Tolerance is the acceptable deviation in a part’s dimensions that allows it to meet its functional requirements. It is one of the first things product teams evaluate when choosing a machining service provider because it directly affects fit, performance, manufacturability, and cost.
As is usually assumed, CNC machining tolerances are not just for precision components; neither is a lower tolerance desired in every application, nor is it limited to linear dimensions. Different applications demand different levels of accuracy, and understanding that is necessary for optimal design and manufacturing.
This guide covers CNC machining tolerance: how it’s classified, what common types are dominant in the industry, how each is specified, what standards govern them, and how you can balance machining cost with tolerance.
What are CNC Machining Tolerances?
A CNC machining tolerance is the permissible variation in a part’s specified dimensions that allows it to meet its functional and assembly requirements. It defines the acceptable difference between the nominal dimension in the design and the actual dimension produced during machining.
Since no manufacturing process can achieve perfect accuracy, tolerances establish limits for size, form, position, geometry, and surface condition.
For example, if a CNC-machined shaft is specified as 100 ± 0.01 mm, the final diameter can range from 99.99 mm to 100.01, and is considered acceptable for use.
Why does Tolerance Matter in Machining?
Tolerance directly influences three engineering decisions:
First, it determines process selection. Based on the required tolerance, you decide whether conventional CNC machining meets the requirement or if precision machining is needed. Basically, it guides which operations are technically and economically appropriate.
Second, tolerance makes interchangeable manufacturing possible. For example, if you have two parts from different production runs that need to mate reliably, tolerance is what guarantees they will.
Third, it accounts for real-world variables. We know that tools wear out over time; machines vibrate, and there are thermal expansions, which could deviate the part from its actual value. A defined tolerance also establishes limits to capture those inconsistencies.
Classification of Tolerances
At a higher level, machining tolerances are classified into dimensional, geometric, and surface tolerances. Each affects part fit, function, and manufacturing performance differently.
Dimensional Tolerance
It defines an acceptable range based on the physical dimensions of a part: length, width, depth, and diameter.
Dimensional tolerance directly affects fit and assembly. For example, shafts, holes, and mating features rely more on dimensional tolerance to ensure proper clearance or interference.
Geometric Tolerance
Geometric tolerance is concerned with geometric characteristics: flatness, straightness, circularity, angularity, perpendicularity, and true position. These controls become important when a part may meet dimensional requirements but still fail functionally due to alignment or form errors.
Surface Tolerance
The more technical term here is surface finish or surface roughness, quantified in a Ra value, which stands for average roughness. Ra, expressed in micrometres (µm), is the average deviation of the surface profile from its mean line, expressed in micrometres.
A lower Ra value indicates a smoother surface, which affects friction, sealing, wear resistance, and appearance. A general machined surface might carry an Ra of 3.2 µm, while a precision ground surface could be as low as 0.2 µm or below.
5 Types of Tolerances (As Per Industry)
In industrial references, you may commonly find these five tolerance types: standard, unilateral, bilateral, limit, and GD&T tolerance.
Standard Tolerance
Standard tolerance refers to the default permissible deviation applied to any dimension on a drawing that does not carry its own explicit tolerance callout. In CNC machining, this is governed by DIN ISO 2768.
ISO 2768 defines four tolerance classes for linear dimensions: fine (f), medium (m), coarse (c), and very coarse (v). The standard also covers geometrical tolerances under three classes: H, K, and L. Most CNC machining shops default to ISO 2768-mK (medium dimensional tolerances with K-class geometrical tolerances) unless otherwise specified.
Here’s how tolerance ranges scale with part size under the medium (m) class:
- 0.5 to 3 mm: ±0.1 mm
- 3 to 30 mm: ±0.2 mm
- 30 to 120 mm: ±0.3 mm
- 120 to 400 mm: ±0.5 mm
Smaller parts allow tighter standard tolerances; larger parts naturally require wider bands. For features requiring tighter control, engineers specify tight tolerances on the drawing.
Unilateral Tolerance
Unilateral tolerance allows variation in one direction only from the nominal dimension. This type is used for mating features (fits) where one part must always stay within a boundary relative to the other, like shafts and holes.
For shaft machining, the maximum size is fixed so the part does not become too large to fit into a mating hole. For holes, the minimum size is fixed so the mating shaft can assemble without interference.
