Getting a part made to the wrong tolerance is an expensive mistake. You might not notice it until assembly and by then, the damage is done. Whether you’re specifying parts for the first time or reviewing a supplier’s quote, properly understanding precision CNC machine tolerances will save you time, money and a fair amount of frustration.
Most guides on this topic focus on the numbers but skip the reasoning behind them. This one covers both.
What does Tolerance Actually Mean in CNC Machining?
Tolerance, in plain terms, is the acceptable amount a finished dimension can vary from what the drawing specifies. No machine, regardless of how expensive or well-maintained, produces a perfect dimension every single time. Temperature fluctuations, tool wear and material movement during cutting introduce small variations.
Tolerance sets the boundaries for how much variation is acceptable before the part is considered out of spec.
So if a drawing calls for a shaft diameter of 25mm with a tolerance of ±0.05mm, the finished part can measure anywhere between 24.95mm and 25.05mm and still pass inspection.
CNC Tolerance Standards: ISO 2768 Explained
The most widely used international reference is ISO 2768, which defines general tolerances for linear and angular dimensions. It saves engineers from having to specify a tolerance on every single feature of a drawing.
ISO 2768-1 covers linear and angular dimensions and splits into four tolerance classes:
- Fine (f): For high-precision work, typically applied to metals
- Medium (m): The most commonly used class for general engineering
- Coarse (c): For lower-precision applications
- Very Coarse (v): Used when large dimensional variation is acceptable
In practice, most CNC shops default to ISO 2768-m (medium) unless otherwise stated. For metal parts where tighter control is needed, ISO 2768-f (fine) is often specified. For plastics, the medium is usually the starting point because plastics respond differently to heat and cutting forces.
As a rough benchmark: standard CNC machining tolerance sits around ±0.127mm (±0.005″) for metal parts. For high-precision work, that figure tightens to ±0.025mm (±0.001″) or better.
How Tight Can CNC Machines Actually Go?
This is where things get interesting. Some CNC machines, under the right conditions, can hold tolerances as tight as ±0.0025mm, roughly a quarter of the width of a human hair.
But ‘can hold’ is different from ‘should hold’. Running a machine at its absolute precision limit costs more, takes longer and requires stricter inspection. It also demands environmental controls: temperature swings in a workshop cause metal to expand and contract, which throws off dimensions at the micron level.
Practically speaking, here’s a useful breakdown by precision level:
- Standard: ±0.127mm (±0.005″) for general components, most industrial applications
- High precision: ±0.025mm (±0.001″) for medical devices, aerospace, tight assemblies
- Ultra precision: ±0.0025mm or tighter for specialist applications, often requiring climate-controlled environments
The takeaway: tighter tolerances are achievable but they come with higher cost and longer lead times. They should only be specified where the function of the part genuinely requires it.
CNC Milling Tolerances vs CNC Turning Tolerances
Milling and turning are both capable of high-precision work but they behave differently and knowing which suits your part’s geometry matters.
CNC milling tolerances are well-suited to:
- Flat surfaces and complex 3D profiles
- Positional accuracy across multiple features (e.g. bolt hole patterns)
- Parts with pockets, slots and irregular geometry
Standard milling tolerances sit around ±0.127mm, with precision milling capable of ±0.025mm or tighter on critical features.
CNC turning tolerances are well-suited to:
- Cylindrical and rotational features like shafts, bores and threads
- Concentricity and roundness requirements
- Diameter control across long runs
For cylindrical features specifically, turning typically achieves tighter tolerances more cost-effectively than milling. A turned shaft can hold ±0.005mm in diameter in many cases, whereas achieving the same with milling would require additional setups and cost.
The practical rule: if your part has critical cylindrical features (bearing seats, press fits, threaded bores), CNC turning services are usually the more economical choice. For complex geometry with precise feature relationships across multiple faces, CNC milling services offer a reliable solution.
Accuracy vs Tolerance in CNC Machining: What’s the Difference?
This distinction trips up a lot of people, including experienced engineers.
Accuracy
It describes how close the machine’s output is to the intended dimension. A machine that consistently produces a 25.03mm diameter when you specified 25.00mm has a systematic error. It’s inaccurate but it may be consistent.
Tolerance
Tolerance is the allowable range of variation. It doesn’t care whether the error is systematic or random, only whether the final dimension falls within the specified limits.
Here’s where it gets practical: a machine with poor accuracy can sometimes still produce parts within tolerance if the range is wide enough. Conversely, a highly accurate machine might still produce out-of-tolerance parts if the tolerance band is extremely narrow and any variation at all causes failure.
When specifying work, you want to think about both. High-accuracy machines are important when you’re working to tight tolerances. You need the machine’s natural tendencies to fall consistently within a narrow band.
