CNC Machining Tolerances: A Practical Guide for UK Design Engineers
Tolerance is one of the most important—and frequently over-specified—parts of a CNC machining drawing.
For a design engineer, adding another decimal place can feel like inexpensive insurance. For the manufacturer, however, that apparently small change can affect tooling, machining strategy, fixturing, inspection time and sometimes even the manufacturing process itself.
The objective should therefore not be to make every dimension as accurate as possible. It should be to make every dimension as accurate as the function requires.
That distinction can significantly affect the cost and lead time of CNC machined components.
What is a CNC machining tolerance?
A tolerance defines the acceptable variation from a nominal dimension.
For example, a shaft dimensioned:
Ø20.00 ±0.02 mm
can measure anywhere between 19.98 mm and 20.02 mm and remain within specification.
Modern CNC equipment is capable of extremely accurate machining, but machine positioning accuracy is only one part of the equation. Actual component accuracy is influenced by material, component geometry, tool deflection, workholding, temperature, number of setups and the method used to inspect the finished feature.
This is why asking whether a CNC machine can “hold 10 microns” is less useful than asking whether a particular feature on a particular component can economically and repeatably be manufactured to that tolerance.
Why tighter tolerances increase CNC machining cost
A tighter tolerance does not necessarily mean dramatically more cutting time.
The additional cost often comes from everything required to reliably achieve and prove the dimension.
That can include additional finishing passes, more rigid workholding, shorter or more specialised cutting tools, reduced feeds, tool-wear compensation, additional setups and increased inspection.
Extremely demanding features can also require processes such as reaming, boring, grinding or EDM rather than conventional milling alone.
Industry guidance similarly recommends avoiding unnecessarily tight tolerances because they can increase manufacturing cost and restrict manufacturing options.
The commercial implication is simple:
Do not apply precision to a component where precision provides no functional benefit.
Use general tolerances for non-critical dimensions
A useful drawing strategy is to establish a sensible general tolerance and individually specify the features that genuinely require tighter control.
General tolerance standards such as ISO 2768 are commonly used for this purpose.
Rather than individually tolerancing every pocket, edge and clearance feature, a drawing can specify a general tolerance while reserving tighter requirements for critical dimensions.
This makes the engineering intent much clearer to both the machinist and inspector.
It also helps prevent a manufacturer interpreting an unnecessarily precise CAD dimension—for example 37.000 mm—as meaning that the feature genuinely requires micron-level control.
Which features normally justify tighter tolerances?
There are plenty of situations where precision genuinely matters.
Typical examples include bearing locations, dowel holes, shaft fits, precision bores, sealing interfaces, alignment features and components forming part of a controlled mechanical stack-up.
Consider a machined enclosure.
The outside dimensions may have considerable freedom without affecting its function. The positions of two locating dowels, however, might determine the alignment of an optical or mechanical assembly.
Applying the same tolerance to both features makes little engineering sense.
A better drawing communicates their different functions.
Hole and shaft fits: specify the fit, not an arbitrary tolerance
Where two cylindrical components interact, ISO fit designations can often communicate design intent more effectively than independently chosen plus/minus tolerances.
A familiar example is an H7 bore used as part of a defined clearance, transition or interference fit.
The correct fit depends on the function of the assembly: whether the shaft must slide freely, locate accurately, be removable or remain permanently retained.
The important point is to start with the mechanical requirement, then derive the tolerance.
Don't begin with the tightest tolerance the supplier says it can manufacture.
Remember tolerance stack-up
Individual dimensions can all be within specification while an assembly still fails.
This happens when tolerances accumulate through a chain of components or dimensions.
Consider several components positioned sequentially between two fixed datums. If each component can vary independently, the worst-case variation of the final position can become considerably larger than the tolerance of any individual part.
Tolerance analysis is therefore particularly valuable around:
bearing assemblies
gear trains
optical systems
seals
shafts and spacers
connector locations
precision mechanisms.
Sometimes redesigning the datum structure is considerably cheaper than tightening every component in the stack.
GD&T can reduce unnecessary precision
Geometric Dimensioning and Tolerancing (GD&T) allows an engineering drawing to control the characteristics that actually affect function.
