Guides / Process
How to Drill Carbon Fiber Without Delamination
Carbon Fiber Machining editorial team · October 21, 2026
Key points
- Delamination at the exit side is caused by drill thrust when the last plies have nothing behind them.
- Sharp diamond-coated or PCD drills with composite geometry, backing on the exit face and a lower feed at breakthrough keep it small.
- Helical interpolation with an end mill gives cleaner holes and tighter sizes than drilling, at a slower cycle.
- Agree an acceptance limit for hole damage before the order, especially on structural parts.
Carbon fiber delaminates when it is drilled because the drill pushes the plies apart. As the tip breaks through the back face, the last plies have no material behind them, and the thrust force peels them away from the laminate instead of cutting them. The result is a ring of lifted plies or fuzz around the exit of the hole. Entry-side peel-up happens too, when the cutting edges pull the top ply upward.
Shops that drill carbon well control four things: the tool, the support behind the part, the feed at breakthrough and the cutting conditions. This guide explains each one and how to check holes when parts arrive.
Why the exit side fails
A carbon laminate is strong along its fibres and weak between its plies, where only resin holds them together. While the drill is inside the part, the plies below the tip carry the thrust. As the tip approaches the back face, fewer and fewer plies remain. At some point the remaining plies cannot carry the thrust and they bend away from the drill and split from the rest of the laminate.
The damage grows with:
- Thrust force, which rises with feed rate and with a blunt or wrong drill point.
- Missing support, when nothing presses the back face.
- Tool wear, because a worn edge pushes instead of cutting.
- Heat, which softens the resin near the hole.
Tooling: geometry and coating
Standard metal-cutting drills are the main cause of delamination. Carbon needs:
| Tool | Where it fits | Notes |
|---|---|---|
| Diamond-coated carbide drill, composite point | Most holes in plate | Brad point, dagger or double-angle point shears fibres at the hole edge |
| PCD drill | High volumes, fits | Longest life, highest cost per tool |
| Diamond-coated end mill, helical interpolation | Larger holes, fits | Low thrust, any diameter from one tool, slower |
| Step drill or drill-ream | Tight fits | Rough and finish in one pass |
| Standard HSS twist drill | Avoid | Wears in a few holes, high exit damage |
Typical shop practice. The shop chooses the tool for your laminate and hole size.
Tool life is the hidden variable. A drill that cuts clean holes for the first fifty can push out the exit plies on the next ten. Good shops track hole counts per tool and change tools on a schedule, not when the parts start to look bad.
Support behind the part
The cheapest fix for exit damage is backing. A sacrificial board of MDF, plastic or aluminium is pressed tight against the exit face, so the last plies are supported while the drill cuts through them. On a router the plate usually sits on a spoil board that does the same job.
For tubes, a mandrel or a plug inside the tube supports the far wall. For molded parts, the trim fixture has backing pads under each hole. If you design a part that a shop must drill after molding, give it datum faces the fixture can locate on.
Feed and speed at breakthrough
Most CNC programs for carbon drill in two stages: normal feed through most of the thickness, then a reduced feed for the last fraction of a millimetre. Lower feed means lower thrust when the plies are weakest. Spindle speeds are high, because diamond tools cut best at high surface speed.
Pecking, where the drill retracts to clear chips, is used on thick laminates to stop dust packing in the hole. It is not a cure for delamination by itself.
Helical interpolation: the cleaner alternative
Instead of a drill, a small end mill can circle down into the hole. The thrust is far lower because the tool cuts sideways. Hole quality is better and one tool makes many diameters. The drawback is cycle time. Shops use it for fits, large holes and parts where exit damage is not acceptable. Ask for it on the drawing when a hole carries a bearing or a precision dowel.
Stacks: carbon with metal
Drilling carbon bonded or bolted to aluminium or titanium in one pass is common in aerospace. Each layer wants a different speed and feed. Titanium chips can scratch the carbon bore as they leave. If your part is a stack, say so in the RFQ. Not every carbon shop drills metal stacks.
How to inspect holes on delivery
Check under good light and, for critical parts, under magnification:
- Exit side: look for lifted plies, fuzz or a lighter ring around the hole. A light ring often means plies have separated below the surface.
- Entry side: look for peel-up of the top ply.
- Size: a pin gauge tells you more than a caliper in a carbon hole.
- Bore wall: loose fibres in the bore point to a worn tool.
Some buyers set a delamination limit: the diameter of the damaged area divided by the hole diameter, often called the delamination factor. Agree the limit before the order. A cosmetic part may accept none. A hidden bracket may accept a small ring if strength is checked.
Worked example (hypothetical)
A camera boom maker sees cracked plies around the clamp bolt holes of a 30 mm roll-wrapped tube. The holes were drilled freehand with a standard drill and no internal support.
The new process: tube held in a split collet, an aluminium plug inside at each hole, a composite drill with reduced feed at breakthrough and a fresh drill every few dozen holes. The bolt holes move from 1 diameter to 2.5 diameters from the tube end. The cracks stop and the holes come out clean on both walls. More on tubes is on the tube machining page.
Design choices that help
- Hole diameter at least the plate thickness.
- Holes at least two diameters from an edge.
- Inserts instead of tapped threads.
- Fits only where needed, and called out so the shop plans interpolation.
These and more are in our design rules. For the full picture of methods and materials, read the buyer’s guide to carbon fiber machining. The CNC cutting cost guide shows how holes and fits show up in a quote, and cross drilling carbon fiber tubes goes deeper on tube holes.
If your part has holes that must be clean on both sides, we match it with shops that drill carbon every day. See drilling and trimming or send your RFQ.
Frequently asked questions
What drill bit is best for carbon fiber?
A diamond-coated carbide or PCD drill with a geometry made for composites, often a brad point, dagger or double-angle point that cuts the fibres at the hole edge before the centre pushes through. Standard 118 degree HSS drills wear in a few holes and push out the exit plies.
Should carbon fiber be drilled wet or dry?
Both are used. Dry drilling with strong dust extraction is common on routers. Wet drilling or mist keeps dust down and cools the tool, but the part must be dried and the coolant must not attack the resin. Shops pick based on the machine and the part.
How do I drill a hole in a carbon fiber tube without cracking it?
Support the tube from inside with a mandrel or a plug, clamp it in a V-block or split collet, use a sharp composite drill and drop the feed before the drill breaks through the far wall. Keep cross holes at least two diameters from the tube end.
Need a quote for a carbon part?
Send the DXF or STEP file. We match you with up to 3 shops that cut carbon fiber every day and reply within 24 business hours.
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