Pipe Bending Basics - The Fabricator

7/30/2019 Pipe Bending Basics - The Fabricator

1/370 The FABRICATOR| www.theabricator.com |SEPTEMBER 2012

Editors Note: Tis is the first article in a series on

press brake operation basics. Te inormation

comes rom the Fundamentals o Press Brake Op-

erations e-Fab (online training program), presented

by Steve Benson o ASMA LLC and available rom

the Fabricators & Manuacturers Association. Ma-

terial also comes rom FMAs Precision Press BrakeCertificate Program, presented by Benson and held

at locations across the country. For more inorma-

tion, visit www.manet.org/training or call 888-

394-4362. Figures are courtesy o ASMA LLC, 2952

Doaks Ferry Road N.W., Salem, OR 97304, 503-399-

7514, www.asmachronicle.com, smartphone www.

asmachronicle.com/mobi.htm.

Bottoming and coining with the press brake

had its heyday. But over the past several de-

cades, air orming has become the industry

standard. So when air orming, how do you select

your bottom die opening? Do you choose a dieopening thats 6 times the material thickness, 8

times, 10 times, or even 12 times?

Te narrower the die opening, the more ton-

nage it will take to bend a part. I youre a new op-

erator, or i youre worried about exceeding ton-

nage limits, you might choose an opening thats

10 to 12 times the material thickness; i youre not

worried about tonnage, you may reach or a die

opening thats only 6 times the thickness.

Almost every press brake comes with a tonnage

chart, and many toolmakers publish inormation

on maximum tonnage or every tool they make.

You use a ormula to calculate tonnage require-

ments or a specific job to ensure you dont push

your machine beyond the tooling load limit as

well as what the press brake manuacturers ram

load limit specifies (see onnage Matters side-

bar). ool placement on the bed, type o bending

operation, and other actors come into play.But in press brake air orming, the die opening

does ar more than afect available tonnage per

oot. So which is the perect die opening6, 8,

10, or 12 times material thickness? A widely held

rule o thumb is that 8 times material thickness

is a perect die opening. Well, this is correct, but

only when theres a one-to-one relationship be-

tween the material thickness and the desired in-

side radius. But i you want to put a 3-in. radius

into 0.036-in.-thick material, that 8-times-materi-

al-thicknes rule o thumb just wont work.

Heres why. In bottoming, the punch profile e-

ectively stamps the material at slightly more thanmaterial thickness. In coining, the punch presses

so hard into the material that it thins the mate-

rial and actually realigns the molecular structure

o the metal, which is why coining is rarely per-

ormed these days, at least on purpose. In air

orming, though, the die opening sets the radius o

the bend.

Tis becomes obvious when you see air orm-

ing in action. o air-orm, the punch descends to

a certain point, but unlike bottoming, the work-

piece doesnt conorm tightly around the punch

radius; rather, the inside radius is produced as a

percentage o the die opening. I you change the

die opening, you change the inside bend radius

you producesometimes significantly. I you

change the inside bend radius, you change the

amount o material elongation within each bend;

the bend deduction (BD) in turn changes your

part, and you will then be unable to orm the part

to the desired dimensions.

Te 20 Percent Rule

Te 20 percent rule shows just how dramatically

small changes in the die opening afect the result-ing inside bend radius. Tis rule states that the in-

side bend radius o an air-ormed part will be 20

percent o the die opening. Unlike most rules o

thumb in the trade, this one isnt based on cold-

roll steel, but instead uses 304 stainless steel as the

baseline. In cold-rolled steel, its 15 to 17 percent,

sot aluminum (50 series) 14 to 16 percent, 304

stainless 20 to 22 percent, and hot-rolled pickled

and oiled 12 to 14 percent.

Here, its still helpul to use 60,000-PSI-tensile-

strength cold-rolled steel as a baseline. I you are

working with another material, you can multi-

ply the tensile by this baseline. So, i your metalis 120,000-PSI tensile, thats twice the tensile

strength o cold-rolled steel. So you multiply 15

percent by 2, and find your inside radius will be

approximately 30 percent o the die opening.

