Dimensioning Rolling Offsets
Figure 1325 identifies the six measurements required to dimension a rolling offset. There are four length dimensions and two angular dimensions. When a rolling offset is part of a configuration similar to that shown in Figure 1328, the lengths of the sides of triangle 3 are applied to the dimensions of the isometric box (see Figure 1325). Notice the SA of triangle 3 in Figure 1328 is equal to the RUN of the box, the ROLL of the box is equal to the SO of triangle 3, and angle X of triangle 3 is the same as the horizontal angle (HOR°).
The RISE is determined by subtracting the lower plane elevation from the upper plane elevation. Rolling offsets are typically fabricated using 45° elbows; therefore, the vertical angle will be 45°. Notice the dimension labeled TRAVEL in Figure 1325. It establishes the true length of the pipe from the upper plane to lower plane across the box. This length is the most difficult to calculate. The values used to determine its length depend on how the pipe enters and exits the isometric box. Figure 1329 shows the two examples of the ways pipes enter and exit the isometric box.
In example A, the pipe enters and exits the isometric box in the vertical plane. Example B shows the pipe to enter and exit the box in the horizontal plane. When a pipe enters and exits in the vertical direction, a 45° angle is formed between the TRAVEL and a dashed line drawn diagonally across the bottom of the box (hypotenuse of triangle 3). When a pipe enters and exits in the horizontal direction, a 45° angle is formed between the TRAVEL and the RUN of the box. Depending on the type, vertical or horizontal, two different 45° right triangles will be formed. The TRAVEL of the pipe becomes the hypotenuse for either triangle. When solving a right triangle whose angle is 45°, the SA and SO will always be equal. As you can see in example A, the length of the dashed line is equal to the RISE of the box, and in example B, the dashed line drawn diagonally across the end of the
box is equal to the RUN of the box. Once the SA and SO lengths of the 45° right triangle are known, Pythagorean's theorem can be used to easily solve the TRAVEL dimension of the pipe. Since 45° is a commonly used angle, Figure 1318 can also be used.
The chart in Figure 1330 converts inches into decimals. Multiplication and division of fractions is simplified using decimal equivalents. This chart is extremely helpful when performing mathematical calculations on a calculator that is limited to decimal input only.
\ 
DECIMALS OF 
A FOOT 
/ 
DECIMALS OF AN  
s 
0" 
1" 
2" 
3" 
4" 
5" 
6" 
r 
8" 
9" 
10" 
11" 
12" 
A® / 
INCH 
.0000 
.0833 
.1667 
.2500 
.3333 
.4176 
.5000 
.5833 
.6667 
.7500 
.8333 
.9167 
  
1/16" 
.0052 
.0085 
.1719 
.2552 
.3385 
.4219 
.5052 
.5885 
.6719 
.7552 
.8385 
.9219 
1/16" 
.0625  
1/8" 
.0104 
.0937 
.1771 
.2604 
.3437 
.4271 
.5104 
.5937 
.6771 
.7604 
.8437 
.9271 
1/ar 
.1250  
3/16" 
.0156 
.0990 
.1823 
.2656 
.3490 
.4323 
.5156 
.5990 
.6823 
.7656 
.8490 
.9323 
3/16" 
.1875  
1/4" 
.0208 
.1042 
.1875 
.2708 
.3542 
.4375 
.5208 
.6042 
.6875 
.7708 
.8542 
.9375 
1/4" 
.2500  
5/16" 
.0260 
.1093 
.1927 
.2760 
.3594 
.4427 
.5260 
.6094 
.6927 
.7760 
.8594 
.9427 
5/16" 
.3125  
3/8" 
.0312 
.1146 
.1979 
.2812 
.3646 
.4479 
.5312 
.6146 
.6979 
.7812 
.8646 
.9479 
3/8" 
.3750  
7/16T 
.0365 
.1198 
.2031 
.2865 
.3698 
.4531 
.5365 
.6198 
.7031 
.7865 
.8698 
.9531 
7/16" 
.4375  
1/2" 
.0417 
.1250 
.2083 
.2917 
.3750 
.4583 
.5417 
.6250 
.7083 
.7917 
.8750 
.9583 
1/2" 
.5000  
9/16" 
.0468 
.1302 
.2135 
.2969 
.3802 
.4635 
.5469 
.6302 
.7135 
.7969 
.8802 
.9635 
9/16" 
.5625  
5/8" 
.0521 
.1354 
.2187 
.3021 
.3854 
.4687 
.5521 
.6354 
.7187 
.8021 
.8854 
.9687 
5/8" 
.6250  
11/16" 
.0573 
.1406 
.2240 
.3073 
.3906 
.4740 
.5573 
.6406 
.7240 
.8073 
.8906 
.9740 
11/16" 
.6875  
3/4" 
.0625 
.1458 
.2292 
.3125 
.3958 
.4792 
.5625 
.6458 
.7292 
.8125 
.8958 
.9792 
3/4" 
.7500  
13/1 (f 
.0677 
.1510 
.2344 
.3177 
.4010 
.4844 
.5677 
.6510 
.7344 
.8177 
.9010 
.9844 
13/16" 
.8125  
7/8r 
.0729 
.1562 
.2396 
.3239 
.4062 
.4896 
.5729 
.6564 
.7396 
.8229 
.9062 
.9896 
7/8" 
.8750  
15/16" 
.0781 
.1615 
.2448 
.3281 
.4115 
.4948 
.5781 
.6615 
.7448 
.8281 
.9115 
.9948 
15/16T 
.9375 
Figure 1330. Inch to decimal conversion chart.
226 Pipe Drafting and Design 1. What is an isometric?
CHAPTER 13 REVIEW QUIZ
2. Which three dimensions found in orthographic views are required when drawing an isometric?
3. What is material takeoff?
4. T F Pipe 14" and above is drawn double line on an isometric.
5. T F Multiple pipes are drawn on a single sheet of isometric grid vellum.
6. T F All isometrics are drawn to scale to show exact size and pound rating.
7. T F Lengths of pipe should be drawn proportionally on an isometric.
8. T F Symbols should be drawn different sizes to reflect a change in pipe size.
9. What is the preferred direction to draw the north arrow on an isometric?
10. What are placed on isometrics to define the pipe's exact routing through a facility?
11. How are isometric offsets formed?
12. To establish proper visual orientation, the indication of_or_angles are included on all isometric offsets.
13. State Pythagorean's theorem. _
14. What are the names of the three sides of a right triangle?
15. Name the six dimensions required on a rolling offset box.
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