A manual sheet metal layout exercise

Creating a perimeter boot is done the same way you would lay out a square-to-round or rectangular-to-round fitting - by using triangulation. As you look at the fitting shown in Figure 1, block out the rectangular offset, as shown in Figure 2. You can see the rectangular-to-round with the rectangular area on a slant. The straight, 4-inch offset is nothing more than a mitered extension to the rectangular opening in the direction desired.

Figure 1 (left) and Figure 1a.
To make the necessary drawings for this exercise, you will need to use "bend" lines (sometimes called "generator" lines). Two elevation views (figures 1 and 1a) and a top view (Figure 3) are shown. The Figure 1a elevation view is the view you need to obtain the true lengths of all your bend lines. Divide the circumference into equal spaces. Twenty-four are used in this problem. You could create as few as 12 spaces. The more bend lines, the more defined or accurate your round will be.

Figure 2

True-length triangles

Figure 3 shows two individual triangles, to be used as the fitting's top and bottom. As shown from the top view (Figure 1), the distance from one-half of the rectangular-to-round is 3 inches. The other half is the 3 inches plus the 2 1/4 inches, for a triangle height of 5 1/4 inches, represented by the bottom of the boot.

Look at the Figure 1a elevation drawing. Use dividers and transfer the distance of points 5 to 6, 5 to 7, 5 to 8, 5 to 9, etc., until all, including 5 to 12, are transferred to the base line of the triangle, as shown. Repeat this with the lower section on the second true-length bar with a height of 5 1/4 inches.

Transfer the distances from the Figure 1a elevation drawing, points 19 to 12, 19 to 13, 19 to 14, etc., until all, including 19 to 18, have been referenced on the base line of the true-length triangle.

Pattern development (Figure 4)

The pattern will be developed in two pieces - the main part of the fitting and a separate piece for the collar. The true-length triangles are only for the top and bottom of one side, so the development would only cover from the left of Line 2-18, which is the center of the boot shown in Figure 1a. Because the round is centered with the rectangle, the right side of Line 2-18 is a mirror image of the left side; we use the same true lengths for the top and bottom on the left and right of the fitting.

All necessary lines for the development of the pattern in Figure 4 may be obtained from the true-length triangles and top view shown in Figure 3.

Draw a line, shown as 19-19 in Figure 4, the width of the rectangular section of the boot. Establish Point 18 in the pattern by setting the dividers to the distance of the true length of Point 19 to Point 18 from the true-length triangle. Once this distance is set, strike an arc near Point 18 as shown, using Point 19 as center.

To keep from repeating this step, do the right side of the fitting as well. Use Point 19 as center and make an arc at Point 18; these will intersect on the center Line 2-18. Using a second set of dividers set at the distance that was used to divide the round into equal spaces, approximately thirteen-sixteenths, for a 6-inch round divided by 24 parts. These dividers will only be used to reference points from 18 to 17, 17 to 16, 16 15, etc., until we have them all, including points 7 to 6.

Figure 3

Establishing points

When you draw an arc from Point 18 to Point 17, using Point 18 as a center, you then have to establish where Point 17 will actually be on the arc. To do this, take the first set of dividers and set them to the true length from Point 19A to Point 17 on the true-length triangle.

Now using Point 19 as center in the pattern, make an arc that intersects the one drawn from Point 18. Remember to do both sides of the fitting to save you from repeating this step.

Now using Point 17 as center, with the second set of dividers, draw an arc near Point 16. This point will be established as you again take the true length from the true-length triangle by setting the dividers from Point 19A to Point 16 on the base line. Then using Point 19 as center in the pattern, create an arc that intersects with the previous arc to establish Point 16.

Repeat this until you have established all points, 18 to 12, using Point 19 as center. This completes the bottom half of the fitting.

To establish the top half, using Point 5 as center, we will continue with the same steps. You will need to locate where Point 5 will be. (Remember the triangulation rule: use two known points to find a third.)

You have two known points, 12 and 19. Use Point 12 as center to make an arc near Point 5. To get the actual length for the arc, look to the true-length triangle and set the dividers from Line 5A to Point 12 on the base line. This only shows that Point 5 will be somewhere in the arc. You need to establish the true length of Line 19-5 and make an arc using Point 19 as center that intersects with the arc drawn from Point 12. This will establish Point 5.

Now this part of the pattern can be completed. Connect points 5 to 11, 5 to 10, 5 to 9, etc., until Line 5-6 has been established.

As shown in the top view (Figure 3), there are two triangular shapes that make up the top; one side is marked as A5-6. Line 6A is 3 inches, as shown. Set the dividers to 3 inches and using Point 6 as center, make an arc near A, as shown in pattern view. Line 5A from the top view is 5 inches. Now reset the dividers to 5 inches, and using Point 5 in the pattern view as center, strike an arc to intersect with the previous arc struck from Point 6. This intersection becomes Point A.

Line 3-4 from the pattern view is drawn parallel to Line 5A, one inch away. Yet to be established are the sides, shown by lines 1-19, 19-5 and 5-4.

Figure 4

Making the sides

This section cannot just be drawn if you expect it to fold together properly. Look at the relationship between Lines 4-5 and Line 5-6. Note that Line 4-5 is perpendicular to Line 5-6 in the elevation view (figures 1 and 1a). In the pattern, you drew a 1-inch line from Point 5 to Point 4, perpendicular to Line 5-6.

Again, you will use two known points, 4 and 19, to locate a third, Point 1. Set the dividers the distance of Point 4 to Point 1 in the elevation view (2 1/4 inches) and make an arc near Point 1 using Point 4 as center. With the dividers set to the height of the back, 4 inches, and using Point 19 as center, make an arc that intersects the previous arc establishing Point 1.

Parallel to Line 1-19, add about half an inch for the seam in the corners. The last step is to draw in the backside of the fitting from Point 19 to Point 1, as shown in Figure 4 and labeled "Back."

The start collar (not shown) is a narrow piece of like metal, approximately 2 inches wide. The length of the start collar would be the diameter of the opening multiplied by 3.141. Add the necessary half-inch allowance for the lap to be riveted. The start collar can be attached by spot welding, pop rivets or button locks, although the most common way is to use a turning machine.

Any of these fittings can be fabricated in a number of ways: the seam at Line 19-1 could have been developed at Line 19-15. Be creative. You'll be surprised how easy many of these become if you break them up into smaller projects and connect them like a puzzle. Do remember to add the necessary allowance for the seams.

A printable worksheet for this and similar projects is available at www.thesheetmetalshop.com.

(For more information on manual sheet metal layout, write to The Sheet Metal Shop Resource Center, 516 Chicago Ave., Waukesha, WI 53188.)