Karwon Composites publishes general size ranges and confirms tolerances against your application at quotation stage.
An enquiry arrives asking for carbon rod at a tolerance that would be ambitious for a ground steel shaft. Somewhere upstream, someone has copied a tolerance from a metal drawing onto a composite one, and nobody has questioned it.
The result is predictable. Suppliers who understand the material quote a longer lead time and a higher price for a grinding operation, or decline. Suppliers who do not understand it accept the tolerance, deliver material that does not hold it, and the problem surfaces at goods inward.
Composites hold dimensional tolerance differently from metals, and the difference is a function of process rather than care.
Why process determines tolerance
In machining you start oversize and remove material until you reach the dimension. The tolerance you can hold is a function of your machine and your patience.
In most composite manufacture you are consolidating fibre and resin into a shape, and final dimensions emerge from the interaction of fibre volume, resin content, thermal expansion, cure shrinkage and tooling. The dimension is an outcome of a process, not a target you approach by removing material.
This is why an as manufactured composite tolerance is looser than a machined metal tolerance, and why the way to tighten it is to add a machining step afterward rather than to try harder during manufacture.
Process by process
Pultrusion. Fibre is pulled through a heated die. The die controls the section, so tolerance is good and consistent along the length, and it is set by die wear and pull conditions rather than by operator skill. As pultruded round rod typically holds a diameter tolerance in the range of a few hundredths of a millimetre to around a tenth, depending on diameter and the mill.
Ground pultruded rod. Take a pultruded rod and centreless grind it. Now you are machining, and tolerances tighten substantially, into the range people associate with precision shafting. It costs more and it removes the outer resin rich layer, which changes the surface. Ground rod is the correct answer for linear guides, shafts and anywhere an exact fit is required. It is unnecessary for a structural member in bending.
Roll wrapped tube. Prepreg is wound onto a mandrel and cured under tape. The inside diameter is set by the mandrel and is therefore the tightly controlled dimension. The outside diameter is the accumulation of ply thicknesses and is looser. Wall thickness varies accordingly.
This has a direct design consequence that catches people out. If you are bonding a fitting inside a roll wrapped tube, you are working to a controlled dimension. If you are clamping something around the outside, you are working to a looser one. Where possible, design to the bore.
Pultruded tube. Both diameters are die controlled, so the outside is better controlled than a roll wrapped tube, but the wall is limited in what the process can produce and the fibre architecture is predominantly axial. Different product, different use case.
Moulded and pressed plate. Thickness tolerance depends on whether the mould is closed and matched, or whether it uses a vacuum bag on one side. A matched die tool controls thickness on both faces. A bagged laminate controls one face and leaves the other to the layup, and thickness variation across a large panel is real.
Flatness on plate is a separate specification from thickness, and it is the one that gets forgotten. A panel can be perfectly within thickness tolerance and still have a bow across its diagonal that makes it useless for a flat bonded joint. If flatness matters, specify it explicitly, because it will not be inferred.
Specify the dimension that matters
The most useful discipline available to a composites buyer: identify which single dimension your assembly actually depends on, and specify that one tightly.
Every additional tight tolerance costs money and lead time. Tolerancing everything to the tightest achievable value is how a straightforward part becomes an expensive one.
Some worked examples.
A tube carrying bending load. Stiffness scales with the fourth power of diameter and the wall contributes far less. Your critical parameter is not a tight diameter tolerance, it is the fibre architecture and consistent wall thickness. Specifying the outside diameter to a fine tolerance adds cost and buys nothing structural.
A tube receiving a bonded insert. Now the bore is critical, because bond line thickness controls joint strength. Specify the inside diameter tightly and let the outside float.
A rod acting as a linear shaft. Now the outside diameter and the surface finish are both critical, and you want ground rod. This is the case where a tight tolerance genuinely earns its cost.
A plate being bonded to a flat surface. Flatness is critical, thickness less so within reason. Specify flatness explicitly.
How tolerance interacts with the rest of the specification
Two effects worth knowing.
Tighter tolerance means more rejected material, which means either higher price, longer lead time, or both, since the mill has to produce more to deliver your quantity. On a small order this can be the difference between a viable job and an unviable one.
Tolerance is stated on a defined measurement condition. Composites move with temperature and, to a smaller degree, with absorbed moisture. A tolerance quoted at twenty degrees is not automatically held at fifty. For most structural applications the movement is small enough to ignore. For precision applications in this climate it is not.
Asking for the right thing
When you send an enquiry, the useful shape is:
- The nominal dimension
- The tolerance you actually need on the one dimension that matters, and why
- Where you are relaxed, stated explicitly so the supplier does not assume tight everywhere
- The surface finish requirement, if any
- The temperature range the part works in, if it is unusual
Point three is the one that saves money. A supplier who knows you are relaxed on outside diameter will quote a standard product. A supplier who has to assume will price the risk in.
Our approach
Karwon Composites publishes general size ranges on our product pages and confirms tolerances against your application at quotation stage, per order and per batch. We do this deliberately rather than publishing a headline tolerance figure, because the achievable tolerance depends on the process, the size, the mill and the quantity, and a single published number would be wrong for most enquiries.
Tell us what the part has to do and which dimension the assembly depends on. If a tighter tolerance is genuinely needed, we will source or specify a secondary operation for it. If it is not, we will tell you that too.