Fabrication Formulas

Formulas:

Bend Allowance (BA)  - The amount of added to the sum of the two leg lengths to obtain the flat pattern length.

 

Bend Deduction (BD)  - The amount removed from the sum of the two flange lengths to obtain a flat pattern.

 

Material Thickness (MT) - The gauge of the material in decimal form. Bend Angle (B<)  - The inside angle between the two legs or flanges of a bend. K-Factor (K)  - The ratio of the position of the Neutral Axis to the Material

Thickness.

 

Y-Factor (Y)  - A constant based off of the K-Factor used by Pro-Engineer in place of a K-Factor.

Inside Radius (IR) - The final formed inside radius after spring back has occurred.

Outside Radius (OR) - The final formed outside radius after spring back has occurred.

Outside Setback (OSSB)  - Distance between the outside tangent points and the apex of the outside mold lines.

 

Inside Setback (ISSB)  - Distance between the inside tangent points and the apex of the inside mold lines.

 

Outside Mold Line (OML) - A line that runs parallel to the outside of the workpiece.

Inside Mold Line (IML) - A line that runs parallel to the inside of the workpiece. Outside Offset (OSOS) - The measurement from the surface of the outside

radius to the apex of the outside mold lines.  When calculating the outside offset use the included bend angle.

 

Inside Offset (ISOS) - Distance between the inside mold lines and the bend line.

 

Inside Through the Material (ISTM) - The distance between the outside and inside mold lines.

 

Inside Shift (ISS) - Distance between the outside mold lines and the bend line.

 

Bend Line Shift (BLS) - The distance from the Outside Mold Line to the original bend line on the flat pattern.  This is used to calculate the back stop location when working off of a flat pattern.

Terminology

Air Bending  - One of the three types of bending for sheet metal where the outside mold line is not pressed against the die.

Air Bend Force Chart  - A chart used to calculate the tonnage required for a bend based on thickness, tooling and length.

Annealing  - Annealing is a treatment for metals where a material is heated above the recrystallization temperature maintained for a period of time and then cooled.  Annealing is used to soften material, relieve internal stresses and improve its cold working prope

rties. Bending - The process of cold working metal to achieve a desired profile. Bend Line - The line across the metal where the punch comes in contact with

the metal and the bend begins. Bump Bending  - Also known as Step Bending, the process for forming a large

radius with conventional tooling by performing a series bends in close proximity. Blanking  - The process of cutting flat patterns from stock sheeting, done typically

with lasers, water jets, plasmas or punch presses. Bottom Bending  - One of the three types of bending for sheet metal where the

radius of the punch tip is pushed into the sheet metal. Box Bending  - The process of bending a 4 sided sheet metal box. Coining  - One of the three types of bending for sheet metal where the punch

penetrates into the sheet metal under high tonnage forming a consistent bend. Cross Break  - Light bends added to sheet metal in order to stiffen its surface. Crowning - The deflection along a bend due to the tooling or brake not being

able to apply equal tonnage along the bend.  Crowning is controlled on modern brakes with internal hydraulic systems which can help equalize pressure.

Curling   - A forming process which leaves a circular, closed loop at the end of the sheet.  This forms a safe edge for handling and stiffens the part’s edge.

Flange Length - The length of the workpiece when measured from the apex to the edge of the bend.

Flat Pattern - The general term for the unfolded, flattened, geometry of a part. Foil - Very thin sheet metal made from typically malleable metals such as

aluminum and gold. Gage, Gauge  - The thickness of the metal organized by numbers, the smaller the

number the thinner the metal. Galvanneal  - Steel which has been galvanized and then subsequently annealed. Galvanized  - In order to prevent rust steel is dipped into molten zinc which alloys

itself with the surface of the steel. Gusset   - A section of the metal inside a bend which is not bent, but rather

forced into the bend in order to stiffen the piece. Hem  - A flange that reaches 180° or more.  Hems can be flattened, left open or in

a variety of tear drop shapes. Jog  - Also known as an offset bend, this is when two bends of the same angle,

