CNC machining tolerance is the level of deviation that you are allowed to have in your machined parts. It provides the tolerable range within which the measurements of a part manufactured can be out of the assigned design. When you want your parts to fit and serve accurately having a small tolerance is essential.
What is CNC Machining Tolerance?
CNC machining tolerance refers to the allowable range of deviation of the actual measured dimensions of the part relative to the basic dimension during the machining process. It considers the aspects such as machine capacities, material characteristics, and environmental parameters that may lead to variations. Tolerances are defined to provide a range in which parts are formed to functional requirements, yet not excessively tight, which may raise production costs.
Standard CNC Machining Tolerance Chart
| CNC Process / Feature | Typical Starting Tolerance | Suitable Application | Conditions to Confirm |
| CNC milling | ±0.005 in (±0.13 mm) | General linear dimensions, pockets, and external profiles | Feature length, pocket depth, tool reach, and number of setups |
| CNC turning | ±0.003–0.005 in (±0.08–0.13 mm) | General diameters, lengths, shoulders, and grooves | Length-to-diameter ratio, wall thickness, material stability, and chucking method |
| Reamed or precision-bored hole | ±0.001–0.002 in (±0.025–0.05 mm) | Bearing seats, locating holes, and controlled mating features | Hole diameter, depth, roundness, surface finish, and inspection method |
| CNC routing | ±0.010 in (±0.25 mm) | Large plates, panels, plastics, and non-critical profiles | Part size, sheet flatness, cutting force, and thermal movement |
| General machined plastic feature | ±0.005–0.010 in (±0.13–0.25 mm) | Non-critical dimensions on stable engineering plastics | Moisture absorption, thermal expansion, residual stress, and clamping deformation |
| Thin wall, deep cavity, or slender feature | Quote-specific | Features susceptible to vibration, tool deflection, or part distortion | Wall thickness, unsupported length, cutting sequence, and stress-relief requirements |
| Multi-setup or large part feature | Quote-specific | Dimensions measured across different machining setups | Datum transfer, fixture repeatability, part weight, and inspection capacity |
How Tolerances Are Shown on Engineering Drawings
Engineering drawings usually express the allowable range of variation through ± values after dimensions, upper and lower deviations, limit dimensions, general tolerances in the title block, and geometric dimensions and tolerances (GD&T).
| Tolerance Expression | Drawing Example | Allowed Dimension Range |
| Symmetric bilateral tolerance | 15.00 ±0.05 mm | 14.95–15.05 mm |
| Asymmetric bilateral tolerance | 15.00 +0.10/-0.02 mm | 14.98–15.10 mm |
| Unilateral tolerance | 15.00 +0.05/-0.00 mm | 15.00–15.05 mm |
| Limit dimensions | 15.05 / 14.95 mm | 14.95–15.05 mm |
| Title block tolerance | X.XX ±0.01 | Determined by the number of decimal places on the drawing |
| General tolerance standard | ISO 2768-m | Looked up based on the nominal dimension range |
| Fit tolerance | ⌀20 H7/g6 | Looked up according to ISO 286 |
| Basic dimension with GD&T | 15.00 in a rectangular box | Does not use ± tolerance directly |
For sizes with upper and lower deviations, the following relationship can be used to determine the acceptance range:
Maximum allowable size = Basic size + Upper deviation
Minimum allowable size = Basic size + Lower deviation
Bilateral and Unilateral Tolerances
Bilateral tolerance allows the dimension to vary on both sides of the basic dimension. For example, ±0.05 mm is a symmetric bilateral tolerance, while +0.08/-0.02 mm is an asymmetric bilateral tolerance. Unilateral tolerance only allows the dimension to deviate in one direction. It is worth noting that during design, a deviation in one direction can affect part assembly and material allowance.
General Tolerances in the Title Block
Engineering drawings do not require tolerances to be specified for each non-critical dimension separately. As designers, you can set a default value in the title block according to the number of decimal places, and of course, you can also reference general tolerance standards such as ISO 2768-1.
