Why PCD is the default for graphite and CFRP
Graphite is extremely abrasive but machines dry; carbon fiber laminates are anisotropic and delaminate under wrong attack angles. Polycrystalline diamond (PCD) brazed or vein-processed onto a carbide body gives 20-100x the life of uncoated carbide in both materials, and keeps an edge radius below 5 microns for clean fiber shearing.
Graphite electrode milling
Graphite wants speed and light chip load. Dust evacuation is the primary challenge.
| Tool dia (mm) | RPM | Feed (mm/min) | Axial DOC | Radial WOC |
|---|---|---|---|---|
| 3 | 20000-30000 | 1500-2500 | 0.5 x D | 0.5 x D |
| 6 | 16000-24000 | 2000-3200 | 0.5 x D | 0.5 x D |
| 8 | 14000-18000 | 2200-3500 | 0.4 x D | 0.4 x D |
| 10 | 12000-16000 | 2200-3600 | 0.4 x D | 0.4 x D |
- Dust control: vacuum shroud at the tool always beats shop-air blow-off
- Tool wear check: measure edge radius every 60 minutes; graphite detail degrades quickly once the PCD edge rounds past 8-10 microns
- Corner strategy: for sharp electrode corners, use a PCD veined tool rather than a brazed segment, because the diamond wraps the full periphery
CFRP trimming and routing
Delamination and fiber pull-out are the failure modes. The countermeasures are diamond edges, high RPM, low chip load and climb milling:
| Operation | RPM | Feed (mm/min) | Strategy |
|---|---|---|---|
| Edge trimming, 6 mm PCD router | 18000-27000 | 1000-2500 | Climb, single pass, full depth if < 6 mm thick |
| Pocketing, 6 mm PCD end mill | 15000-22000 | 800-1800 | 0.3 x D axial steps |
| Drilling with PCD or diamond-coated drill | 6000-12000 | 0.05-0.10 mm/rev | Peck if depth > 3 x D, exit support preferred |
Three rules that prevent 90 percent of CFRP failures
- Climb mill everything: conventional milling lifts the top ply; climb milling presses it down
- Support the exit side: backing board or sacrificial plate under through-holes and edge trims
- Never re-cut dust: stalled dust in the cut acts as lapping compound and destroys the edge within minutes
Machine requirements checklist
- 20k+ RPM spindle (30k ideal for small tools)
- Air blast plus extraction at the spindle nose
- Sealed, dust-protected linear guides on older machines
- Tool holders with runout below 5 microns at nose
Summary
Graphite: high RPM, moderate feed, aggressive dust extraction, veined PCD for detail work. CFRP: climb milling, high RPM, low chip load, supported exits. In both materials, PCD is not a luxury but the economical choice once you account for tool changes and scrapped detail.
