Maya Ellison
Bending metal plastically involves achieving a permanent deformation without causing any damage to the metal piece. This process is typically done with expensive tools called brakes, but it can also be done without any equipment. The type of metal must be considered, as some metals like brass, copper, and bronze become brittle when heated and are more prone to breaking. Other metals like steel and aluminum exhibit springback, which is the release of elastic strain, and require a greater bend angle to achieve the required bend. To bend a metal piece, one must calculate the bend allowance, which accounts for the expansion that will occur on the outside of the bend angle.
Characteristics and Values for Bending Metal Plastically
| Characteristics | Values |
|---|---|
| Tools | Vise and hammer, pipe bender, pliers, crescent wrench, blowtorch, c-clamps, 2x4 wooden blocks, rubber/plastic mallet, or metal brake |
| Metal Type | Steel, aluminum, brass, copper, bronze, pot metal |
| Metal Properties | Thickness, grain direction, ductility, resistance to fracture, yield strength, melting point |
| Techniques | Mark bend line, stabilize long sheets, apply heat, calculate bend allowance, use leverage, soften metal, annealing |
| Precautions | Avoid bending hot brass, prevent scratching, ensure sufficient vise security, avoid bending aluminum too close to melting point |
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What You'll Learn
Bending sheet metal by hand
To bend sheet metal in a vice, start by marking a line on the metal where you want to bend it. Put your wooden or metal form blocks in the vice and clamp your sheet of metal between the forms. If your sheet metal is long, ask a friend to stabilize the part hanging out from the form blocks so it doesn't affect the bend. Then, tap your metal along the bend line with a rubber or plastic mallet until the sheet bends to the angle you want. If you're working with a thick sheet of metal, apply heat from a blowtorch along the bend line to soften it.
The bending you will be doing will warp your sheet metal, so it is physically bigger. To calculate the expansion that will take place on the outside of your bend angle, you will need to know your bend allowance. You can find your bend allowance with the following formula: (π/180) x B x (IR + K x MT) = bend allowance (BA), where B is the angle complementary to your desired bend angle (1 to 180 degrees), MT is the thickness of the material, IR is the inside radius, and K is the K-factor. To ascertain your K-Factor, inside radius, and material thickness, you should use a Bend Allowance Chart.
In the world of sheet metal fabrication, creating parts that can be easily bent by hand offers a range of benefits, from reducing manufacturing costs to enabling quick prototyping and on-site adjustments. There are various patterns you can use to make parts hand-bendable, but they all have one crucial element in common: they reduce the amount of material on the bend line. For example, by understanding the principles behind hand bending and utilizing tools like Onshape's custom FeatureScript, you can easily create parts that can be adjusted on-site or used for rapid prototyping.
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Using a blowtorch for thick metal
Bending thick metal often requires the use of a blowtorch to apply heat to the seam of the bend line, softening the metal and facilitating the bend. This process should be done with caution as many types of metal have extremely high melting points, and applying a blowtorch incorrectly could damage the metal or equipment.
Firstly, you must calculate your bend allowance. This will help you to understand the expansion that will take place on the outside of your bend angle. The formula for this is: (π/180) x B x (IR + K x MT) = bend allowance (BA). B is the angle complementary to the desired bend angle, MT is the thickness of the material, IR is the inside radius, and K is the K-factor. You can find the K-factor, inside radius, and material thickness using a Bend Allowance Chart.
Next, mark a line across the stock of the metal with a soapstone pencil to indicate the centre of the bend. Clamp the stock in a vise, ensuring that the vise is tightened sufficiently so that the metal does not slip or bend incorrectly.
Light the blowtorch and adjust it to a neutral flame. Play the flame back and forth along the line, keeping it directly on the line to ensure a straight bend. When the area along the line is bright orange, turn off the blowtorch. Press down on the end of the metal with one hand, applying uniform and firm pressure to bend it on the line.
If the angle is not correct, reheat the metal and adjust the bend. Once the metal has cooled, use a combination square set with a protractor head to check the angle of the bend.
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Calculating bend allowance
Calculating the bend allowance is crucial to getting a good quality finished part from your press brake. Bend allowance is defined as the arc length of the neutral axis through the bend. It tells us how much extra length is generated by the bend deforming.
