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SHEET METAL FABRICATION DESIGN GUIDE
The fundamentals of Sheet Metal Design
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Sheet Metal Introduction
Basic Principles
Forming Basics
Laser Cutting
Tolerances
Features
SHEET METAL INTRODUCTION
These basic sheet metal fabrication guidelines include important design considerations
to help improve part manufacturability, enhance cosmetic appearance, and reduce
overall production time.
Basic Principles
Sheet Metal Fabrication is the process of forming parts from a metal sheet by punching,
cutting, stamping, and bending.
3D CAD files are converted into machine code, which controls a machine to precisely cut
and form the sheets into the final part.
Sheet metal parts are known for their durability, which makes them great for end
use applications (e.g. chassis). Parts used for low volume prototypes, and high volume
production runs are most cost-effective due to large initial setup and material costs.
Because parts are formed from a single sheet of metal, designs must maintain a uniform
thickness. Be sure to follow the design requirements and tolerances to ensure parts fall
closer to design intent and cutting sheets of metal
FORMING BASICS
Bending
Bending is a process whereby a force is applied to sheet metal which causes it to bend at
an angle and form the desired shape. Bends can be short or long depending on
what the design requires.
Bending is performed by a press brake machine that can be automatically or manually
loaded. Press brakes are available in a variety of different sizes and lengths (20-200 tons)
depending on the process requirements.
The press brake contains an upper tool called the punch and lower tool called the die
between which the sheet metal is placed.
The sheet is placed between the two and held in place by the backstop. The bend angle is
determined by the depth that the punch forces the sheet into the die. This depth is
precisely controlled to achieve the required bend.
Standard tooling is usually used for the punch and die. Tooling material includes, in order of
increasing strength, hardwood, low carbon steel, tool steel and carbide steel.
Parts to be bent are supplied as flat patterns with bending information. Sometimes bend
positions are etched with bend notches, or these notches can be cut out to show the
benders where to bend.
Once the laser has cut the flat parts out they can be sent for bending. A press brake forms
the flat pattern into a bent part.
Critical Dimensions
The following are some terminology that are used in sheet metal. Designers need to
adhere to machinery guidelines when designing for bending. Bends can be
characterised by these parameters. Some critical dimensions that need to be considered
when setting up sheet metal in CAD software are sheet metal thickness, the k-factor,
and bend radius. One needs to check that these factors are consistent with the tooling
that will be used in manufacturing. This manufacturing guide gives important
guidelines for good design practice.
Bend line– The straight line on the surface of the sheet, on either side of the bend, that
defines he end of the level flange and the start of the bend.
Bend radius – The distance from the bend axis to the inside surface of the material,
between the bend lines.
Bend angle – The angle of the bend, measured between the bent flange and its
original position, or as the included angle between perpendicular lines drawn from the
bend lines.Sometimes specified as the inside bend radius. The outside bend radius is
equal to the inside bend radius plus the sheet thickness.
Neutral axis – The location in the sheet that is neither stretched nor compressed, and
therefore remains at a constant length.
K-factor – The location of the neutral axis in the material, calculated as the ratio of the
distance of the neutral axis T, to the material thickness t. The K-factor is dependent
upon several factors (material, bending operation, bend angle, etc.) and is greater than
0.25, but cannot exceed 0.50. K factor = T/t
Bend allowance – The length of the neutral axis between the bend lines or the arc
length of the bend. The bend allowance added to the flange lengths is equal to the
total flat length.
K-Factor
The K-factor is the ratio between the the neutral axis to the thickness of the material.
Importance of the K-factor in sheet metal design
The K-factor is used to calculate flat patterns because it is related to how much
material is stretched during bending. Therefore it is important to have the value correct
in CAD software. The value of the K-factor should range between 0 – 0,5. To be more
exact the K-factor can be calculated taking the average of 3 samples from bent parts
and plugging the measurements of bend allowance, bend angle, material thickness and
inner radius into the following formula:
Some basic K-factor values are shown here. Use these as a guideline.
K-factor chart
Radius
Soft / Aluminum
Soft / Aluminum
Hard / Stainless Steel
0-t
.33
.38
.40
t. - 3*t
.40
.43
.45
3*t. - >3*t.
.50
.50
.50
0 - t.
.42
.44
.46
t. - 3*t.
