CNC Machining Tolerances: How to Specify Them Without Increasing Part Cost
Tolerance is one of the most important—and frequently over-specified—parts of a CNC machining drawing.
A component may only need one accurately located bore, a controlled bearing fit and a flat mating face to perform correctly. Yet it is surprisingly easy for a drawing to place unnecessarily tight tolerances across almost every dimension.
The result isn't necessarily a better component.
It can instead mean additional machining operations, slower feeds, more setups, temperature control, extra inspection and potentially processes such as reaming, grinding or EDM. All of those can increase both price and lead time.
For design engineers and purchasing teams sourcing CNC machined components in the UK, the better approach is to identify which dimensions actually control function and apply tighter tolerances selectively.
What is a CNC machining tolerance?
A tolerance defines how far a manufactured dimension is permitted to deviate from its nominal value.
For example:
25.00 ±0.10 mm
allows the finished dimension to measure between 24.90 mm and 25.10 mm.
By comparison:
25.00 ±0.01 mm
permits only one tenth of that variation.
Both dimensions may be entirely achievable, but they don't impose the same manufacturing requirement.
The tighter tolerance reduces the available process variation. Depending on the feature, achieving it consistently may require more careful machining, additional finishing operations and more extensive inspection.
This is why tolerances should ideally be based on the functional requirements of the assembly rather than simply specifying the smallest value a supplier can manufacture.
How tight can CNC machining tolerances be?
There isn't one universal tolerance for CNC machining.
Capability depends on factors including:
component geometry and size;
material;
feature type;
machine and tooling;
number of setups;
thermal stability;
workholding;
measurement method; and
whether secondary processes are available.
As a useful industry reference rather than an Axion-specific capability promise, Protolabs publishes ISO 2768-based CNC machining tolerances and states that tighter requirements can be applied to individually specified features. Its UK guidance also notes that particularly demanding tolerances can require secondary processes including grinding, polishing or EDM.
The important question therefore isn't:
“What is the tightest tolerance CNC machining can achieve?”
It is:
“What tolerance does this feature need for the product to work reliably?”
That distinction can have a significant effect on manufacturing cost.
Use general tolerances for non-critical dimensions
A well-structured engineering drawing doesn't need an individual ± tolerance beside every dimension.
General tolerances can instead establish acceptable variation for dimensions that don't have a specific functional requirement.
ISO 2768 is commonly used for this purpose. Commercial CNC machining services, for example, use ISO 2768 tolerance classes for otherwise unspecified dimensions.
A drawing might therefore define an appropriate general tolerance in its title block while applying individual tolerances only to critical features.
That immediately tells the machinist something valuable:
these dimensions are important; the remainder can be manufactured using the agreed general tolerance.
It can also make drawings substantially easier to interpret.
Tighten tolerances where the function requires them
There are several areas where tighter control can be entirely justified.
Bearing and shaft fits
A bearing bore, shaft journal or precision bush interface may require a defined ISO fit rather than an arbitrary bilateral tolerance.
The required fit should be chosen according to the assembly's function: clearance, transition or interference.
Using the appropriate fit communicates design intent far more effectively than simply making every nearby dimension ±0.01 mm.
Locating features
Dowel holes and other locating features frequently determine the alignment of an assembly.
Their diameter and relationship to other features may therefore matter considerably more than surrounding cosmetic geometry.
This is a good application for geometric tolerancing.
Sealing surfaces
O-ring grooves, gasket faces and other sealing features can require control of dimensions, geometry and surface texture.
Here, simply specifying a linear tolerance may not describe everything the function requires.
Mating components
Where two machined components interface, identify the dimensions that actually determine their relationship.
Tolerance analysis—or a tolerance stack for an assembly—can help establish how much variation each component can accept while still meeting the final functional requirement.
Don't use ±0.01 mm as a default
A drawing covered in ±0.01 mm dimensions can look precise.
Manufacturing it is another matter.
A dimension that could comfortably vary by ±0.1 mm may provide no additional product performance when controlled to ±0.01 mm, yet the tighter requirement can reduce manufacturing flexibility.
The manufacturer must not only produce the feature but demonstrate that it meets the drawing.
Inspection therefore becomes part of the cost equation.
A loose clearance feature might be quickly verified using conventional metrology. A tightly controlled relationship between multiple features may require a CMM or a more detailed inspection strategy.
