Slide 003: Post-Processing, Finishing & Quality Verification

Slide Visual

Slide Overview

This slide covers the operations that happen after the CNC router completes its program: tab removal, edge finishing, deburring, quality measurement, and documentation. These post-processing steps transform a rough CNC output into a finished part.

Instruction Notes

Machine Shutdown Procedure

Before touching the workpiece: 1. Wait for spindle to reach full stop (VFD deceleration takes 5-15 seconds) 2. Verify all axis motion has ceased 3. Leave dust collection running for 2+ minutes to clear residual dust 4. Do NOT retract Z manually if the program has a proper end sequence — the G-code should include a safe Z retract (G28 Z0 or G0 Z1.0) before M5 (spindle stop) and M30 (program end)

Tab Removal

Tabs hold parts in place but must be removed cleanly:

Tool	Best For	Surface Quality	Speed
Flush-cut hand saw	Most materials	Good — minimal sanding needed	Medium
Sharp chisel	Wood tabs	Excellent if skilled	Fast
Oscillating multi-tool	Thick tabs, hard materials	Good	Medium
Band saw	Thick tabs	Requires cleanup	Fast
Utility knife	Thin tabs (<0.04")	Variable	Fast

Technique: Score the tab line first, then cut. Support the part to prevent grain tearout at the tab location. Sand the tab witness mark flush with 120-grit → 220-grit sandpaper.

Edge Finishing

CNC-routed edges often need post-processing: - Wood: Sand through grits — 120 → 180 → 220. Use a sanding block for flat edges, flexible pad for curves. Break sharp edges with a 45° light sanding pass or chamfer - Acrylic: Flame polishing (carefully!) or progressive sanding (400 → 600 → 800 → 1200 → buffing compound) for optically clear edges - Aluminum: Deburr with a deburring tool, chamfer file, or countersink bit by hand. Remove all sharp edges — aluminum burrs cause cuts - MDF: Seal edges with sanding sealer or thin CA glue before painting — MDF end grain absorbs finish unevenly

Quality Verification

Measure and document every critical dimension:

Measurement Tools & Accuracy: | Tool | Resolution | Best For | |------|-----------|----------| | Digital calipers | 0.001" (0.01mm) | Widths, depths, hole diameters | | Tape measure | 1/16" | Overall dimensions, non-critical | | Square | Visual check | 90° corners, perpendicularity | | Depth gauge | 0.001" | Pocket depths, counterbores | | Pin gauges | Exact size | Hole diameter verification |

What to Measure: - Overall X/Y dimensions (compare to CAD nominal) - Pocket depths (compare to programmed depth) - Hole diameters (check for oversize from tool deflection) - Corner radii (should equal tool radius for inside corners) - Surface finish (visual/tactile inspection) - Flatness (check for warping after cutting — stress relief in material can cause this)

Documentation

For each job, record: - Material type and dimensions - Tool used (type, diameter, flute count) - Speeds and feeds (RPM, feed rate, stepdown) - Measured vs. programmed dimensions - Any issues encountered and how they were resolved - Photos of finished part

This data builds institutional knowledge and helps optimize future jobs.

Key Talking Points

1. The job is not done when the machine stops — post-processing is part of the process
2. Tab removal quality often determines final part quality
3. Different materials require different finishing techniques
4. Always measure and document — this builds your feed/speed knowledge base
5. Deburring is not optional — sharp edges are a safety hazard on every material

Learning Objectives (Concept Check)

• [ ] Demonstrate proper tab removal using at least 2 methods
• [ ] Perform quality measurements and compare to programmed dimensions
• [ ] Document a completed job with all required data points

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Last Updated: 2026-03-19