For example, if a shaft diameter has tolerances written as 25 +0.00 / -0.023mm, then you may find hole dimensions specified as 25 +0.023 / -0.00mm.
Bilateral Tolerance
In bilateral tolerance, dimensional variation is allowed in both directions from the nominal dimension. A dimension called out as 25 ±0.025mm means the part can measure anywhere between 24.975mm and 25.025mm and still be within spec.
The deviation does not have to be equal on both sides, either. For instance, 25 +0.010 / -0.020mm is still bilateral; it just distributes the tolerance band unevenly around the nominal.
It’s one of the most common tolerances in CNC machining because many dimensions do not require directional control. You may find it used for general dimensions of slot widths, plate thickness, bracket lengths, and non-critical mating features.
Limit Tolerance
Limit tolerance defines the acceptable dimension directly by stating the maximum and minimum permissible values.
For example, instead of writing a shaft size as 20 ± 0.05 mm, the drawing may specify it directly as 19.95 mm to 20.05 mm. This removes ambiguity on the shop floor and is common in high-volume production drawings.
GD&T Tolerance
GD&T (Geometric Dimensioning and Tolerancing) is best understood as a functional control system rather than just another tolerance type. It is used when dimensional tolerance alone is not enough to ensure a part performs correctly.
For example, a hole may have the correct diameter but still fail if its position is slightly off from the intended centerline. In such cases, GD&T controls the geometric relationship between features to ensure proper alignment, assembly, and functional performance.
It defines requirements for form, orientation, location, and runout, helping engineers control how features relate to one another in real applications.
GD&T is commonly applied under standards such as ASME Y14.5 and ISO GPS and is widely used in aerospace, automotive, medical devices, and precision engineering, where functional accuracy matters equally as dimensional accuracy.
How to Balance Machining Cost and Tolerance
Tighter tolerances directly increase machining cost because they require finer tooling, slower feed rates, stricter process control, and in some cases, secondary operations such as grinding. They may also demand higher-precision CNC machines and more frequent inspection during production.
That is why design engineers should not apply the same tolerance across every feature. For example, for a bore that mates with a bearing shaft, define tight control because the function depends on a proper fit. A cosmetic feature on the same part can follow general tolerances such as ISO 2768-m without requiring additional precision operations.
Over-specifying tolerance increases cost, while under-specifying may lead to assembly or functional issues.
Tolerances for CNC Machining Services at FastPreci
If you are a product designer or engineer looking for a manufacturing partner that can hold your defined part tolerances, FastPreci covers that. We offer CNC Milling, CNC Turning, and CNC Grinding, each capable of hitting the tolerance ranges your design requires.
| Machining Operation | Tolerance |
| CNC Milling | ±0.01 mm (Linear Dimensions) ±0.005 mm (Holes) |
| CNC Turning | ±0.002 mm |
| CNC Grinding | ±0.001 mm |
For CNC milling, we follow ISO 2768-m or ISO 2768-f as the general baseline, unless the design calls for something specific. If your part has tighter requirements or custom tolerance callouts, share your drawing with our team, and we will take it from there.
We help to identify where tight tolerances are necessary and where standard tolerances can be applied to reduce cost without affecting function. Reach out to FastPreci today to discuss your project requirements and get a tailored quote.
FAQs
What is a standard machining tolerance?
A standard machining tolerance is the default permissible deviation applied to dimensions on a drawing that do not carry an individual tolerance callout. In CNC machining, this is based on the standard ISO 2768.
How does tolerance affect machining costs?
For tighter tolerances, you need finer tooling, slower feed rates, and advanced CNC machines, which increases both machining time and cost. On the contrary, standard tolerances are achievable with the standard options without any additional cost.
Does material affect tolerances?
Yes, Harder materials (like tool steel) can hold tighter tolerances because they resist deflection during cutting. Whereas, softer materials are more prone to thermal expansion and tool pressure, which can push the part outside its tolerance band.
Is tight tolerance desired in every application?
No. Tight tolerance is only necessary where function depends on it, for instance, mating features or bearing fits. Applying it across every dimension adds cost without adding value. That’s why experts recommend going with standard tolerances in non-critical features.
