A prototype can look perfect on screen and still fail when two parts are assembled. A mounting hole sits slightly out of position, a shaft binds in its housing or a cover needs force to close. Often, the problem is not poor machining but an unclear tolerance specification.
For UK engineers and buyers sourcing CNC machined prototypes, the aim is to specify enough accuracy for the component to work, without paying for precision that brings no practical benefit. This guide explains the figures, shows how tolerances affect cost and sets out what to include in a drawing and quotation request.
What is a CNC machining tolerance?
A CNC machining tolerance is the permitted variation from a specified dimension or geometry. If a drawing calls for a 20.00 mm width with a tolerance of ±0.10 mm, an acceptable measured width can range from 19.90 mm to 20.10 mm, subject to the drawing's measurement and acceptance requirements. The total tolerance band is 0.20 mm.
The distinction matters: tolerance is not a promise that every part will measure exactly 20.00 mm. It is an agreed range within which the component is considered acceptable. A nominal size alone does not tell a machinist what variation is permitted.
Dimensional tolerances control sizes and distances. Geometrical tolerances control characteristics such as flatness, perpendicularity, position and run-out. A hole might have the correct diameter but still prevent assembly because its centre is in the wrong place.
CNC machining tolerances UK: a practical comparison table
The following values are illustrative planning bands, not a universal industry standard, a price list or a statement of Attwood PD's guaranteed capability. Actual achievable tolerances depend on the material, feature, part size, machining method and inspection arrangement.
| Example bilateral tolerance | Total permitted variation | Possible application | Likely cost implication |
|---|---|---|---|
| ±0.20 mm | 0.40 mm | Non-critical covers, clearance features or initial form trials | Often comparatively economical |
| ±0.10 mm | 0.20 mm | Many general machined dimensions on suitable parts | Common starting point for discussion |
| ±0.05 mm | 0.10 mm | More controlled mating features or mounting interfaces | More process control and checking may be needed |
| ±0.02 mm | 0.04 mm | Selected accurate bores, locations or interfaces | May require specialist tooling, setups and inspection |
| ±0.01 mm | 0.02 mm | Carefully selected high-precision features | Feasibility review and potentially additional operations |
Do not apply a single row to every feature on a component. A straightforward external dimension on a compact aluminium part is a different challenge from a deep pocket in a flexible plastic housing. Very tight figures should always be confirmed against a specific drawing before an order is placed.
Why do tighter CNC tolerances cost more?
Reducing the permitted variation leaves less room for normal process effects, including tool wear, cutting forces, temperature changes and movement when a component is clamped or released. A supplier may need to adjust how the part is made and how its dimensions are verified.
Machining time and process control. Additional finishing passes, smaller cuts, slower feeds or more frequent tool checks can increase machine time.
Fixturing and operations. A feature that must be accurately located relative to several faces may require carefully designed workholding or another setup. Reaming, boring or grinding may be appropriate for particular critical features.
Inspection. A ±0.02 mm callout is of little use without a suitable measurement method. Precision gauges, a coordinate measuring machine (CMM), controlled conditions or documented reports can add time and expense.
Scrap and delivery risk. As the tolerance band narrows, variation that would once have been acceptable may cause rejection or rework. This matters especially for one-off prototypes where a replacement can affect a test date.
There is no reliable universal percentage by which a change from ±0.10 mm to ±0.05 mm increases a quote. One short bore may be simple to control while the same requirement across many surfaces can transform the job. The useful question is: which particular feature needs the extra accuracy, and why?
Which features genuinely need tight tolerances?
Start with function rather than applying precision throughout the drawing. Examples that may deserve closer control include:
- Bearing seats and shaft fits, where clearance or interference affects operation.
- Locating holes and dowel positions, where misalignment can stop an assembly fitting.
- Sealing faces, where flatness and surface finish can be as important as size.
- Sliding or guided interfaces, where too much clearance creates play and too little causes binding.
- Critical datum surfaces, which establish how the component locates during assembly and inspection.
A decorative outside face, generous clearance slot or non-functional recess may need far less control. A prototype bracket, for instance, could have relatively relaxed overall dimensions while two dowel holes need carefully defined size and positional relationships.
Also consider tolerance stack-up. If two dimensions in a chain each have a possible variation of ±0.10 mm, their worst-case contribution to that chain can be ±0.20 mm, assuming they add in the same direction. Several individually acceptable parts can therefore create an unacceptable assembled result. Design the tolerance around the full assembly, not just one drawing.
How to specify tolerances on a drawing
A 3D CAD model describes nominal geometry. For functional interfaces, a controlled 2D drawing or an agreed model-based specification helps make acceptance criteria unambiguous. Include:
- A sensible general tolerance for dimensions without specific callouts, agreed with the manufacturer.