Because alloy characteristics can vary, the

rule covers a range o percentages, so you can

start with the median value and adjust as neces-

sary over time. Say you have three test pieces o

16-gauge cold-rolled steel (all with the same grain

direction) and bend them using a 132-in.-radius

punch using the median percentage value or

three diferent die openings. A 0.473-in. die open-

ing may produce an inside bend radius o 0.70

in. A 0.551-in. opening produces an inside bend

radius o 0.082 in. And a 0.984-in. opening may

produce an inside bend radius o 0.147 in. Tats

a huge radius variation, and the die width changed

by about hal an inch.

Bend Deduction Basics

Each bend elongatesits oten calledgrowth or

stretch, though elongate is the technically accu-

rate term. Because o this, you must deduct cer-

tain amounts o that value rom the flat blank size

so that when the material bends, it elongates to

the dimension required by the print. Te elonga-

tion occurs because the bends neutral axis shitstoward the inside radius.

Each bend consists o a bend angle and inside

bend radius. I two bends on a part have the same

bend angle and inside radius, both will have the

same bend deduction. I another bend on the part

has a diferent angle/inside radius combination, it

will require its own bend deduction calculation.

Although modern bend deduction charts are

relatively accurate, older charts have serious vari-

ances. More than 100 bend deduction charts have

been published over the decades, and not one o

them agrees entirely with another. Say youre put-

Dissecting benddeductions and

die openingsWhy precise

bend deductions anddie openings matter


2/3The FABRICATOR| www.theabricator.com |SEPTEMBER 2012 71

ting a 0.063-in. radius in 0.060-in-thick cold-rolled

steel. One chart may give a bend deduction o

0.106, another 0.136thats a diference o 0.030

in. Tis might not be a big deal i youre working

with loose tolerances and have a limited number

o bends. But in a part with multiple bends, that

0.030-in. variance will stack up, eventually making

it impossible to orm a part to print.

rue, customers may not care about the inside

radius. Te part print may speciy a specific radius,

but as long as the part matches up and works as

intended, theyre probably not about to get outtheir radius gauges to measure each bend. But i

you determine the precise bend deduction need-

ed or a particular radius, you efectively make

manuacturing easier and reduce the number o

inherent mistakes. Hence, your throughput goes

up and you are much more productive.

o start, you need to know what happens to

sheet metal when it bends. During each bend, the

materials neutral axiswhere the material nei-

ther compresses nor expandsshits inward to-

ward the inside radius. Te bend allowance is the

length o the bend as measured on the materials

neutral axis.

Here is where geometry and trigonometry come

into play. Te ormula may look intimidating at

first, but its actually pretty straightorward: Bend

allowance = [(0.017453 Inside bend radius) +

(0.0078 Material thickness)] Bend angle. You

can insert the value o the inside bend radius as

determined by the 20 percent rule. For the bend

angle, you use the complementary angle up to 90

degrees. I a bend is more than 90 degrees, use the

included angle (see Figure 1).

So whats behind these numbers, exactly? Well,

0.017453 is pi (, or 3.14) divided by 180, a basic

geometry unction that converts numbers to de-

grees around a circle; 0.0078 is pi divided by a neu-

tral-axis actor in Machinerys Handbook. Tis or-

mula uses a K-actor value o 0.044 in.; during the

bend, this determines the location where the neu-

tral axis will move during the bend. For instance,

in 0.060-in.-thick material, you multiply that thick-

ness by 0.044 in. (0.060 x 0.044) and get 0.026. Tis

means the neutral axis will move to 0.026 in. rom

the inside surace o the bend.

Yes, that 0.044 number may change slightly with

the material0.048 in. or certain stainless grades,0.042 in. or certain aluminum gradesbut or

most precision sheet metal work, using 0.044 in.

or the neutral axis will suce.

Next, you need to know the bends outside set-

back, which is the distance rom the apex, where

the two planes o the bend intersect on the inside

suraces, to the tangent point o the bend, where

flat metal transitions to curved metal. Heres the

ormula, easily workable with any scientific calcu-

lator: Outside setback = [tangent (complementary

bend angle / 2)] (Material thickness x Inside bend

radius).

