Carbon fiber plate CNC cutting guide: tools, methods, dust

Why cutting carbon plate is different

A carbon fibre reinforced plate is not a uniform material like metal or wood; it consists of hard, brittle fibres in a comparatively soft resin. That structure changes cutting behaviour in several ways (Teti, 2002; Sheikh-Ahmad, 2009):

• Abrasiveness: carbon fibre quickly dulls a tool’s cutting edge. A dull tool crushes the material instead of cutting it, and fibre protrusion, fraying and heat at the edge increase.
• Chip form: there is no continuous chip as in metals. The fibres fracture into fine dust and fragments, which affects both tool life and workplace safety.
• Surface plies: in laminates, the top and bottom plies are held to the layers below only by resin. Cutting forces can lift these plies and start delamination at the edge.
• Heat: resin softens at high temperature. If the temperature in the cutting zone approaches the resin’s glass transition temperature (Tg), the edge shows melted resin, burn marks and surface damage.
• Fibre orientation: as the angle between the cutting direction and the fibre direction changes, so do the cutting forces and the edge quality. Different edges of the same part can therefore show different edge quality.

Cutting methods

CNC routing

The most common way to cut parts from carbon plate. Complex contours, pockets and holes can be machined in one set-up with high dimensional accuracy. Success depends on the right tool, suitable cutting parameters and effective dust extraction.

Abrasive waterjet

Cuts with high-pressure water and abrasive particles; there is no heat effect and thick plates can be cut. In laminates, interlaminar damage can occur where the jet first pierces the plate, so the pierce point is often placed outside the part or pre-drilled. The edge can have a slight taper, and cut parts need drying.

Laser

Because fibre and resin have very different thermal properties, laser cutting makes the resin recede along the edge and leaves charring and a heat-affected zone. It is therefore generally not recommended for structural parts.

Diamond blades and hand tools

Usable for straight cuts and prototype work. Edge quality depends largely on the operator, and dust is hard to control.

Choosing the tool

Tool material

• Solid carbide: economical, but dulls quickly against abrasive carbon fibre. Suited to small batches and test cuts.
• Diamond-coated carbide (CVD): a diamond coating on a carbide body keeps the tool sharp far longer. The most common choice for routing composites.
• PCD (polycrystalline diamond): cutting edges made of diamond tips; long-lasting and preferred in series production. Geometry options are more limited.

Tool geometry

• Compression router: the lower part of the tool cuts upwards and the upper part downwards. The two helix zones push the fibres into the plate and leave a clean edge on both faces. The transition point must sit within the plate thickness, so the depth is set according to the tool geometry.
• Diamond-cut (burr) composite router: many small cutting teeth formed by crossing left- and right-hand helix flutes. It spreads the cutting force and is widely used for routing composites.
• Up-cut router: evacuates chips well but can lift fibres on the top face.
• Down-cut router: leaves a clean top face but can cause fraying on the bottom face and pack chips into the kerf.

Tool diameter sets the smallest inside corner radius you can cut: an inside corner smaller than the tool radius cannot be routed. Small-diameter tools allow fine detail but break more easily and run at lower feed.

Cutting parameters

Feed rate, spindle speed and feed per tooth are related as follows:

Feed rate (mm/min) = spindle speed (rpm) × number of flutes × feed per tooth (mm)

If the feed per tooth (chip load) is too low, the tool rubs instead of cutting; heat rises and the tool dulls faster. If it is too high, cutting forces grow and the edge frays or the tool breaks. Starting values come from the tool maker’s tables for composite materials and are then tuned with test cuts.

General principles:

• Letting the tool dwell in one place builds up heat and leaves burn marks; keep dwell to a minimum at corners and entry points.
• Prefer ramped or helical entry to plunging straight down.
• Climb and conventional milling affect edge quality differently; compare both directions in a test cut. Where dimensional accuracy matters, follow a roughing pass with a light finishing pass.
• Dry cutting with strong dust extraction is common practice. An air blast cools the tool and clears chips from the cutting zone.

Workholding

The plate must be flat and fully supported; vibration directly degrades edge quality. A common solution is a spoilboard on a vacuum table. In full-depth cuts, the tool goes slightly into the spoilboard.

