View Full Version : how to make offset

10-07-2007, 02:06 PM
does anybody have a problem with cutting a mitter and a duct using the owl formula? it seems everytime I use the formula the length is shorter ofcourse. how do i fiugre my loss on that length and it also seems that my offset is a little short of what i needed or longer than i need. is there a more efficient way for figuring out an offset for a straight duct aka field cut?

10-07-2007, 04:37 PM
If your measurement for the finished length is not taken in the same plane as the offset (ie: hold the tape measure on an angle, not level or plumb), you will end up short.

Your measurement needs to be the true length of the piece that you need- just like using triangulation in shop layout. Then add allowances for duct connections.

10-08-2007, 07:08 PM
ironman.............this is one way to do it...

10-08-2007, 09:12 PM
thanks danski0224 & bordontn2

10-14-2007, 06:14 PM
I've never been formally trained... what is the "owl" formula?

10-14-2007, 06:48 PM
Offset X Width divided by length (I really never heard this expression either) but we learned the same method as I explained here in the library in a similar fasion as bordon did above

Here is another resource link to what I know as Offset x Height divided by length.


10-14-2007, 07:44 PM
LeadHead............chances are if you had been formally trained you would have never heard of the O.W.L. method.
That was probably developed by someone with no book larnin'..Like the short cut for laying out
square to rounds using the rolation method..No radial lines or trianglulation there.......

10-16-2007, 06:11 PM
Sorry...I didn't read the lower script in your drawing bordontn2. I get it now...O*W/L. Duh!.

10-21-2007, 05:24 PM
I know this is not the proper way to fab a field offset ,BUT also another way to field offset a piece of duct is to : Find a very straight wall, put the piece of ductwork against wall the side that needs the offset ,pull away from the wall at the top the offset you need , be 4"-6" etc. keeping the bottom against the wall .And then measure the gap from the floor to the bottom of duct .This is the amount that needs to be cut off at a angle back to nothing on the other end.Take your bending tongs and straighten out the ends.This will work with a offset up to 8-10" within 4ft.I KNOW THIS IS NOT THE CORRECT WAY but it will get you out of a bind on the job when something is in the way .This can be adapted to work with ductmate or 'S' and drives