If your board’s edge is already generally flat and square just perform the squareness test on it.


Holding the square against the board’s reference face, sight down the board’s edge as you point it at a light source and gently lower the square’s blade to the board’s edge and observe the reading. You will have one of three readings: 1) the edge is square to the reference face (no light between the blade and the board’s edge), 2) the right side of the board’s edge is higher than the left side of the board’s edge (light is showing under the blade on the left side of the board’s edge), or 3) the left side of the board’s edge is higher than the right side of the board’s edge (light is show under the blade on the right side of the board’s edge).  Do this same test all along the length of the board about every 6-8 inches, noting the readings.  Typically, the edge will not be square on the first test, but you do get lucky every now and then. 




Let’s assume the first test indicates that the edge is not square; what now?  Using a pencil or pen mark the edge’s high side by running a line along the edge on the high side.  We are now going to bring the high side (marked with the line) down to the low side then retest for squareness.  



This is a good time to discuss the blade’s position in your plane when squaring a board’s edge.  The plane’s blade must be set parallel to the sole of the plane.  If instead the blade is skewed relative to the plane’s sole, you will be introducing that skew onto the edge of your board and you will never get the edge square to the reference face.  How to set you plane’s blade parallel to the sole is beyond the scope of this article but see any of Rob’s YOUTUBE videos on the subject.  In addition to the blade being parallel to the sole, the blade should only project out the bottom of the plane by a few thousands of an inch or less.  We want very fine, thin shavings.      


We will take advantage of the design of the plane’s throat to bring down the high side of the board’s edge.  Looking at the bottom of the plane, the plane’s throat does not extend all the way to the plane’s edges.  On each side of the throat there is about a ¼ inch of the sole.  



Place the plane on the board’s edge, align the side of the plane so that it is flush with the board’s face on the low side of the edge.  This will keep the plane’s blade from cutting on the low side of the edge, but it will cut on the high side of the edge. 



Take three to four passes, taking care to hold the plane square to the reference face.  Observe the shaving as it is ejected up through the throat.  The shaving’s width should be less that the width of the board; you are not taking a “full-width” shaving yet because you are preventing the plane from cutting on the low side of the board’s edge. 



After the last pass, readjust the plane’s position on the board’s edge by centering the plane on the edge. 



Keeping the plane square to the reference face, continue taking passes until a full width shaving is ejected through the throat.  This should only take one to three passes.



Retest the edge for squareness.  If the edge is not square, repeat this process until it passes the square test.  Hint: As you get closer and closer to a square edge retract your plan blade so you take a thinner and thinner shaving.


Once the board’s edge passes the square test, check the board’s edge for flatness using one of the flatness tests described earlier.  One of the characteristics of a plane is that as you continue to plane an edge it will naturally introduce a hump in the board.  If the flatness test reveals such a hump, which is common, the procedure to remove the hump is to take a small pass (4-6 inches) with your plane out of the middle of the edge of the board.  Continue taking passes out of the middle of the board increasing the size of each successive pass until you take a pass along the entire length of the board.  Once you take this “full length” pass observe the shaving, looking for a full width shaving.  Once you achieve a full length and a full width shaving, retest for squareness and flatness. 


Keep repeating these two processes until your edge passes the square and flat tests.  While there are a lot of things going on simultaneously in this process, it is easy to master in just a few tries.  Once the technique is mastered, speed comes quickly.  Soon you will be reaching for you hand plane rather than your earplugs when it's time to true a board’s edge. 


Good luck.


Luther Shealy