Unit 17: Computer aided manufacturing (CAM)
LO3: Be able to set-up and operate a CNC machine to
produce components
Preparation and production quality using CNC
Instructions and answers for tutors
These instructions should accompany the OCR learner resource activity which supports Cambridge
Technicals in Engineering Level 3.
These tasks are about the 3D model and the quality of the machined component. The tasks are similar but not
identical to the Learning Outcome 4 activity where the quality of 3D printing is considered. For additive manufacture
the quality is an interaction between the output from CAD and the settings of the 3D printer. For subtractive
manufacture the surface quality and accuracy is an interaction between the output from CAD and the choice of tool
and path types.
These activities offer
These activities offer
opportunities for
opportunities for maths
English skills
skills development.
development.
Suggested timings:
Activity 1: 2 hours
Activity 2: 4 hours
Activity 3: 2 hours
Suggested supporting resources:
Software help sheets
Diagrams of machine strategies
Version 1
Activity 1
Introduce learners to the need to export files from CAD to CAM and that Subtractive Manufacturing
methods require geometrical information contained in the STL mesh in order to create tool paths.
Mention that other file formats are available for Additive Manufacturing.
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 CNC. 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. Solidworks) 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.
Version 1
Activity 2
This task assumes you have suitable 3-axis milling machines or routers.
Provide the learners with appropriate CNC software that contains a variety of machine toolpath
strategies and that allows simulation of the machined component. Boxford’s ‘GeoCAM’ or Denford’s ‘VR
CNC Milling 5’ are suggested as they are wizard based and fairly straightforward to use. If you have
different machines, the Boxford software will output in a variety of file formats including G-codes for
Activity 3.
The optimum strategies will mainly depend on the geometry of the component. The optimum finish will
depend on a combination of the density of the STL mesh, roughing and finishing paths, tool size and
type, speeds and feeds. The simulation should give a good idea of the surface finish expected.
Activity 3
Learners may need assistance at setting up the machine and zeroing the tools. Provide suitable
materials for manufacture. Discuss the outcome with learners and whether it is as expected from the
simulation in Activity 2.
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Version 1
Unit 17: Computer aided manufacturing (CAM)
LO3: Be able to set-up and operate a CNC machine to
produce components
Preparation and production quality using CNC
Learner activity sheet
Activity 1
STL format is a surface model of the component made up of a triangular mesh. The size of the triangles
can be altered before exporting to give the optimal resolution of the component. 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 make on a CNC machine 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 size.
Version 1
CAD
model
STL settings
(Deviation, Angle)
[coarse]
Output file size
(binary)
Output file size
(ASCII)
Comments
[medium]
[fine]
Activity 2: Toolpath choice
The most important factor in the outcome of a milled model or part is the strategy for removing material.
This will vary depending on the geometry of the part and the material being used. See the summary of
common toolpaths at:
http://www.grzsoftware.com/manual/toolpath_settings.htm
Your tooling strategy will determine not only what your model looks like, but also the time it will take to
cut. A more efficient set of toolpaths can dramatically improve overall finish while decreasing cutting
time.
This task is about processing your STL files from Task 1 in CNC software and simulating the output to
decide on the appropriate strategy or strategies for machining the component. Suggested software is
Boxford’s GeoCAM or Denford’s VR CNC Milling 5. Work in pairs or small groups to process your STL
files using a range of toolpaths for roughing and finishing (separately and together). Complete the table
below to record your findings.
STL
File
Roughing strategy
Offset Waterline
Raster Profile
Rest
Combined
with
Finishing strategy
Comments on finish and
accuracy
Raster Profile
Offset
True Spiral
Z Level
Combination
uses ‘Z Level Milling’ for the
steep areas and then
‘Raster Profile Milling’ for
the shallow areas of the
model
Rest
Activity 3
Select one of the best strategies from Activity 2 to manufacture one of the components on a 3-axis mill or
router.
Version 1