Sheet Metal Fabrication Techniques

Sheet Metal Fabrication Techniques

Sheet metal fabrication processes are very common practices in manufacturing. There are tons of kinds of products that we produce with these processes. Also, inside metalworking, sheet metal fabrication is a very big professional area that engineers, and people can develop their skills around this area.

Here we take a look at a small glance at the general characteristics of sheet metal fabrication processes.

Sheet metal fabrication.
Sheet metalworking exterior part.

What is Sheet Metal Fabrication? 

As you understand from the name, sheet metal fabrication processes include the processing of thin metal sheets with various applications. These applications also include cutting, shearing, and forming processes. 

To say a process is a ‘sheet metal fabrication’, we need to understand what kinds of metals that we consider ‘sheet’ 

Sheet Metals

As you guess that the concept of ‘sheet’ is about the thickness of the work metal. The boundary is 6mm in thickness if we want to consider workpieces as ‘sheets’. 

In sheet metal fabrication processes, the thickness range is between 0.4mm(1/64″)to 6mm(1/4″). After the 6mm, we consider the metal workpieces generally ‘plates’. To obtain sheets from metal plates, they roll the metal into sheets to decrease the thickness of the workpiece. 

Types of Metals in Sheet Metal Fabrication

Sheet metal cutting.
Cutting.

In this case, the type and alloying elements of sheet metal fabrication processes are very important. So, as you understand, the metal workpieces must be formable. So, we use the low-carbon steels mainly in sheet metal fabrication processes.

Types of Products Produced with Sheet Metal Fabrication Processes 

Sheet metal forming.
Sheet metal forming(Image Source: https://careersmart.org.uk/occupations/sheet-metal-workers).

The most important product of sheet metal fabrication processes is the exterior bodies of automotive products such as trucks, cars, etc. Furthermore very high technology processes that they use to produce the different shapes of exterior metals of trucks and cars. 

Also, they produce some of the interior structural parts of locomotives, cars, and airplanes from them or plate metals. 

In daily life, they produce office and home furniture parts and structures, and metal exteriors of household appliances from these techniques. 

Properties of Sheet Metal Fabrication Parts

As you understand that there are lots of kinds of areas that we use these parts. This is because of some of the superior characteristics and features of sheet metal fabrication parts and manufacturing techniques. 

  • High Strength Parts: Parts that we produce from sheet metals are strong enough to withstand all the external effects such as forces, impacts, etc. Because of it, sheet metal fabrication parts that they produce from relatively thick sheets that they use as structural parts. Not only steel parts they are using. For example aluminum sheet metal applications are also very common for automotive and beverage can applications. 
  • Dimensional Accuracy: Parts that they can produce can with sheet metal fabrication processes have very good dimensional accuracies in which they prefer them as exterior and interior structural and appearance parts. 
  • Good Surface Finish: Also, this is the biggest reason these parts thet we use on exterior parts of automotive and household appliances. 
  • Low-Cost Production: They use the production methods in sheet metal fabrication processes are relatively low cost if we compare with other metal part production methods. So, we can produce relatively complex and accurate parts in a serial manufacturing manner, in a much cheaper way. 

Working Temperature in Sheet Metal Fabrication

They apply sheet metal fabrication processes at room temperature in general. They make shearing, shaping, and cutting processes generally in room-temperature environments. But, they can use high temperatures for thick and brittle metals. 

Sheet Metal Cutting Processes

They process them them in various ways. One of these ways is cutting operations. We produce various kinds of parts with cutting processes. 

Sheet metal punching.
Sheet metal cutting illustration(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition).

In most basic aspects, cutting takes place by shearing a sheet metal stripwith the use of cutting blades. One of these cutting blades generally makes a reciprocating motion to cut the strip. We call this moving blade a ‘punch’. And we call the stationary blade a ‘die’. So, there is a small clearance between punch and die, at the easy cycle of motion of the punch, the strip is cut. 