but opposite direction, are used to create a ‘z’ shaped profile. Kink - A light bend typically between 5° and 15° which is used to stiffen a flat

piece of metal. Large Radius Bending  - Also known as R Bending, large radius bending is when

the inside radius is greater than 8 times the material thickness. Leg - Length of the workpiece from the edge to the outside tangent point of the

bend radius. Neutral Axis - An imaginary line within the bend where the material goes

through no compression or stretching during the bend process. Obtuse Angle  - A geometry term for an angle which is greater than 90°. R Bending  - Bending with an inside radius greater than 8 times the material

thickness. Reflex Angle  - A geometry term for an angle which is greater than 180° Sharp Bend - When the radius of the bend is less than %63 of the material

thickness, seen commonly with hemming applications. Spring Back  - The amount to which the workpiece resists bending by returning to

its original shape. Step Bending  - Also known as bump bending, the process for forming a large

radius with conventional tooling by performing a series bends in close proximity. Straight Angle  - A geometry term for an angle which is equal to 180°. Tolerances   - General dimensioning and tolerances of bends and sheet metal. Tooling   - General term for the dies, punches and holders found on press brake

equipment. Work Piece - The general term for the sheet metal part being bent.

Design GuidelinesBends

Bends are the most typical feature of sheet metal parts and can be formed by a variety of methods and machines which negate the absolute need for some of the below tips. 

However for typical parts meant to be cost effective and easily produced the following tips should be useful.

The minumum flange length is based on the die used to bend.  Consult and Air Bend Force Chart to determine typical minimum flange lengths.

When multiple bends are on the same plane try and design the part so the bends all face the same direction.  This will prevent the need for the operator to flip the part.  This also benefits man leaf and panel benders which can only bend one direction per setup.

Avoid large parts when possible, and especially large parts with small or detailed flanges.  Chasing a large part through each bend can be dangerous and exhausting for an operator.  This also makes you vulnerable to reduced part accuracy.

Always consult a tooling profile chart when developing your part.  Know the tools available in your shop or the standards if you are outsourcing production.  Specialized tooling cen be very expensive.

Counterbores & Countersinks

While thinner gauge sheets won’t often be countersunk there are a few guidelines to try and follow on thicker sheets to preserve the strength of the material and prevent deformation fo the features during forming.

The distance between two countersinks should be kept to at least 8 times the material thickness.

To ensure strength the distance between a countersink’s edge and the edge of the material should be 4 times the material thickness.

There should be at least %50 contact between the fastener and the surface of the countersink.

To prevent any deformation of the hole the edge of the countersink should be at least 3 times the material thickness from the tangent point of the bend.

Curls

When adding a Curl to the edge of a sheet the following guidelines will ensure that no special tooling is required.

The outside radius of a curl can be no smaller than 2 times the material thickness.  This will create an opening with a 1 material thickness radius.

A hole should be at least the radius of the curl plus material thickness from the curl feature.

A bend should be at least the radius of the curl plus 6 times the material thickness from the curl feature.

Dimples

The diameter of a dimple should be no more than 6 times the material thickness. The inside depth of a dimple should be no more than the inside radius. A hole should be at least three times material thickness away from the edge of

the dimple.  Or the inside radius of the dimple plus 3 times material thickness. From the part’s edge, dimples should be at least 4 times material thickness plus

the radius of the dimple. From a bend, dimples should be at least 2 times material thickness plus the

dimple radius plus the bend radius. From another dimple, dimples should be 4 times material thickness plus the

inside radius of each dimple.

Embossments & Ribbing

Embossments and offsets should be measured to the same side of material unless it is necessary to hold an outside dimension.

For round embossments or ribs, maximum depth is equal to the internal radius of the embossment.

For flat embossments, the maximum depth is equal to the inside radius plus the outside radius.

For V embossments the maximum depth is equal to 3 times material thickness. Embossments should be at least 3 times material thickness from a hole’s edge. Between two parallel ribs, minimum distance is 10 times material thickness plus

the radius of the ribs.

Extruded Holes

Between two extruded holes, distance should be at least 6 times material thickness.

