07 Computer-controlled machining

Assignment

Group assignment:
- Complete your lab's safety training
- Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
- Document your work to the group work page and reflect on your individual page what you learned
Individual project
- Make (design+mill+assemble) something big

Overview：

Previously, my experience with interlocking furniture was limited to manually cutting cardboard, which was often simple and rough. This week, I learned a lot of new digital fabrication techniques. A major highlight was completing the entire workflow—from 3D design to CAM toolpath generation—seamlessly within Fusion 360, which proved to be incredibly convenient.

I also discovered the concept of Dogbone joints, which was entirely new to me. Since a circular CNC bit cannot cut sharp 90° internal corners, I used the Nifty Dogbone plug-in in Fusion 360. It made adding these necessary clearances incredibly simple and automated, ensuring my press-fit joints could be fully seated. I chose 15mm Birch Plywood for my project to ensure structural stability for a "large-scale" object, using a 6mm Flat End Mill.

Group Assignment

Workflow :

1.Safety Training & PPE Preparation➡ 2. Characterization: Speeds, Feeds & Setup ➡ 3. Comb Test for Tolerances

Safety Training

Pre-Operation & Risk Management:

• Supervision & Training: I learned that only trained individuals are allowed to operate the machine. Untrained people must never work on the CNC without professional supervision.

• The "Buddy System": I must always operate the Machine with a partner. Having a second pair of eyes to double-check the setup and files is crucial for catching potential errors before they become dangerous.

• Machine State & Cleaning: The machine must be powered off during loading. Cleaning the bed and rails is a critical fire prevention measure, as accumulated wood dust is highly flammable.

• Air Pass (Path Verification): Before the actual cut, I performed an "Air Pass" (Z-zero 50mm above the board) to verify the movement and ensure the spindle wouldn't hit any screws or clamps.

• Pre-Cutting Safety Check: Turn on the vacuum switch to activate the dust extraction system before the machine begins cutting, to guarantee continuous dust removal throughout the entire process.

• E-Stop Physical Location: I identified the physical location of the Emergency Stop button and ensured it was placed in an easily accessible spot (e.g., outside the glass enclosure) for instant reaction.

Operational Safety Rules:

• PPE & Attire: Eye and ear protection are mandatory. I must wear closed-toe shoes at all times and ensure long hair is tied back to prevent it from being caught.

• No Gloves & Rail Safety: Never wear gloves while operating the machine, as they can be caught by the spinning spindle and pull your hand in. Similarly, never place hands on the rails, as the machine can move unexpectedly in any direction.

• Material Security: I must ensure the plywood is properly secured before cutting. I am also aware of the risk of small parts coming loose after being cut; if not secured with Tabs, they can be thrown forcefully by the bit.

• No Unattended Operation: Never leave the machine running unattended. A spinning tool generates significant friction and heat, posing a fire risk.

Fire Prevention & Technical Optimization:

• I learned to minimize fire risks by using correct chip loads, sharp bits, and double-checking all toolpath files. I must always be prepared to pause or stop the cut immediately if anything sounds or looks incorrect.

Lab Material Inventory:

6mm x 2440mm x 1220mm Plywood (8 sheets)
15mm x 2440mm x 1220mm Plywood (6 sheets)
Test Tool: 6mm Flat End Mill

Characterization: Speeds, Feeds & Setup

I performed cutting tests on the 15mm board to find the optimal settings:

Spindle Speed: 24000 RPM
Feed Rate: 20000 mm/min

These are the actual operating values displayed and used on the machine.

Stepdown (Depth of Cut): 12mm
Runout & Kerf: The actual measured kerf was 6.1mm. I will use a 0.1mm tolerance compensation in my designs.

Setup: Post Processing, Machine Definition, Tool Library

Overview: Within the Manage tab of the Manufacture workspace, I configured the Machine, Tool, and Post Libraries. By manually adding parameters based on our specific lab equipment, I established a digital twin environment. This allows for direct simulation to visualize results and verify post-processing before actual machining.

