7. Computer Controlled Machining¶

Group Assignment¶

1. Safety Guidelines¶

First, the instructor introduced us to safety rules:

Eye Protection: Always wear safety goggles.
Ear Protection: Protect your ears from the noise generated by the machine.
Avoid Accessories: Do not wear watches, rings, or bracelets to prevent entanglement hazards.
Avoid Loose Clothing: Oversized or dangling clothes, such as scarves, can get caught in the machine’s moving parts.
Wear Closed-Toe Footwear: Protect your feet from potential hazards.

In case of an emergency, press the Emergency Stop (E-stop) Button. Multiple emergency stop buttons are strategically placed around the workshop for quick access.

2. Theoretical Session¶

The instructor explained essential theoretical concepts and how to use the software.

CNC Milling and Cutting Tools Guide¶

1. CNC Milling Machine & Its Components¶

CNC Router vs. CNC Milling Machine¶

CNC Router: Used for wood, plastics, and soft materials; operates at higher speeds.
CNC Milling Machine: Designed for harder materials like metals; operates at lower speeds but with higher precision.

The CNC milling machine we are using ShopBot PRS Alpha:

Key Components of CNC Milling Machines¶

Spindle: Rotating component that holds and drives cutting tools. For example, the ShopBot PRS Alpha features a collet and collet nut.
Tooling: Various cutting tools, such as end mills and drills, are used depending on the material and desired outcome.

Tool Size¶

Determines capability. Large bits remove more material but lack detail; smaller bits provide finer resolution.
- Overall Length: Longer bits reach deeper but may be less rigid.

Stickout: Distance from the collet to the bit’s tip. The more stickout, the less rigid a tool is. If it sticks out too far, the bit is prone to be bent by the cutting force.

Cutting Length: Defines how deep a bit can cut. Cutting depth should not exceed flute length to prevent overheating.
Shank Diameter: The non-cutting end that fits into the collet.
Cutting Diameter: Determines the resolution of cuts. A larger diameter increases rigidity and allows deeper cuts.

It is impossible to cut out features that are smaller than the cutting diameter because bits are cylinders (except for V-bits).

Common Router Bits¶

Drill Bit
End Mill
V-bit

Tip Shapes¶

Flat (Square) Tip: Forms nearly 90-degree angles at the tip.
Ball-Nose Tip: Has a rounded end for smooth finishes.
Chamfer (V-bit) Tip: Features a sharp conical shape for beveled edges.

2. Milling Tool Materials¶

High-Speed Steel (HSS): Affordable but wears faster.
Carbide: More expensive, lasts longer, and is better for high-speed machining.

3. Milling Tool Types and Geometries¶

End Mills¶

Flat Endmill: Used for roughing and finishing flat surfaces.
Ball Nose Endmill: Ideal for 3D contouring and smooth finishes.

Flutes and Teeth¶

Teeth refer to the cutting edges, and flutes are the grooves formed between teeth. As the bit rotates, teeth are responsible for cutting materials off, while flutes help evacuate the chips (namely removed materials) from the workpiece.

Flutes/Teeth

Number of Flutes

The number of flutes on your router bit impacts the work speed and the surface finish of your product. Having more flutes offers two main advantages. First, it adds to the strength of the bit, which means the bit can be fed into the workpiece faster and work on harder material. Secondly, bits with more flutes tend to give a better surface finish.

Type of Flutes - Straight Flutes: Stronger, work at high speeds, but provide a rougher finish. Best for wood and plastics.
- Spiral Flutes: Remove chips efficiently, leave a smoother surface, but are weaker and require lower speeds.

Upcut vs. Downcut Tools¶

Upcut: Pulls chips upward, good for metals but may cause splintering in wood.
Downcut: Pushes chips downward, creating cleaner cuts in soft materials.

4. Tool Performance Factors¶

Deflection: Tool bending due to cutting forces, leading to inaccuracies.
Hangover: Tool extension from the holder, affecting stability.
Runout: Tool wobble, affecting cut precision and tool life.

5. Milling Techniques & Operations¶

Up Milling vs. Down Milling¶

Up Milling (Conventional): Cutter moves against feed direction, causing more heat and force.
Down Milling (Climb Milling): Cutter moves with the feed, producing a smoother finish.