To calculate the bend allowance, you need to know the K-factor, bend radius, bend angle, and material thickness. The K-factor is a property of the material and the equipment, representing the ratio between the thickness of the metal and the neutral axis. The bend radius is the inside radius of the bend, while the bend angle is the angle you've decided to bend your flange. The material thickness is simply the thickness of the material you've selected.
The formula for calculating the bend allowance is:
Bend Allowance = Angle * (π / 180) * (Radius + K-factor * Thickness)
Or
Bend Allowance = [(0.017453 × Inside radius) + (0.0078 × Material thickness)] × Bend angle, which is always complementary.
By plugging in the values, you can calculate the bend allowance value, which tells you the length of the neutral axis through the bend. For example, if we use the values from the previous example, we get a bend allowance value of 0.1326”.
After calculating the bend allowance, you can then calculate the bend deduction, which tells you how much length should be removed from the flange to account for the stretch that occurs during the bending process. The bend deduction value can be calculated as:
Bend Deduction = (Outside setback x 2) – Bend allowance
By following these steps and using the appropriate formulas, you can ensure that your bends are of the correct size and in the right places.
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Grain direction and cracking
Grain direction is one of the most important factors to consider when bending a metal piece plastically. It influences everything from a material's yield strength to the severity of cracking that could occur on the outside surface of a bend. The grain direction can be observed by looking at the surface of the metal through a microscope or a high-magnification magnifying glass.
The grain direction can affect the bend angle, bend radius, bend allowance, and bend deduction. Bending with the grain can increase the likelihood of cracking, lower the required tonnage, change the springback, and sometimes lead to tearing. It is best to keep the inside bend radius at or above the bend minimum, especially if the bend is longitudinal (with) the grain direction. With the bend line perpendicular or diagonal to the grain, the inside bend radius can be slightly smaller.
To alleviate cracking, make the inside bend radius as close to the material thickness as possible. This means making the ratio of the inside bend radius to material thickness as close to a one-to-one relationship as possible. A smaller radius pulls the material tightly around the bend, pulling the grains apart and causing cracks. Using a larger bend radius can help prevent cracking.
Additionally, the outside radius of the bend is more prone to cracking when bending parallel to the grain direction. The smaller the inside radius is when bending parallel to the grain, the greater the chances of severe cracking. It takes more force to bend a piece of material when the bend line runs across the grain. Not all materials have a grain direction; copper has no grain, while mild cold-rolled steel has a pronounced grain.
It is important to note that the grain direction must be considered before any bending process is carried out. As a general rule, the grain line should run perpendicular to the bend to avoid potential cracking or fracture. In cases with multiple bends, causing the grain line to run parallel to the bend, it is crucial to consider the material choice, thickness, and radius size to limit cracking.
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Annealing brass and copper
Brass and copper can be annealed in a similar way. The process involves heating the metal to a certain temperature and then allowing it to cool. This removes the residual strains from cold working.
To anneal brass and copper, you need to heat it to a certain temperature, which is often described as a "cherry red" or "red hot" colour. Some sources suggest that you only need to heat the metal to just below this point. It is important to note that overheating the metal can cause damage, so care should be taken to avoid applying too much heat. Once the metal reaches the desired temperature, it is considered annealed.
The next step is to allow the metal to cool. Some sources suggest quenching the metal in water, while others recommend slow cooling by letting it air cool. Quenching can help to clean the metal by removing oxide scaling, but it may also introduce stresses into the material due to the thermal shock. Therefore, slow cooling is recommended if you are concerned about the potential for stress or if you are working with brass-plated steel, as the plating may come off during quenching.
It is important to note that brass and copper alloys have different annealing temperature ranges. For copper alloys, the soft annealing temperature is between 300°C and 650°C, while for brass alloys, it is between 425°C and 650°C.
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Frequently asked questions
You can use expensive sheet metal bending tools, called brakes, to bend sheet metal. However, you can also use a vise and hammer, or a blowtorch.
First, calculate your bend allowance, then mark a line on the metal where you want to bend it. If you're using a vise, put wooden or metal form blocks in the vise and clamp your sheet of metal between them. If your sheet metal is long, get a friend to stabilize the part that's not clamped. Then, tap the metal along the bend line with a rubber or plastic mallet until it reaches the desired angle.
If you're working with a thick piece of metal, you can apply heat from a blowtorch along the bend line to soften it. However, be cautious as many kinds of fabricated metal have extremely high melting points. Additionally, some metals like brass, copper, and bronze get hard and brittle when shaped, so they are more likely to break unless annealed.