From edge to extruded hole, distance should be at least 3 times material thickness.

From bend to extruded hole, distance should be 3 times material thickness plus bend radius.

Gussets

Gussets are used to strengthen a flange without the need for secondary processes such as welding.  While gussets will almost always require custom tooling some basic guidlines should help.  Be sure to consult with your factory’s Brake Press department to learn what they are equipped to bend.

45° gussets shouldn’t be designed to be more than 4 times material thickness on their flat edge

For holes, the distance between the gusset and the hole’s edge should be at least 8 times material thickness.

Hems

Hems are used to create folds in sheet metal in order to stiffen edges and create an edge safe to touch.

For tear drop hems, the inside diameter should be equal to the material thickness.

For open hems, the bend will lose its roundness when the inside diameter is greater than the material thickness.

For holes, the minimum distance between the hole’s edge is 2 times the material thickness plus the hem’s radius.

For bends, the minimum distance between the inside edge of the bend and the outside of the hem should be 5 times material thickness plus bend radius plus hem radius.

Holes / Slots

Distance from outside mold line to the bottom of the cutout should be equal to the minimum flange length prescribed by the air bend force chart.

o Rule Of Thumb: 2.5* Material Thickness + Bend Radius. When using a punch press the diameter of a hole should always be equal to that

of your tooling and you should never use a tool who’s diameter is less than that of the material’s thickness.

o Rule Of Thumb: Never design a hole smaller than .040” Diameter unless laser cutting.

When using a punch press holes should be at least 1 material thickness from any edge.  This prevents bulging along the edge.

Lances & Louvers

Formed lances and louvers will almost always require specialized tooling so be sure to understand what is available to you before designing the feature.

The minimum depth of a lance should be twice the material thickness and at least .125”

If the lance if formed with standard tooling be sure that the length of the bend is dividable by a standard set of Sectionalized   Tooling .

From a bend, lances should be at least 3 times material thickness plus bend radius, however the actual minimum is often much greater than this and driven by the tooling profile.

From a hole, lances should be at least 3 time material thickness from the edge of the hole.

Notches & Reliefs

The minimum width of a notch is equal to the material thickness and at least .04”.  This is negated if the blank is being cut by a Laser System in which case the minimum is only the kerf of the laser.

When determining the length of a notch it is very important to understand the tooling used to cut the notch.  When possible the notch should be equal to a multiple of the punch’s length in order to prevent nibbling from occurring.

From a bend, the minimum distance is 3 times material length plus the bend radius.

When fabricating with a Punch Press the minimum space between two notches should be at least 2 time material thickness and at least .125”

Welding

Welding by hand should be restricted to gauges thicker than 20 gauge. Spot welding should be used for joining equally thick co-planar surfaces.  The

arm geometry and throat depth of the spot welder will be a limiting factor. Welded joints should be designed with as tight of tolerances as possible to

remove the need for a welder to add wire. Wire material should always be the same as the material being welded.

Plating

Sharp edges and corners will typically receive about twice as much as the plating material because of the current density in these areas.

If possible tap and thread after plating, else assume that the material will grow up to 4 times the typical platting thickness, compensate pitch and depth accordingly.

Avoid recessed areas which are difficult to reach. Because the parts are going to be hung from hooks and dipped it is beneficial to

design hanging holes into your part rather than leaving the decision to the plater.  These holes can be small, just enough to get a wire hook through.  These holes will also give you control over how the part is positioned when it is dipped.

In addition to hanging holes design drainage holes.  Knowing the orientation of the part from your hanging holes make sure the part can be easily cleaned after plating.

Assume all areas of the part will be plated, masking is not recommended.

 

5 Most Useful Sheet Metal Design TipsDesign for manufacturability is a very useful concept in today’s sheet metal design industry. A sheet metal design should ideally take care about all the aspects of sheet metal manufacturability. This article will talk about five such important sheet metal design tips.

Bend Relief: Bend relief is the notch that needs to be created for sheet metal bending. For better clarity see the picture below:

 

The flange which does not have relief will result in a greater amount of distortion or tearing of the adjacent material.