.46
.47
.48
3*t. - >3*t.
.50
.50
.50
Air Bending
Bottom Bending
Wall Thickness
Parts need to maintain a uniform wall thickness throughout. Generally capabilities of of
0,9mm – 20mm in thickness are able to be manufactured from sheet (<3mm) or plate
(>3mm) but this tolerance depends mainly on the part.
When considering sheet metal thickness, a single sheet with punches (holes) is a good rule
of thumb. Some features such as countersinks are doable but counter bores and other
machined features are difficult to produce as they require post machining.
BENDING
Bend Radius
Sheet metal bend brakes are used to bend material into the parts desired geometry. Bends
that are in the same plane need to be designed in the same direction to avoid part re
orientation, to save both money and time.
Keeping the bend radius consistent will also make parts more cost-effective. Thick parts
tend to become inaccurate so they should be avoided if possible. Small bends to large.
Consistent Orientation
Inconsistent Orientation
Springback
When bending a piece of sheet metal, the residual stresses in the material will cause
the sheet to springback slightly after the bending operation. Due to this elastic
recovery, it is necessary to over-bend the sheet a precise amount to achieve the desired
bend radius and bend angle. The final bend radius will be greater than initially formed
and the final bend angle will be smaller. The ratio of the final bend angle to the initial
bend angle is defined as the springback factor, KS. The amount of springback depends
upon several factors, including the material, bending operation, and the initial bend
angle and bend radius.
Dimensions:
To prevent parts from fracturing or having distortions, make sure to keep the inside
bend radius at least equal to the material thickness
Bend Angles:
A +/- 1 degree tolerance on all bend angles is generally acceptable in the industry.
Flange length must be at least 4 times the material thickness.
Rule of thumb
It is recommended to use the same radii across all bends, and flange length must be at
least 4 times the material thickness.
Minimumflange Length, b
This is the minimum length of the The bend must be supported all the way until the bend
is complete the flange must be long enough to reach the top of the die after it’s been fully
formed. Brake press operators should know the minimum flange lengths for their tooling
before attempting bends that may not work and while it is possible to calculate the
minimum flange having an Air Bend Force Chart on hand certainly makes it more
convenient.
Material Thickness, t
The thickness of the material is not proportional to the tonnage like the v opening.
Doubling the thickness does not mean doubling the tonnage. Instead the bending force
is related by the square of the thickness. What this means is that if the material
thickness is doubled the tonnage required increases 4 fold.
Work Piece Length, L
Like the v opening the tonnage required is directly related to the length of the work
piece. Doubling the work length means doubling the required tonnage. It should be
noted that when bending short pieces, under 3” in length, the tonnage required may be
less than that which is proportional to its length. Knowing this can prevent damaging a
die.
Air Force Bending Chart
The Air Force Bending chart is a chart showing the tonnage used for bending different
thickness sheet metal. It is useful for sheet metal designers as it specifies the bend
radius and tooling to be used for different thicknesses. It is shown here for mild steel.
Designers can use this as a guide when designing the minimum flange length possible
with the tooling for different V blocks as well as the bend radius. The following charts
are based on the Armada Air Force bend guide.
Bend Relief
When a bend is made close to an edge the material may tear unless bend relief is given.
Bend 1 shows a a tear relief.
Bend 2 shows a rectangular relief cut into the part, the depth of the relief should be greater
than the radius of the bend. The width of the relief should be the material thickness or
greater.
Bend reliefs are utilised where a bend extends on an edge. The relief notch is added to
prevent tearing. Bend reliefs will be no deeper than the material thickness plus the bend
radius.
Bend Height
Bend height Sheetmetal bend height should be at least twice the thickness of the
sheetmetal plus the bend radius
H=2t + r
If the bend height is too small this will result in deformation and low bending quality.
Forming Near Holes
When a bend is made too close to a hole the hole may become deformed. Hole 1 shows a
hole that has become teardrop shaped because of this problem.
To save the cost of punching or drilling in a secondary operation the following formulas can
be used to calculate the minimum distance required:
For a slot or hole < 25mm in diameter the minimum distance to Hole 2 centre:
D = 2t + r
As a rule of thumb the distance from the outside of the material to the bottom of the cutout
should be equal to the minimum flange length as prescribed by the air bend force chart
D = 2,5t + r
When using a punch press, or laser cutting, holes should never be less than that of the
material thickness.
