Rules

Posted by Easy WoodCraft on

Jan 14, 2016

If you told me I could have only one mea­suring tool in the shop, Id be hard-pressed to decide between the 6" metal rule that lives in one apron pocket and the 12-foot Starrett tape measure in the other. Although you might think I'd go for the tape measure, I'd stick with the metal rule. Why?
It's more accurate, not only for laying out joints, but also for checking them: A tape measure just doesn't slip into a tongue or rabbet cut in the edge of a board like a metal rule does. Fortunately, I don't have to make that call.

Type of Rules

Woodworkers can choose from a wide variety of rules made of either metal or wood (top photo). I much prefer a metal rule because its thinness reduces problems with parallax associated with the thicker wooden rules. In my shop I have a 6", 12", 18", and 1-meter metal rule. Another specialty rule is a hook rule (top inset). It has a plastic hook on one end that catches the edge of the work-piece for super-accurate mea­suring. When buying a rule, check to make sure that the graduations are etched into the metal and not simply printed on (middle drawing). Printed graduations can wear off over time.

CHECKING A RULE

For a rule to have any worth, it must be accu-rate. Check the graduations of your rules against each other—you may be surprised to find that they re not the same. The two rules in the photo at near right are off almost 1/32". This is an excellent reason to use a single rule throughout a project; if you use multiple rules and tape measures, you may inject errors that cannot be traced. Also, check your rules for flatness (photo below right). Set a rule on a known-flat surface and place a pair of flat scraps (called winding sticks) on the rule. Sight along the tops of the sticks to check for twist; replace any rules that aren't absolutely flat.

LAYOUT TIPS

Block of Wood

A common cause of inaccuracy when using a metal rule is in measuring in from the edge of a work-piece. The problem is positioning the rule so that it's absolutely flush with the edge. Many woodworkers use their finger as a stop and press the rule into their finger. Good idea, but the rule will often go past the edge when pressed into the resilient flesh. A better solution is to use a scrap of wood as a stop. I always have a few small scraps of MDF (medium-density fiber-board) lying around just for this. MDF is super-flat and resistant to warp. Just press the scrap firmly against the edge of the work-piece and then butt the end of the rule up against the scrap.

Center of A Board

Here's a quick tip for finding the center of a board with a rule. Position one end of the rule flush with the edge of the work-piece (the scrap block tip mentioned above works great here). Then pivot the rule until a number that's easily divisible by 2 rests directly over the opposite edge. Divide this number in half and mark the center.

Dividing A Board Equally

Quick: You need to divide a board that's 9 5/8" wide into five equal parts. The solution? Modify the "center of a board" trick above. Place an end of the rule flush with one edge of the work-piece. Pivot the rule until a number (10, in this case) that's easily divisible by the desired units rests directly over the opposite edge. Then mark to create equal spacing.

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