Fit Tolerances for Holes and Shafts
For holes and shafts that need to fit together, your drawings can use tolerance band designations such as H7, g6, h6, etc. These letters and numbers cannot be directly converted into a fixed ± value. The actual limits depend on the nominal size range and the fit standards referenced in the drawings.
GD&T Feature Control Frames
The dimensional tolerance mainly controls the size of the feature and cannot fully control its shape, direction, and position. GD&T uses feature control frames to express such requirements. For example:
location degree | ⌀0.20 | A | B | C
This annotation indicates that the controlled features must be located within a position tolerance zone with a diameter of 0.20 mm relative to the references A, B, and C. Flatness, verticality, parallelism, profile, and roundness must also be expressed using their respective geometric characteristic symbols.
Dimensional Tolerances vs. GD&T
In fact, these are two tolerance standards for parts. Dimensional tolerances primarily control the size of part features or the distance between two features, such as hole diameter, shaft diameter, length, and thickness. Geometric Dimensioning and Tolerancing (GD&T) controls the shape, direction, profile, location, and runout of features.
What Dimensional Tolerances Control
Dimensional tolerance specifies the upper and lower limits of allowable variation for a specific dimension of a part. It is suitable for controlling the fitting dimensions and overall size of parts. If measurement of the part’s dimensions is needed, quality inspection personnel can use tools such as calipers and micrometers.
What GD&T Controls
GD&T uses unified geometric symbols, feature control frames, and datum systems to define allowable tolerances. It does not replace all dimensional tolerances but controls the geometric state of the part beyond dimensional requirements.
| GD&T Category | Common Control Items | Primary Purpose |
| Form | Straightness, flatness, circularity, cylindricity | Controls the shape of an individual feature itself |
| Orientation | Parallelism, perpendicularity, angularity | Controls the orientation of a feature relative to a datum |
| Location | Position | Controls the precise location of holes, slots, shafts, or other features |
| Profile | Line profile, surface profile | Controls complex curved surfaces or overall contour |
| Runout | Circular runout, total runout | Controls variation of a rotating part relative to a datum axis |
Among them, flatness or roundness usually does not require a datum, while position tolerance, perpendicularity, and circular runout do require a clear datum. You need to establish a reference system that can be jointly used during part machining, assembly, and inspection.
How to Choose the Right Tolerance?
This is a topic that many people want to understand. When choosing CNC machining tolerances, you should start from the functional and assembly requirements of the part, and then verify it in conjunction with materials and structures, manufacturing capabilities, and testing methods.
Excessively wide tolerances may cause interference, looseness, leakage, or positioning deviation. Overly tight tolerances may increase finishing, fixturing, inspection, and scrap costs without necessarily improving part performance.
| Functional Requirement | Features to Control | Recommended Tolerancing Method |
| Two parts must slide freely relative to each other | Maximum and minimum assembly clearance | Hole/shaft dimensional tolerance or clearance fit |
| Two parts must remain rigidly connected | Minimum and maximum interference | Interference fit and assembly stress verification |
| Locating pin align accurately with hole | Hole diameter, hole location, and datum relationship | Dimensional tolerance and geometric tolerancing |
| Bearing must be seated stably | Bore diameter, roundness, cylindricity, and surface roughness | Dimensional tolerance, GD&T, and roughness requirements |
| Sealing surfaces must contact evenly | Flatness, profile, and surface quality | GD&T combined with surface roughness |
| Rotating part run smoothly | Coaxiality relationship and rotational error | Position, circular runout, or total runout |
| General exterior or non-mating surfaces | Basic dimension and appearance | General dimensional tolerance |
Classify Critical and Non-Critical Features
On the same drawing, the dimensions of parts should not all use the same strict tolerances. Designers can classify them into the following levels based on the consequences of deviation:
- Critical Features: Out of tolerance can lead to assembly failure, leakage, interference, and safety risks. You should specify tolerances separately on the drawings and define inspection methods.
- Important Features: The critical features of components will affect their performance, service life, wear, noise, etc. You should allocate tolerances based on actual functions and verify them during the trial production phase.
- Non-critical Features: Tolerances for non-critical features of parts that vary within a certain range will not affect the function of the part. You can use title block tolerances or applicable general tolerance standards.