Small parts can come loose at the end of a cut, hit the tool or escape the vacuum. To prevent this, small holding tabs are left on the contour, or a thin skin is left on the final pass and the part is separated afterwards. Tab marks are cleaned up by sanding or a light router pass.

Drilling holes

Drilling is where delamination most often occurs in composite parts. At the entry face, the drill pulls the top plies up along its flutes and peels them; at the exit face, the thrust force pushes the last plies apart. Exit delamination starts once the thrust force exceeds a critical value, so any measure that lowers thrust reduces damage (Hocheng and Tsao, 2005; Liu et al., 2012).

• Use sharp drills designed for composites (for example brad-point or double-angle point geometries).
• Put a rigid backing board on the exit side.
• Reduce the feed as the drill approaches the exit.
• For large or precise holes, use helical interpolation with an end mill; thrust is low and the diameter can be set on the CNC.

In 3D woven plates, binder yarns tie the surface plies to the layers below, which reduces their tendency to separate. A backing board and controlled feed remain good practice all the same.

Preparing the DXF file

• Save the drawing at 1:1 scale in millimetres.
• Draw the outer contour and holes as closed polylines or circles. Remove open ends, overlapping lines and duplicate lines.
• Give inside corners a radius of at least the tool radius. Where a sharp inside corner is needed, add a tool-diameter relief (dog-bone) at the corner.
• Holes to be routed must be larger than the tool diameter; smaller holes are drilled.
• State dimensional tolerances and edge quality expectations on the drawing or in the order note.
• Place the parts within the plate boundaries and leave a machining margin at the plate edge.

Safety and carbon dust

Dust from machining carbon plate carries two separate risks. The first is electrical: carbon dust conducts electricity and can cause short circuits when it reaches machine electronics, drives or sockets. The second is health-related: fine dust can be respirable and can irritate skin and eyes.

• Extract dust at the source with a hood close to the tool and a filtered collector that retains fine dust.
• Where possible, cut inside an enclosure.
• Operators wear particulate respiratory protection of a suitable class (FFP2 or FFP3), safety glasses and gloves.
• Clean the machine and surroundings with a filtered vacuum instead of compressed air, which lifts the dust back into the air.

Follow the occupational health and safety rules on dust exposure that apply in your country; in Türkiye these are set out in the Dust Control Regulation (Tozla Mücadele Yönetmeliği) and related legislation.

Inspection after cutting

Check the cut part along its edges for fibre protrusion, fraying, burn marks and lifting of the surface plies. Verify dimensions with callipers or a coordinate measuring machine. Sharp edges can be lightly sanded; for parts that will work in humid conditions, sealing the edges with resin reduces moisture ingress. When preparing test specimens, the specimen preparation principles in ASTM D5687 can be followed.

In practice

The OWC carbon plate measures 500 × 250 × 6.5 mm and its resin has a glass transition temperature (Tg) of 140 °C; the edge temperature during cutting must stay below this value. You can machine the plate on your own equipment, or upload your DXF file to the CNC cutting service to see the price; the parts are cut from the plate and shipped.

Frequently asked questions

References

• Sheikh-Ahmad, J. Y. (2009). Machining of Polymer Composites. Springer.
• Teti, R. (2002). Machining of composite materials. CIRP Annals, 51(2), 611–634.
• Hocheng, H., Tsao, C. C. (2005). The path towards delamination-free drilling of composite materials. Journal of Materials Processing Technology, 167(2–3), 251–264.
• Liu, D., Tang, Y., Cong, W. L. (2012). A review of mechanical drilling for composite laminates. Composite Structures, 94(4), 1265–1279.
• Davim, J. P. (ed.) (2010). Machining Composite Materials. ISTE / Wiley.
• ASTM D5687/D5687M. Standard Guide for Preparation of Flat Composite Panels with Processing Guidelines for Specimen Preparation. ASTM International.
• Tozla Mücadele Yönetmeliği (Dust Control Regulation, Türkiye). Resmî Gazete, 5 November 2013, No. 28812.