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Carbon Fiber Part Design Guidelines for CNC Machining
Ten rules that keep a carbon part cheap to cut and strong in use. Check your DXF against them before you send it.
Design rules at a glance
Typical values for woven and quasi-isotropic plate on a CNC router. They are planning values, not limits. A shop can go past most of them, at a price. The shop that quotes you confirms what it can hold.
| Feature | Rule of thumb | Why |
|---|---|---|
| Hole diameter | At least the plate thickness | A 2 mm plate takes 2 mm holes easily. Smaller holes are possible but break more tools. |
| Hole to edge | 2 x hole diameter or more | Measured from hole centre to the edge. Closer than that, the edge can crack under a fastener. |
| Inside corner radius | R1.5 mm or larger | Inside corners copy the cutter radius. R1.5 lets the shop use a 3 mm tool, which cuts faster and lasts longer. |
| Slot width | 2 mm or wider | 1 mm tools exist but are slow and fragile. Wider slots cost less. |
| Bridge between cutouts | 1.5 x thickness, minimum 2 mm | Thin bridges flex during cutting and can delaminate. |
| Profile tolerance | ±0.1 mm typical | ±0.05 mm is achievable on routers with good fixturing. Only call it where it matters. |
| Hole tolerance | ±0.05 mm drilled | Bearing fits need interpolated or reamed holes with PCD tools. Ask for it on the drawing. |
| Thickness | Set by the plate | Machining does not change thickness. Press-cured plate is often ±0.1 to ±0.2 mm. Check the plate datasheet. |
| Countersinks | Leave 1/3 of thickness straight | A countersink that runs through most of a thin plate leaves a knife edge that cracks. |
| Threads | Use inserts | Threads cut into carbon strip out under load. Use bonded or press-fit inserts, or through bolts with nuts. |
Preparing a DXF for carbon cutting
- Scale 1:1, units in millimetres, one part per file or a clearly labelled nest.
- Closed outlines only. No duplicate or overlapping lines, they make the router cut twice.
- Separate layers for through cuts, pockets and engraving, named plainly.
- Holes drawn at finished size. Say which ones are fits and which are clearance.
- Add a PDF with thickness, plate type, cosmetic side, edge finish and quantity.
Cosmetic side and edges
Carbon plate often has one gloss face and one peel ply or matte face. Mark the side that will be seen. Routing leaves a sharp, slightly rough edge. Ask for sanded, chamfered or sealed edges if people will handle the part, and sealed edges if it will see moisture or fuel.
Threads, inserts and fasteners
Threads cut straight into carbon strip under load and crack the plies around them. Use one of three options instead:
- Through bolts with washers under head and nut to spread the load.
- Bonded inserts in aluminium, stainless or titanium, set in epoxy after machining.
- Press-fit or flanged inserts made for composites, in holes sized to the insert maker's table.
Keep fastener holes at least two diameters from any edge and from each other. On thin plate, washers matter more than torque.
Which plate to specify
For most brackets and frames, woven plate (twill, plain) is the default. Ask for UD or quasi-isotropic plate when stiffness per weight matters most, and forged carbon when the part has 3D shape or many small features.
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