SHEETMETAL DESIGN

Sheetmetal design introduction:

Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material. Plate, sheet and foils are words used to describe the classification of metal depending upon its thickness. Thicknesses can vary significantly; although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate. Metal is passed between rolls under extreme pressure to make it thinner and longer in the direction in which it is moving. The amount of pressure that is applied decides which of the three categories the resulting aluminum product will belong.  

Followings are the few characteristic of sheetmetal over machining:

1)      Function:  Most low stress (and many high stress) components requiring moderate stiffness can be created from sheetmetal. Sheetmetal is particularly effective for parts that function as containers, chutes and gates, but can effectively be used for mounting brackets as well.

2)        Attachment method:  Sheetmetal parts are typically welded (permanent), riveted (semipermanent), or connected via fasteners (removable) to other parts. Since sheetmetal parts are typically too thin to be threaded (i.e. too thin to achieve the required 5 threads of engagement), attachment holes should be designed as thru holes.

3)       Mechanical properties: Sheetmetal parts are typically lighter than their billet machined counterparts. In general, simple sheetmetal parts require looser tolerances and are less stiff. Sheetmetal parts requiring welding should be created from steel, because thin aluminum is much more difficult to weld (because of its higher thermal conductivity).

4)       Manufacturing properties:  Sheetmetal parts are typically cheaper and faster to produce than their billet machined counterparts.

More things to be known about sheetmetal design:

·         The raw material for sheet metal manufacturing processes is the output of the rolling process. Typically, sheets of metal are sold as flat, rectangular sheets of standard size. If the sheets are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal process is to cut the correct shape and sized ‘blank’ from larger sheet.

·          Design of sheet metal components should be such that it would minimize scrap loss and die cost, and improve efficiency

·         The cost in sheet metal forming operation can be reduced by using thinner sheets if the strength and rigidity are increased by bending and forming into ribs configuration

·         Major considerations for the metal selection are the types of metal and their thickness. Protocase Inc. offers carbon steel and stainless steel as standard offerings

Thumb Rules:

      Design 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. Followings are the important thumb rule in sheet metal design.

      Bend Relief:Bend relief is the notch that needs to be created for sheet metal bending. The flange which does not have relief will result in a greater amount of distortion or tearing of the adjacent material. To prevent tearing relief should provide at bend. As per sheet metal design thumb rules, the length 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 greater than or equal to the sheet metal thickness.

  

     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 too small it may break during operation.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.The distance between the sheet metal bend line and edge of the hole should be equal or  greater than four time 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 minimum bend radius for mild steel sheetmetal should be equal to the thickness of Sheetmetal. But for harder material it’s difficult to achieve result in rupture takes place at outer setback. 

 Minimum Flange Width: Minimum flange width should be equal or more than four times sheetmetal thickness. Otherwise, the tool will create marks on the sheet metal surface while manufacturing. 

   

     Bend should be perpendicular to the sheetmetal grains, parallel to grain results in tendency to crack. If bend perpendicular to grain not possible, prefer for 45° to the grain direction.

Bends:

• The minimum flange length is based on the die used to bend.  

• The minimum height of a bent flange is directly related to the material thickness, bend radius, and length of bend.

• Two Bend with same direction results in good accuracy and reduces setup timing. Bends should be tolerance  +/-1.5° at a location adjacent to the bends

• Avoid large parts with small or detailed flanges, which reducing 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 can 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 sheet and prevent deformation of 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 4 times the material thickness from the tangent point of the bend.

• The maximum depth is 3 times the material thickness at an angle of the hardware.

Curls:

• The outside radius of a curl can be no smaller than 2 times the material thickness.  

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

Dimples:

• The diameter of a dimple should be no more than 6 times the material thickness.

• The minimum distance between Dimple edge and Hole edge should kept 3 times the material thickness 

• From the part’s edge, dimples edge should be at least 4 times material thickness

• From a bend edge, dimples edge should be at least 3 times material thickness.

• Distance between two dimples should be 4 times material thickness plus the inside radius of each dimple.

• The maximum diameter should be six times the material thickness, and a maximum depth of one-half the inside diameter.

• The minimum distance that a dimple should be from a hole is three times the material thickness plus the radius of the dimple.

• The minimum distance that a dimple should be from the edge is four times the material thickness plus the inside radius of the dimple.

