Introduction to Additive Manufacturing

Question 1

What are are additive, subtractive and formative manufacturing?

Answer 1

Additive: Materials manipulated such that successsive pieces combine gradually to grow the part (layer upon layer)

Subtractive: Material removing from a solid block until desired shape is reached (milling for example)

Formative: Mechanical forces applied to material to form it into desired shape (bending, moulding/casting)

Question 2

What are the generic steps of AM process?

Answer 2

1. CAD
2. STL convert
3. File transfer to machine
4. Machine setup
5. Build
6. Remove
7. Post-process
8. Application

Question 3

In CAD what is a wireframe, a surface and a solid?

Answer 3

Wireframe: set of vertices and edges (doesnt describe object interior)

Surface: set of vertices,edges and faces (doesnt describe object interior)

Solid: Set of vertices, edges, faces and volumes (fully describes object)

Question 4

From a CAD wireframe, a surface and a solid, what is the best for AM and why?

Answer 4

Solid is best as contains data that fully describes object.

Surface geometries can be used but only if surface is ‘watertight’, fully describes surface and solid volume can be deduced

Wireframe cannot be used

Question 5

What is an STL file?

Answer 5

Mesh of tessellated traingles applied to the surface of a CAD model, determines accuracy of part.

Contains the (x,y,z) position of each vertex with a vector description of surface normal (pointing away from solid)

Format allows geomerty to be sliced to create process controll data for layers.

Question 6

What are the advantages and disadvantages of AM?

Answer 6

Adv:

• Design freedom (complex geometries)
• Integrated assembly: combines assemblies into single parts, reduces overall prodcut cost
• No tooling cost: suitable for prototyping and low volume production

Disadvantage:

• Resolution (surface finish)
• Choice of materials is limited

Question 7

What are two limitations of STL files?

Answer 7

1. Hard to edit: they contain an approximation of the original 3D surface and not the surface itself so its easier to reconstruct an STL file from scratch than modify an existing one
2. Limited Resolution: User needs to increase number of triangular planes that approximate a surface. Even with highest resolution, STL format not as low fidelity as parametric surface models.