DFMPro for Sheet Metal Design Guidelines

DFMPro provides pre-configured design guidelines DFM rules for design which helps to develop better quality parts at reduced cost. Some of the common sheet metal design guidelines included are minimum distance between holes, cutouts, slots, distance between holes, cutouts, slots to part edge and bends, multiple bends in same direction, minimum bend radius, minimum radius of rolled hem, open hem and tear drop hem, minimum flange width, minimum size of slots, holes and many other design guidelines.

DFMPro sheet metal design guidelines are highly  customizable and provides the flexibility to configure the available sheet metal design rules as per organization specific design rules. It also provides the ability to add new design for manufacturing rules easily into its existing database

Sheet Metal Design Guidelines

The section below contains examples of some design guidelines for sheet metal. These guidelines may help companies to avoid rejections and rework due to engineering errors leading to higher cost of quality and delay in the delivery to customer.

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.

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.

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.

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.

Embosses are small, shallow formed projections on the surface of stamped parts. During this operation, stretching is the main deformation mode resulting in high tension. Thereby the metal is subject to excessive thinning or fracturing. Consequently, the depth of the embossed feature is restricted by the material’s thickness and ability to stretch in addition to the emboss geometry.It is recommended that the maximum depth of embossment be less than or equal to three times material thickness.

Minimum bend radii requirements can vary depending on applications and material. For aerospace and space applications, the values may be higher. When the radius is less than recommended, this can cause material flow problems in soft material and fracturing in hard material. Localized necking or fracture may also occur in such cases. It is recommended that minimum inner bend radius should be at least 1 times material thickness.

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

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.

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. DFMPro sheet metal design 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

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.