• The minimum distance that a dimple should be from a bend is two times the material thickness plus the inside radius of the dimple plus the radius of the bend.

Gussets

Gussets are used to strengthen a flange; following are the guide lines for gussets.

• 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.

• The width and depth, recommended at an angle of 45 degrees, is directly proportional to the radius and 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 and Hem is 3 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.

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”

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

• The minimum width of an open lance is two times the material thickness or 3.00 mm (0.125 inch), 

• The minimum width of a closed lance is two times the material thickness or 1.60 mm (0.06 inch),.

Notches & Reliefs:

• The minimum width of a notch is equal to the material thickness and at least .04”. .

• 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.

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

• The maximum length for a straight/radius end notch is equal to five times the width.

• The maximum length for a V notch is equal to two times the width.

• The minimum distance from a notch to a bend in a parallel plane is eight times the material thickness plus the radius of the bend.

• The minimum distance from a notch to a bend in a perpendicular plane is three times the material thickness plus the radius of the bend.

• The minimum distance between two notches is two times the material thickness or  3.200 mm (0.125 inch)

Equations In Sheetmetal Design:

K-Factor – The ratio of the position of the Neutral Axis to the Material Thickness

Bend Allowance (BA) – It is used to calculate the total length of flat pattern. In other word, Bending allowance is an additional sheet length which given at bend portion. We can get equation of BA by following assumptions,,

BA=arc length, 

Arc length of any bend= (angle in rad) X (radius of neutral axis)

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

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.

Sheet Metal Forming Processes And Process Terminology: 

(Good Designer Should Know About the Manufacturing Process before Start the Actual Design of Product)

      Sheet metals are widely used for industrial and consumer parts because of its capacity for being bent and formed into intricate shapes. Sheet metal parts comprise a large fraction of automotive, agricultural machinery, and aircraft components as well as consumer appliances. Successful sheet metal forming operation depends on the selection of a material with adequate formability, appropriate tooling and design of part, the surface condition of the sheet material, proper lubricants, and the process conditions such as the speed of the forming operation, forces to be applied, etc. A numbers of sheet metal forming processes such as shearing, bending, stretch forming, deep drawing, stretch drawing, press forming, hydroforming etc. used for specific purpose and the requisite shape of the final product.

Sheet metal operations can be classified as follows.

•Shearing operations- Shearing, Blanking, Piercing, Trimming

•Tension operations- Stretch forming.

•Compression operations- Coining, Sizing, Hobbing

•Tension and compression operations- Drawing, Bending, Forming, Embossing

Shearing:

   Irrespective of the size of the part to be produced, the first step involves cutting the sheet into appropriate shape by the process called shearing. Shearing is a generic term which includes stamping, blanking, punching etc. When a long strip is cut into narrower widths between rotary blades, it is called slitting. Blanking is the process where a contoured part is cut between a punch and die in a press. The same process is also used to remove the unwanted part of a sheet, but then the process is referred to punching. Similarly, nibbling, trimming are a few more examples of cutting process using the same principle of shearing process

Piercing :

     Cutting out holes in a blank strip or a semi-finished component using a press tool is called piercing. The cut out piece is scrap or slug.

Bending:

     Bending is the operation of deforming a flat sheet around a straight axis where the neutral plane lies. In this process Metals take a permanent deformation if they are stressed beyond their elastic limits. It is a very common forming process for changing the sheets and plates into channel, drums, tanks, etc. Spring back is a major problem during bending of sheets that occurs due to elastic recovery by the material causing a decrease in the bend angle once the pressure is removed. The springback can be minimized by introducing excess amount of bending so that the finished bending angle is the same after the elastic recovery. However, a careful estimate of the elastic recovery based on the mechanical behavior of the sheet material is necessary to achieve the same.

Stretch Forming:

     It is a method of producing contours in sheet metal. In a pure stretch forming process, the sheet is completely clamped on its circumference and the shape is developed entirely at the expense of the sheet thickness. The die design for stretch forming is very crucial to avoid defects such as excessive thinning and tearing of the formed part. The stretch forming process is extensively used for producing complex contours in aircraft and automotive parts.

Deep Drawing:

      Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter. This can be achieved by redrawing the part through a series of dies. The metal flow during deep drawing is extensive and hence, requires careful administration to avoid tearing or fracture and wrinkle. In deep drawing process type of material and thickness plays major role: Slightly thicker materials can be gripped better during the deep drawing process. Also, thicker sheets have more volume and hence can be stretched to a greater extent. However, the drawing force will increase with the sheet thickness. The percentage elongation property or ductility of the material is an essential quality for materials to be used for deep drawing.