If we take a closer look at the cutting phase of strips in more detail, the shearing action starts with the plastic deformation of the strip. While the punch is moving forward, penetration of the strip occurs. After the required motion of the punch, a fracture occurs.

Also, if we take a closer look at the sheared edge of the strip, we will see a characteristic edge like in the figure. You can see the rollover, burnish, fractured zone, and burr shapes.

Types of Sheet Metal Cutting in Fabrication Operations

Blanking and punching.
Blanking and punching(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition).

All the sheet metal cutting operations may depend on the same principles. But according to the process parameters and produced part shapes, they can be classified into different types of operations.

Shearing

The first type of sheet metal cutting operation is shearing. And also, this is the most classical type of cutting operation in which there is a punch and die.

Blanking

The difference in the blanking operation is that all the surrounding of the sheared part is cut from stock. Furthermore, they are cutting all the edges. Here, the strip is scrap, the produced part is called a blank.

Cutoff and parting.
Cutoff and parting(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition).

Cutoff

The cutoff is a sub-type of the shearing operation in which we use all strips to produce cutoff parts without any scrap or very little scrap.

Parting

Parting is also another sub-type of shearing in which parts and scraps produce in a specific pattern.

Slotting

Slotting is a type of blanking operation in which they produce rectangular slot shapes. 

Perforating

This is also another blanking type in which we obtain perforated sheet metal geometries. In here, small blankings are scraps. 

Slotting, perforating and notching.
Slotting, perforating, and notching(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition).

Notching

From a side of a metal strip, they cut small pieces in notching operations to obtain desired strip shapes. 

Trimming

They cut the excessive sheet metal sections with trimming operations. Also, in other metal processing operations, they use trimming operations. 

Shaving

They use shaving operations to obtain more accurate dimensional sizes by using very small clearances. 

Importance of Clearance in Sheet Metal Cutting in Fabrication Operations

As we stated above, clearances are very important in sheet metal cutting in fabrication operations. Also, clearance means the distance between the die and punch. In an ordinary sheet metal cutting process, they should design clearance to obtain desired cut section. 

If the clearance is small, the cutting forces will be higher and double-burnishing may occur. And also, if the clearance is very high, excessive burrs can occur. 

So, an engineer must design the clearance properly to obtain the desired results. 

Sheet Metal Stamping Fabrication Processes

Stamping dies.
Metal stamping dies(Image Source: https://www.thomasnet.com/articles/custom-manufacturing-fabricating/understanding-metal-stamping/).

The most used sheet metal fabrication process is the stampşng process. In the most basic explanation, there is a stamping press and stamping die in these pieces of machinery. So, the stamping press is making a reciprocating motion on stamping dies. And also, they place virgin sheet metal on stamping dies. In the cycle of reciprocating motion of stamping press on stamping die and the workpiece, they produce a sheet metalworking product. 

Because of this characteristic, sheet metal stamping processes are very suitable for mass production. 

What is Sheet Metal Bending Fabrication?

Sheet metal bending operations are another manufacturing operation around other production techniques. The sheet metal cutting operations and bending operations are the two main categories that all the sheet metal fabrication techniques. 

And also, in bending operations, they shape the flat sheet metal fabrication workpieces with deformation without any cutting or separation. 

In bending operations, they stretch the outer surface of the metal and the inner surface of the workpiece is compressed. Also, there is no change in thickness. Plastic deformation takes place after the application of bending force or load. 

Two Types of Sheet Metal Bending Fabrication Processes

We apply sheet metal bending processes with dies and punches. So, dies are tooling that gives the required bending shapes to sheet workpieces and punches are the tools that give primary forces or loads to shape the metals. 

There are generally two types of straight bending operations for sheet metal fabrication workpieces; V-bending and edge bending. 

V-Bending of Sheet Metal Fabrication Parts

V-bending.
V-Bending.

In V-bending operations, they design the die with a groove on which a metal workpiece that we place. And the punch also has the same positive shape as this v-shaped die. When the punch makes the required motion on-die, the sheet metal fabrication workpiece takes the shape of the die. They use various kinds of angles from very narrow to very wide.

