Measure and mark for better accuracy
There's more to measuring and marking than stretching a tape across a board and drawing a pencil line. By using the right tools and accessories, you'll greatly improve your precision and even speed up your work. Fortunately, these improved results don't require fancy gizmos or staring through a jeweler's loupe.
Be sharp to make your mark
Let's focus first on the most basic item -- what you mark with -- and how it affects accuracy. The thick lead of a carpenter's pencil draws a line 1⁄16 " wide, below. Cutting to one side of the line yields a far different result than cutting to the other side. Accurate layout of parts and joinery requires the fine line drawn by a pencil with a 5H lead, available at office-supply stores. The harder lead sharpens to a finer point and holds that point longer.
Save your carpenter's and no. 2 pencils for writing notes and marking machined surfaces, below, where bolder marks are easier to spot at a glance, letting you know what operations are completed and which need to be done on a board.
A pencil won't leave permanent marks on a workpiece; the marks erase or sand away easily. To avoid leaving a mark in the first place (such as when marking a finish-sanded part), apply a piece of masking tape and write on the tape.
When marking a line that will be cut away or hidden by other parts, switch to a marking knife, top photo. The beveled face and flat back of the knife put the cutting edge right next to a straightedge for a surgically precise mark. And a sharp marking knife severs the wood fibers, creating a shallow kerf -- the ideal starting point to register the blade of a chisel or the teeth of a handsaw. The kerf creates a shadow, which is easier to see on the workpiece than a pencil line.
To use a marking knife, place a steel rule on the "keeper" piece and, with the flat face of the knife against the rule, draw the knife along. If the knife should stray off course, the mark ends up on the waste piece.
Establish a definitive rule
Every country has a bureau of standards, which maintains a set of incredibly precise measuring instruments against which other measuring devices are calibrated. Carry this concept into your workshop. Choose a precise, finely etched 12" steel rule for the bulk of your measuring tasks. A quality combination square provides not only a rule; the head, with 90° and 45° angles built in, increases the tool's versatility [see Proving a square has the right angle, below]. For measurements longer than 12", use only one 12' tape measure. Anything longer is just extra bulk to carry. And check the tape against the rule to make sure they agree, below.
Now that you've established which rule rules the roost, make sure everything else in your shop agrees with it; for example, the rip-fence indicators on your bandsaw and tablesaw, below, and any other rulers. If other rulers don't measure up, relegate them to the house.
A metric rule can come in handy, too, especially if calculations with imperial dimensions give you a headache. For example, determining one-half, one-fourth, one-fifth, or three times 77⁄8 " brings out the pencil and paper compared with working with its metric equivalent, 20cm.
Proving a square has the right angle
Calling a tool a square doesn't make it square. To ensure that yours lives up to its billing, do this simple test with a piece of straight-edged scrap.
With the head of the tool to one side, draw a line the length of the blade. Flip the square and draw a second line next to the first. If the second line parallels the first, two photos below, the square is true. If the lines slant away from each other, the square needs adjustment or replacement.
After determining that your square is square, protect it from drops and bumps that could compromise its accuracy.
Put accurate tools to use
After choosing quality measuring and marking tools, use these simple techniques to get the most accurate results.
First, select reference edges and faces and measure from them as often as possible. For example, when laying out a series of drawer openings along a cabinet's stiles, always measure from the same end of each stile. After marking the locations, measure between the marks to double-check your accuracy.
When marking, make sure you sight straight down on the ruler. Working to one side throws off what appears to be an accurate mark, below.
To mark a dimension, draw a "V" extending from the ruler instead of a single tick mark. A single line can end up angled, causing confusion over which end is the real dimension. To extend a line or transfer it around an edge, place your knife or pencil on the tip of the "V" and gently slide your square or ruler up to it. Then use moderate pressure and draw the pencil or knife across the workpiece once. Repeated passes only widen the mark, reducing accuracy.
More ways to mark accurately
Maintain the proper orientation of nearly identical parts as you mark them by indicating which surfaces are the top, bottom, left, right, front, back, inside, and outside as needed. For several pieces needing identical layout marks, such as matching mortises in opposing table legs, save time and improve accuracy by clamping the pieces together and marking across all of them at once, right.
Finding the center of a workpiece is simple: Measure the width, then divide that number in half. To confirm your math, measure in that distance from each edge and make a mark, below. If the marks fall on top of each other, you've found dead center. If not, adjust the measurement by half of the amount between them and try again.
In some instances, the most accurate measurement comes from avoiding a ruler or tape. For example, when fitting a divider between two rails in a face frame, below, measuring and then transferring that dimension to the workpiece invites at least two chances for error to creep in. Instead, place the workpiece against the opening and mark the dimension directly onto the workpiece.
Apply these methods to your work and watch the improvement.