using a protractor or compass for accuracy. Always

Therefore, a straight section of pipe 89 9/32 inches long,

draw your lines toward the inside of the bend. Measure

after proper bending, would fit the dimension AC of

a distance along line DE that is equal to the bend radius

figure 16-4.

(point E). At point E, draw a line parallel to line AB.

Repeat the same process for the other leg of your piping

section to get line FG, point G, and a line parallel to line

BC. The point where the two lines you have constructed

When you get ready to lay out boltholes in a flange,

cross is the center of the bending circle and should be

check your plan for the number of boltholes needed and

labeled X.

the diameter of the pitch circle. A pitch circle is the

2. Lay out the bend section by constructing lines

circumference of the circle that will connect the center

that are at right angles from lines AB and BC that cross

points of the boltholes. A pitch chord is the distance

from the center of one bolthole to the center of the next.

at point X. Label these points H and I. Now set your

compass to the bend radius and strike an arc that touches

Here are the steps that you should use to lay out the

points H and I on lines AB and BC. This arc is your bend

boltholes in any given flange:

section.

1. Scribe the pitch circle using the radius given in

3. Use a protractor to determine angle X. Angle

the plan.

X should be 40 degrees. Use the formula (bend section

2. Determine what the pitch chord is to be. This

= 2 × pi × radius × angle + 360) to determine the total

may be done geometrically or mathematically, or it may

length of the bend section. In this case, the bend section

be listed in the plan.

equals 2 × 3.1416 × 24 × 40 ÷ 360 or 16.76 inches.

3. Divide the circle into the desired number of

4. To find the TDL, you still need to know the

equal parts as called for in the plan.

length of the remaining straight sections. You divide

When the circle has been accurately divided, the

angle X (40 degrees) by 2 for an angle of 20 degrees.

points on the circumference of the circle will locate the

Now refer to table 16-1 to find the tangent for an angle

centers of the boltholes. You can locate the center of the

of 20 degrees or 0.3640. Multiply the tangent (0.3640)

boltholes with a pair of dividers by adjusting the points.

by the bend radius (24) and you get 8.74 inches or the

But this trial-and-error method is time consuming and

distance from BH and BI. Next subtract 8.74 inches

not very accurate. Use the proper geometric

from the distance of AB (54 inches) and from BC (36

construction described in the following paragraphs, and

inches) or 45.26 inches for AB and 27.26 for BC,

you can locate the bolthole centers more accurately and

respectfully.

in less time.

5. The final step is to add the length of the legs to

The following problems illustrate the constructions

the length of the bend section (45.26" + 27.26" + 16.76"

used to lay out flange boltholes.

= 89.28") to get the TDL in inches. At this point, you

may convert the decimal measurement into a fraction

PROBLEM I--Lay out a five-hole flange. (See

for easier use in measuring a section of pipe.

fig. 16-5.)

16-6

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