As per sheet metal design thumb rules, the depth of bend relief should be greater than or equal to the inside bend radius of the bend and the width of the bend relief should be the same as the sheet metal thickness or more.

Minimum Hole Size for Sheet Metal: While placing the holes in your drawing please keep in mind that, you should not make holes small enough to break the tool. As you reduce the size of the sheet metal hole, smaller size punches will be required. If the size of the punch becomes to small it may break during operation.

Sheet metal design rule of thumb in this case is: The diameter of the hole should be equal or more than the thickness of the sheet metal.

Minimum Clearance Between a Hole and Bend: It is very important to maintain enough clearance between a hole and bend for sheet metal design, or else the hole will get deformed like below:

The sheet metal design rule of thumb in this case is: The distance between the sheet metal bend line and edge of the hole should be two times or greater the thickness of the sheet metal.

Minimum Sheet Metal Bending Radius: Minimum sheet metal bend radius depends on the selection of tool and the process. The more ductile the sheet metal, the smaller the inner bend radius is possible.

The sheet metal design rule of thumb in this case is: The minimum bend radius for mild steel sheet metal should be equal to the thickness.

Minimum Flange Width: While specifying flange widths in your drawing, please ensure that the width of the flange does not go below four times the thickness of the sheet metal. Otherwise, the tool will create marks on the sheet metal surface while manufacturing.

Related Readings

Overview of Sheet Metal Design using Pro Engineer : Pro engineer is considered to be the leading software for sheet metal design. Like surfacing of catia, sheet metal design module of pro engineer is the unique selling feature of the software.

How to Design Sheet Metal Forming Components in Pro Engineer : Form Feature: Pro Engineer is developed in such a way that the feature operations in ProE are actually similar to real life manufacturing operations. Form feature in Pro Engineer is identical to the real life sheet metal forming operations (where components are produced by sheet metal punch and Die press tools

Sheet Metal Component Design Guide

<<<   Please note that this page is under construction   >>>

The aim of this guide is to provide designers with simple hints and tips to allow them to design sheet metal components that are easy to manufacture, and therefore cost effective, whilst maintaining maximum precision and quality.  

Designing Folded Components

Before reading this section it is recommended that you take a look at How Sheet Metal Bending Works to fully understand how the pressbrake machine operates and the different types of tooling.  The majority of folded parts at Hydram are formed using air bending, and the following guidelines assume that air bending is to be used.  Air bending uses the minimum amount of force, which maximises the capabilities of the pressbrake and minimises wear on the tooling. 

Design Considerations for Folded Sheet Metal Components

Bend radius (inside bend radius)

With air bending, the inside radius is predominantly determined by the die opening or V-width.  It is preferable for any given material to use the largest practical die opening to minimise the force required, which results in a larger radius.  Often a smaller radius is desirable and a good rule-of-thumb is to use a minimum inside radius equal to the material thickness.  Dies are manufactured in particular sizes and this limits the choice of radii for a particular material.  For this reason generous tolerances on the inside bend radii should be allowed for in the design (typically +/- 30% of the required dimension).

Folding aluminium can be difficult as it has a tendency to crack, particularly when bending parallel to the material grain.  Softer alloys such as 1050, 3103, 5083 and 5251 are less problematic and can be folded in a similar manner to mild steel.  Harder alloys, such as those in the 2XXX series, may require minimum bend radii between four to eight times material thickness to avoid cracking.

Minimum flange length

The recommended minimum flange length would be at least four times the material thickness. The limit on small flanges obtainable on the pressbrake is determined by the die opening or V-width.  Small flanges approach the edge of the die opening and can slip under the top tool as it penetrates.  This makes it physically impossible to produce the bend in one operation.  A smaller flange is only possible with additional work, such as forming a larger flange and then machining to size, which makes it a costly feature.

Bend relief

If a bend is too close to material on an adjacent edge the material is likely to tear. Picture 1 illustrates the problem.  To prevent tearing, either the bend to edge distance should be increased, as in Picture 2, or bend relief should be provided.  Picture 3 shows bend relief cut into the part.  The relief length should be greater than the radius of the bend and the width of the relief should be at least the material thickness.

