A carbon part looks finished the moment it comes out of the mould. It isn’t. The edges still have to be trimmed, holes drilled, cut-outs milled. This is exactly where the hard part begins. Anyone who has watched a cutter travel through a laminate of carbon fibres understands why we look so closely at this step with our suppliers.
Carbon eats tools
Carbon fibre is hard and extremely abrasive. Every cut grinds away at the tool, and an ordinary carbide cutter goes blunt quickly. That is why the work is done with diamond-coated tools – and even those only last for a limited number of parts. Add the wrong spindle speed or too high a feed rate, and the edge frays or the laminate separates into layers. That is called delamination, and a part with delamination at the edge is scrap.
Then there is the dust. Carbon dust is fine, electrically conductive and anything but harmless to your health. That is why we mill wet. The coolant binds the dust, cools the cutting point and flushes the chips away. It makes the job cleaner, but not easier.
The real sticking point: the part must not move
The biggest issue in milling isn’t the tool at all – it’s the fixturing. A carbon part is stiff, but thin and often curved. If it moves even minimally during the cut, the edge vibrates, the laminate tears and dimensional accuracy is gone. So the part has to sit rock-solid – and it has to do so without the clamping force itself leaving pressure marks or cracks. Two approaches have proven themselves.
Powerful magnets
The classic solution we rely on with our suppliers is very strong magnetic clamping systems. Carbon itself is not magnetic, of course. What is held is the fixture – the nest the part sits in. These nests are fixed onto a magnetic plate, and the holding force is enormous. The advantage is obvious: fast changeover, no bolting, the table surface stays clear and the force is spread over an area rather than through individual screws. For many geometries this is the robust, economical solution.
Increasingly common: pneumatic vacuum fixturing
With thin, strongly curved or delicate parts, however, classic clamping reaches its limits. Where do you grip a filigree part without deforming it? For exactly this, we increasingly see pneumatic solutions at our partners, where the part is drawn in and held by vacuum. The fixture is then a custom-made vacuum nest that follows the contour of the component and pulls it down over a large area. No point loads, no marks, and even thin shells stay put.
The catch: it costs more. Every geometry needs its own nest, plus a pump, seals and a properly tuned system so nothing draws air. That investment has to pay off over the quantity. For small runs or single parts, magnetic clamping is often the better choice; for demanding contours and higher volumes, you run more smoothly with vacuum in the end.
Where tolerance comes from
It’s easy to underestimate how tight the requirements on carbon parts can be. For add-on parts, the look may just need to be right – that’s often enough. But as soon as a part is bolted, fits into an assembly or carries a load-bearing function, we’re talking tenths or even hundredths of a millimetre. Hole patterns have to match the mating part, cut-outs have to line up, edges have to sit precisely.
The tricky bit: tolerance isn’t decided by the tool alone, but precisely at the fixture. If the part sits slightly distorted under tension in the nest, you mill to the exact millimetre – in the wrong place. The hole pattern is correct in the machine, and still doesn’t fit the mating part, because the part springs back once released. That is why the nest has to reproduce the component contour exactly and hold the part in its natural position without bending it. Temperature plays a part too: mill too hot and the material works, and as it cools the edge shifts. Machining wet keeps not only the dust under control, but the dimensions as well.
What ends up on the inspection report is therefore the sum of tool, feed, cooling and, above all, the fixture. That is why we agree the tolerances with our suppliers in advance and tune the fixture to them. A part that has to hold 0.1 mm on the drawing needs a nest that actually allows exactly that.
Why we look so closely at this
In the end, the fixturing decides whether a part comes out of the machine to size or lands on the scrap pile. That is why we talk to our suppliers not only about material and laminate lay-up, but also about the fixture. Which clamping concept fits the geometry, the quantity and the budget? We settle that question before the first chip, not after. For our customers that means: you get a part that fits, without nasty surprises in the process.
Further reading: Why we manufacture carbon tooling from steel and aluminium rather than composite is explained in our article on carbon fibre part manufacturing in Asia. We’ve covered the difference between genuine carbon and forged carbon separately. For an overview of our carbon sourcing, see our parts sourcing page.
