Focus This Week

This week is focused on Computer-Controlled Machining. The goal was to characterize feeds, speeds, and toolpaths on a large-format CNC router, and design, mill, and assemble a large-scale project. I designed and fabricated a plywood Go-Kart seat and battery support frame using a ShopBot CNC router, configured cutting toolpaths with Vectric Aspire (profiling, pocketing, and dogbone fillets for press-fit joints), and documented the assembly.

Group Assignment — CNC Router Characterization

The group assignment was to test run our large-scale CNC router, testing safety procedures, runouts, and calculating the kerf, joint clearances, feeds, and speeds for plywood milling. The complete group characterization log is available on the Fablab Dilijan Group Assignment Page.

CNC Machine Specifications & Settings

• CNC Router: ShopBot PRSalpha 96-48 (cutting bed size 2440mm x 1220mm).
• Milling Tool: 1/4 inch (6.35mm) downcut two-flute carbide endmill (downcut prevents surface splintering of plywood veneers).
• Spindle Speed: 12,000 RPM.
• Feed Rate: 120 inches/min (approx 50 mm/s).
• Plunge Rate: 60 inches/min (approx 25 mm/s).
• Stepover: 40% (for pocket cuts).

Individual Assignment — Fabricating a Plywood Kart Seat

To fit within the Go-Kart chassis, I designed an ergonomic, curved plywood seat that snaps together using press-fit slot joints and is locked with wooden wedges. I used a 15mm thick Baltic Birch plywood sheet.

Step 1: CAD Design and CAM Toolpaths

I modeled the seat assembly in Fusion 360, making the slots parametric. I then exported the DXF vectors into Vectric Aspire to generate the toolpaths:

• Pocket Toolpaths: Cut at 5mm depth for the interlocking tabs, using a pocket offset strategy.
• Profile Toolpaths: Cut through the full 15mm sheet in 3 passes (5mm depth per pass) to cut out the seat panels. I added tabs (4mm thick, 15mm long) to keep parts from moving during the cut.
• Dogbone Fillets: Because round router bits cannot cut sharp inside corners, tabs won't fit into standard rectangular slots. I applied Dogbone fillets (radius 3.175mm) to all inside corners, allowing the tool to clear out the corners.

Step 2: Machine Setup and Operation

1. Securing Sheet: Screwed the 15mm plywood sheet onto the MDF spoiler board, placing screws carefully away from the cutting vector paths.
2. Zeroing Spindle: Used the aluminum Z-zero contact plate to set spindle height. Zeroed X and Y coordinates at the bottom-left corner of the sheet.
3. Cutting: Turned on the dust collector and the vacuum pump. Started the spindle and loaded the toolpath file.
4. Post-Processing: Used a chisel to cut the holding tabs, sanded all edges to remove splinters, and rounded the corners.

Step 3: Assembly

The seat panels interlock using slot-and-tab joints. I tapped them into place using a rubber mallet. The joints fit snugly, locked tightly by tapered wedges, providing a rigid, glue-less seating structure.

Original Design Files

Download the DXF vector designs and CRV project files for the CNC seat:

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Week 7 — Summary

This week focused on large-format CNC machining. Here is a summary of the accomplishments: