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TUTORIAL: Learn offsets fast and easy.
July 2001
http://search.sheetmetalworld.com/fuseindex.cfm?fuseaction=showprofile&companyid=2127&searchtype=ven">
As in all fittings you need to establish a starting point, so draw a horizontal line on the paper, somewhere near the bottom (A,C) now draw a line perpendicular to line A,C. and establish one end of the fittings, line A, B. , The fitting in the illustration is 18" long (But this could be any length) so we need to make a mark 18" on line A,C and draw another line perpendicular to A,C (line C,D) to establish the length of the fitting. Being that lines A,B and C,D are reference lines you will want to lightly extend these up a ways until you establish the other points. Our fitting is going to be 12" in height and 18" long, so mark the reference line for the height of the fitting (3) at 12" up from line A,C and on line C,D
Our offset is 6" ( this could be any height) so measuring up from the bottom reference line A,C you need to make a reference mark at (1) on line A, B the height your offset will be. From point (1) you need to measure up the height you want your fitting ( in our case we measured up 12") and mark it at (2) on line A,B
Now comes the important steps, in in order to find the reference points to place our trammel points on, to draw the arch, we need to divide the height of the fitting in half , find the mid point, (points 4 and 5)
Draw a line from (4, 5),we need to now bisect line (4, 5) Place one end or your dividers on 5 and draw an arch. Do the same with 4 , Where the two arches meet draw a line straight through to establish (1a ) point 1a becomes the midpoint for line (4,5)
Now we need to do the same thing as we did above by finding the midpoint between (1a, 4) and (1a, 5). Once you establish these midpoints you will be able to than draw a new line perpendicular to (line 4,5). Extend these perpendicular lines to (P)and the intersection will be at the lines (A,B and C,D) 
Once you establish (P) this is where you place one end of your dividers or trammel points to draw your arches, NOTICE that we use the same point (P) for the two arches.
This will work for any offset, as long as the height of the fitting stays the same (p1 and p2 have to stay the same on both ends)
This pattern does not include the wrapers.
(c)2001 The Sheetmetal Shop. Reproduced with kind permission.
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