Picture 1    Picture 2    Picture 3

Bend relief has a number of benefits.  The bigger the relief, the easier it is to align the component over the tooling reducing both setup and running costs.  Relief also prevents crack propagation which is particularly important if the component is subject to vibration as existing cracks can grow rapidly.   In these situations it is best to avoid creating relief with sharp corners so that that finished component will be more durable.

Forming near holes

It is recommended that holes are positioned away from bends to avoid distortion.  If a bend is too close to a hole, the hole would become deformed during the bending operation.  

Picture 1    Picture 2

A hole required so close to the bend would therefore need to be created after bending with a secondary drilling operation.  To avoid this additional expense, any holes or slot edges can be positioned so that they are clear of the die opening when the bend is formed.  As a rule-of-thumb this equates to a distance three to four times material thickness from the bend line.

Bend edge distortion

Picture

The above picture illustrates the distortion created at the edge of a bend as material thickness increases and bend radius decreases.  If this distortion is unacceptable then the component can be relieved as shown in the picture.

Guidelines for Drafting & Dimensioning Folded Sheet Metal Components

Dimension the part in a single direction where possible.  Due to the sequential nature of the forming process, and the fact that a dimensional variation is introduced at each bend, dimensioning in a single direction parallels the process and helps to control tolerance accumulation.

Allow a more generous tolerance on flange lengths (+/- 0.2mm) as tighter tolerances, while achievable, will make the part more expensive.  Often tolerances in the drawing title block may be unnecessarily tight for certain dimensions and angles, while appropriate for others.

Avoid dimensioning bend radii where possible.  Each subcontractor will have their own tooling preference, and this determines the bend radii on the part.  If bend radii are important then whenever possible use the same bend radius for all of the bends on the part.  This helps the subcontractor minimise set-ups and reduce costs.  In any event, a generous tolerance should be allowed to allow a maximum choice of tooling (typically +/- 30% of the required dimension).  Note that the resulting radius should not vary within

a batch of components made with the same tooling despite the wide tolerance allowance.

Generally, dimensioning should be done from a feature to an edge.  Avoid feature-to-feature dimensions over two or more planes.  Feature-to-bend dimensions may require special fixtures or gauges.

Outside dimensions should be used unless the inside dimension is critical.  However, offsets and embosses should be dimensioned from the same side of the material unless the overall height is critical.

Only the absolutely critical dimensions should be highlighted as such.  Placing excessively high tolerances and redundant critical dimensions on a drawing can dramatically increase the cost of the part.  

If a blank development is required on your drawing, then mark it up as "For Information Only".  Developments provided by third parties are often created without consideration for the actual pressbrake tools to be used during production, which will influence the accuracy of the finished part.  To maximise quality, precision subcontractors will generate their own blank development to suit their tooling, ensuring that the formed component matches the requirements.

The Art of Sheet Metal Design: Tips for Accurate Bending Allowance CalculationsWritten by:  Suvo • Edited by: Lamar Stonecypher Updated Aug 24, 2010

Taking accurate sized developed sheet metal initially is extremely important to reach the accurate final sheet metal product. This sheet metal design guide will talk about calculation of accurate developed length by calculating bending allowance and K factor.

Typical Sheet Metal Bending Process

Sheet metal bending process can be briefly jotted down as:

Developed sheet metal size is obtained from drawing. Developed sized sheet metal is cut out from large sheet by punching operation. Bending brake is used to bend the sheet metal piece to the required shape and

angle.

Bending Allowance Calculation

Bending allowance is the input for calculating developed sheet metal size. How? See the snap below:

 

If you have a sheet metal bending product (as shown in above picture) with two legs of length “X” and “Y” and you unbend it, you will see that total length of the unbend sheet will NOT X + Y rather, the length will be X + Y + BA. Where, BA is bending allowance.

The formula for calculating sheet metal bending allowance is (Please refer to the above picture):

BA= Bend Angle * (Π/180)* (R+ K factor* T)……Eqn.1

Where,

BA is Bend allowance.

