As woodworkers, we need to assess four primary types of measurements. They are: alignment (straightness), planar accuracy (twist), angular accuracy (angles) and linear dimension (length). In order to adequately address them all, every woodworker should have at least one of each of the following tools.
A straightedge assesses straightness or flatness. Originally shop-made, it has been replaced by metal versions. The bevel-edge type is preferable to those with thick edges because the narrower the contact face, the easier it is to see inaccuracies in your work.
Twist is usually called winding by woodworkers. A board or assembly in winding has its ends twisted in opposite directions. To check for twist, you need to sight across a pair of winding strips. They are the basis for measuring planar accuracy — there is no alternative. Winding strips are simply a pair of shop-made, parallel-edged straight pieces of wood of the same dimension.
Squares and Gauges
The measuring tools we use to determine angles fall into three sub-groups: try squares, which measure 90°; miter squares, which measure 45°; and sliding bevels, which measure variable angles. A popular alternative to the try square is the combination square borrowed originally from the machinist’s toolbox. Combination squares have several features not associated with try squares. First, the blade is graduated so that it works also as a rule. Second, the blade slides. Finally, the stock measures 45° as well as 90°, or multiple angles if it is the revolving style.
Graduated scales, which include metal tapes, rules and calipers, measure linear dimensions. For most bench work in furniture making, the 12" and 24" rules are both useful sizes to own.
The Vernier caliper, also borrowed from the machinist’s toolbox, is an excellent gauge for measuring the depth of grooves, stopped holes or stopped mortises. The main jaws can be used to measure outside dimensions as well as inside dimensions of grooves or tenons. It’s a highly precise measuring tool.