? calculation bending EMT
- Thread starter Denis
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
if I am understanding correctly, there isn't really a formula. But you could measure the distance from the star point to the arrow, add 24, add (24-5) and that should be correct - I think. I've never done it that way. I would just make a 90 24 to the back of the bend and measure and cut the other end to length. ..
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- Chapel Hill, NC
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Are you looking for a^2 + B^2 = C^2-- the pythagorean theorem
K8MHZ
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- Electrician
Good one.
OP needs to scroll down a bit to get to the 'bender gain' chart. I think gain is what he is trying to figure out.
K8MHZ
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- Electrician
The PDF only has a gain chart for rigid. For EMT, you can figure the gain by making a test bend. But, using the rigid spec (should be close using the same bender) the gain for 1/2" is 2 5/8 inches. That means if you start with 4 ft. of pipe and bend a 90, the lengths of the legs will be 2 5/8" longer, so to make the piece come out correctly, you would cut the gain amount off the pipe before you bend it.
Knowing how to do this is pretty much required for threaded pipe. With EMT you can cut after bending.
mgookin
Senior Member
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- Fort Myers, FL
If the radius is consistent, then the length of pipe required for the curve is 1/4 of the circle circumference if you go 90 degrees.
(2 pi r) / 4
(2 pi r) / 4
K8MHZ
Senior Member
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- Electrician
Ah, but which circle? The inside of the bend, the center or the outside of the bend?
Also, the math doesn't take into consideration the stretching of the pipe from doing the bending. Gain is on a bender is measured by comparing actual run distance to resultant pipe length and using the difference for doing gain calculations.
Gain will not only vary from bender to bender, but from one batch or type of pipe to another. The charts will get you close, but to be sure you really should bend up a 90 and measure it.
Regarding highlighted, the length of each leg would not be 2-5/8" longer. The total of both legs would be 2-5/8" longer than the original straight length, i.e. 4' 2-5/8".
If you were using a "center-mark" bender, such as some one-shot benders, each leg would end up 1/2 the gain longer.
For a hand bender, you can usually calculate using the deduct value as the radius to get close, i.e. the outside of the bend... but minus 1/2 the diameter would likely be closer.
Bends are a combination of elongation, compression, and deformation through the bend (i.e. cross section is neither circular nor elliptical after bending). That's also why power benders typically use the centerline to do the measuring.
I agree.
K8MHZ
Senior Member
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- Electrician
Correct. I meant the total. I thought about that after I posted and I should have been more clear.
K8MHZ
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- Electrician
Hmm...I never thought about that. So my 1/2" bender with a 5" deduct is close to a 5" radius? That would mean the bends would be about radius of a 10 inch plate. Interesting.
Yeah, but on most jobs, you'll likely be doing enough bends that it's easier and more accurate to just measure before and after to determine actual gain... rather than taking the time to do the calculation and still be off a little.
electrofelon
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- Cherry Valley NY, Seattle, WA
I've done a ton of emt and never run into the situation in the op. Interesting. Good comment on the rmc, never considered the difficulty of putting a piece of pipe with a 5 foot bend in the threader. Definitely agree that there are differences between benders. I've got one that when bending with the star point I have to add 1/4 inch. Then my 1 1/4 90s always come out 1/4 inch longer, but they may be because I'm light so there is a little mush as the bend starts. I would like to do rigid more often but it hardly ever comes up. Helped out for a few days at a food processing plant and it was all stainless rigid and stainless fs boxes. Can't imagine how much that cost. Definitely wanted to keep your screw ups to a minimum
Can't say never myself...
I will measure remnants to see if one's long enough to get what I need out of it, to minimize scrap.
K8MHZ
Senior Member
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- Electrician
I've done a ton of emt and never run into the situation in the op. Interesting. Good comment on the rmc, never considered the difficulty of putting a piece of pipe with a 5 foot bend in the threader. Definitely agree that there are differences between benders. I've got one that when bending with the star point I have to add 1/4 inch. Then my 1 1/4 90s always come out 1/4 inch longer, but they may be because I'm light so there is a little mush as the bend starts. I would like to do rigid more often but it hardly ever comes up. Helped out for a few days at a food processing plant and it was all stainless rigid and stainless fs boxes. Can't imagine how much that cost. Definitely wanted to keep your screw ups to a minimum
Having to bend 1 1/4 pipe with a hand bender should be considered cruel and unusual punishment.
If you add a 1/4 inch, why would you not expect the piece to be 1/4 inch longer??
I sure hope they weren't making you use hand benders on SS pipe.
K8MHZ
Senior Member
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- Electrician
Consider having to make a piece with multiple bends, like a 4 bend saddle with a 90 or two. Doing that with threaded pipe takes a bit of skill, bit is routine for some hands.
Sure glad to have my No-Dog when I come up against work like that!!
Even gladder to have a good pipe vise and a porta-pony!!!
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