Edge Bending of Sheet Metal Parts

Bending.
Image Source:http://mold-technology4all.blogspot.com/2011/08/bending.html

In edge bending operations, we squeeze the workpiece between a pressure pad and the die. Also, the section of the workpiece is desired to be bent and hung from the edge of the die. A punch makes a reciprocating motion to bend the sheet metal fabrication workpieces. 

Furthermore, the bending angles are changing between 0 to 90 degrees. The angles above 90 degrees can be very burdensome and costly. 

Comparison of These Two Types

In terms of cost and manufacturing speed, edge bending is much more applicable for very high production rates. But the cost the tooling of edge bending is more expensive than the v-bending technique. 

Engineering Analysis and Calculations of Sheet Metal Bending Operations

If we must design a sheet metal bending operation for a specific workpiece, we must calculate three parameters to obtain a healthy production; Springback, Bend Allowance, and Bending Force.

Calculation of Springback

In plastic deformation processes, especially for metals, there is a phenomenon that we call spring back. If we deform a ductile metal is plastically under pressure or force, when we release the pressure or force, a portion of the plastic deformation also relieved according to the last shape under the load. 

In bending operations, we must calculate this effect accurately to obtain the required shape in tight tolerances.

Also, we refer to the spring back as the increase of the included angle of the workpiece after the application of an included angle of the forming tool. We can calculate it with this formula; 

Spring back formula for sheet metal bending.

a’ is the included angle of the workpiece and at is the included angle of the tooling.

You can use this quick calculator to make your springback calculation for your bending operation. The use of the calculator is very simple. Just enter the required values inside the brackets then click on the ‘Calculate!’ button. If you want to make another calculation, click on the ‘Reset’ button and then re-enter the new values.

Sheet Metal Springback Calculator












How to Compensate the Springback?

The compensation of spring back is very simple actually. Overbending is a method to compensate for the springback of workpieces in which the angle of the tooling is smaller than the actual angle. So, we call the other method bottoming in which they squeeze sheet metal fabrication parts at the end of the production step.

Calculation of Bend Allowance

The bend allowance factor is important for the situation in which the bending radius is small relative to the workpiece thickness. The neutral axis of the workpiece will shrink because of the stretch of the bending section of the sheet metal fabrication. Because of this stretch, we call this length change bend allowance and we must calculate it must to meet the required dimensional expectations from parts. 

You can calculate the bend allowance with this formula again; 

Bending allowance for sheet metals.

In this formula, ‘α’ is the bending angle of the workpiece, ‘R’ is the bending radius and ‘t’ is the thickness of the workpiece. 

If the 2t>R, ‘K’ must have a 0.33 value. If the 2t<R, ‘K’ must have a 0.50 value. These are the standardizations of the value of the ‘K’. 

Use the calculator to calculate the bend allowance quickly. 

Calculation of The Bending Force

Bending force is maybe the most important sheet metal bending process parameter we calculate wisely. Because the calculation of bending force is related to the power consumption of the whole machşnery in each step of bending operation. Power means money that we will spend on this process. 

The calculation of the required force is very simple. And also, you need to use the formula below to calculate the force required for the bending operations. 

Force.

Kf is a constant that we must select according to the type of bending. So, in general, we select Kf value 1.33 for V-bending operations and 0.33 for edge bending operations. Also, ‘TS’ is the tensile strength of the metal. ‘w’ is the width of the sheet metal part and the ‘t’ is the thickness of the part. ‘d’ is the die opening dimension which is;

the mouth opening length of the v-bending dies and the distance between the die and punch for edge bending.

Die and punches.
Illustration of the ‘d’ distance(Image Source: Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition, pg 453)

You can use the calculator below to calculate the required bending force for the sheet metal bending operations.

Sheet Metal Bending Force Calculator


















NOTE: Please take care of the units when you are doing your calculations to obtain correct results. A good engineer must use consistent sets of units for calculations.

