To find the length of a rafter by the use of the table, first find the required pitch, at the left end of the table. Opposite this and under the graduation on the edge representing the run in feet, will be found the length of the rafter; e.g., a rafter having a run of 12' with a ¼ pitch, is 13' 5" long, one with a run of 11' and a ⅓ pitch, is 13' 28⁄12", one with a run of 7' and a ⅝ pitch, is 11' 26⁄12" long, etc.
When the run is in inches, the readings are for 1⁄12 of the run in feet: e.g., a rafter with a run of 12" and a ¼ pitch is 135⁄12", one with a run of 11" and a ⅓ pitch, is 133⁄12". Where the run is in both feet and inches, find the feet and the inches separately; and add together; e.g., a rafter with a run of 11' 6", and a ½ pitch, is 15' 68⁄12" + 86⁄12" = 16' 32⁄12".
The lumberman's board-rule, Fig. 199. To measure wood by it, note the length of the board in feet at the end of the measure. The dot nearest the width (measured in inches) gives the B. M. for lumber 1" thick.
The try-square, Fig. 200, which is most commonly used for measuring the accuracy of right angles, is also convenient for testing the width of a board at various places along its length, for making short measurements, and as a guide in laying out lines with a pencil or knife at right angles to a surface or edge. The sizes are various and are indicated by the length of the blade. A convenient size for the individual bench and for ordinary use has a blade 6" long. It is also well to have in the shop one large one with a 12" blade.
| Fig. 200. Try-Square. | Fig. 201. Miter-Square. | Fig. 202. Sliding-T Bevel. |
In testing the squareness of work with the try-square, care must be taken to see that the head rests firmly against the surface from which the test is made, and then slipped down till the blade touches the edge being tested, Fig. 203. The edge should be tested at a number of places in the same way: that is, it should not be slid along the piece. The try-square is also of great use in scribing lines across boards, Fig. 204. A good method is to put the point of the knife at the beginning of the desired line, slide the square, along until it touches the knife-edge; then, resting the head of the square firmly against the edge, draw the knife along, pressing it lightly against the blade, holding it perpendicularly. To prevent the knife from running away from the blade of the try-square, turn its edge slightly towards the blade.
Fig. 203. Using the Try-Square.