The "shoe boot" (Figure 1) is typically found in ventilation work, primarily in the exhaust (suction) side, removing fumes or debris from worksites.
Practical Design of Exhaust and Blower Systems, written by E.W. Favalora in 1935, explains some of the history behind this fitting. In the chapter "Loss in branches or tees," it says: "Branches or tees entering a main at right angles to the flow of the main should never be installed." It says that this type of fitting should only be used in tight areas where there is insufficient space and installing a 45-degree piece of duct cannot be done.
It explains that right-angle tees do not have any satisfactory suction because the angle they connect to at the main, and those used interfere with the main airstream. Branches should, whenever possible, intersect at a 45-degree angle to the direction of the airflow in the main. By creating a larger opening in the duct, as shown by developing the oblong shoe boot, you can simulate the 45-degree angle with the transition to the duct.
This article is about how to develop the shoe boot. Practicing these fittings may strengthen your pattern-following skills.
Shoe boots consist of two separate pieces, besides the adjoining duct, for three separate patterns. All of them need attention to the intersection of the cylinder or duct that's shown in Figure 2.
Two of the patterns are of the boot itself; the third pattern would be the duct opening to which the boot would be attached. To develop the patterns, you must establish the points to where the pieces of each fitting are joined to each other. To accomplish this, many lines must be projected from View A to View B.
ViewsConcentrate on Figure 2 for a moment, the elevation and end views of the assembled shoe boot. By drawing lines from the end view (View A) to the front view (View B), you can establish points 1', 4', 5' and 8', where X and Y will be joined to Z. However, you do not have enough information to correctly draw the lines between points 1' to 4' and 5' to 8' in Figure 2, View B. To correctly draw this line, more lines must be projected from the profile of each cylinder (duct) as shown in Figure 3.
When developing the profiles of each duct, A and B, it is up to you how many sections per quarter you would like to work with. This article uses three for clarity; the more sections your profile is divided into, the more defined your line of intersection between points 1' to 4' and points 5' to 8' will be. Profile C is not divided the same as Profile A or Profile B; it is the end view of Z and only there to help define where Duct X intersects Z.
Once the profile has been divided into equal sections, draw lines perpendicular to the ends of the duct to where these lines intersect the joining duct, as shown by the projecting lines from profile A down to Profile C.
Now establishing the new points 2 and 3, extend these same lines from where they intersect on profile C. Draw horizontal lines parallel to Duct Z as shown. These lines, projected from points 2 and 3, will help establish the points between 1' to 4' and between 5' to 8' as we extend the lines established from Profile B.
Point 5To establish Point 5', draw a line from Point 5a parallel to Line 8a-8'. Point 5' is established where this parallel line intersects the horizontal line extended from Point 4'. To establish the points between 5' and 8', drop lines 6 and 7 down from the profile (B) to Line 5a-8a as shown. From where Point 6 and Point 7 meet Line 5a-8a, draw a line from each parallel to Line 8a-8'.
Now all the necessary points have been established to begin developing the patterns.
All three patterns will be laid out using parallel-line development. Begin with section Y in Figure 4. Remember, with parallel-line development, all lines project out perpendicular to its plane.
Start with known points such as Point 4' and extend a line perpendicular to Line 8a-8; repeat this with the other points, 5a through 6a, and 5' through 8'. Draw a line parallel to Line 8a-8' as shown and label this Line 8a-8'. This can be at any distance. Add parallel lines B, C and D, as shown.
To determine the distance between A-B, B-C and C-D, you need to take the distance of each cord or division from Profile B - the length of Point 5 to Point 6, Point 6 to Point 7 to Point 8. If you follow these points down from Profile B to the top of Y (5a to 8a), you will see they are the same length; however, you can't see them in true length.
Perpendicular linesSet your dividers or trammels to the true length from 8 to 7 in Profile B. Using 8a as center, draw an arc intersecting the perpendicular line drawn from Point 7a, as shown. This intersection becomes 7a in your pattern. Repeat these steps from 7a to 6a, and from 6a to 5a. You could repeat these steps again for the lower 5' to 8'; however if you were to draw lines parallel to points A, B, C and D, beginning from 7a, 6a and 5a, where they intersect the lines extended from 5', 6', 7', and 8', these would be your points.
Set your trammelsOnce you have established points 5a and 5', you can set your trammels to the length of 5a to 4' from Y, and using 5a as center on your pattern, strike an arc near Point 4'. Reset the trammels to the length of 5' to 4' from Y, and using 5' as a center on your pattern, make an arc that intersects with the previous arc drawn that establishes Point 4' on your pattern. This completes half the pattern. Now using Line 8a-8' as the center, trace this pattern as a mirror copy to finish the complete layout. If you are going to be flaring this pattern out along points 4' to 8' and 5a to 8a, you will need to add the necessary allowance for this step.
Figures 5 and 6 show how to develop the pattern for the intersecting duct and the pattern for where the opening will need to be in the main duct.
SpacingAgain, you should make lines perpendicular to the piece you're developing a pattern for (shown as shaded area in Figure 5). Draw 12 lines parallel to View B, equally spaced to the distance that was used to divide the Profile B and project all the lines from points 1 to 4 and 5a perpendicular to the fitting in View B. Take note that points 1, 2, 3, 4, 5a and 8a in Figure 5, View B, are all shown in true length.
Transfer these heights to View A using your trammels or dividers. You may want to add the necessary allowance to Line 5a-4. This will give you material to allow section Y to be pop-riveted.
Make the openingNow you will make the opening where the duct will be joined together (shown in Figure 6). Section Z on the drawing is the piece that needs an opening. Begin by drawing a line parallel to Z, as shown by Line A-Aa which is the length of the section Z. Draw two lines perpendicular to A and Aa, and to B and Ba.
The distance from Point A to Point B is the total circumference of section Z. The opening can be placed anywhere in this area; it only depends on where you want the seam to be. Project all points down, as shown in Figure 6, points 1 through 8. Draw a center line (shown in red) at a right angle to Line AB. On each side of the center line, there will be three more lines drawn parallel to the center line. The distance from each line to the next space, Nos. 1, 2, and 3, will be transferred from the End View also shown as Nos. 1, 2 and 3. You will need to add to your pieces any material necessary for seams. If you do not need the entire duct developed, you may eliminate the length of Line AB and just work on the opening that you can use for a pattern.
(For more information on manual sheet metal layout and exercises, visit www.thesheetmetalshop.com).