What is The Sheet Metal Drawing Fabrication?

In drawing operations, we generally obtain cup-shaped, box-shaped sheet metal structures. These structures are generally complex, concave-shaped structures that we can produce with drawing operations easily. 

To do it, we place a workpiece upon a blank hole and we attach it with blank holders. Also, upon the hole and the workpiece, a punch makes the reciprocating motion to make the workpiece take the shape of the blank hole. 

With this very basic mechanism, beverage cans, cooking appliances, ammunition shells, etc. that we produce. 

Basic Principles of Sheet Metal Drawing Fabrication Operations

Die and blankholders.
Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition, pg. 455.

Above all, if we take a look at the basic principles of sheet metal drawing operations, we can introduce the basic parameters below. 

‘Dp’ is the diameter of the punch which makes reciprocating motion in the sheet metal drawing mechanism. 

Also, ‘Rp’ is the corner radius of the punch and the ‘Rd’ is the corner radius of the die. These radius values are very important to obtaining successful drawing operations. Without these radius values, we obtain hole-punching which is technique from cutting operations that we expect. 

So, the blank holders are applying ‘Fh’ force to the workpiece. ‘Db’ is the diameter of the workpiece sheet metal. 

We must design a specific amount of clearance ‘c’ between the punch and die. This clearance value is generally 10% greater than the diameter of the thickness of the workpiece. 

As you understand from the mechanism that there will be a gradual bending occurring at the beginning of the operation. When the punch moves and starts to deform the workpiece, bending occurs at the edges of the dies. While the punch moves to the bottom of the die, straightening of the cup-shaped part’s wall occurs. 

Because of the force application of the blank holders, friction takes place between the workpiece and the die. We must design this friction correctly to obtain a successful sheet metal drawing operation. 

Severity Of The Sheet Metal Fabrication Drawing Operations

There are some basic calculations related to the severity of sheet metal drawing operations. So, one of these calculations is the drawing ratio, DR. We calculate the drawing ratio as the ratio of blank diameter to punch diameter. And also, in general, we define the upper limit of the drawing ratio as 2.00. The greater the ratio means a severe drawing operation. 

Eliminating Wrinkling

Wrinkling is one of the most encountered defects in drawing operations. But, we can eliminate it with a very simple calculation. We must calculate the thickness-to-diameter ratio to overcome this problem. Also, we express the thickness to diameter(t/Db) ratio as a percentage and it must be over 1%. With the increasing thickness to diameter ratio, the tendency to wrinkle decreases.

What Is The Sheet Metal Stretch Forming Fabrication Process?

Sheet metal stretch forming processes have very basic mechanisms. Also, grippers hold the sheet metal workpiece by grippers and stretched with tension. The applied tensile stress on the workpiece is above the yield point of the metal. Because of this fact, the deformation is plastic. 

After the tensile stress applications, the workpiece is forced to bent over a positive die to obtain the required shape. And also, after the release of the workpiece from grippers, a little spring back takes place. 

Calculation Of The Required Stretch Force

The Stretch force that we require to stretch the workpiece above the yield point that we can calculate easily with a very basic formulation. 

Stretch for in sheet metal fabrication.

The formulation is like the above. In this formula, ‘F’ is the required force, which equals the multiplication of the ‘L’ which is the length of the workpiece in the direction perpendicular to stretching, ‘t’ which is the thickness of the workpiece, and ‘Yf’ flow stress of the workpiece material. 

Furthermore, you can use the calculator below to calculate the required force for sheet metal stretch forming processes.

Stretch Force Calculator















The use of the calculator above is very simple. You just need to enter the required values inside the brackets then click on the ‘Calculate!’ button to see the result. If you want to make another calculation, just click on the ‘Reset’ button then re-enter the values. 

What is Roll Bending?

Roll bending.
Roll bending schematic illustration.

Roll bending operations are mainly used for the production of large tanks and large containers that are produced by large sheet metal fabrication workpieces. With the help of the set of rolls, large workpieces are drawn through these rolls. At the end section of these rolls, the sheet metal fabrication workpiece comes out as a curvature structure. 