Minimum Distance from extruded hole to
part edge
Extruding metal is one of the most extreme pressure applications in press working and
generates lot of friction and heat. If an extruded hole is too close to the part edge, it can
lead to deformation or tearing of the metal. It is recommended that the minimum distance
between the extruded holes to part edge should be at least three times the thickness of
sheet.
Minimum Distance Between Extruded Holes
Certain distance should be maintained between two extruded holes in sheet metal
designs. If extruded holes are too close it can lead to metal deformation. It is
recommended that the minimum distance between two extruded holes should be six
times the thickness of sheet metal.
Minimum Hole Diameter
The diameter of the hole in sheet metal part should not be very small, small holes are
created by piercing operation and for manufacture small holes, small sizes punches are
required. Small hole size in sheet metal requires smaller size punching tool which may
leads to break during the operation. It is recommended that the diameter of the hole should
be equal or more than the thickness of the sheet metal.
LASER CUTTING
Laser cutting is a type of production that uses a laser to cut different metals. The laser
has a high energy beam which easily burns through the material. Laser cutting can be
used on materials such as metal, aluminium, plastic, wood, rubber, etc. Lasers use
computer numerically controlled programming (CNC) to determine the shape and
position ls of the cutouts. Material thicknesses of up to 20mm can be lasercut. There
are advantages and disadvantages in using lasercutting. CO2 lasers are more
traditional, and can cut thicker materials but do not deliver such an accurate cut as fibre
lasers. Fibre lasers can generally cut thinner materials and have much higher cutting
speeds than CO2 .
Advantages and Disadvantages
Advantages of lasercutting over cutting mechanically include better workholding,
reduced workpiece contamination, better precision and reduced chance of warping as
the heat affected zone is small. Some disadvantages are that lasercutting does not
always cut well with some materials (for example not all aluminium) and it is not
always consistent. Despite the disadvantages lasercutting is highly efficient and cost
effective.
Tolerances
General Tolerances
If a drawing or specification sheet has not been provided by the customer, we will
manufacture the product from the model to the specifications listed here. Sharp edges
will be broken and deburred by default. Critical edges that must be left sharp should be
noted and specified on a print.
Tolerances
Forming and Bending:
+/- 0,4 mm
Bend to hole or feature:
+/- 0,2 mm
Linear dimensions excluding locations to bend
+/- 0,1 mm
Diameters with inserts
+/- 0,06 mm
Angularity
+/- 2 degrees
Surface roughness
+/- 3,2 micrometers
Material Restrictions
Materials that are not suitable for lasercutting include mirrored or reflective materials,
Masonite boards, composites containing PVC.
Acceptable Materials
Generally the following materials are suitable for lasercutting: metal, stainless steel,
some thicknesses of aluminium, wood and some plastics.
Localized hardening
Localised hardening takes place on the edges where the where the laser has cut. This
hardening produces a durable and smooth edge without the need for finishing after
using the laser cutter
Distortion
A heat-affected zone (HAZ) is produced during laser cutting . In carbon steel, the higher
the hardenability, the greater the HAZ. Distortion from laser processing is a result of
the sudden rise in temperature of the material near the cutting zone. Distortion is also
created by the rapid solidification of the cutting zone. In addition, distortion also can be
attributed to the rapid solidification of material remaining on the sides of the cut.
Kerf
During laser cutting a portion of the material is burnt away when the laser cuts through,
leaving a small gap. This ‘gap’ is known as the laser kerf and ranges from 0.08 – 0.45mm
depending on the material type, thickness and other conditional factors. A minimum
distance of 1-2mm between parts needs to be left to avoid accidental crossover cutting.
It is also advised to keep parts 2-5mm away from the edge of the material due to some
sheets being warped or slightly off in their sizing. One should always cut parts in the
boundary of the sheet size and not use the sheet edges as a border.
TOLERANCES
Wall Thickness
Because Sheet Metal parts are manufactured from a single sheet of metal the part
must maintain a uniform wall thickness. Sheet metal parts with a minimum of 0.9mm to
20mm in thickness can be manufactured.
Hole Diameter
When designing parts for laser cutting one should not make holes smaller than the
thickness of the material.
