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In manufacturing, producing interchangeable parts that can fit in assemblies requires consistency. However, absolute uniformity is practically impossible due to microscopic variations in material and process.
This is where tolerance comes in—it defines an acceptable range of variation for a part’s dimensions, ensuring it functions properly in its intended application. It provides a buffer between the ideal dimensions and what’s realistically achievable.
This detailed guide uncovers CNC machining tolerances, including their types, calculations, and how international standards guide their specification.
What is a Machining Tolerance?
Variations in tools, materials, and processes always lead to small deviations. To account for these, we define an acceptable range of dimensions within which the part can still work as intended.
A machining tolerance sets the minimum and maximum values a part can have while remaining functional. A tighter tolerance brings the part closer to the exact specification, while a broader range permits more variation.
For clarity, here’s an example: if a shaft has a machining tolerance of 20 ± 0.01 mm, the final part can be anywhere between 19.99 mm and 20.01 mm and still fit and function as required.
How Machining Tolerances Are Expressed?
All machined parts have some numerical values engraved on them – these basic terms can help you understand what they mean:
- Basic Size: The theoretical size calculated for the part. It’s the size shown on blueprints by designers.
- Actual Size: The size of the finished part. Ideally, it should be equal to the basic size. But in reality, it’s slightly smaller or larger.
- Upper and Lower Limits: Since the actual size isn’t exactly the basic size, limits are set to define the acceptable range within which the part can still be functional.
- Tolerance: The difference between the upper and lower limits, represented as a plus, minus, or both next to the basic size that indicates allowed deviation.
Why do Machining Tolerances Matter?
Defining machining tolerances for parts serves many purposes like:
Consistency in Manufacturing
Tolerance establishes a standard range for making, instead of aiming for an unattainable perfect size. Without tolerances, each part would require extensive fine-tuning, which would slow down production and increase project complexity.
Managing Margin of Error
Tool wear, material inconsistencies, temperature changes, and machine calibration errors are inevitable. By setting a tolerance, manufacturers can keep these errors within a controlled range.
Fit and Functionality
Tolerances also influence the final look and structural integrity of parts. Consistent tolerance control avoids visible gaps, uneven surfaces, or misalignments within mating assemblies.
Saving and Controlling Costs
Tighter tolerances require precise CNC machining, which can increase both the time and cost of production. You can balance quality with cost efficiency by avoiding the expense of producing unnecessarily precise parts.
Types of CNC Machining Tolerances
The tolerances can vary based on size, configuration, and standards followed. Some machining tolerance types popular in the industry are:
1. General/Standard Tolerance
General tolerances are a common way to define acceptable variations in a part’s dimensions during CNC machining. They can either reflect what a machine shop can produce without extra cost or be applied to an entire drawing, unless specific tolerances are mentioned for certain features.
Standard tolerances cover linear measurements (lengths), angular measurements, and features like chamfers or rounded edges. They are divided into four classes: very Coarse (v), Coarse (c), Medium (m), and Fine (f). Each class provides a range of acceptable variations for different size brackets, as defined by standard ISO 2678.
| Permissble deviations in mm for ranges in nominal lengths | f (fine) | m (medium) | c (coarse） | v (very coarse) |
|---|---|---|---|---|
| 0.5 up to 3 | ±0.05 | ±0.1 | ±0.2 | – |
| Over 3 up to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| Over 6 up to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| Over 30 up to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| Over 120 up to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| Over 400 up to 1000 | ±0.3 | ±0.8 | ±2.0 | ±4 |
| Over 1000 up to 2000 | ±0.5 | ±1.2 | ±3.0 | ±6 |
| Over 2000 up to 4000 | – | ±2.0 | ±4.0 | ±8 |
2. Unilateral Tolerance
In unilateral tolerance, the allowed variation is only in one direction, above or below the nominal size.
For example, if a part’s diameter is 50 mm with a tolerance of +0.02 mm, the diameter can only increase to 50.02 mm but not decrease. This type of tolerance is specified for parts that need a clearance fit (shaft in a bore, pin a hole).
3. Bilateral Tolerance
Bilateral tolerance permits variation in both directions from the nominal dimension. For instance, a diameter specified as 50 ± 0.02 mm can range from 49.98 mm to 50.02 mm.
This tolerance provides more flexibility in manufacturing, as minor deviations in both directions are acceptable. It’s common for parts where symmetry is important, like fasteners or rotating components (shafts).
4. Limit Tolerance
Limit tolerance specifies the upper and lower limits without using a nominal dimension. For example, a part might be defined with limits of 49.98 mm and 50.02 mm. The part must fall between these two values.
5. Geometric Dimensioning and Tolerance (GD&T)
GD&T is an advanced system that communicates allowable variations in part geometry using standardized symbols and definitions. It is a proper framework with a broader set of tolerances to control form, orientation, location, and profile
- Location Tolerance: Location encompasses true position tolerance, concentricity, and symmetry. It defines how far a feature can deviate from its true position.
