Figuring out the straight length of a flatbar after its been rolled into a circle

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Lets say there is a 1"X1/4"  flat bar that has been rolled and has a diameter of 20". So visually, this flat bar is in the shape of a circle now. I am instructed to build one more of these starting from a straight piece of 1"X1/4" flat bar. Lets say i have a magical break press that will only bend the straight flat bar once it senses the required flat length of the existing rolled flat bar.Is this correct? I take the diameter of the existing rolled flatbar (20") and X that by 3.14 (pie). That math gives me a circumference of 62.8". Does this mean that in order to replicate that circle i would need to cut my flatbar to 62.8"? would that at least be very close? Does this math make sense?
Reply:It's pretty much a trial and error process. I would guess you would need to cut at least a quarter inch off the length to get it to come out. The other factor is the flat spot that occurs. Rolling it right then tacking the ends and rolling it more will get some of the flat spot out , but some will remain.
Reply:The diameter to measure is the center of the steel to the center of the steel.The reason for this is in theory the center is the neutral axis, the outer portion stretches and the inner shrinks. So the true bar measurement is shorter than you have currently figured, as MJD noted.Basically, (19.75)(3.14) = 62.015"Note: it's diameter minus 1 thickness.I usually cut long, bend, trim to fit since I rarely have more than one or two to make.Dave J.Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~ Syncro 350Invertec v250-sThermal Arc 161 and 300MM210DialarcTried being normal once, didn't take....I think it was a Tuesday.
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Originally Posted by SlowCruiser

Lets say there is a 1"X1/4"  flat bar that has been rolled and has a diameter of 20". So visually, this flat bar is in the shape of a circle now. I am instructed to build one more of these starting from a straight piece of 1"X1/4" flat bar. Lets say i have a magical break press that will only bend the straight flat bar once it senses the required flat length of the existing rolled flat bar.Is this correct? I take the diameter of the existing rolled flatbar (20") and X that by 3.14 (pie). That math gives me a circumference of 62.8". Does this mean that in order to replicate that circle i would need to cut my flatbar to 62.8"? would that at least be very close? Does this math make sense?
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Originally Posted by MinnesotaDave

The diameter to measure is the center of the steel to the center of the steel.The reason for this is in theory the center is the neutral axis, the outer portion stretches and the inner shrinks. So the true bar measurement is shorter than you have currently figured, as MJD noted.Basically, (19.75)(3.14) = 62.015"Note: it's diameter minus 1 thickness.I usually cut long, bend, trim to fit since I rarely have more than one or two to make.
Reply:

Originally Posted by SlowCruiser

Lets say there is a 1"X1/4"  flat bar that has been rolled and has a diameter of 20". So visually, this flat bar is in the shape of a circle now. I am instructed to build one more of these starting from a straight piece of 1"X1/4" flat bar. Lets say i have a magical break press that will only bend the straight flat bar once it senses the required flat length of the existing rolled flat bar.Is this correct? I take the diameter of the existing rolled flatbar (20") and X that by 3.14 (pie). That math gives me a circumference of 62.8". Does this mean that in order to replicate that circle i would need to cut my flatbar to 62.8"? would that at least be very close? Does this math make sense?
Reply:Ok nice. So by using the center to center diameter you suggested instead, does ever other step i listed still follow correctly?
Reply:Thanks everybody who commented. I'm 20 and trying to get a grip on the basics so if anything this will get me thinking more clearly
Reply:Def good info for me as well. Never had to do this myself.   I too would like to know how you can further calculate the cut length and avoid possibly over cutting the material to length and doing unnecessary grinding.Dynasty 200DX   (2014)Millermatic 211 (2015)Optrel 864   (2014)Smith Medium duty MBA 30510 (Xmas 2014)Tennsmith 16ga 4ft finger brake (2015)Trailblazer 325 EFI and excel

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Reply:Not scientific but wrap a string on the outside, lay it straight and measure. This will be close.
Reply:Ummmm...Why not just wrap a tape measure around it?America Needs AMERICA'S Oil!!!"Global warming is the greatest scam in history ...There is no run away climate change. The impact of humans on climate is not catastrophic. Our planet is not in peril."--John Coleman, Founder of The Weather Channel
Reply:old time blacksmith's who weren't math wiz's used this type of traveler to determine iron wagon tire lengths. the metal tires were flat stock rolled into a circle,forge welded and shrunk onto the wooden wheel. this is more or less your application.

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Reply:

Originally Posted by Weld one

Not scientific but wrap a string on the outside, lay it straight and measure. This will be close.
Reply:These are mostly horrible answers.Yeah, we can all cut long, roll and then trim to fit. But he was asking for the mathematical calculation to use.I am terrible at math. I have to REALLY sit down and do the calculations. Even then my wife tells me I am wrong. Granted, theory is different than practice. I think we should be versed in both.Lincoln Power Mig 210MP MIGLincoln Power Mig 350MP - MIG and Push-PullLincoln TIG 300-300Lincoln Hobby-Weld 110v  Thanks JLAMESCK TIG TORCH, gas diffuser, pyrex cupThermal Dynamics Cutmaster 101My brain
Reply:Reading closely again I see that people have commented above on the same thing. I'll leave my somewhat redundant post below since it has a nice picture I stole on the web.

-----When you bend something the outside will stretch and the inside will compress.So the length of the original bar is somewhere in the middle, called the neutral axis.So the thickness of the material has to be taken into account.