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 properties.

• 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 bending process in 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 bending process in 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 larger 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 work piece 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.

Reverse Engineering

Introduction

(Reverse engineering was often used during the Second World War and the Cold War. It is often used by military in order to copy other nation’s technology, devices or information, or parts of which, have been obtained by regular troops in the fields or by intelligence operations)

Engineering is the profession involved in forecasting, designing, manufacturing, constructing, and maintaining of products, systems, and structures. At a higher level, there are two types of engineering: forward engineering and reverse engineering

→Forward engineering is the traditional process of moving from high-level abstractions and logical designs to the physical implementation of a system..

In some situations, there may be a physical part without any products technical details, such as drawings, bills-of-material, or without engineering data, such as thermal and electrical properties. The process of duplicating an existing component, subassembly, or product, without the aid of drawings, documentation, or computer model is known as Reverse engineering. ←

Reverse engineering (RE) is the process of taking something (a device, an electrical component, a software program, etc.) apart and analyzing its workings in detail, usually with the intention to construct a new device or program that does the same thing without actually copying anything from the original.

Reverse engineering (RE) : A systematic methodology  for analyzing  the design of an existing device or system, either as an approach to study the design or as a prerequisite for re-design.

To accomplish this task, the engineer needs an understanding of the functionality of the original part and the skills to replicate its model and characteristics in details. In the fields of mechanical engineering and industrial manufacturing, reverse engineering refers to the method of creating engineering design and documentation data from existing parts and their assemblies.

The new analytical technologies, such as three-dimensional (3D) laser scanning and high-resolution microscopy, have made reverse engineering easier, but there is still much more to be learned. Several professional organizations have provided the definitions of reverse engineering from their perspectives. It has been incorporates in appropriate mechanical design and manufacturing engineering standards and multiple realistic product constraints with broad knowledge in multiple disciplines such as:


• Applying knowledge of mathematics, engineering, and science in data analysis and interpretation. 
• Using process, techniques, instruments, and tools in reverse engineering applications 
• Conducting appropriate experiments and tests to obtain the necessary data in reverse engineering. 
• Identifying, formulating, and solving issues related to reverse engineering.
• Understanding legal and ethical responsibilities pertinent to reverse engineering. 
• Assessing and evaluating documents and fostering attainment of objectives of a reverse engineering project. 

·        

Reverse engineering can be viewed as the process of analysing a system to:

1. Identify the system's components and their interrelationships
2. Create representations of the system in another form or a higher level of abstraction
3. Create the physical representation of that system

Reverse engineering is very common in such diverse fields as software engineering, entertainment, automotive, consumer products, microchips, chemicals, electronics, and mechanical designs. For example, when a new machine comes to market, competing manufacturers may buy one machine and disassemble it to learn how it was built and how it works. A chemical company may use reverse engineering to defeat a patent on a competitor's manufacturing process. In civil engineering, bridge and building designs are copied from past successes so there will be less chance of catastrophic failure. In software engineering, good source code is often a variation of other good source code.

In some situations, designers give a shape to their ideas by using clay, plaster, wood, or foam rubber, but a CAD model is needed to enable the manufacturing of the part. As products become more organic in shape, designing in CAD may be challenging or impossible. There is no guarantee that the CAD model will be acceptably close to the sculpted model. Reverse engineering provides a solution to this problem because the physical model is the source of information for the CAD model. This is also referred to as the part-to-CAD process.

Another reason for reverse engineering is to compress product development times. In the intensely competitive global market, manufacturers are constantly seeking new ways to shorten lead-times to market a new product. Rapid product development (RPD) refers to recently developed technologies and techniques that assist manufacturers and designers in meeting the demands of reduced product development time. For example, injection-moulding companies must drastically reduce the tool and die development times. By using reverse engineering, a three-dimensional product or model can be quickly captured in digital form, re-modelled, and exported for rapid prototyping/tooling or rapid manufacturing.