- Profile Tolerance: It controls the shape of a feature by specifying permitted deviations from its true geometric profile. The profile could be a line or surface.
- Form Tolerance: It specifies allowable variations in the shape of features without regard to their location or orientation. It further has sub-types: straightness, flatness, circularity, and cylindricity.
Orientation Tolerance: This tolerance controls the angle between features for a perfect alignment. For instance, its sub-type, perpendicularity tolerance ensures that a hole is drilled perpendicular to a surface.
What to Consider When Defining Tolerance of Parts
When defining tolerance for parts, you have to keep in view all factors that can influence the final dimensions of the parts:
Material Properties
The material’s mechanical properties can influence the level of tolerance achieved in CNC machining. The first factor is the material’s hardness. Softer materials are harder to machine precisely because they change shape when the cutting tool touches them.
The second one is abrasiveness. Coarse materials wear out the cutting tools faster, which makes it harder to maintain accuracy. As the tool wears down, the precision decreases, and the tool needs frequent replacements to keep up with the required tolerance.
Thermal stability also matters. Some non-metals lose their shape as heat builds up during machining. This limits the types of processes you can use on those materials, as they can warp when exposed to high temperatures.
Cost
The tighter the tolerance, the higher the cost. For instance, achieving a tolerance of ±0.01 mm requires slower machining speeds, multiple tool passes, and careful adjustments to prevent errors. This extends the time for each part and raises the labor involved.
Moreover, specialized jigs are needed to maintain such tight limits, which further adds to production costs. With tighter tolerances, there’s always a risk of producing parts that fall outside the acceptable range, leading to potential scrapping or rework.
Machining Method
Tolerance varies for different CNC machining methods and types of equipment used. Here’s a list of tolerances of CNC processes carried out at Richconn:
| CNC Machining Processes | Tolerance |
|---|---|
| CNC Turning | ±0.002mm |
| CNC Milling | ±0.002mm |
| 5-Axis CNC milling | ±0.002mm |
| Wire EDM Machining | ±0.001mm |
| 3D printing | ±0.2mm |
Inspection
Inspecting parts with tighter tolerances becomes increasingly difficult as the precision required rises. Conventional measuring tools, like calipers or micrometers, may not provide the necessary accuracy for these tight limits.
The fine-tuned tolerance parts demand specialized inspection equipment – for instance, you may need high-precision coordinate measuring machines (CMMs) or laser measurement systems, to verify their dimensions.
How to Determine Machining Tolerances?
Now, how do you determine the right machining tolerance for your parts? There are two main approaches:
Self-Calculation
If you’re well-versed in design engineering, self-calculation is a possibility. Look at the part’s intended use, the machining method, and applicable international standards. Decide if a higher or lower tolerance is needed without compromising functionality or standards.
Right CNC Manufacturer
Outsourcing to a manufacturer like Rich Conn is another option. Richconn has been providing CNC machining and 3D printing services for over two decades. We have a team of dedicated experts and professionals who can design your parts to meet international standards.
Standards for CNC Machining Tolerances
Tolerances, like other machining aspects, are governed by some standards. International standards that define machining tolerances are:
ISO 2768
This standard defines general tolerances for linear and angular dimensions, chamfers, and radii. It helps simplify the process by providing default tolerances across four classes (fine, medium, coarse, and very coarse) based on the part’s dimensions, reducing the need for detailed tolerance notes. Rich Conn largely follows this standard.
ISO 286
It’s a geometrical product specification standard (GPS) that focuses on tolerances for cylindrical surfaces and parallel plane distances, often used in systems like shafts and holes that require precise fits. This standard is needed for parts that need controlled clearances or interference fits.
ISO 5459
Another GPS standard; provides rules for the use of datums, which are reference points, lines, or precisely located and oriented relative to other parts or assemblies.
ISO 129-7
A sub-part of ISO 129 that covers technical product documentation. ISO 129-7 specifically defines how dimensions and tolerances should be indicated on drawings. It basically helps in standardizing the documentation of technical drawings.
ANSI B4.1
This is an American standard for preferred limits and fits for cylindrical parts, defining the acceptable variations in size and how well different parts will fit together. It’s used extensively in U.S.-based industries, offering detailed charts for various fit classes, from loose to tight.
ASME Y14.5
It is the authoritative standard for geometric dimensioning and tolerancing (GD&T) in the U.S. It defines all GD&T types (form, orientation, position), symbols, and rules. Precision-critical industries like automotive, and aerospace utilize it.
Conclusion
In the past, precision wasn’t as critical, but today’s mechanical systems -especially in aerospace and automotive—demand parts that fit and function perfectly. This makes machining tolerances more important than ever.
Although tight tolerances are crucial in certain industries, at the same time, it’s necessary to balance the need for precision with cost considerations to avoid unnecessary over-engineering.