At this point you know the outside setback and

the bend allowance. Youre now ready to calculate

bend deduction: again, the amount o material

deducted rom the flat blank to account or ma-

terial elongation during each bend (see Figures 2

and 3).

Heres the ormula: Bend deduction = (2 Out-

side setback) - Bend allowance. With the bend de-

duction in hand, you can determine the appropri-

ate flat blank size, and program the press brake

so that the backgauge fingers accommodate or

material elongation during each bend.

Note that, as shown in Figure 4, you cant add

ComplementaryAngle Included

Angle

OSSB

BD = 2 x OSSB B A

BA

Dimension to Apexx

Leg

Leg OSSB

OSSB

BendAllowance

y

x + y [(OSSB + OSSB) BA] =Flat blank length

Leg

Leg BendAllowance

Leg + Leg + BA = Flat Blank Length

FIGURE 1 In the bend allowance calculation, you usethe complementary angle up to 90 degrees. I a bend ismore than 90 degrees, use the included angle.

FIGURE 2 Every bend has two outside setbacks (OSSB).So to calculate the bend deduction, multiply the OSSB by2, and then subtract the bend allowance (BA), which isthe length o bend along the neutral axis.

FIGURE 3 Tis shows the basics o flat blank develop-ment. Dimension x and y go to the apex. o account orbend elongation, add x and y, and then subtract thebend deduction. Te bend deduction is double the out-side setback (OSSB + OSSB) minus the bend allowance.

FIGURE 4 Tis flat blank calculation doesnt work be-cause it does not consider the bend deduction.

Tonnage MattersWhen you choose a die opening, you still change the available tonnage, o course. o ensure an ap-

plication isnt pushing the tonnage limits o your machine or tooling, you need to know how much

tonnage a job really takes.

For this, its back to the math. Note how in the ollowing equation the material thickness is squared.

A little more material thickness can go a long way in increasing the required tonnage.

[(575 Material thickness squared)/ V die width]/12 Material actors Bending method actors =

onnage per inch. In this calculation, air orming mild steel is the baseline. Other bending methods

and materials are above or below this baseline.

Once you know the tons per inch, you can calculate the tonnage needed or the job at hand: Te

length o the bend ons per inch = otal tonnage.

All this is rooted in a 90-degree bend in a standard die. Maximum tonnage isnt obtained imme-

diately. In most cases, about 80 percent o total tonnage is achieved within the first 20 degrees o

bend angle. In other words, even with a slight angle, a bend can put immense pressure on tooling

and equipment.

As always, tooling and press brake manuacturers have the final word when it comes to the ton-

nage capabilities o their equipment. You should nevereverexceed the maximum allowable ton-

nage or a tool set or press brake.

Material factors

t Mild steel, 1.0

t Copper, 0.5

t H series aluminum, 0.5

t 6 aluminum, 1.28t 304 stainless, 1.4

Method factors

t Air orming, 1.0

t Urethane toolingoperations, 4.0

t Bottom bending, 5.0+

t Coining, 10+


3/372 The FABRICATOR| www.theabricator.com |SEPTEMBER 2012

one bend leg, the bend allowance, and another leg to come up with a valid

flat blank (Leg + Leg + Bend allowance). Tats because this does not take

material elongation into account. Tis is why calculating the bend deduction

is so important.

Te Bedrock of BendingGeometry and trigonometry calculations like these represent the bedrock o

press brake operation. O course, modern controls and sotware can auto-

mate a lot o these calculations. But in your abrication career, such modern

equipment may not always be available. Moreover, just because a machine is

old doesnt mean it cant be productive.

And this is only the beginning. As a press brake operator, you have so many

variables to worry about. What type o bend is itsharp, radius, or proound?

Whats the grain direction? Te list goes on. (Editors note: Tese topics and

more will be covered in uture installments o this series.)

Most important, i you know the math, you know exactly what goes into

each bend. Charts and rules o thumb are useul, but in precision metal abri-

cation, you should, well, beprecise. Te more knowledge you have, the better

and more productive you can be.

The more knowledge you have, the

better and more productive you can be.

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Pipe Bending Basics - The Fabricator