5.4 Layout: Straight Lines
D.M. Donner
There are two types of lines in layout, straight lines and arcs. Remember, we are performing semi-precise layout where we employ different tools and techniques to achieve the goal of accuracy. A better understanding of these factors will aid you in creating a quality layout.
Steel Rules
The most basic tool used to create a straight line is a rule. Machinists prefer steel rules which come in differing lengths and widths, and they can be rigid or flexible. The six-inch rule sits nicely in a shirt pocket and can be used for quick, semi-precise measurements. A steel rule is made from quality steel that won’t rust and has numerical increments engraved along its edges. Graduations are in inches, metric, fractional, or decimal.
The steel rule has four edges on which graduations are etched. Graduations can be in 8ths, 16ths, 32ths and 64ths of an inch, as well as 10ths, 50ths and 100ths. The metric ruler has 1/2 mm and 1 mm graduations. Some rulers come with markings on the short end of the ruler to fit into tight spaces.
To layout a straight line with a steel rule, the operator must visually line up the edge of the rule with a layout mark. This step is the most critical. A misplaced ruler will produce a misplaced scribe line, so take time to align the ruler accurately.
Once the ruler is in place, use a sharp scribe to drag along the edge of the ruler, being careful not to move the ruler during the scribing process. Tilt the scribe to ensure the point is against the edge of the ruler.
Firm Joint Caliper
Another low-tech method for creating a straight line parallel to an edge is to use a special type of firm joint caliper referred to as an odd leg caliper, where one leg is straight and used for scribing and the other leg is bent and designed to follow the edge of the stock. A firm joint caliper is a comparison device that has two legs that are held together by friction.
Parallel refers to the relationship between two items in which the two items sit side-by-side with equal distance between them at all points. For example, a layout line and the edge of the stock can be arranged in parallel.
To create a straight layout line using an odd leg caliper, the edge of the stock must be flat and free of burrs. Draw file the edge if necessary to achieve this condition. Draw filing is a finish filing process intended to remove a small, controlled amount of material and achieve a smooth finished surface.
Keeping the file perpendicular (at a right angle) to the stock, drag the file in the direction of the cutting edges on the file, and inspect the edge for high/low spots. Layout dye or permanent marker can be applied to the stock edge to make it easier to see the high and low spots. This is illustrated in the image above.
An odd leg caliper must be set using a ruler to establish the distance from the edge of the stock to the straight layout line. This is the first opportunity for error, so take your time and do it accurately.
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