Calculate the Required Fit
When selecting hole and shaft tolerances, you need to analyze both mating parts simultaneously, rather than determining the tolerances of the hole and shaft separately.
For clearance fit:
Minimum clearance = Minimum hole diameter − Maximum shaft diameter
Maximum clearance = Maximum hole diameter − Minimum shaft diameter
Check the Tolerance Stack-Up
When the assembly result is determined by multiple dimensions, even if each dimension is qualified, cumulative error may still lead to the final assembly dimension being out of tolerance.
In worst-case analysis:
Worst-case total tolerance = |T₁| + |T₂| + … + |Tn|
Where, T₁ to Tn are the bilateral tolerance values of each constituent dimension in the tolerance chain.
Match the Inspection Method to the Tolerance
Any tolerance must have a reliable verification method.
- Whether to use calipers, micrometers, go/no-go gauges, profilometers, or coordinate measuring machines (CMM);
- Which drawing datums to use during inspection;
- Parts need to be inspected after machining completion or after surface treatment;
- Whether critical dimensions require 100% inspection or sampling inspection;
- Whether first article inspection reports, dimensional reports, or material certifications are required.
How Tight Tolerances Affect Cost and Lead Time
Tight tolerances will increase CNC machining costs and lead times, but the degree of impact depends not only on the tolerance values, but also on part features, materials, geometric structure, number of setups, and inspection methods.
More Machining Operations
Parts with normal dimensions may be completed through one rough machining and finish machining operation. However, parts with tight tolerance features may require splitting the process into rough machining, semi-finish machining, and final finish machining.
Through rough machining, most of the material on the workpiece will be removed, which allows machining stress and heat to be released. Performing finish machining after the part has stabilized helps reduce final dimensional drift. For precision holes, shafts, or sealing surfaces, you may also need to add operations such as reaming, precision boring, grinding, honing, or lapping.
Slower Cutting and Additional Tool Passes
Parts with tight tolerances often require uniform finishing allowances. And during the machining process, the machine tool must also set a smaller depth of cut and more stable cutting parameters. These measures can improve the dimensional consistency of the parts, but will also extend the processing time. Especially for parts with deep cavities, thin walls, or elongated features, feed rates may need to be reduced further. This is also to prevent the tool from deflecting and vibrating, thereby affecting the quality of the part.
Specialized Tooling and Workholding
For some parts, universal fixtures may not provide sufficient repeatability and positioning accuracy. In such cases, manufacturers need to design soft jaws, dedicated locating pins, vacuum fixtures, etc.,to complete the clamping work.
These specialized fixtures not only need to be manufactured, but also require trial assembly and repeatability validation. For thin-walled parts that are prone to deformation, fixtures must also control clamping force while providing adequate support.
Higher Inspection Costs
The smaller the tolerance of parts, the more complex the detection methods become. Calipers are well-suited for parts with ordinary dimensions. For parts with tight tolerances, you may also need to use professional inspection instruments such as micrometers, pneumatic gauges, coordinate measuring machines (CMM), roundness testers, contour measuring instruments, and so on.
How to Reduce Cost Without Sacrificing Function
- Only tighten critical features
- Use functional datums
- Use GD&T correctly
- Shorten the tolerance chain
- Use standard fits
- Avoid cross-clamping tolerances
- Clarify final acceptance state
- Confirm inspection requirements in advance
ISO and ASME Standards to Reference
These two tolerance standards, I believe you are very familiar with them. This article provides an overview to help you read and understand. ISO and ASME standards provide a unified language for dimensions, tolerances, datums, fits, and geometric controls in engineering drawings. Additionally, your project should establish a dominant standard system. Although ISO and ASME are similar in some symbols and concepts, their default rules, terminology, and interpretations are not completely identical.