Bend Angle represents the angle to which sheet metal has bend (here it is 90 degree).

R is inner bend radius.

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K factor= t/T……….Eqn.2

So, if you got K factor value now, you can easily calculate bending allowance.

Calculating K factor

There are standard charts available with sheet metal design handbook for calculating K factor (ideally it should be 0.5). But the most accurate way to find out K factor is reverse engineering of a small but same sheet metal sample piece. Procedure is as below:

Now by referring Fig.1 and Fig.2 we can write:

X= X’ - (tan (A’/2))*(T+R)…..Eqn.3

Y=Y’- (tan (A’/2))*(T+R)……Eqn.4

Since, all the items of right hand sides of the Eqn.3 and Eqn.4 are known so we can get the values of X and Y of the sample.

We also know the developed length of the sample (P). By doing P – (X+Y), we can get the bend allowance (BA) for the sample. Now, use Eqn.1 and you will get K factor. This K factor will be same for the

actual product. Use this K factor value to calculate bending allowance for main product.

Conclusion

For getting accurate output of sheet metal bending process you need to calculate accurate sheet metal developed length. And for calculating accurate sheet metal developed length you need to accurately calculate sheet metal bending allowance and for bending allowance calculation you need to have accurate K factor value. The procedure discussed in this sheet metal design guide is the simplest way to calculate bend allowance with accepted accuracy.

he bend radius should be kept the same for all radii in the part to minimize set up changes. Bend radius guidelines are as follows:

  •

For most materials, the minimum inner radius should be at least 1 material thickness.

  •

As a general rule, bending perpendicular to the rolling direction is easier than bending parallel to the rolling direction. Bending parallel to the rolling direction can often lead to fracture in hard materials, thus bending parallel to rolling direction is not recommended for cold rolled steel > Rb 70, and no bending is acceptable for cold rolled steel > Rb 85. Hot rolled steel can be bent parallel to the rolling direction.

  •

The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or

operator due to slippage.

  •

Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should be located a minimum of 3 times the stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line.

  •

Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.

  •

Parts should be inspected in a restrained position, so that the natural flexure of the parts does not affect measurements. Similarly, inside dimensions in an inside bend should be measured close to the bend.

he bend radius should be kept the same for all radii in the part to minimize set up changes. Bend radius guidelines are as follows:

  •

For most materials, the minimum inner radius should be at least 1 material thickness.

  •

As a general rule, bending perpendicular to the rolling direction is easier than bending parallel to the rolling direction. Bending parallel to the rolling direction can often lead to fracture in hard materials, thus bending parallel to rolling direction is not recommended for cold rolled steel > Rb 70, and no bending is acceptable for cold rolled steel > Rb 85. Hot rolled steel can be bent parallel to the rolling direction.

  •

The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.

   Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should

• be located a minimum of 3 times the stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line.

  •

Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.

  •

Parts should be inspected in a restrained position, so that the natural flexure of the parts does not affect measurements. Similarly, inside dimensions in an inside bend should be measured close to the bend.

Introduction

In drawing, a blank of sheet metal is restrained at the edges, and the middle section is forced by a punch into a die to stretch the metal into a cup shaped drawn part. This drawn part can be circular, rectangular or just about any cross-section.

Drawing can be either shallow or deep depending on the amount of deformation. Shallow drawing is used to describe the process where the depth of draw is less than the smallest dimension of the opening; otherwise, it is considered deep drawing.

Drawing leads to wrinkling and puckering at the edge where the sheet metal is clamped. This is usually removed by a separate trimming operation.

Design Considerations

 •

Round shapes (cylinders) are easiest to draw. Square shapes can also be drawn if the inside and outside radiuses are at least 6 X stock thickness. Other shapes can be produced at the cost of complexity of tooling and part costs.

• The corner radiuses can be reduced further by successive drawing operations, provided there is sufficient height for the draw.

• Perpendicularity can be held to ±1º, flatness can be held to 0.3%. This can be improved by performing extra operations.