# Design For Manufacturing and Assembly(DFMA) principles of Sheet Metal Work

by Dasa . 15 Dec 2020

## Purpose:

DFMA includes Design For Manufacturing (DFM) & Design For Assembly (DFA). DFMA ensures that the Design of products(components & assemblies) is easy to manufacture, easy to assemble, and highly functional at a lower cost.

## Scope:

DFMA intends to challenge the Design and achieve the above-said purpose.

## When:

During concept generation, we can apply DFMA. And DFMA can be continuously involved in the preliminary design stage and detailed design stage. Therefore, it is advisable to start DFMA well before tooling has begun.

## Who:

All stakeholders like engineers, manufacturers, and materials to be involved in the process.

## Principles:

Every industry has its guidelines for DFMA, and in recent days, many associations provide guidelines for various products. We have covered a few of the basic procedures in this article.

DFMA can be applied in various processes as listed below

Manufacturing
Design
Material
Environment


## Manufacturing:

Below are things to be considered while choosing a manufacturing process,

a. Low capital (Cost of tooling & machining)
b. Low-cost machining operations
c. Understand the existing machinery and available tools on the shop floor.


Compared to the above common points, below is the list of topics to be considered related to sheet metal work.

# Estimate the cost of **Individual dies** for **profile-shearing**.
# Compare the price between cut-off dies, part-off dies, and blanking dies.
# Compare the percentage of scrap material between cut-off dies, part-off dies, and blanking dies.
# Compare the cost of Individual dies & Progressive dies for piercing, forming, and shearing operations.
# Estimate the cost of Individual dies for bending operations.
# Estimate the pressing force and selection of Mechanical press.
# Compare the cycle time and processing cost between Individual dies and Progressive dies.
# Study the requirement of Laser cutting.
# Selection of Press Brake and tools for various part lengths. And estimate the cost of press brake operation.
# Study of types of equipment involved in the assembly of components.
# Material Handling.


## Design:

Below are things to be considered in the Design of components and assemblies.

### Part level:

a. Use GD&T and choose the correct tolerances, surface finish, and materials.
b. Design for loose tolerances, which gives low-cost tooling, and fewer rejections.
c. Eliminate screw & fasteners wherever possible.
d. Try to use unique thickness, hardware, and components from standard parts.
e. Minimize part count by combining two are more components wherever possible.
f. Avoid finishing operations, where ever possible.


### Assembly level:

a. Self orienting/locating parts, better-joining methods, and better assembly sequence make the assembly easier and faster.
b. Create modular assemblies, and they are used to create various configurations.
c. Choose better-joining methods like self-lock, screws, self-tapping screws, fasteners, weld nuts, and adhesives.
d. Consider symmetric parts/sub-assemblies, which reduces errors in the orientation of parts.
e. Verify the interference of components and assemblies.


Compared to the above common points, below is the list of topics to be considered related to sheet metal work.

Part Width: Parts having parallel edges on the outer profile can be designed to define the width of the raw material sheet.

Holes & Cuts: Small holes and cuts should be avoided, which requires fragile punches.

Corner Radius: The corner radius should be greater than or equal to twice sheet metal thickness.

Distance between features: The distance between any two feature edges should be equal to or more than twice sheet metal thickness.

Hole sizes: The hole diameter should be equal to or greater than the sheet metal thickness. (To avoid higher punch load & increase the life of punch and sheet metal)

Hole location: The distance between a bend line/sheet edge to any of the hole edges should be a minimum of two times the sheet metal thickness. (To avoid deformation of holes in bending)

Slots: Slots parallel to a bend line should have a distance of 4 times the sheet metal thickness between the edge of the slot and bend line.

Bend radius: Minimum bend radius should be equal to the sheet metal thickness.

Flange width: Minimum flange width should be equal to four times the sheet metal thickness.

Bend relief: The length of the bend relief should be equal to or greater than the inner radius of the bend. On the other hand, the width of the bend relief should be equal to or greater than the sheet metal thickness. (To avoid torn)

Corner relief: Apply corner relief, where ever required. The relief dimension should be greater than or equal to twice the sheet metal thickness.

Grain: Lugs and tabs formed parallel to sheet metal grain are likely to get cracks.

Chamfer: Chamfer on corners reduces the spring-back effect.

Bend: Bend on edges reduces the metal tearing & bend-on bend, reduces the spring-back impact.

Collar/Hole Flanges: Collar around a pierced hole increase the stiffness. The flange height should be two to three times the sheet metal thickness, ensuring the permissible Elasticity of the material.

Coining: Coining around a flared hole increases the strength.

Sharp bends: Avoid sharp bends & corners, which can easily tear.

Forming & Lancing: The length of forming/lancing should be at least four times the height, ensuring the maximum tensile strain that the material can withstand.

Ribs: The inside radius of the ribs should be a minimum of 2 times the sheet metal thickness, ensuring the permissible Elasticity of the material is limited to 20%.

Tolerances: Feature tolerances should be given based on the size of the part. Bigger parts should have higher tolerances.

## Material:

Below are the essential material properties to be considered in DFMA

a. Mechanical
b. Thermal


Compared to the above common points, below is the list of topics to be considered related to sheet metal work.

### Part level:

a. Cost
b. Scrap value
c. Elasticity
d. Maximum tensile strain
e. Surface finish & Finish coating
f. Thermal insulation


Low carbon steel is widely used in sheet metal works because it has a high maximum tensile strain and maximum Elasticity at a low cost compared to any other material.

### Assembly level:

• Same/different types of materials joining together in an assembly and evaluate their effect.

## Environment:

Ensure the Product is suitable for the environment. Below are the essential environmental factors which affect the functionality of a product.

a. Corrosion,
b. Humidity,
c. Snow,
d. High wind,
e. Rain


## Keywords:

Product: It refers to components and assemblies.

Concept Generation: A mechanical engineering design phase, which deals with the product requirement, the solution to the requirement, macro-level information about the mechanism and materials involved in the solution.

Preliminary Design: It is a bridge between concept generation and detailed Design, which deals with concept evaluation, schematic diagrams, layouts, architectural diagrams, and structural diagrams.

Detailed Design: A mechanical engineering design phase deals with the engineering drawing for components, assembly, and products.

Material Handling: It is the movement, storage, protection, and control of the material.

Finishing operations: It includes material appearance, surface finish, removal of left out materials, and protection from rust/corrosion.

Sub-assemblies: The Product or the top-level assembly is divided into many small assemblies, and each small assembly is called a sub-assembly.

Modular assemblies: Sub-assembly of a product used multiple times or used to create various product configurations.

Bend relief: It is an incision cut along the sides of a planned bend.

Corner relief: It is a cut on the corners to prevent the sheet metal from unwanted deformation.

Grain Structure: Rolling direction of metal being manufactured into a sheet.

Collar: A small project around a piercing hole in sheet metal.

Coining: Sheet metal bending operation in which, with high pressure, the punch crosses the neutral axis of the sheet metal thickness. In sheet metal coining, the bend radius is always equal to the punch radius.

Finish coatings: It includes painting, corrosion coating, and rust coating.

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