With the different arrangements of rolls, the radius of curvature can change. Different kinds of stock thicknesses also can be rolled with the roll bending operations. 

The inverse of the roll bending processes is also used as ‘roll straightening’ to non-uniform and non-straight workpieces. With the series of rolls, non-uniform and non-straight sheet metal stocks can be made straight with the help of rolls. 

Roll Forming Operations in Sheet Metal Fabrication

Roll forming.
Roll forming schematic illustration.

These operations are very good serial production applications of certain sheet metal fabrication parts. In roll forming, rolls have specific shapes and if the stock passes through these rolls, they will get the shape of the rolls with very long shapes. 

Several rolls are used to give the required shape to the sheet metal contours. Firstly, it starts with the thinning of their stock. With the rolls of the middle side, a partial form is given. And then, at the end of the rolls, an infinite length of the required contour from the workpiece is obtained. Ater that, that infinite part can be cut according to the required lengths of the product. 

In roll forming processes of workpieces, metal channels, gutters, metallic pipes, and tubings with seams are produced in a serial production manner. 

What is The Sheet Metal Spinning Spinning? 

Conventional spinning.
Conventional spinning(Image Source: https://www.rocheindustry.com/metal-spinning/).

Axially symmetric and long sheet metal fabrication parts are produced with sheet metal spinning operations. With proper mandrels, the workpiece is forced to take the shape of the mandrel gradually. This force is applied by roller tools which exert a point force on the workpiece while the workpiece is spinning. 

There are three types of spinning operations. And also, these types are; conventional spinning, shear spinning, and tube spinning. 

Conventional Spinning Operations in Sheet Metal Fabrication

You can remember satisfactory Instagram videos of conventional spinning operations. We explained conventional spinning operations above. The force exertion can be done via an NC controller which provides better production quality. Also, manual spinning operations are applied. 

Axisymmetric mandrels are generally used. Starting sheet metal disc is placed at the edge of the mandrel with a clamp fixture, and the workpiece starts spinning via an electric motor. And also, force is gradually applied to start from the edge of the mandrel, as you see in the illustration. 

The sheet metal thickness does not change in conventional spinning operations. Because of that, the finished part’s diameter is smaller than the workpiece’s. 

The most important advantage of the conventional spinning operation is the production of the very large cone or cup-shaped parts in low quantity. Alternative shaping processes over conventional spinning may be more expensive because of the costs of big dies. But, conventional spinning mandrels can be produced from low-cost materials such as wood or other easily shaped materials. 

Shear Spinning Operations

Shear spinning in sheet metal applications.
Shear spinning(Image Source: https://www.researchgate.net/figure/Schematic-illustration-of-the-shear-spinning-process-9_fig1_270469200).

The main difference between the shear spinning operations from conventional spinning operations is the thinning of the workpiece with the application of shear deformation. Other names are used such as spin forging, flow turning, or shear forming for shear spinning operations. In aerospace, large parts such as rocket noses are produced with shear spinning operations. 

Calculation of the final thickness of the shear spun part is very simple. So, you just need to multiply the starting thickness of the work part by the sin of the angle of the mandrel.

Tube Spinning

Tube spinning.
Internal and external tube spinning(Image Source: IQS Directory).

Tube spinning is a shear spinning operation in which the starting part is a tube other than a disk. So, the starting tube is placed on a mandrel and the thickness of the tube is reduced with the roller tool over the mandrel. Also, with this thickness decrement, the length of the tube increases. The same shearing principle is valid for tube spinning operations. Different profile-shaped tubes can be produced with tube spinning operations. 

Conclusion

Finally, these are the most general characteristics of sheet metal working processes. You can find detailed information about sheet metal working processes in Mechanicalland!

Above all, if you add extra information or if you have questions, please leave your comments and questions below! Your precious feedbacks are very important to us!

External Link: Researchgate.

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