Common Milling Operations¶

Face Milling: Cutting flat surfaces.
Slot Milling: Creating grooves or slots.
Pocket Milling: Removing material inside a defined area.
Contour Milling: Following curved or complex paths.
Thread Milling: Cutting internal or external threads.
Helical Milling: Making spiral features.

6. Cutting Parameters¶

Feed & Speed¶

Spindle Speed: Tool rotation speed (RPM).
Cutting Speed (Vc): constant.

Feed Speed: Speed at which the workpiece moves (mm/min).
Feed per Tooth “\:f_z”
Number of Flutes “z”

Formulas and definitions for milling

Depth of Cut & Step Over¶

Depth of Cut: The vertical thickness of material removed per pass.
Step Over: The lateral distance between tool passes.

Using CNC Milling Machines¶

Design Creation: Create your design using CAD software.
Toolpath Generation: Use CAM software (e.g., VCarve) to generate G-code and upload it to ShopBot software.
Machine Setup: Secure the workpiece, install cutting tools, and set the machine’s origin.
Execution: Run the machine, monitor for issues, and ensure smooth operation.

For more details on ShopBot models, refer to their official documentation:
- ShopBot Support & Resources
- ShopBot PRS Alpha Manual

Dust Extraction System (Duct Fan) We learned how to operate the duct fan, which must be turned on before any cutting to prevent dust buildup and ensure clean working conditions.

Emergency Switches We located and practiced using both emergency switches on the machine, which must be used immediately in any unsafe situation.

Defining Zero Positions Lastly, we defined the zero positions of the cutting tool on the X, Y, and Z axes and cleaned the tool holder.

Securing the Stock to the Bed We aligned and clamped the material (usually MDF) tightly on the CNC bed. We also learned how to use screws safely and checked that the bit path wouldn’t collide with any fasteners.

Joint Test¶

Using VCarve¶

First we should defined the board dimensions accurately to match the actual material size.
Opened the .dxf file and adjusted its placement within the workspace to optimize material usage.
Added tabs and fillets (dog bone type) to improve assembly and avoid sharp corners:
Ungrouped the design.
Joined elements to enable modifications.
Ensured dog bone fillets were applied correctly to all internal corners.

Selected the appropriate tool and parameters:
Checked spindle speed, feed rate, and plunge rate.
Verified cutting depth to avoid cutting into the spoilboard.

Set the allowance offset to -0.5 mm to ensure a tight fit.
Simulated the toolpath to verify correctness and avoid collisions or excessive material removal.
Reviewed and optimized cutting strategies, such as climb cutting for cleaner edges.
Saved the final toolpath file for use in ShopBot software.

Using Fusion 360 CAM¶

First Joint Test

Designed a simple press-fit joint using parametric constraints with adjustable clearance. Used Fusion 360 CAM to generate toolpaths with a 4 mm flat endmill. Ran the test cut and adjusted until I found the correct fit.

Second Test – Slot Fit Joint

Designed a full-slot-fitting joint to test how parts insert vertically into pockets. Programmed it using CAM with dog bone fillets added for internal corners. After testing a few clearances, found the ideal tolerance for a snug, non-forceful fit.

Additional Test

Material Handling Used saws to trim the plywood boards before mounting them on the CNC.

ShopBot Software¶

Opened the ShopBot software to load the toolpath file.
Checked the preview settings to ensure proper orientation and scale.
Loaded the file saved from VCarve and verified all toolpaths were correctly assigned.
Ensured correct material placement by aligning the design within the work sheet.
Set up the machine:
Zeroed the X, Y, and Z axes.
Secured the material using clamps to prevent movement during cutting.
Checked tool tightness and bit sharpness.
Positioned the tool at the correct location in the real workspace using the control panel.
Final checks before cutting:
Verified spindle speed and cutting depth.
Ensured the dust collection system was properly set up.
Started the cutting process and monitored the machine closely for any issues.
After cutting, removed the parts carefully and sanded edges for a smooth finish.

Files - Half Joint Test

Last update: April 3, 2025