Drilling & trimming carbon fiber and composites
Carbon fiber destroys uncoated carbide and tears if you treat it like metal. Here's how to drill, route, and trim composites cleanly — no delamination, no uncut fibers, no fiber pull-out — straight from a RobbJack engineering webinar.
The four problems to design out
Every composite — random fiber, prepreg, intermediate layers, vacuum-bag — behaves a little differently, so there's no single “best” tool. The goal is always the same, though: eliminate these four.
Delamination
Layers separating at the hole or edge, especially on entry and exit.
Uncut fibers
Fuzzy, torn fibers left at the cut instead of a clean shear.
Fiber pull-out
Fibers ripped out of the resin rather than sheared through.
Poor tool life
Carbon fiber is brutally abrasive — uncoated carbide dulls in inches.
Start with rigidity — it's the most overlooked factor
When the part vibrates, tool life suffers and the process becomes unpredictable. The more rigid the workpiece and fixture, the longer the tool lasts, the faster you can run, and the more repeatable the results. Time spent maximizing fixture rigidity up front pays huge dividends later.
The best fixture is a vacuum fixture matched as closely as possible to the part. When that's not practical — short runs, or a family of parts on one fixture with corners hanging unsupported — simple fixes work: sandbags to damp an un-rigid area, or strapping and taping unsupported corners down. Anything that kills vibration makes the whole process reliable.
Hole making: three drill types
Solid carbide
Manual work or a handful of holes onlyCheap, but abrasive composite wears the edge fast and hole quality falls off quickly. Not a production answer.
Diamond-coated
Limitless geometry, lower-plastic-content compositesA carbide blank with the cobalt leached at the surface and diamond grown on the edges — so you can put any geometry on it. Typically 10–20× the life of solid carbide. Tip options: an 8-facet tip with a double angle that reams out delamination, a traditional 4-facet single angle, or an elliptical "bullet" point that spreads the cut over a larger area.
PCD (polycrystalline diamond)
Higher-plastic-content, longest life, resharpenableGround (not grown) edges start and stay sharper than diamond-coated. New solid-tip PCD — a ~3 mm thick solid diamond brazed to carbide — gives carbide-like geometry freedom and can be resharpened. This filled the old gap between carbide and diamond-coated.
Case study: 90,000 holes in an aircraft acoustic panel
Acoustic treatments for aircraft engine nacelles and blocker doors need thousands of tiny holes — here, 90,000 holes at 0.050" diameter in a carbon-fiber sandwich, round and free of delamination. Ten to fifteen tooling suppliers were tested; RobbJack's solution won, delivering up to 80,000 holes in fiberglass and 40,000 holes in carbon fiber with a single tool.
Three things separated success from failure:
- 1Diamond-coating adhesion to the carbide — without it, you just have an expensive carbide tool.
- 2Consistent spindle RPM — a speeder that dropped RPM caused variable chip load and erratic life.
- 3Tool handling — chips before the spindle kill performance.
Trimming & routing: shear, don't grind
The single most common mistake is running a PCD or diamond-coated tool the way you'd run a carbide burr — high RPM, slow feed, light multiple passes. That's a grinding action: it builds heat, melts resin, and wears the tool. Diamond edges are sharp; they want to get in and out fast and shear the material in one full-depth pass. Shearing generates far less heat than grinding.
So the counter-intuitive rule is lower RPM, higher feed, cut to net shape in one pass. Depth of cut up to about 2× the tool diameter per pass. For trimming, PCD's straight flute puts a neutral force on the part (no up- or down-shear) and resharpens; diamond-coated wins where you want more flutes or in lower-plastic-content material.
| Tool | Surface speed | RPM (full slot) | Feed |
|---|---|---|---|
| 3/8" (9.53 mm) router | 800–1,000 SFM | 8,150–10,000 RPM | ~80 IPM (~2,000 mm/min) |
| 1/2" router | 800–1,000 SFM | ~6,100 RPM | high feed, one pass |
Starting points — slower RPM than most shops run, which is exactly what gives the longest life and kills abrasive wear. Many customers run faster once the process is dialed in.
Tool handling
Diamond is hard but brittle. Never use contact measurement — no micrometers or calipers; use vision systems or microscopes. For a probe Z-offset, put a plastic shim or a piece of paper between tool and part and compensate. Store tools in tubes, not loose in cassettes, and train operators — a chip before the spindle means no performance.
Coolant
Coolant is usually dictated by the part or aircraft spec, so you may not have a choice. But where it's allowed — flood, an air chiller, a cold-air gun, even shop air — tool life can go up as much as fivefold. The trade-off is you must control the dust and the environmental issues that come with it.
Engineered tools that eliminate operations
Because RobbJack builds these from a solid carbide blank, one tool can do what used to take several — and being solid carbide, it can be diamond-coated for long life. A dovetail tool that dovetails andtrims the part in one operation has saved customers millions over a part's production run. A composite reamer-countersink with carbide threads (rather than brazed-on high-speed steel) can be diamond-coated end to end, so it reams and countersinks in one pass and holds tighter tolerances.
Real results
- 10–20×
- diamond-coated drill life vs solid carbide
- 40,000
- holes per tool in carbon fiber (perforation)
- 13,000 in
- linear inches of trimming per PCD tool
Keep going
Have a composite that's fighting you?
Every laminate is different. Send us the material and the problem — we'll engineer the tool.
Talk to an engineer