Factors that Affect CNC Machining Tolerance
Tolerances aren’t just a function of the machine. Several variables interact to determine what’s realistically achievable:
Material Properties
Metals like aluminium and steel are dimensionally stable during machining. Plastics, especially softer ones, can flex, distort from heat or spring back after cutting. This makes tight tolerances harder to hold on non-metallic parts.
Tool Condition and Wear
A worn cutting tool introduces variation. As the tool degrades, its geometry changes slightly, which affects how it removes material. For tight-tolerance work, tool inspection and replacement schedules matter.
Thermal Expansion
Metal expands as temperature rises. In a warm workshop, a part that measures correctly at 20°C may read differently at 25°C. Ultra-high-precision work often requires temperature-controlled environments for this reason.
Part Geometry
Thin walls and long, slender features are harder to hold to tight tolerances. They deflect undercutting forces, introducing errors that a solid, chunky part wouldn’t experience.
Fixturing and Setup
How a part is held during machining affects the result. Poor fixturing introduces vibration and movement. Good fixturing is invisible when everything goes right and obvious when something goes wrong.
Tolerance Types You’ll Encounter on Engineering Drawings
Bilateral tolerance
The most common type. Variation is permitted in both directions from the nominal value. Example: 30mm ±0.1mm means 29.9mm to 30.1mm are both acceptable.
Unilateral Tolerance
Variation is only permitted in one direction. Example: 30mm +0.000/-0.200 means the part can only be smaller than nominal, not larger. Useful where one direction of deviation would cause interference.
Limit Tolerance
The drawing simply states the upper and lower limits directly: 29.80 / 30.00. No calculation needed. You either fall within range or you don’t.
GD&T (Geometric Dimensioning and Tolerancing)
GD&T goes beyond simple dimension control. It specifies geometric characteristics like flatness, perpendicularity, true position and concentricity using standardised symbols. It’s more complex to interpret but gives the machinist (and inspector) more precise control over what actually matters for the part’s function.
For assemblies where multiple parts interact or where tight geometric relationships exist between features, GD&T is often worth the added complexity. For precision CNC engineering work, it’s frequently the standard.
How to Choose the Right CNC Tolerance for Your Part
This is where most guides go vague. Here’s a more direct way to think through it.
Start with the function
Ask what happens if this dimension is off. If the answer is “nothing critical,” you probably don’t need a tight tolerance. If the answer is “the part won’t assemble” or “the seal won’t hold,” you need to specify carefully.
Only tolerance features that matter
Applying tight tolerances to every dimension on a drawing increases cost and machining time with no benefit. Identify the features that actually affect fit, function, or safety.
Consider the mating part.
If a shaft goes into a bore, the tolerances on both components need to work together. This is where understanding fits (clearance fit, interference fit, transition fit) becomes relevant.
Talk to your machinist early
Machinists will tell you whether a tolerance is achievable on the available equipment and often whether a slightly relaxed tolerance would make the part significantly cheaper without affecting performance. That conversation is worth having before, not after, the quote.
Avoid unnecessarily tight tolerances
Tight tolerances demand slower feeds, more setups, additional inspection and sometimes specialist equipment. The difference between ±0.05mm and ±0.01mm can double the cost of a batch. Only go tighter when the part genuinely needs it.
Need precision-machined parts that actually hit spec?
Talk to our engineering team about your tolerance requirements. We’ll tell you what’s achievable, what it’ll cost and how to get the best result from your design.
FAQs
What is the tolerance of a CNC machine?
CNC machines vary, but most production machines hold standard tolerances of around ±0.127mm (±0.005″). High-precision machines can achieve ±0.025mm or tighter. At the extreme end, specialist equipment operates to ±0.0025mm, though this requires controlled environments and significantly increases cost.
What is the standard tolerance for CNC machining?
The most widely referenced standard is ISO 2768, with the Medium (m) class being the default for most general machining work. In numerical terms, ±0.127mm (±0.005″) is the common benchmark for metal parts. Plastics typically use a wider tolerance of around ±0.254mm (±0.010″) due to their material behaviour.
What is the difference between accuracy and tolerance in CNC machining?
Accuracy refers to how close a machine’s output is to the intended target dimension. Tolerance is the acceptable range of variation in the finished part. A machine can be accurate on average but still produce occasional out-of-tolerance parts, or it can be inaccurate but consistent. The two concepts address different aspects of machining quality.
What tolerance can CNC machines achieve?
Standard CNC machines routinely hold ±0.127mm. Precision CNC machining reaches ±0.025mm to ±0.01mm. Ultra-precision applications, typically aerospace, medical and optical industries, can require ±0.005mm or tighter, which usually involves specialist equipment and environmental controls.
How do I choose the right CNC tolerance?
Start with the function of the feature. Only specify tight tolerances where the part’s performance actually depends on them, like mating surfaces, press fits, bearing seats and sealing interfaces. For non-critical geometry, use standard ISO 2768 tolerances to keep costs down. When in doubt, speak to the machinist before finalising your drawing.
