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TUTORIAL: Learn how to layout simple transitions
July 2001
http://search.sheetmetalworld.com/fuseindex.cfm?fuseaction=showprofile&companyid=2127&searchtype=ven">
This tutorial will show you how to layout a 14/10 to 14/8 transition with the bottom flat and total length will be 12".
Above, The first thing you need to do is cut a piece of steel 12" wide and approx.. 36" long. We need to consider the way we need to connect the fitting to the other duct. We are going to use S-Slip & drive connections on this one.These connectors require 1" so we need to scribe a line along the top and bottom at 1" This will give us the required allowance.
We will make this transition in 2 piece, The one piece will contain the bottom and 2 sides. Now we know the duct work is 14" wide so we will start by Making 2 lines (A) 14" apart from each other ( Keeping them somewhat centered on the steel ) you'll find out that if you don't you will not have enough steel to layout the sides
Now that you have the bottom layout done, you need to determine the sides and you know that 1 side is 10" x 14"( 14" being the bottom and 10" being the sides)so make a mark 10" from line (A) to line (C). Do this on both sides as I did above.
Now that you have the 14" x 10" done you need to do the same thing for the 8" side (On the top of the plan) Mark your line 8" from line (A) to line (B) on both sides.
Now you need to connect points (C) & (B) and you may start to see the fold out view of the bottom and the 2 sides...
Now that we have the 2 sides and bottom we need to consider how we will connect the top. There are several different methods. The 2 most common are the Pittsburgh and the snap lock. Most Pittsburgh machines require 1" allowance and the lock former requires 1 5/16". We will say we are going to use the Pittsburgh. So we need to add a line ( D,E) parallel to lines(B,C) out by 1"
Once this is done you can cut along lines (D,C) and notch in about 1" as shown above. 
Here ( above ) you should have something that looks like this once you cut out the sides that you don't need. 
The next thing you need to do is cross brake all three sides (This helps reduce noise in the ductwork) You should cross brake any duct larger than 8".
To make the top of the fitting you must find the true length, being line (B,C) plus the 2" extra for the s-slips. This piece will be 14"wide plus the allowance for the 1/4" or size you need depending on what type of connection your using. (c) 2001 The Sheetmetal Shop. Reproduced with kind permission.
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