Configured the machine library with a 5000 mm/min max feedrate to match our lab hardware.

Comb Test for Tolerances

Overview: To verify the actual fit and ensure safety, I executed a rigorous testing process. During file preparation, I used Fusion 360's Design mode for parametric modeling and added Dogbones in the Manufacturing Model. During machine preparation, I followed safety protocols by resetting coordinates, enabling vacuum adsorption, manually locating the XYZ origin, and drawing a Frame to verify the physical cutting area. This synergy between software and hardware helped determine the optimal tolerance.

Testing file Workflow: CAD Parametric Design ➡ Edit Manufacturing Model: Nesting & Dogbone ➡ Setup & WCS ➡ Toolpath & Simulation ➡ Export Post Process

CAD Parametric Design:In Design mode, I designed a set of test slots ranging from 14.7mm to 15.3mm with 0.1mm increments based on the 15mm plywood thickness.

Manufacturing Model Optimization:

Create Nesting Surface: Sketched a rectangle matching the board size as a base in the Manufacturing Model.
Component Arrangement: Arranged test models on this surface to ensure nesting efficiency and grain alignment.
Add Dogbones: Applied Dogbones to the arranged models without altering the original design to ensure clean corners.

Setup & Alignment: Created a new Setup based on the nested surface, accurately setting the Work Coordinate System (WCS) to match the physical board 2440x1220mm placement.

Toolpath & Simulation:

Tab Settings: Added Tabs to the outer boundaries to prevent small part movement during cutting.
Precision Control: No Tabs were added inside the slots to ensure flush mating surfaces for accurate results.
Simulation: Performed a full simulation to verify the toolpath boundary and check for potential collisions.

Export Post Process

Right-click the Setup in the browser bar and select Post Process.

Configuration: Select the Grbl/gerbil processor to match the lab's CNC software.
Output: Check units (Metric/mm), then click Post to export .nc or .gcode files for the machine console.

Machine Preparation & Operation Workflow:

Reset Axes all 0➡ Vacuum & Check ➡ Manual WCS Locating ➡ Draw Frame ➡ Sim & Cut

Environment Initialization: Reset the coordinates of all axes in the control program. Turn on the vacuum pump and manually press the edges of the wooden board to confirm that it is firmly adsorbed and there is no warping.
Positioning: Move the spindle to the origin corresponding to the layout, and manually zero the X and Y axes. Use a tool setter or manual mode to set the Z - axis height. Note: If the plywood is uneven or warped and vacuum suction isn't enough, use screws or nails in safe zones to physically flatten and secure the board, ensuring consistent cutting depth and preventing movement.
Physical Range Verification (Frame): Activate the Frame function, visually observe and confirm that the tool head's movement trajectory is completely within the physical boundaries of the wooden board.
Operation and Part Retrieval: After setup, carry the keyboard to the external physical pause button. Press F9 to start and F10 to pause. Maintain a constant watch from a safe corner, never leave the machine unattended, and be ready to pause for safety. After cutting, extract the test piece to verify 14.9mm as the optimal fit.)

Result :

14.9mm: ✅ Optimal Fit
Physical Measurement: Observed a material deviation of 0.1-0.2mm in thickness on the sides.
Fit Validation: Despite slight material unevenness, the joint sections achieved high precision with no significant thickness or clearance errors.

Individual Assignment

Design Phase (Parametric Design for 15mm Plywood)

Workflow: Define Gap Variable & Parameters ➡ Sketching & Modeling➡ Create Manufacturing Model ➡ Apply Dogbones➡ Arrange/Nesting

Design Phase: I designed two Bar Stools using parametric modeling in Fusion 360. The height of each bar stool is set at 700 mm. For the seat part, I designed two different sizes to create a distinction. To ensure the stools are sturdy enough for daily use, I used 15 mm Plywood.