Bends
Bends in sheet metal are manufactured using sheet metal brakes. A +/- 1 degree
tolerance on all bend angles. Other standard bend radii available, some of which will
add additional cost to your part, include:
0.9mm – 1.2mm
1.8mm – 2.4mm
3.8mm – 5.0mm
7.5mm – 10mm
15mm – 20mm
Curls
Curl Feature Guidelines
Curling sheet metal is the process of adding a hollow, circular roll to the edge of the sheet.
The curled edge
provides strength to the edge and makes it safe for handling. Curls are most often used to
remove a sharp
untreated edge and make it safe for handling. It is recommended that: The outside radius of
a curl should not be smaller than 2 times the material thickness.
A size of the 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
Countersink Holes
Machined and formed countersinks are possible after lasercutting. Machined counter sinks
are created with a drill press while formed counter sinks are created with punch press
tooling. Countersink depths should me no more than 0,6mm the material thickness.
Countersink Tolerance
Countersink Tolerances:
Both machined and formed countersinks are available-conical holes cut into a
manufactured object allowing a screw, nail, or bolt to be inserted flush with the surface.
We recommend the major diameters of countersinks measure between 2.3mm and
12.7mm using one of the following standard angles: 82°, 90°, 100°, and 120°. Tolerance
for formed countersink major diameter is
+/- 0.254mm.
Countersink Tolerances
Machined countersink major diameter
+/- 0,254 mm
Machined countersink minor diameter
2/3 thickness
Formed countersink major diameter
+/- 0,381 mm
Formed countersink minor diameter
+/- 0,381 mm
Countersink Dimensions
The distance between countersink centres should be kept to 8 times the thickness of the
material
The distance between the bend line and countersink centre should be kept to a minimum of
3 times the material thickness and 4 times the material thickness from an edge.
Hems: The Principle of Hemming
Hems are folds at the end of a part to create a rounded edge.
There are various methods for producing sheet metal flattening The hemming process is
usually done in two steps: acute-angled is bend hemming of the envelope. For the
hemming process a high compaction pressure is required. The process develops a large
axial force. This force affects the material longitudinally of the machine.
Hem Feature Guidelines
Open and closed hems can be formed as required. The tolerance of a hem is dependent
upon the hem’s radius, material thickness and features near the hem. It is recommended the
minimum inside diameter equals the material thickness and the hem return length is 4
times the thickness. Closed hems are folds at the end of a part to create a rounded edge.
The tolerance of a hem is dependent upon the hem’s radius, material thickness, and
features near the hem. It is recommend that the minimum inside diameter equals the
material thickness, and the hem return length is 6 times material thickness.
Hemming is nothing but to fold the metal back on itself. In Sheet Metal hems are used to
create folds in sheet metal in order to stiffen edges and create an edge safe to touch. Hems
are most often used to remove a sharp untreated edge and make it safe for handling. Hems
are commonly used to hide imperfections and provide a generally safer edge to handle. A
combination of two hems can create strong, tight joints with little or minimal fastening.
Hems can even be used to strategically double the thickness of metal in areas of a part
which may require extra support. It is recommended that:
For tear drop hems, the inside diameter should be equal to the material thickness.
For open hem the bend will lose its roundness when the inside diameter is greater than the
sheet metal thickness.
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: Dimensions
Keep hole and slot diameters at least as large as material thickness. Higher strength
materials require larger diameters.
Clearances
Holes and slots may become deformed when placed near a bend. The minimum
distance they should be placed from a bend depends on the material thickness, the
bend radius, and their diameter. Be sure to place holes away from bends at a distance
of at least 2.5 times the material’s thickness plus the bend radius. Slots should be
placed 4 times the material’s thickness plus the bend radius away from the bend. Be
sure to place holes and slots at least 2 times the material’s thickness away from an
edge to avoid a “bulging” effect. Holes should be placed at least 6 times the material’s
thickness apart.
Notches & Tabs Feature Notches
Notches must be at least the material’s thickness, whichever is greater, and can be no
longer than 5 times its width. Tabs must be at least twice times the material’s thickness or
3.2mm, whichever is greater, and can be no longer than 5 times its width.
