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

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

1. Solderer
Join Date
Oct 2016
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6

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

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?

2. Master Welder
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central Wis.
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Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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.

3. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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.

4. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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?

You tend to need more length than just diameter times 3.14

The reason is the thickness of the metal. The inside up against your die, is going to compress, the outside is going to expand, the actual size is somewhere in between. My thought is if you roll it and it comes out almost an eighth inch in diameter too large, you can just grind a quarter inch off one end and your are good to go. If you come up short it sucks.

If the metal is one eighth thick, then you should add one eighth to the diameter before you multiply it by pi. I use 3.143 as pi. But that difference of pi would only amount to 1/32" difference in the length of the metal to make a 20 inch circle.

Sincerely,

William McCormick

5. Master Welder
Join Date
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central Wis.
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4,328

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

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.
It looks more official when formulas are involved. Makes sense tho, kind of like bend allowance on sheet metal. No matter the formula, a test piece is always best.

6. WeldingWeb Craftsman
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Dec 2013
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Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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?
Cut 2 pieces 63 inches long, bend both and throw the original away. I roll these type of things and always cut long & trim. If you want them more precise make a mandrel to shrink them onto.

7. Solderer
Join Date
Oct 2016
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6

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

Ok nice. So by using the center to center diameter you suggested instead, does ever other step i listed still follow correctly?

8. Solderer
Join Date
Oct 2016
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Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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

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

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.

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

Not scientific but wrap a string on the outside, lay it straight and measure. This will be close.

11. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

Ummmm...

Why not just wrap a tape measure around it?

12. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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.

13. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

Originally Posted by Weld one
Not scientific but wrap a string on the outside, lay it straight and measure. This will be close.
I use drywall tape it doesn't stretch.
John

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14. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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.

15. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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 R<2T C=.33T, R=2T-4T C=.4T, R=>4T 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 11:49 PM.

16. Solderer
Join Date
Oct 2016
Posts
6

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

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 R<2T C=.33T, R=2T-4T C=.4T, R=>4T 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.
Thank you i appreciate this and everybody else who has answered! this is not a real job for me i just made it up so that i could get exactly this type of feedback. I just want to further my understanding of fabrication

17. Solderer
Join Date
Dec 2015
Posts
9

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

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.

Join Date
May 2007
Location
Western Australia
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Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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.

19. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

Originally Posted by steve45
Ummmm...

Why not just wrap a tape measure around it?
Tape measure works well too. There are 1/4'' wide ones for this so it lays flat ; wider ones may not follow the radius . A cloth sewing tape is good too.

20. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

Originally Posted by Joker11

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.
I used to make a lot of round pipe. And I can assure you that if I had to make a lot of the same sized pipe, I would create a test piece, a test strip, using the method I suggested. And it would come out perfect almost every time. And then I would start shearing up whole sheets of metal to make pipe.

But if you are asking these questions you do not know all the little nuances, that creep into it. What many people are saying is that there is a center line. However that center line may not be exactly in the center by actual testing and building.

So if your piece is a quarter of an inch too long, there is no problem at all, you just trim that one piece down to an exact fit. That takes perhaps 30 seconds. Most guys that are doing these projects have one piece of metal, so they get one shot.

Especially if you have a piece or object to fit the band to, and you want a tight fit you may want a close fit before you weld because the weld will shrink it substantially.

If you want it exact or loose, you probably want it a little large because the weld will shrink it. So although these might seem like horrible answers that is how it is done in the real world. Most plasma cutters are off a bit anyway so you end up with stuff that does not fit from a quarter million dollar machine.

Sincerely,

William McCormick

Join Date
May 2011
Posts
378

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

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

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23. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

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?
Sorta. You'll need to account for the distance that the flat stock extends PAST the center roller in the Ring Roller that you'll use.

24. Solderer
Join Date
Dec 2015
Posts
9

25. Re: Figuring out the straight length of a flatbar after its been rolled into a circle

Originally Posted by hommeacier