Week summary
This week we focused on Computer-Controlled Machining using a large-format CNC router. We began with a safety training session where we reviewed the use of personal protective equipment (PPE), emergency procedures, workspace conditions, and machine safety zones. After that, we worked as a team to understand the CNC workflow: tool installation, material placement, machine zeroing, board fixation, calibration, and machining execution.
To evaluate machining quality and press-fit behavior, we produced two test pieces: a square test and a comb test. These helped us analyze fit tolerance, internal corners, tabs, dogbones, material response, and how feeds, speeds, and pass depth affect the final cut.
Quick data
- Topic: Computer-Controlled Machining (CNC)
- Students: Carmen Elena Gutierrez, David Avila, Esteban Valladares, Jianfranco Bazan, Mario Chong, Rocio Maravi, Grace Schwan, Cindy Crispin, Jennifer Wong
- Machine: ShopBot PRSAlpha 96-48
- Software: Aspire, ShopBot3
- Material: Plywood, 18 mm
- Tool: 6 mm end mill, 2 flutes, up/down cut
Assignment and deliverable
- Complete lab safety training
- Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths
- Document the work on the group work page
- Reflect on the individual page what was learned during the process
Goal
The goal of this group assignment was to understand the workflow of large-format CNC machining and to safely operate the machine while testing the most important variables that affect the fabrication result. We focused on machine setup, material fixation, alignment, toolpath verification, feeds and speeds, and press-fit tolerance.
Safety training
Before operating the CNC machine, we received safety training through a virtual session and also an in-person induction at FabLab UNI. During this training we learned the correct use of PPE, machine safety zones, emergency stop procedures, the startup system, and safe interaction with both the control software and the physical controls of the machine.
We also reviewed the meaning of safety signs used in workshops and laboratories:
- Red: danger or emergency situations
- Blue: mandatory actions such as wearing PPE
- Yellow: caution or potentially hazardous areas
- Green: safe conditions or first-aid equipment
This training helped us understand that safe CNC work begins before the machine is turned on: with a clear workspace, correct clothing, awareness of the risks, and respect for procedures.
PPE checklist
- Safety glasses
- Hearing protection
- Gloves when appropriate
- Closed shoes
- Suitable long clothing / protective clothing
- Helmet if required by the lab
Evidence gallery
Personal protective equipment (PPE):
Safety signage in the CNC milling machine area:
Equipment table
| Tool | Model / Type | How we used it |
|---|---|---|
| CNC router | ShopBot PRSAlpha 96-48 | Used for large-format CNC machining and test cutting on plywood |
| Cutting tool | 6 mm end mill, 2 flutes, up/down cut | Used to cut the square and comb press-fit tests |
| Spindle wrenches | Fixed wrench set | Used to tighten and loosen the spindle nut during tool installation |
| Collet / tool holder | CNC collet | Used to hold the milling bit securely in the spindle |
| Drill / screwdriver | Wireless drill-driver | Used to fix the plywood board to the sacrificial bed with screws |
| Material | Phenolic plywood, 18 mm | Material used for machining tests and press-fit evaluation |
| Fasteners | Wood screws | Used to secure the material and avoid movement during machining |
Safe workspace and protective measures
Before turning on the machine, we checked that the workspace was clear and that the sacrificial bed was clean. We also verified that there were no objects interfering with the machine path or the operator’s movement.
- Safety line clearly marked
- Machine area controlled and free of obstacles
- Dry chemical fire extinguisher available nearby
- Emergency stop button identified before operation
- PPE used by all operators
Machine, material, and tooling
The machine used was a ShopBot PRSAlpha 96-48. The material was a plywood board with 18 mm thickness, fixed to the sacrificial bed using screws. The cutting tool used in this test was a 6 mm end mill, 2 flutes.
Machining parameters
- Tool type: End Mill
- Diameter: 6.0 mm
- Flutes: 2
- Cut type: Up/Down cut
- Pass depth: 3.0 mm
- Stepover: 3.0 mm (50%)
- Spindle speed: 15,000 rpm
- Feed rate: 4500 mm/min
- Plunge rate: 1200 mm/min
- Estimated passes: 6 passes for 18 mm thickness
What we tested
- Material fixation on the sacrificial bed
- Alignment of the board relative to the machine bed
- Initial zeroing in X, Y, and Z
- Recalibration after fixing the material
- Feeds, speeds, and plunge rate
- Depth per pass and toolpath behavior
- Tabs and dogbones for internal geometry
- Press-fit tolerance using test pieces
Workflow
- Create vector design in CAD
- Import the design into Aspire
- Define material dimensions and origin
- Select 6 mm end mill
- Configure tool parameters
- Generate 2D Profile Toolpath
- Add tabs and dogbones
- Simulate the toolpath
- Export the file for ShopBot
- Fix the material to the bed
- Set zero and recalibrate after fixation
- Run the machining process in ShopBot3
Evidence gallery
Design and machining tests
Square test
The square test was used to evaluate the internal fit of a smaller square inside a larger frame. The design had an outer size of 150 mm x 150 mm and an internal square cutout of 50 mm x 50 mm.