Bend notches
Notching is a shearing operation that removes a section from the outer edge of the
metal strip or part. In case, distance between the notches to bend is very small then
distortion of sheet metal may take place. To avoid such condition notch should be
placed at appropriate distance from bend with respect to sheet thickness. Notching is a
low-cost process, particularly for its low tooling costs with a small range of standard
punches.
Clearances
Notches must be at least 3.175mm away from each other. For bends, notches must be
at least 3 times the material’s thickness plus the bend radius. Tabs must have a
minimum distance from each other of 1mm or the material’s thickness, whichever is
greater.
Recommendations for Notch Feature:
Notch width should not be narrower than 1.5 * t.
Length of notches can be up to 5 * t. Recommended corner radius for notches should be
0.5 * t.
Notches must be at least the material’s thickness or 0.04”, whichever is greater, and can
be no longer than 5 times its width. Tabs must be at least 2 times the material’s
thickness or 0.126”, whichever is greater, and can be no longer than 5 times its width.
FEATURES
Corner Fillets
Filleting or rounding the corners of sheet metal is done in order to provide a smooth finish.
Fillets remove sharp
corners making them easier to handle and preventing cuts and scratches.
A fillet is usually designed to be ½ the material’s thickness and filleting makes parts more
cost-effective.
Relief Cuts
Relief cuts help parts fall closer to design intent to avoid “overhangs” and tearing at
bends. Overhangs become more prominent for thicker parts with a smaller bend radius,
and may even be as large as ½ the material’s thickness. Tearing may occur when bends
are made close to an edge.
Dimensions
Relief cuts for bends must be at least one material’s thickness in width, and must be
longer than the bend radius.
FAQ
How Important is Material Thickness in Sheet Metal Design?
Maintaining a uniform wall thickness is crucial in sheet metal design. The capabilities
typically range from 0.9mm to 20mm in thickness. The guide emphasizes the
importance of considering punches (holes) and other features like countersinks when
determining thickness, as some features may require post-machining.
What are the Key Considerations for Bending in Sheet Metal Design?
Bending is a critical process in sheet metal fabrication. The guide details the
importance of factors like bend radius, bend angles, and springback. It recommends
keeping the inside bend radius at least equal to the material thickness and maintaining
a +/- 1-degree tolerance on all bend angles. The guide also discusses the significance
of consistent orientation and minimum flange length in bending.
Can You Explain the Role of the K-Factor in Sheet Metal Design?
The K-factor is vital in calculating flat patterns in sheet metal design. It relates to the
material stretch during bending. The guide provides a range for the K-factor (0 – 0.5)
and offers a chart with basic K-factor values for different materials and bending
methods.
What Are the Guidelines for Laser Cutting in Sheet Metal Design?
Laser cutting is a precise method used in sheet metal fabrication. The guide outlines
the advantages, such as high precision and reduced chance of warping, and the
limitations, like inconsistency with certain materials. It also discusses aspects like kerf,
material restrictions, and the heat-affected zone.
How Do Tolerances Impact Sheet Metal Design?
Tolerances are critical for ensuring the precision of sheet metal parts. The guide
provides general tolerances for various aspects of sheet metal fabrication, including
forming, bending, and linear dimensions. It emphasizes the need for accuracy to meet
design specifications and functional requirements.
What Should Be Considered When Designing Features Like Holes and Slots?
The guide advises that hole and slot diameters should be at least as large as the
material thickness. Placement near bends should be carefully considered to avoid
deformation, with specific minimum distances recommended based on material
thickness and bend radius.
How Are Notches and Tabs Used in Sheet Metal Design?
Notches and tabs are essential for joining and aligning parts in sheet metal design.
The guide provides recommendations for their dimensions, placement, and clearances
to ensure proper function and avoid material distortion.
What Are the Best Practices for Hemming in Sheet Metal Design?
Hemming involves folding the metal back on itself to stiffen edges and create safe
handling. The guide suggests minimum dimensions for open and closed hems and
emphasizes the importance of considering the hem’s radius and material thickness in
the design.
How Do You Ensure Safety and Quality in Sheet Metal Design?
Ensuring safety and quality involves several design considerations, such as using
corner fillets to remove sharp edges, applying relief cuts to avoid overhangs and
tearing, and adhering to recommended dimensions and tolerances. The guide provides
detailed guidelines to help designers create safe, high-quality sheet metal parts.
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