Reasons for reverse engineering:

1. The original manufacturer no longer exists, but a customer needs the product
2. There is inadequate documentation of the original design
3. The original design documentation has been lost or never existed
4. Some bad features of a product need to be designed out. For example, excessive wear might indicate where a product should be improved
5. To strengthen the good features of a product based on long-term usage of the product
6. To analyse the good and bad features of competitors' product
7. To explore new avenues to improve product performance and features
8. To gain competitive benchmarking methods to understand competitor's products and develop better products
9. The original CAD model is not sufficient to support modifications or current manufacturing methods
10. The original supplier is unable or unwilling to provide additional parts
11. The original equipment manufacturers are either unwilling or unable to supply replacement parts, or demand inflated costs for sole-source parts
12. To update obsolete materials or antiquated manufacturing processes with more current, less-expensive technologies
13. Learning about a competitor’s latest research by capturing data to secure as much information as possible to understand its capabilities.

Reverse engineering enables the duplication of an existing part by capturing the component's physical dimensions, features, and material properties. Before attempting reverse engineering, a well-planned life-cycle analysis and cost/benefit analysis should be conducted to justify the reverse engineering projects. Reverse engineering is typically cost effective only if the items to be reverse engineered reflect a high investment or will be reproduced in large quantities. Reverse engineering of a part may be attempted even if it is not cost effective, if the part is absolutely required and is mission-critical to a system.

Reverse engineering of mechanical parts involves acquiring three-dimensional position data in the point cloud using laser scanners or computed tomography (CT). Representing geometry of the part in terms of surface points is the first step in creating parametric surface patches. A good polymesh is created from the point cloud using reverse engineering software. The cleaned-up polymesh, NURBS (Non-uniform rational B-spline) curves, or NURBS surfaces are exported to CAD packages for further refinement, analysis, and generation of cutter tool paths for CAM. Finally, the CAM produces the physical part.

It can be said that reverse engineering begins with the product and works through the design process in the opposite direction to arrive at a product definition statement (PDS). In doing so, it uncovers as much information as possible about the design ideas that were used to produce a particular product. Reverse engineering was originally a crucial tool to gain military advantage and latterly for commercial analysis and gain.

Steps in Reverse Engineering Process:

Identify The Purpose: When you are ready to reverse engineer a product, begin by recording your purpose in your engineer’s notebook. What do you want to learn about the product? Think about questions to ask, area of research, people to contact, and tests to be completed. It’s important that you keep accurate and detailed documentation throughout the entire reverse engineering process. Engineer’s notebook will provide evidence of your process, thoughts, and findings. Whenever possible, you should add supporting documentation, such as annotated sketches. Ultimately, notebook will support your findings and may serves as evidence to support legal proceedings or a patent application.

•           What is the purpose of this product?

•           How does it work?

•           What market was it designed to appeal to?

•           List some of the design objectives for the product.

            •           List some of the constraints that may have influenced the design

In disassemble process components will be disassemble to examine the theories and predictions, this steps dealing with the following questions,
How does it work?
How is it made?
How many parts?
How many moving parts?

Analyse The Elements:

Data capture involved with the scanning strategy- selecting the correct scanning technique, preparing the part to be scanned, and performing the actual scanning to capture information that describes all geometric features of the parts such as, steps, slots, pockets and holes. Three-dimensional scanners are employed to scan the part geometry, producing clouds of points, which defines the surface geometry. These scanning devices are available as dedicated tools or as add-ons to the existing computer numerically controlled machine tools. There are two distinct types of scanners, contact and non-contact.

Analysis is the most crucial part of the reverse engineering process. During the analysis step, engineers attempt to answer all of the questions originally posted. There are four main categories of product analysis,

-Functional

-Structural

-Material

-Manufacturing

Analysis requires detailed research of each category. Research results are recorded in engineer’s notebook along with sketches and digital photos to provide clarity for detailed information. During analysis, some products may require partial reassembly to observe the interaction of functional components.

• Carefully reassemble the product.
• Operate the device and record observations about its performance in terms of functionality (operational and ergonomic) and projected durability

Material Analysis: The choice of material greatly affects a parts performance, and the material properties must be correctly matched to the parts application. Analysing materials requires understanding basic material properties (mechanical, electrical, thermal, chemical, optical, acoustical), because materials are also identified by scientific properties.

After the analysis, Report preparation process tabulate following things

• Inferred design goals
• Inferred constraints
• Design (functionality, form (geometry), and materials)
• Schematic diagrams
• Lists (materials, components, critical components, flaws, successes, etc.)
• Identify any refinements that might enhance the product’s usefulness.
• Upgrades and changes