Tolerance Design: Given the actual thickness variations of the 15mm plywood, I set the thickness parameter to Material_Thickness = 15mm.
For the tolerance test, I changed the size of the slots connecting the seat and the legs to 14.9 mm. However, I didn't modify the tolerance of the slots on the legs because there is more contact and friction area at the slots on the legs, and a size of 1 5mm is more appropriate.
Corner Treatment: Using the Nifty Dogbone plugin, I applied T-Bone fillets with a 6.05 mm Tool Diameter and additional 0.05.
Arrange/Nesting: I used the Arrange tool to lay out the components within the 2440mm x 1220mm workspace.

CAM & Toolpath Planning

Workflow: Create Setup➡ Set WCS Origin➡ 2D Contour ➡ 2D Pocket ➡ Simulation➡ Post Process

2D Contour

2D Pocket

Simulation & Post Process

Critical Error Analysis

Through iterative testing and consultation with Gemini, I identified two critical areas for error prevention:

Operation	Goal	Bottom Height Setting	Rationale
2D Pocket	Partial Depth	From: Selected Contour(s) Offset: 0mm	Follows the CAD model depth (9.8 mm) relative to the top surface.
2D Contour	Full Cut-through	From: Stock Bottom Offset: -0.5mm	Never calculate from the Top! Referencing the Bottom ensures a clean cut regardless of stock thickness.
- Processing Order: 2D Pocket must run BEFORE 2D Contour. If the outer profile is cut first, the part becomes loose, causing vibration or displacement when attempting the inner pocketing.

Specific Toolpath Parameters

2D Pocket (Inner Grooves):
- Stepdown: Enabled Multiple Depths with a 2.0mm Maximum Stepdown.
- Result: The 5mm depth was cleared in 3 passes, resulting in a very smooth pocket floor and efficient chip removal.
2D Contour (Outer Profile):
- Stepdown: Single Pass (15mm). With a rigid machine and sharp end mill, a single pass provided superior edge finish and efficiency.
- Tabs: Added 5mm x 3mm Tabs to secure the parts during the final cut.

Machining & Assembly

Workflow: Cutting ➡ Manual Fix ➡ Sanding ➡ Assembly

I have placed the mechanical settings in the group assignment. The content regarding parameter modifications in the assembly part has been presented in the previous text.

Critical Reflections on Failures

Pocket Reference Error: Initially set Pocket Bottom to Stock Bottom, causing the tool to "float" above the material. Corrected it to reference Selected Contours.
Processing Order Error: Ran Contour before Pocket. Once the part was detached, it shifted during pocketing, causing a mis-cut.
- Lesson: Always Pocket before Contour.
Dimension Mismatch: The leg slots didn't align with the footrest due to a CAD inspection oversight.
- Solution: Performed a manual saw cut to fit the parts.

Manual Fix

Sanding & Assembly

Final

Perfect！

File

Test f3d

Chair f3d

Conclusion

The machining process involved a learning curve, as certain issues only became apparent after the initial cuts, requiring some manual intervention. However, once the Pocket depth references were corrected and the leg-to-footrest alignment was resolved through manual adjustments, the remainder of the fabrication proceeded smoothly.

The final tolerances proved to be exceptionally precise. Following sanding and assembly, my classmates tested the stool and confirmed its impressive stability. It has now found a permanent place in my home setup; its height is a perfect match for my standing desk, making it the ideal companion for a quick rest during long working sessions.

Bottom Height is absolute: Verify if cutting down from Top or up from Bottom.
Sequence is safety: Complete internal features before the final outer cut.
Physical Check: Digital models require a final assembly/interference check before milling.
Bottom Height is absolute: Verify if cutting down from Top or up from Bottom.
Sequence is safety: Complete internal features before the final outer cut.
Physical Check: Digital models require a final assembly/interference check before milling.