NC Grinding

Workshop Details

• Instructors: Kat, Neil
• Location: CBA shop
• Machine: Genmitsu ProVerXL430 V2 Tabletop CNC

Project Goal

The goal was to embed an RFID tag inside a rock and seal it with a 3D-printed lid. To achieve this, I needed to create a cavity in the rock for the RFID sticker using CNC stone grinding techniques.

Recommended Materials

• Flat slabs in granite or marble
• Ceramic or stone tiles
• Materials with uniform surface height

Materials to Avoid

• Organic shapes/river rocks: Irregular surfaces make zeroing difficult
• Layered slate: Delaminates and breaks diamond bits
• Thick materials (>1.5"): Limits Z-clearance
• Surfaces with irregular heights: Cannot maintain consistent depth

Material Properties

• Harder rock: Requires slower spindle speed
• Softer rock: Can handle faster spindle speed

Software Setup

• FreeCAD: Download development version
• Universal GCode Sender: Download UGS
• Reference: Adaptive Tool Path Tutorial

FreeCAD Workflow

1. Import or Create Model: Import STEP file or create sketch in FreeCAD
2. Extrude Below Z0: Extrude the sketch to negative value so it goes under Z0 (tool is at bottom of stock, not top)
3. Set Up Stock: Configure stock as bounding box below Z0
4. Configure Tools:
   • Go to CAM Workbench → Tool Library → Open Library Editor
   • Add Tool → Select Endmill Shape → New Toolbit
   • Set Tool Diameter (measure your actual diamond bit diameter)
   • Save the toolbit
5. Create CAM Job:
   • Output: GRBL/Gerber
   • Click stock as bounding box
   • Pick the face to work on
   • Add tools from library
6. Adaptive Clearing:
   • Select Adaptive operation
   • Click Apply
   • Use CAM Simulator to preview toolpath
7. Speeds and Feeds:
   • Cutting Speed: Based on material hardness
   • Feed Rate: 100 mm/min (recommended starting point)
   • Plunge Rate: Slower than feed rate
   • Spindle Speed: 500 RPM (for rock)
   • Stepover: Adjust based on bit diameter
8. Post Process:
   • Go to Post Processor → Select GRBL
   • Choose save location
   • Click Post Process

Important Notes

• If using Fusion 360, delete G28 G91 Z0 lines in Universal GCode Sender (warning symbols will appear)
• Export as GRBL format
• Pay attention to scale and depth in relation to adaptive clearing

; Recommended Settings for Rock
G21 ; Set units to millimeters
G90 ; Absolute positioning
M3 S500 ; Spindle on at 500 RPM
F100 ; Feed rate 100 mm/min

Work Holding Challenges

Securing irregularly shaped rocks proved to be one of the biggest challenges:

Solutions That Work

• Clamps: Work well with large flat slabs
• Custom clamping jigs: Similar to wax milling clamps

Failed Attempts

• Clay support: Water softens clay too much
• Silicone mold around rocks: Insufficient rigidity, vibration issues (see Attempt 02 below)

Potential Solutions

• Marine adhesive for temporary bonding
• Hot glue rock/mold to flat backing material (wood or marble tile)
• Custom clamp designs for small irregular shapes

Zeroing Sequence

1. Secure the rock: Clamp firmly, ensuring no movement
2. Set X0 Y0: Corner or center of design area
3. Set Z0: Use paper method at top of material surface
4. Verify: Jog to X0 Y0 Z0 — bit should just touch surface
5. Check clearance: Ensure machine won't collide with clamps

Water Management

• Pour water until it lightly covers the rock surface
• Water captures silica dust and creates a slurry
• Since material is rock, slurry can be safely disposed in CBA sink
• Never run dry — this creates dangerous silica dust

Cutting Process

Load the G-code in Universal GCode Sender and begin the cut. Grinding rock produces excellent dust suspended on the water surface.

Setup

• Used the adaptive clearing toolpath on an actual river rock
• Clamped the most stable flat surface
• Set X/Y/Z so cutting region was as level as possible
• Note: There was a dimple in the center of the indent area

Problems Encountered

1. Layered Structure: Rock contained multiple thin sedimentary layers
2. Unstable Cutting: Bit drifted and couldn't cut a clean circle
3. Tool Damage: Diamond bit connector section was ground away

Root Causes

• Feed rate too high: 250 mm/min was excessive for layered rock
• Material hardness: Exposed sediment layer was too hard and abrasive
• Bit design: Fragile connector section couldn't withstand forces

Solution

Switched to a different diamond bit without a fragile connector section and reduced feed rate to 100 mm/min.

Concept

Since achieving a perfectly flat surface is essential, I created a soft silicone mold to stabilize the irregular rock shape.

Mold Creation Process

1. Placed rock surface down in a container
2. Poured Oomoo silicone to create negative mold
3. Let cure overnight
4. Removed rock, leaving perfect mold cavity

Results

✅ Successes

• Surface flatness problem solved: Mold successfully held rock geometry
• Excellent cutting quality: Surface finish was very good
• Repeatable positioning: Rock could be removed and replaced accurately

❌ Problems

• Mold too soft: Vibrated and shifted during machining
• Distorted cut shape: Movement caused geometric inaccuracy
• Clamping insufficient: Standard clamps couldn't prevent vibration

Clamping Attempts

Method 1: Weight Distribution

• Placed heavy marbles on top and right of mold
• Result: Reduced but didn't eliminate movement

Method 2: Wood Backing

• Placed wood blocks around mold for support
• Result: Slight improvement but still vibration issues

Recommended Improvements

After discussing with Dan, we identified several better approaches:

1. Hot glue mold to backing: Attach mold or rock to flat material (wood or marble tile) to prevent shifting
2. Wax-milling clamps: Use the same clamp type from wax grinding to secure irregular shapes
3. Custom small-rock jig: Design specific clamping system for small stones
4. Harder mold material: Consider using harder casting materials for mold

Technical Insights

• Feed rate matters: Start slow (100 mm/min) and increase carefully
• Material knowledge crucial: Layered rocks are problematic for diamond bits
• Flat reference essential: All zeroing depends on flat, stable surface
• Water is mandatory: Never compromise on dust suppression
• Tool selection critical: Bit geometry must match material forces

Work Holding Lessons

• Irregular shapes require custom solutions
• Soft molds need rigid backing to prevent vibration
• Standard clamps work best with flat slabs
• Hot glue temporary bonding is underrated

Process Improvements

• Test toolpath on scrap material first
• Measure actual tool diameter before generating toolpath
• Start with conservative speeds and feeds
• Monitor bit wear constantly
• Keep water level consistent throughout cut

Hygiene Technology Discussion

During the recitation, we discussed the intersection of technology and community development:

Key Questions

• How do you ensure you're having conversations with communities, not just placing infrastructure?
• How can we help indigenous people produce sustainable products within their community without waste?
• How can we help indigenous people continue their ancestral knowledge?
• What are communities actually worried about?

Field Experience: Peru

• What people value cannot be known from one-time visits
• Community is important: Introducing people to your community builds trust
• Goal: Help people test their products, not just build products for them

Designing Infrastructure

What does it mean to design infrastructure?

• Start with people, not machines
• Physical labs are entirely volunteer-based, maintained through local managers
• Technology should enable communities, not replace their knowledge

Files

• FreeCAD Project File and G-code Toolpath

Recommended Settings Summary

Parameter	Value	Notes
Feed Rate	100 mm/min	Starting point, adjust based on material
Spindle Speed	500 RPM	For hard rock
Plunge Rate	50 mm/min	Slower than feed rate
Stepover	50% tool diameter	Adjust for finish quality