For production quantities, that additional time is repeated across the batch.
The effect becomes particularly important when moving from prototype quantities into regular production.
Think about tolerance across multiple setups
A feature's manufacturability isn't determined by its nominal tolerance alone.
The relationship between features matters too.
Imagine two precision holes machined in the same setup. Their relative position can often be controlled efficiently because the component remains in one fixture and coordinate system.
If the relationship instead spans features requiring the component to be removed, repositioned and machined from another orientation, additional sources of variation are introduced.
This is one reason multi-axis machining can be valuable for complex components. Axion uses 3-, 4- and 5-axis CNC milling, allowing suitable geometries to be produced with fewer setups.
A DFM review can sometimes identify ways of changing datum structures or feature geometry so critical relationships are easier to manufacture and inspect.
Tolerance isn't the same as surface finish
These requirements are sometimes confused.
Dimensional tolerance controls the permitted variation in size or geometry.
Surface roughness describes the texture of the machined surface and is commonly expressed as an Ra value.
A component can therefore be dimensionally accurate while having an inappropriate surface texture—or have an extremely smooth surface while being dimensionally incorrect.
Only specify a demanding Ra value where the application requires it.
Typical examples might include:
sealing surfaces;
bearing or sliding interfaces;
cosmetic faces;
optical or precision interfaces; and
areas requiring a particular coating or subsequent process.
Applying an unnecessarily fine surface-finish requirement across an entire component can introduce additional machining or finishing operations without improving its function.
Remember the effect of finishing
The drawing should also account for what happens after machining.
Anodising, plating, polishing, grinding, bead blasting and other finishing processes can affect surfaces differently.
If a finished component contains a critical bore, threaded feature, sealing surface or electrical contact area, tell your manufacturing supplier which dimensions are critical after finishing.
It is much easier to plan masking, machining allowance or secondary operations before production starts than to resolve a fit problem after an entire batch has been finished.
Axion's CNC milling service includes post-processing options such as anodising, polishing, powder coating and grinding, so these requirements can be considered as part of the overall manufacturing route rather than as an afterthought.
Give your CNC supplier both STEP and drawing data
For most precision-machined components, the 3D CAD model and 2D drawing serve different purposes.
The STEP model communicates the component geometry efficiently.
The engineering drawing communicates requirements that aren't always unambiguously contained in the model, including:
critical tolerances;
datums and GD&T;
threads;
fits;
surface-finish requirements;
material specification;
finishing;
inspection requirements; and
other manufacturing notes.
This is why Axion asks customers to include both PDF drawings and STEP files when requesting a quotation.
Providing both also gives the manufacturing engineer an opportunity to spot conflicts before production.
A practical tolerance strategy
For a new CNC-machined component, a sensible drawing review is:
Identify the features that determine whether the component functions.
Establish the permissible variation of those features from the actual assembly requirement.
Use appropriate fits or GD&T where they communicate the requirement better than ± dimensions.
Apply a suitable general tolerance to non-critical dimensions.
Specify surface roughness only where function or appearance requires it.
Consider coating and finishing thickness when defining critical finished dimensions.
Review tolerance stacks across mating components.
Ask your machining supplier to review the design before committing to production tooling or a large batch.
The last point is particularly useful.
A machinist may be able to identify a tolerance that is straightforward to achieve with a small design change—or an expensive requirement that provides no practical benefit.
Design for manufacture before requesting production pricing
Tolerance decisions made during CAD development affect more than machining accuracy.
They influence tooling, workholding, machine selection, inspection and sometimes the entire manufacturing process.
Axion combines CNC machining with mechanical design and DFM support, allowing the manufacturing route to be considered while the design can still be changed. Our machining capability covers aluminium, stainless steel, titanium, copper, brass and engineering polymers across prototype and production quantities.
For purchasing teams, this can also make quotations easier to compare.
Rather than asking several suppliers to interpret an over-constrained drawing differently, a clearly toleranced specification establishes exactly what needs to be manufactured and inspected.
Need a CNC machining quote?
If you have a component ready for manufacture—or a drawing you'd like reviewed before release—send Axion Precision your STEP model and PDF engineering drawing.
We can review the geometry, material, tolerances, finish and quantity together and identify potential DFM opportunities before manufacture.
