Straight Offset vs. Mitered Offset

[Shon]

Hopefully this is the right forum.

Which is better? I've heard both sides....

Mitered is better because the actual dimension doesn't change.

Mitered is not better because it increases the actual offset in a shorter distance.

Any thoughts?

Shon

[bordontn]

I believe this is what Shon is refering to..see attached drawing..
8" offset on a 12" duct within 16" Drawn both ways. I think I would prefer
the mitered offset..Not an expert speaking,tho.......
"smart too late; old too soon"
bordontn

[Doublecut]

I prefer the mitered offset as well. If i remember correctly anything offseting more than 15 degree should be mitered or if one wants to do one better go with a radius offset.

[Shon]

bordontn nailed it with that drawing. Is there anyway to calculate the CFM that can be moved through each offset, according to the degree of the angle?

Just food for thought.

Shon

[vezfriend]

hey buds, i'd like to know more or learn a lot regarding mathematical solution involving air duct fabrication. In my country there are no graphic calculator so far issued or on sales for consumption, I used my old scientific calculator that took me almost an hour to do a layout of one fitting( regardless of size & metal thickness). Would you or is there someone who could help me or teach do a little mathematical solutions on solving my problems regarding sheet metal layout. :idea:

[LeadHead]

I don't have one of these but, try this. http://www.sheetmetallayoutcalculator.com/

[MattM]

In the bottom drawing the cross-section area drops a considerable amount off its original size. (See his 10-3/8ths measurement?) The actual restriction is also a factor of length.

[bordontn2]

MattM...That top drawing...You could probably add 35-45 ft.in duct length. 12x12 @ .08 would be 800cfm/850fpm
The lower drawing @ .08 would be 800cfm/950fpm at that point..still have to add 35-45ft.for the offset restriction....
Sumpin like that. A longer radius offset would only add about 10 ft. to your total length.........a radius 45° is worth 5 ft.
bordontn2

[thehammer]

About the 2 offsets in the picture....WHY??? To make an O-Gee offset you would need to be 20" long. Can't you find 2 more inches on each side? Thats what I would do instead of these "fudge it" fittings, The bottom "Choked" fitting is for those who are idiots, lazy or folks who are pretending to know sheet metal.(who ever told you it was better than the mitered one)
Sorry you need to figure on a lot more than 45'of restriction, it has "lost" 1/6 of it's area being "Choked".
Calculate your offset "loss"on top of that . Both are ugly,
In short- both those offsets are crap. Com'on people lets show a little pride in our work, make a proper O-Gee offset , forget the butcher shop procedures.

:roll:

[sheetmental]

if one has the room to make it longer, that's great. if one has the option of making it radius, that's great too. but the fact is, as a shop guy, we're rarely given any options. usually you've got some guy trackin mud into the office at the end of the day, carrying a piece of cardboard with a list of fittings someone scribbled for him to drop off, and he needs them yesterday. you can call the foreman, but he's got 15 reasons why he can't cut the straight back a little or he doesn't have room for this or that alternate method. sometimes, it's actually true, lol. a couple days later, the foreman calls and wants to know why they don't fit. you try to find a polite way to tell him you made what he asked for, because if you tell him his guy can't f/m worth a darn it causes problems. so then he measures it himself, asks for the same thing only 2"shorter this time, you make it again and the process repeats itself over and over again. that's how it goes. you make what they tell you to make, even if it's retarded.

[cactassdupree]

I've gotten some of those end of the day we needed it yesterday. Well pal take it one day at a time. ENJOY! your work. tell them you need a working sketch, or you'll do the best U can with the info you have. :) dupree

if u never slow down / u never grow old

[tinhead]

thehammer is correct,there is no replacement for an ogee offset,however in practice,you dont get them very often,usually only on exposed work. the last contractor i worked for encouraged field fab and taught the "owl method" to all installers.(offset x width / length = cutback) this produces a decent looking set, and is only limited by how much it stretches the dimension across the end of the duct. if that will not fit,time to miter,or order your fitting.

[cactassdupree]

thehammer is correct,there is no replacement for an ogee offset,however in practice,you dont get them very often,usually only on exposed work. the last contractor i worked for encouraged field fab and taught the "owl method" to all installers.(offset x width / length = cutback) this produces a decent looking set, and is only limited by how much it stretches the dimension across the end of the duct. if that will not fit,time to miter,or order your fitting.


I think if you take half off each piece they will fit up perectly. That's the trick to the OWL.

:) dupree

[tinhead]

the way it was shown to me,you open and flatten your drives(for a broadway set) on one piece of duct,then remove your cutback dimension from opposite sides on opposite ends of same tube. gives a parallelogram shaped(top view) duct. more cutback gives a longer "s" than original, if it grows too much to connect,combine two or more tubes to increase length,reducing cutback,if space permits. hope that clears things up a bit. . . is much easier to show it, as compared to describing the process. admittedly, my geek-fu is weak,not sire how to attach a drawing,much less generate one.

[dman543]

Some Shops that try to keep there overhead down are limited in equipment, but still take pride in making the best produce that they can, my shop is that way . But I still do the best I can with what I have . About the 2 offsets in the picture....WHY??? To make an O-Gee offset you would need to be 20" long. Can't you find 2 more inches on each side? Thats what I would do instead of these "fudge it" fittings, The bottom "Choked" fitting is for those who are idiots, lazy or folks who are pretending to know sheet metal.(who ever told you it was better than the mitered one)
Sorry you need to figure on a lot more than 45'of restriction, it has "lost" 1/6 of it's area being "Choked".
Calculate your offset "loss"on top of that . Both are ugly,
In short- both those offsets are crap. Com'on people lets show a little pride in our work, make a proper O-Gee offset , forget the butcher shop procedures.

:roll:

[tinman97030]

I think if you take half off each piece they will fit up perectly. That's the trick to the OWL.

:) dupree
move the cut a few inches in from each end and make the cuts. If it is really severe, add an inch to the bottom of each cut and s-lock the joint. When I do this I only cut three sides and swing the ends in either s-lock or a nice looking patch.