best practices for sheet metal part design

e-mug Technologies Pvt. Ltd, Aerospace Division - DEDC for B/E Aerospace Inc.,

BEST PRACTICES FOR SHEET-METAL PART DESIGN

Prepared By: Mahammad rafi Bagawan


Material selection

When selecting material for your design, try to choose the most efficient and common type and thickness of material.

If possible, try to select similar material being used in other areas of project. Thicknesses to .188 and above usually are considered plate. Is it Structural or Non-Structural? This determines what material should be used. Customer requirements. Analyze function of the part. The part function may effect

the: material choice, surface finish.

Here are the most common types used in SPG:

Aluminum sheet: 6061 T6 Stainless Steel sheet: 301 , or Full hardSteel sheet: 4130 N


Design of sheet metal Internal Features:

Cutting out the features of a sheet-metal part can be generated several ways besides with a CNC punch press. Laser or Plasma cutters are often used.

When a design can be improved by adding a shape that would be best made by a laser cutter, it should be considered. Keeping designs as simple as possible will help keep costs to a minimum.

Thru Holes: Per FAA requirements the edge of

a hole/slot described above can not be closer than two times its diameter, plus 0.030 from hole center to an edge. On rare occasions this is not feasible. In this case testing will

approve design.


The diameter of hole needs to be 1.5 times thickness of the material or larger to be punched and it is 1.0 times material thickness for laser cut parts.

If a smaller hole is needed, an additional process to drill hole will be

required, avoid this when ever possible.

Material = .134 Thick.134 x 1.5 = .201

.201 is the minimum diameter we can punch.So, these .125 diameter holes would need to be

drilled.


.259.257 - .2611/4

.194.192 - .1963/16

.162.160 - .1645/32

.132.129 - .1331/8

.097.097 - .1003/32

Common Punch SizeHole Diameter RangeRivet Diameter

Holes for Rivets: Rivet holes need to be 2 times there diameter away from edges or other features with the exception of nut plates.

Model Rivet Holes to the Following Common Punch Sizes:

Tapped Holes: The use of tapped threads in sheet-metal parts is discouraged and would need engineering manager approval. The most common solution is the use of riveted on Nut Plates to get enough thread engagement.


Slots: The use of slots can be beneficial in some circumstances.

Keep width to a standard thru hole size, length is not as critical as several increments can be made by a punch to achieve required length.

.446 X .840.427 X .820.384 X 1.354.370 X .840

.321 X 1.024.321 X .962.250 X 1.000.281 X 1.531

.218 X .500.213 X .500.202 X .390.200 X .320

.188 X .500.156 X .500.131 X .171.130 X .320

Slots:


SQUARES .250 .270 .375 .500 .625 .750 1.00 2.00

1.00 sq with .250 corner radii

.500 sq with .125 corner radii

1.49 X 2.640.500 X 1.600.250 X 2.000

.250 X 1.00.125 X 1.00.125 X.250

.093 X .250.070 X .250.062 X .500

Standard Winston Manufacturing Punch Sizes

RECTANGLES:


Lightening Features: Keep a .25 distance away from other features and edges. A lesser amount of larger cut-outs is usually better that a greater quantity of smaller ones.

Be aware of the integrity of the part when placing them.


Countersunk Holes:

1. Use Standard Counter Sunk sizes. 2. Verify the countersink angle matches the angle of the fastener.3. Primary hole may enlarge due to thin material and countersink angle.4. Model & Dimension what is expected. 5. Counter Sunk: 60, 82, 90, 100 and 120 Degrees

Sheet-metal part

Machined part may need to match the countersink of the part above. Model what is needed/expected.


Pockets & Profiles:

Internal Features: Keep internal features, holes, pockets and cut-outs a minimum of 1.5 times material thickness from each other and part edges.

Note: This only applies if it is NOT a Structural Fastener Feature!!

Similar Features: When designing keep features similar wherever possible. Meaning try to keep holes, radii and slots consistent if possible. This will save tool change operations.