These include position, flatness, perpendicularity, parallelism, circularity and runout.
Used correctly, GD&T can provide a manufacturer with more freedom than conventional ± dimensioning while controlling the feature that matters to the assembly.
For example, the exact X and Y dimensions of several mounting holes may be less important than their true position relative to the functional datums.
Axion's CAD and mechanical design service includes production drawings and GD&T, making DFM review particularly useful before a component reaches production.
Surface finish is not the same as dimensional tolerance
Another common drawing issue is treating surface finish and dimensional accuracy as though they are interchangeable.
They describe different characteristics.
A dimension controls the size or geometry of a feature. Ra describes the average surface roughness.
General CNC machining can produce relatively fine surfaces, with roughly Ra 6.3 µm to Ra 0.8 µm covering a typical range depending on process and requirements. Protolabs, for example, lists less than 1.6 µm Ra as its typical CNC surface finish.
Axion can also provide polishing, grinding, anodising and other post-processing alongside CNC milling.
A bearing seat or sealing surface may require both dimensional and surface-finish control.
A hidden clearance pocket probably does not.
Again, specify the requirement where it adds functional value.
Material matters
The same tolerance can present very different manufacturing challenges depending on the material.
Aluminium alloys such as 6082 and 7075 generally machine very well, which is one reason they are widely used for precision components.
Stainless steels introduce different cutting forces, heat and tool-wear considerations.
Engineering polymers such as PEEK and acetal present another set of challenges, including thermal expansion and material movement.
Large, thin components can also distort as material is removed, regardless of how accurate the CNC machine itself is.
Axion machines aluminium, stainless steel, titanium, copper, brass and engineering polymers including PEEK and acetal, so material selection can form part of the DFM discussion rather than being considered independently from tolerancing.
Be careful with tolerances across multiple setups
Feature relationships are generally easier to control when they can be machined in the same setup.
Turning a component over, moving it to another machine or re-establishing a datum introduces another opportunity for variation.
This is one reason multi-axis machining and mill-turn equipment can be valuable for complex precision components.
Axion's CNC milling capability includes 3-, 4- and 5-axis machining, while its turning capability includes live tooling for features such as flats, cross holes and off-centre machining without transferring the component to a separate milling operation.
During DFM review, changing a datum or feature orientation can sometimes remove an entire setup.
That can improve repeatability and reduce cost.
A practical tolerance strategy
Before releasing a CNC component for quotation, ask four questions.
What dimensions actually control function? Identify interfaces, alignment features, fits and tolerance stack-ups.
What happens if this dimension varies? If ±0.1 mm has no effect on assembly or performance, there may be little reason to specify ±0.01 mm.
Does the drawing communicate the functional datums? A machinist needs to understand which features relate to one another.
Can the critical features be manufactured in the same setup? Small design changes can sometimes make this possible.
This is where involving the manufacturer before drawings are frozen can pay for itself.
CAD model plus engineering drawing: send both
For straightforward components, a STEP model may contain almost everything required to manufacture the geometry.
For precision components, the 2D engineering drawing remains extremely useful because it communicates information that geometry alone cannot reliably convey.
That includes critical tolerances, fits, GD&T, surface finish, threads, material specifications, finishing requirements and inspection notes.
When requesting a quotation from Axion Precision, sending both the STEP model and PDF engineering drawinggives the manufacturing team the clearest picture of the requirement.
Better tolerancing can mean better parts for less money
Precision manufacturing is not about applying the smallest possible tolerance to every dimension.
It is about controlling the characteristics that determine whether the component works.
A well-toleranced drawing tells the manufacturer where precision matters—and where it doesn't.
That can simplify machining, reduce inspection, improve supplier communication and ultimately make a component easier to manufacture repeatedly.
Need a CNC machining quote?
Axion Precision supports UK engineering teams from prototype through production with CNC milling, CNC turning, CAD/DFM support and finishing.
Send us your STEP file and PDF drawing and we can review the component for manufacturability as part of the quotation process.
Request a CNC machining quote from Axion Precision.
