Unit 17: Computer aided manufacturing (CAM)

LO4: Be able to produce components using additive manufacturing techniques

Preparation and production quality using 3D printing

Instructions and answers for tutors

These instructions should accompany the OCR learner resource activity which supports Cambridge Technicals in Engineering Level 3.

Activity 1

Introduce learners to the need to export files from CAD to CAM and that Additive Manufacturing methods require more parameters than CNC. Highlight the need for a format to describe the component in geometrical values (STL) and that some Additive methods need colour information as well (VRML/3XD formats). Show that both these formats are inadequate to carry full information (e.g. texture and material type) and that the industry is now developing 3MF as a future proof format.

In this task, learners will work with STL format as this is currently available to export from all 3D CAD software. Learners will explore how the size of the triangles in an STL mesh can be altered before exporting to give the optimum resolution of the part. They will find that the density of the STL mesh affects the size of the file exported; the finer the mesh the larger the file (Binary STL files are always smaller than ASCII).

This task could be carried out in pairs to stimulate discussion. Assist the learners to select suitable 3D CAD models for 3D printing based on the machine(s) you have available and avoid models that will need support material. The table would be expected to show larger file sizes for finer STL settings. Discuss with learners the need for different settings according to whether the output is for a prototype or a final product. Learners can use CAD software (e.g. solid edge to display a faceted model preview; alternatively they could use Meshmixer or MeshLab in wire frame view or netfabb Studio in repair mode.

Notes

Most CAD programs have a choice of STL settings or an optimal output for a given 3D model as shown in:

http://www.redeyeondemand.com/stl-file-format/ or http://3dprintingsystems.com/export-to-stl/
(note that the Pro Engineer (Pro/E) settings apply also to PTC Creo).

Advice for Solidworks includes the following:

·  Move the Deviation to adjust the deviation (chordal) Tolerance, which controls whole-part tessellation. Lower numbers generate files with greater whole-part accuracy.

·  Move the Angle to adjust the angle Tolerance, which controls smaller detail tessellation. Lower numbers generate files with greater small-detail accuracy, but those files take longer to generate.

Activity 2

This task assumes you have a 3D printer that uses FDM.

Learners will need access to the 3D printer software or could initially use Repetier Host (which is free of cost) for evaluating the settings. The obvious outcomes are that denser fill settings will use more material and take longer to make, but discussion could also include the value of the component and the type of finish needed for its function. Less dense structures may be useful to reduce the component mass. STL files with finer meshes will allow better finish but this may be overridden by the layer thickness setting. Learners may be surprised by the amount of time needed to produce the components.

Activity 3

Possible responses:

Feature / Additive / Subtractive
Material choice / Limited but improving
(note consumer/industrial differences) / Wide choice
Ease/difficulty of use / Easy to set up / Needs experience
Time required / Depends on layer thickness but can be very slow / Depends on surface area to be machined
Quality of finish / Layered needs treatment after manufacture / High quality
Accuracy / Depends on nozzle size and temperature / Highly accurate
Complexity of part / No limit on complexity / Limited for 3 axis machines (no overhangs)
Design changes / Easy to switch / Needs setting up
Cost / Plastics inexpensive / Depends on material
Waste / Very little (supports only) / Large amount and can be difficult to recycle

Unit 17: Computer aided manufacturing (CAM)

LO4: Be able to produce components using additive manufacturing techniques

Preparation and production quality using 3D printing

Learner activity sheet

Activity 1

STL format is a surface model of the part made up of a triangular mesh. The size of the triangles can be altered before exporting to give the optimal resolution of the part. Most CAD programs have a choice of STL settings or an optimal output for a given 3D model as shown in:

http://www.redeyeondemand.com/stl-file-format/ or

http://3dprintingsystems.com/export-to-stl/

(note that the Pro Engineer (Pro/E) settings apply also to PTC Creo)

Advice for Solidworks includes the following:

·  Move the Deviation slider to adjust the deviation Tolerance, which controls whole-part tessellation. Lower numbers generate files with greater whole-part accuracy.

·  Move the Angle slider to adjust the angle Tolerance, which controls smaller detail tessellation. Lower numbers generate files with greater small-detail accuracy, but those files take longer to generate.

Select suitable 3D CAD models that you will use to 3D print and use the “export STL function” in the CAD software to explore the settings available and the resulting file sizes. Save the files and right click to examine Properties to find the file size. Display the file using software such as Meshmixer (in wire frame view) to show the mesh.

CAD model / STL settings
(Deviation, Angle) / Output file size (binary) / Output file size (ASCII) / Comments
[coarse]
[medium]
[fine]

Activity 2:

The software used to prepare your STL files for printing will have a number of settings for control of layer size (resolution), fill density and fill type. Changes in support material are omitted for now. This task is about exploring the settings to see their effect on (1) time taken for printing, (2) amount of material required and (3) the surface finish obtained. It is suggested that you work in small groups and divide the changes in settings between you. Record your findings on a chart similar to that below.

You will be able to ascertain how much material is needed (this may be in print preview).

If you use Repetier Host software or similar you will be able to examine the layers after slicing to see the internal structure of the model to be printed, the estimated printing time and the amount of material required.

STL File / Settings / Time needed / Material amount / Finish quality / Comments
Layer size
Fill type
Fill density
Etc.

Finally you will want to print the component in order to see the real finish, although this may take some time and you may have to return to this task later.

Activity 3

Having completed these Activities and those from Learning Outcome 2 you are in a position to compare Subtractive and Additive Manufacturing processes. Discuss this in pairs and complete the table below to summarise the differences.

Feature / Additive / Subtractive
Material choice
Ease/difficulty of use
Time required
Quality of finish
Accuracy
Complexity of part
Design changes
Cost
Waste

Version 1