On a large bend radius in relation to the material thickness the neutral line is in the middle.On tight bends it is closer to the inside of the bend. I found this as a guideline - Basic Diemaking Eugene Ostergaard R4T C=.5T R=Inside Bend Radius, T=Thickness of material, C=Location of neutral axis.Anyway, back to the question - 1"X1/4" flat bar that has been rolled and has a diameter of 20".Since it's a large bend, radius 10" while the thickness of the bar is only 1/4" then neutral line is in the middle.Is the 20" outside or inside diameter? Outside? That means the center of the circle is 20"-1/4" = 19.75". Length of the bar is then 62" (length x 3.14). Inside? That means the center of the circle is 20"+1/4" = 20.25". Length of the bar is then 63.6" (length x 3.14).If the material thickness in relation to the bend would have been greater, then the formula above would have to be adjusted because the neutral axis would not be in the center anymore.For instance say taking a 3/8" rod and bending it into a 1" ring.Last edited by Pete.S.; 02-27-2017 at 10:49 PM.
Reply:

Originally Posted by Pete.S.

Reading closely again I see that people have commented above on the same thing. I'll leave my somewhat redundant post below since it has a nice picture I stole on the web.

-----When you bend something the outside will stretch and the inside will compress.So the length of the original bar is somewhere in the middle, called the neutral axis.So the thickness of the material has to be taken into account.

On a large bend radius in relation to the material thickness the neutral line is in the middle.On tight bends it is closer to the inside of the bend. I found this as a guideline - Basic Diemaking Eugene Ostergaard R4T C=.5T R=Inside Bend Radius, T=Thickness of material, C=Location of neutral axis.Anyway, back to the question - 1"X1/4" flat bar that has been rolled and has a diameter of 20".Since it's a large bend, radius 10" while the thickness of the bar is only 1/4" then neutral line is in the middle.Is the 20" outside or inside diameter? Outside? That means the center of the circle is 20"-1/4" = 19.75". Length of the bar is then 62" (length x 3.14). Inside? That means the center of the circle is 20"+1/4" = 20.25". Length of the bar is then 63.6" (length x 3.14).If the material thickness in relation to the bend would have been greater, then the formula above would have to be adjusted because the neutral axis would not be in the center anymore.For instance say taking a 3/8" rod and bending it into a 1" ring.
Reply:Slow Cruiser,to directly answer your original question... "Figuring out the straight length of a flat bar after it's been rolled into a circle."     >Tape accurately (with a FLAT tape) the OUTSIDE of the rolled ring. SUBTRACT from this amount,the metal thickness X 3.142 (PI). This gives you the original straight line length of the flat bar.      >If you tape the INSIDE circumference, ADD the thickness X PI.
Reply:It's called pitch circle diameter. PCD (OD - t) x pi , ( ID+t) x pi. It's accurate. You must also set the ends before you roll.
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Originally Posted by steve45

Ummmm...Why not just wrap a tape measure around it?
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Originally Posted by Joker11

These are mostly horrible answers.Yeah, we can all cut long, roll and then trim to fit. But he was asking for the mathematical calculation to use.I am terrible at math. I have to REALLY sit down and do the calculations. Even then my wife tells me I am wrong. Granted, theory is different than practice. I think we should be versed in both.
Reply:I carry a body measuring tape for clothing. It curls up real small and is 60" long do I can get pretty far. Also doesn't stretch Sent from my 2PS64 using Tapatalk
Reply:https://www.amazon.com/Lufkin-954FT-.../dp/B000ZUDNPE    6x material thickness for id
Reply:

Originally Posted by SlowCruiser

Lets say there is a 1"X1/4"  flat bar that has been rolled and has a diameter of 20". So visually, this flat bar is in the shape of a circle now. I am instructed to build one more of these starting from a straight piece of 1"X1/4" flat bar. Lets say i have a magical break press that will only bend the straight flat bar once it senses the required flat length of the existing rolled flat bar.Is this correct? I take the diameter of the existing rolled flatbar (20") and X that by 3.14 (pie). That math gives me a circumference of 62.8". Does this mean that in order to replicate that circle i would need to cut my flatbar to 62.8"? would that at least be very close? Does this math make sense?
Reply:


Your reply is WRONG.
Reply:

Originally Posted by hommeacier



Your reply is WRONG.Flats are easily removed by backrolling using the common power or hand crank slip roll.Last edited by hommeacier; 04-22-2017 at 10:14 PM.
Reply:My mistake. you are correct Broccoli. I was referring to DGM's reply just before yours.
Reply:

Originally Posted by steve45

Ummmm...Why not just wrap a tape measure around it?
Reply:Same here. I have a few years in a fab shop and I'm just now starting out on my own. I still have my day job as a jr Project Manager but I plan on starting my own company in a few years. I just turned 20 Friday.
Reply:The same you figure the flat pattern for any round tube or pipe ..Outside diameter then minus 1 material thickness times 3.14 / Below chart does not account for weld shrink !.Then add 1 material thickness for weld shrink /this is only a Typical factor number / weld joint design and welding technique can make a difference also..Any measuring tool going around the outside diameter is not going to be accurate. unless its a "Pi Tape''better off measuring across 6 equal places and then take average dim' provided it's pretty round ..For your example:Measure outside diameter then minus 1 material thickness times 3.14Then  Add' 1 material thickness .So ...... Lets say there is a 1" X 1/4" flat bar that has been rolled and has a diameter of 20".Assuming the 1/4" is the thickness, Right ?.20" (outside dia') minus .250 (1 material thickness) = 19.75 then times Pi (3.14) = 62.046 then plus .250 (1 material thickness) = 62.296

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