1.562-1.5721.5621.625-1.6411.6251.500

1.438-1.4481.4381.500-1.5161.5001.375

1.281-1.2911.2811.344-1.3601.3441.250

1.156-1.1641.1561.219-1.2351.2181.125

1.031-1.0391.0311.094-1.1051.0941.000

.906-.9140.906.938-.9500.9380.875

.781-.7890.781.812-.8240.8130.750

.656-.6630.656.688-.6980.6880.625

.531-.5380.531.562-.5720.5630.500

.453-.4600.453.469-.4790.4690.437

.391-.3970.391.406-.4160.4060.375

.328-.3340.328.344-.3540.3440.321

.266-.2720.270.281-.2900.2810.250

.206-.2110.210.221-.2280.221#10 (.190)

.180-.1850.183.196-.2030.203#8 (.164)

.154-.1590.156.170-.1770.173#6 (.138)

.141-.1460.144.156-.1630.162#5 (.125)

.120-.1240.125.128-.1350.132#4 (.112)

.106-.1100.109.116-.1220.116#3 (.099)

.094-.0980.097.102-.1080.105#2 (.086)

.081-.0850.083.089-.0950.093#1 (.073)

.067-.0710.070.078-.0820.078#0 (.060)Close Fit RangeModel Hole To:Free Fit RangeModel Hole To:Fastener Size

AVAILABLE B/E PUNCHES ROUNDS


Forming Sheet-metal

Air Bending is the most common forming operation. Air Bending is defined as: Forming process to bend metal with a Brake

press. The process is done with the punch touching the work piece and the work piece not bottoming in the lower cavity, or bottom die.


Standard Acceptable Bend Radii:

Once material and thickness has been selected reference Bend Radii chart to see minimum bend radius for your choice.

You can select a larger common radius, but not a smaller one unless manufacturing approves.

Once bend radius has been selected use this radius for all bends in part unless absolutely necessary.

Additional radii will require additional set-ups which will increase cost of the part.

Select radii from chart. Do not deviate from this unless management and/or manufacturing approval is obtained. Keep radii to standard sizes


Acceptable Bend Radii


Stepped radii:Stepped radii can be done for radii over 0.75 but are very costly to produce in volume. See Figure below for an illustration of stepped radii in a part.

Avoid radii over .75 if possible. Bends like the one above are made by Stepping (slightly bending part at several locations to achieve required radii) or custom tool is needed. This process is time consuming or expensive but may be necessary.


Internal features can be placed up to tangent line of bend radius with minimal deformation.

If this is acceptable note on drawing that deformation is acceptable in this area.

If a fastener is going to be installed thru hole close to the bend consider the head of fastener, washer or bolt will not go into the radius.

Features may be designed up to tangent line but consider if fasteners are going to be put into them. Minimal bulging may be present.


Minimum Flange Length:

There are limits to how small a flange can be without additional operations.

If it needs to be smaller than noted in table it will be formed longer and require an additional operation to cut it down to size.

Refer to Minimum flange length charts for your material. Flange dimensions are to outside. In this case .30.


Minimum Flange Lengths


Flange edge & Bend line are not parallel.

Flange edge & Bend line are not parallel.

Whenever possible keep bends parallel with flange edge.

That makes forming easier since they form off of the brake press back stop; if not parallel a scribe line or fixture is needed adding cost.


Chamfer on flanges should be designed as shown. See Figure below If they go closer to bend the will bulge out because of

tooling restraints. If bulging is acceptable, note on drawing.

Chamfer Rule

Bringing a chamfer below the 2R+T rule will cause this area to bulge out.

SLIGHT BULGING ACCEPTABLE IN

THIS AREA

2R + T

Refer to Bend Radii charts for correct bend radius for your material selection.


Boxed corners: If two flanges, 90 apart from each other are needed then you may need a bend relief.

The main function of a bend relief is to reduce cracking in corners. Make relief to common size punch. Dimension as a reference

dimension in case manufacturing wants to change size.

Boxed corners

Select bend relief radii large enough to cross all bend lines and keep to common size punch diameter.


Rules of Thumb

Keep holes and pockets a minimum 1.5 X Material thickness away from edges. Reference bend radii charts to select correct radii for material type. Reference chart for minimum flange lengths. Hole/pocket can be made up to tangent line of bend radii but consider fastener

head, washer or minimum deformation. Punch / Laser diameter needs to be greater than 1.5 X part thickness to punch /

laser. If less than this, the hole will need to be drilled. Avoid tapped holes in sheet-metal. Critical dimensions needed for profile, feature location, and bend radii. Keep bends parallel to edges of part. Model features with standard tooling, i.e. holes, slots, and cutouts. When designing countersunk holes, model/dimension what is expected.


Sheet metal Manufacturing Process followed in SPG-WS


Step 1: Raw material to overall size

CNC Turret head

Step 2: Fix the raw material in CNC Punch press

CNC punch die arrangement


Step 3: They will take out flat pattern


Step 4: They will remove all sharp corners and burrs from flat pattern in deburring station.

Burrs

DEBURRING STATION


CNC punch press

Back stoppers

Air Bending Brake Press Dies

Step 5: Air Bending CNC press machine is used for Forming Operations.


Operation being done Different Kinds of Brake Press Dies

Measuring flange length and angle value

best practices for sheet metal part design

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