1. Objective

The objective of the group assignment for the Computer-Controlled Machining week is to understand the operation, calibration and safe use of the CNC machine available in the Fab Lab. During this activity the team analyzes the machine behavior, verifies its precision, and determines the optimal parameters for cutting different materials.

This assignment allows the group to experimentally determine key machining parameters such as:

• Machine runout and mechanical precision
• Alignment of the spindle and cutting bed
• Workpiece fixturing techniques
• Appropriate cutting speeds
• Feed rates for different materials
• Toolpath strategies
• Material response to machining

2. Lab Safety Training

Before operating the CNC machine, all students completed the laboratory safety training. This training ensures that every user understands the potential risks associated with large-scale digital fabrication machines.

Main Safety Topics Covered

• Emergency stop procedures
• Proper machine startup and shutdown
• Use of personal protective equipment (PPE)
• Dust extraction and ventilation
• Handling of cutting tools and bits
• Safe positioning of the operator
• Fire prevention during machining

Personal Protective Equipment (PPE)

• Safety glasses
• Hearing protection
• Closed shoes
• Dust mask when cutting MDF or composite materials

3. CNC Machine Used

For this group assignment we used the CNC router available in our laboratory. This machine allows large-format machining and is suitable for cutting materials such as plywood, MDF, acrylic, and soft metals.

Main Machine Components

• Spindle motor
• Collet and tool holder
• Machine bed
• Vacuum or screw-based fixturing system
• X, Y and Z axis linear motion system
• CNC controller

4. Testing Machine Runout

Runout refers to the deviation of the cutting tool from a perfectly centered rotation. Excessive runout can produce inaccurate cuts, poor surface finish and premature tool wear.

To measure the runout, a dial indicator was placed near the tool tip while rotating the spindle manually.

Procedure

1. Install a straight end mill in the collet.
2. Place a dial indicator touching the tool shaft.
3. Rotate the spindle slowly by hand.
4. Observe the variation in the dial indicator reading.

Observation

A minimal runout value indicates correct alignment of the spindle and tool holder. Higher values may indicate worn collets or spindle misalignment.

5. Machine Alignment

Machine alignment ensures that the spindle is perfectly perpendicular to the work surface. Poor alignment can produce uneven cuts and dimensional inaccuracies.

Alignment Check

• Verify spindle perpendicularity
• Check flatness of the sacrificial board
• Surface the CNC bed if required

6. Workpiece Fixturing

Proper fixturing is critical in CNC machining. The workpiece must remain completely immobile during cutting to ensure precision and safety.

Common Fixturing Methods

• Screws
• Clamps
• Vacuum table
• Double-sided tape

For our test we used screws to secure the plywood sheet to the CNC bed.

7. Speeds and Feeds Testing

Speeds and feeds determine how fast the tool rotates and how quickly it moves through the material.

Testing these parameters helps identify the best configuration for clean cuts and efficient machining.

Parameter	Description
Spindle Speed	Rotation speed of the cutting tool (RPM)
Feed Rate	Speed at which the tool moves through the material
Plunge Rate	Speed at which the tool enters the material
Depth of Cut	Thickness removed per pass

8. Materials Tested

Different materials respond differently to machining operations. During the tests we analyzed cutting behavior for common Fab Lab materials.

• Plywood
• MDF
• Acrylic

9. Toolpaths

Toolpaths define how the cutting tool moves to remove material. Different strategies are used depending on the geometry and machining goal.

Toolpath Types

• Profile cutting
• Pocket cutting
• Adaptive clearing
• Drilling operations

10. Results and Observations

After performing the different tests, the group was able to identify optimal machining parameters for the CNC machine.

• Stable spindle speed for plywood cutting
• Optimal feed rate without burning the material
• Proper fixturing technique to avoid vibration
• Recommended toolpath strategies for efficient cutting

These parameters will be used later during the individual assignment to fabricate large CNC objects.

11. Conclusion

The group assignment allowed us to understand the relationship between machine configuration, material properties, and tool behavior.

By testing runout, alignment, fixturing methods, speeds, feeds, materials and toolpaths, we developed a practical understanding of CNC machining workflows.

This knowledge is essential for producing accurate and safe large-scale digital fabrication projects.

Objective

The objective of the individual assignment is to design and fabricate a large object using a CNC machine.

For this assignment a **large wooden table** was designed using press-fit joints and manufactured using plywood sheets. The design process was carried out using Autodesk Fusion 360 for CAD modeling and assembly validation.

Step 1 – Identify the Components

The first step of the design process was to define the structural components that form the table.

The table consists of three main structural elements:

• Table surface (top panel)
• Table legs
• Cross supports (traverses)

These components provide structural stability and allow the table to be assembled using press-fit joints without additional hardware.

Step 2 – Define Material Thickness

After identifying the components, the next step was to define the material used for fabrication.

The table was designed to be fabricated using:

• Plywood
• Material thickness: 15 mm

All joints were dimensioned considering this thickness, which is critical for proper press-fit assembly.

To ensure a good fit between the parts, a tolerance of:

• 0.2 mm tolerance

was incorporated into the slot dimensions.

Step 3 – Design the Components

Each table component was designed in Fusion 360 using parametric sketches.

The main components created were:

• Tabletop panel
• Two side legs
• Two structural cross beams

The parametric approach allows easy adjustment of the dimensions and material thickness.

Step 4 – Generate Dogbone Joints

CNC milling tools are circular, which means that internal corners cannot be perfectly square.

To solve this limitation, **dogbone joints** were added to all internal corners of the slots.

This modification allows the rectangular parts to fit perfectly during assembly.

Dogbone fillets were added with a tolerance of:

• 0.2 mm clearance

Step 5 – Assembly Simulation

Once all components were created, the next step was to test the assembly in Fusion 360.

This simulation allows verification of:

• Correct slot dimensions
• Structural alignment
• Assembly feasibility
• Stability of the final structure

All components were positioned using joints and constraints to simulate the real assembly process.

Step 6 – Prepare CNC Files

After validating the design, the parts were exported for CNC machining.

The workflow included:

1. Flatten components in a single plane
2. Create the sheet layout (nesting)
3. Export DXF files
4. Generate toolpaths in CAM software

Toolpaths used included:

• Profile cutting
• Tabs for part stability
• Dogbone corner machining

Step 7 – CNC Machining

The parts were cut using a CNC router.

Main machining parameters included:

Parameter	Value
Material	Plywood
Thickness	15 mm
Tool	6 mm flat end mill
Tolerance	0.2 mm

Step 8 – Assembly of the Table

After machining, the parts were removed from the board and assembled manually.

The press-fit joints allowed the structure to be assembled without screws or glue.

The final structure proved to be stable and structurally sound.

Final Checklist – Fab Academy Requirements

• ✔ Designed a large object using CNC machining
• ✔ Used parametric CAD design
• ✔ Designed press-fit joints
• ✔ Applied material thickness constraints
• ✔ Added machining tolerances
• ✔ Implemented dogbone joints
• ✔ Simulated assembly in CAD
• ✔ Generated CNC toolpaths
• ✔ Fabricated the object using a CNC machine
• ✔ Documented the complete process

Conclusion

This assignment demonstrates the complete workflow required for large-scale digital fabrication using CNC machining.

Through the design and fabrication of the table, it was possible to understand the importance of material parameters, joint design, machining tolerances, and structural assembly.

The use of parametric modeling in Fusion 360 allowed efficient modification of the design and ensured accurate fabrication.