- Individual tolerances on critical dimensions, rather than making the entire part unnecessarily precise.
- Datums and geometrical controls where position, form or orientation affects assembly.
- Material grade, surface finish and any coating requirements, particularly on interfaces affected by finishing.
- Drawing revision and inspection requirements, so manufacture and approval refer to the same version.
In UK engineering drawings, ISO 2768-1 is one established reference for general linear and angular tolerances. For hole and shaft fits, ISO 286-1 provides the basis of the ISO fit system. These are different tools for different specifications, not interchangeable shorthand.
Be careful with legacy notes that specify ISO 2768-2 for general geometrical tolerances: that part was withdrawn and succeeded by ISO 22081:2021. If an inherited drawing refers to older standards, agree how it should be interpreted before production.
How material, geometry and finishing affect accuracy
Aluminium, stainless steel and engineering plastics behave differently under cutting forces and temperature changes. Thin aluminium walls may deflect during milling. Stainless steel can demand careful control of cutting conditions. Some plastics can move under clamping pressure or change dimensions with temperature and moisture. None of these materials has one meaningful tolerance capability independent of the part design.
Feature geometry matters as much as material. Deep narrow pockets, long slender shafts, thin ribs and features reached with long tools are often harder to hold accurately than accessible, well-supported surfaces. Milling and turning can each deliver controlled results, but the correct process depends on the component shape and the features being controlled.
Ask whether the quoted tolerance applies before or after finishing. Anodising, plating or another surface treatment may alter final dimensions or mating behaviour. Attwood PD can help coordinate surface finishing with the machining specification so that the finished component, rather than just the freshly cut part, meets the intended function.
What inspection should prototype buyers request?
Inspection needs to match the risk. For a non-critical development housing, routine dimensional checks may be sufficient. For a fit-critical assembly or approval-stage prototype, ask which dimensions will be recorded and which instruments or methods will be used.
Options can include calipers or micrometres for suitable features, plug or pin gauges for holes, and CMM measurement for more complex geometrical relationships. Measurement uncertainty matters when a requirement approaches the capability of the measuring system. Agree whether you need a dimensional inspection report, first-article evidence, material traceability or checks on every part rather than samples.
These arrangements should be established before quotation, not assumed from a statement such as 'precision machined'. Attwood PD's quality and compliance support can help align inspection and documentation with the project's real requirements.
Prototypes, small batches and production: should the tolerance change?
For an early concept model, the priority may be overall fit, handling or appearance. A functional prototype needs enough accuracy to make test results meaningful. Later validation parts may need production-intent materials, fit requirements and repeatable inspection, especially if the design will move into low-volume or ongoing manufacture.
Tightening every dimension for the first prototype can waste budget and delay learning. Equally, making an early test part too loosely may hide problems or create failures unrelated to the design. Record which features are essential to the test and carry those requirements into later revisions. If a future design will use injection moulding, discuss how the machining-based validation relates to the eventual moulded part, since the processes have different dimensional behaviours.
A buyer's checklist before requesting a CNC quote
- Supply a STEP file or other suitable 3D model alongside a revision-controlled drawing where necessary.
- Identify critical fits, datums, positional requirements and inspection points.
- Separate functional tolerances from cosmetic preferences.
- State the material grade, required finish, quantity and intended use.
- Explain whether the parts are for concept evaluation, functional testing or repeat production.
- Ask the supplier to flag expensive or difficult callouts and suggest alternatives where appropriate.
The aim is not to loosen tolerances indiscriminately. It is to put precision where it protects performance and leave sensible manufacturing freedom elsewhere.
Frequently asked questions
What is a standard CNC machining tolerance in the UK?
There is no single tolerance guaranteed by every UK CNC supplier. Figures around ±0.10 mm are a useful starting point for discussing some general machined features, but the quotation and drawing must define what actually applies.
Can CNC machining achieve ±0.01 mm?
It can be achievable for selected features under suitable conditions, but it should never be assumed for a whole component. Geometry, material, size, process and verification determine feasibility.
Are tighter tolerances always better?
No. The best tolerance is the one that reliably achieves the required fit or function. Extra precision without a functional reason can increase cost and lead time.
Is a CAD file enough to request a CNC quote?
A CAD model is often enough for an initial discussion, but a drawing or equivalent controlled specification is advisable where critical fits, geometrical controls or inspection evidence matter.
A practical next step
Attwood PD supports UK teams with CNC machining for plastic and metal components, from functional prototypes to ongoing supply. If you are unsure whether a tolerance is necessary or commercially sensible, share your model, drawing and assembly requirements. A review of the critical features can help establish a manufacturing and inspection plan before committing to the parts.