- Dogbones were added to internal corners
- Tabs were added to keep the inner part stable during machining
- Useful for checking internal corner accuracy and fit behavior
Comb test
The comb test was used to compare slot widths and identify the best press-fit according to the real material thickness and machining tolerance. Its size was 420 mm x 100 mm.
- Slots ranged from 18.00 mm to 20.00 mm
- Increment: 0.25 mm
- Dogbones and tabs were included
- Used to determine the most suitable joint tolerance
Reference dimensions of the comb test
| Slot number | Width (mm) | Observation purpose |
|---|---|---|
| 1 | 18.00 | Very tight fit |
| 2 | 18.25 | Tight fit |
| 3 | 18.50 | Press-fit comparison |
| 4 | 18.75 | Medium fit |
| 5 | 19.00 | Reference around real material thickness |
| 6 | 19.25 | Slightly loose fit |
| 7 | 19.50 | Loose fit |
| 8 | 19.75 | Very loose fit |
| 9 | 20.00 | Clearance fit |
CAM preparation in Aspire
Square test prepared in Aspire with tabs and dogbones
Comb test toolpath with slot variations for tolerance evaluation
Square test prepared in Aspire with tabs and dogbones
Comb test toolpath with slot variations for tolerance evaluation
Toolpath configuration notes
The toolpaths were created in Aspire and exported for ShopBot machining. We used a 2D Profile Toolpath and added tabs where needed to keep the pieces attached during the cut. Dogbones were also included in internal corners to compensate for the round geometry of the end mill and improve internal assembly.
Toolpath strategy:
- Software: Aspire
- Machine control: ShopBot3
- Operation type: 2D Profile Toolpath
- Tool: 6 mm end mill
- Pass depth: 3 mm
- Feed rate: 4500 mm/min
- Plunge rate: 1200 mm/min
- Spindle speed: 15000 rpm
- Tabs: enabled
- Dogbones: added in internal cornersMachining process
Machine zeroing and calibration before machining
Square test cut
Comb test cut
Results
The machining process was completed successfully and without major issues such as tool breakage, material displacement, or severe cutting errors. The square and comb tests allowed us to analyze press-fit behavior and understand how small differences in slot width affect assembly.
One important observation was that after placing and fixing the board with screws, the machine had to be recalibrated, especially on the X and Z axes. This helped maintain precision before starting the cut.
Material behavior
The plywood produced slightly rough edges after machining. This was related mainly to the layered structure of the material and its natural tendency to chip. It was not a machine failure, but rather a material characteristic that should be considered in future design and finishing decisions.
Possible improvements for future work include using better-quality plywood, adjusting toolpath direction, fine-tuning feeds and speeds, adding a finishing pass, or sanding the edges after machining.
Test results gallery
Square test result used to verify internal fit and geometry
Comb test used to compare slot tolerances and press-fit behavior
Assembly check to compare how the joints behave in real conditions
Problems and solutions
No critical problems occurred during the machining process. However, we identified a few important observations that helped us improve the workflow:
- Recalibration after fixturing: after screwing the board into place, the machine reference had to be checked again to ensure precision.
- Rough edges: due to plywood structure and chipping tendency, the edges were not completely smooth and may require finishing.
- Internal fit: dogbones were necessary in internal corners because the round end mill cannot create a perfectly sharp corner.
These observations reinforced the importance of machine setup, geometry adaptation, and understanding how material properties affect the final result.
What we learned as a group
- Safe operation starts before machining begins
- Proper material fixation is essential for accurate results
- Toolpath simulation helps prevent machining errors
- Feeds, speeds, and pass depth strongly affect cut quality
- Press-fit tests are essential for tolerance evaluation
- Dogbones improve internal assembly in CNC joints
- Material behavior must be considered during design and finishing
Tips and recommendations
- Always verify the workspace before turning on the machine
- Use PPE during the whole process
- Simulate the toolpath before machining the real material
- Recheck zeroing after fixing the material
- Add tabs to internal pieces to keep them stable
- Use dogbones in internal corners for better fit
- Consider a finishing process for plywood edges
Evidence gallery
Conclusion
This group assignment allowed us to understand and document the workflow of large-format CNC machining using the ShopBot PRSAlpha 96-48. Through this exercise, we practiced safety procedures, prepared material and toolpaths, configured machining parameters, and executed a successful test cut on plywood.
The experience helped us connect digital design with physical fabrication and gave us a clearer understanding of how machine setup, tool selection, parameter configuration, material behavior, and safety practices influence the final result. The square test, comb test, and dogbone-adjusted joints made the assignment especially useful for evaluating real press-fit conditions for future CNC projects.
Final group documentation after completing the CNC machining tests
Reference
The comb test reference used in this practice was adapted from Fab Academy documentation and adjusted to our own machine, material thickness, and machining parameters.
Download files
- CNC design file (.dxf) Download