ISO Standards for CNC Machining Drawings
| ISO Standard | Primary Control Content | When to Reference |
| ISO 2768-1 | Linear and angular dimensions not individually toleranced | When the drawing needs to uniformly specify general tolerances for common dimensions |
| ISO 22081 | General geometrical specifications and general size specifications | When establishing generic requirements for geometric features not individually controlled |
| ISO 1101 | Geometric tolerance symbols, feature control frames, and annotation rules | When using position, flatness, perpendicularity, profile, or runout |
| ISO 5459 | Datums and datum systems | When the drawing uses datums such as A, B, C to control orientation or location |
| ISO 2692 | Maximum material requirement, least material requirement, and related rules | When size features such as holes or shafts use material condition requirements |
| ISO 286-1/2 | Linear size tolerances, tolerance zones, and hole/shaft fits | When selecting standard fits such as H7, g6, h6 |
| ISO 14405-1 | Size specification and interpretation for linear sizes | When it is necessary to clarify how hole diameter, shaft diameter, width, and thickness are defined |
| ISO 21920 series | Specification and evaluation of profile-method surface texture | When the drawing needs to specify surface requirements such as Ra, Rz |
ASME Standards for CNC Machining Drawings
| ASME Standard | Primary Control Content | When to Reference |
| ASME Y14.5 | Dimensioning, GD&T, d atum and material condition rules | When North American projects or customers require ASME GD&T |
| ASME Y14.100 | General requirements for engineering drawing practices | When standardizing drawing format, views, notes, and referenced documents |
| ASME B4.1 | Preferred limits and fits for cylindrical parts | When designing inch-based hole/shaft fits |
| ASME Y14.41 | Digital product definition data | when using 3D models as the dominant manufacturing definition |
| ASME B46.1 | Surface texture, roughness, and waviness | When the drawing needs to specify or evaluate surface quality |
| ASME Y14.5.1 | Mathematical definition of the geometric principles in Y14.5 | When measurement software, algorithms, or complex digital verification require a rigorous mathematical model |
Importance of Knowing Tolerance in CNC Machining
Ensure Part Fit
You ensure proper part fit through knowledge of tolerances, avoiding assembly problems. Proper dimensions mean that components fit and work without colliding with each other or leaving gaps.
Maintain Functionality
Part functionality is maintained by following designated tolerances. Accurate machining eliminates failures, guarantees dependable performance, and facilitates optimal functioning in any mechanical systems.
Reduce Material Waste
You reduce scrap through tolerances. Precise machining minimizes errors and rework, which saves material and enhances overall production efficiency.
Improve Quality Control
Tolerances are monitored to improve quality control. Standard dimensions allow you to achieve standards, minimize defects, and provide high-quality parts to customers.
Optimize Production Costs
You control production costs through appropriate tolerance choice. Do not have over restrictive specifications that add to machining time and cost without value addition.
Facilitate Interchangeability
Part interchangeability is achieved through tolerance standards. Components work across assemblies, allowing components to be easily replaced, repaired, and standardized.
Enhance Machine Performance
By observing tolerances, you safeguard machine performance. Proper machining minimizes the stress on the machinery and equipment preventing damages and increasing the service life.
Common Tolerances in CNC Machining Processes
General Tolerances
General tolerances are used when parts are not of high preciseness. These standard limits allow small variation of dimensions, making production straightforward and affordable. With general tolerances, you are guaranteed offunctionality and you no longer over-complicate the machining process, so they are used in the daily mechanical components and non-critical parts.
Limit Tolerances
Limit tolerances are defined as the maximum and minimum permissible dimensions of a part. This makes each piece fit within an exact range. Limit tolerances allow you to stop defective parts in the production process, maintain precision during assembly, and save on material wastage without compromising on product quality.
Unilateral Tolerances
You have a unilateral tolerance to cause the deviation in one of the directions of the nominal dimension. This method keeps important features above or below a limit. With unilateral tolerances, you dictate machining performance, assure fit and functionality, and eliminate unnecessary precision that can be costly or time-consuming.
Bilateral Tolerances
You use the bilateral tolerances to allow variation above and below the nominal size. This flexibility allows you to strike balance between manufacturing efficiency and functional parts. With bilateral tolerances, you can live with small machine or material differences, provide consistency, and minimize rework or scrap when checking quality.
Geometric Dimensioning & Tolerancing (GD&T)
With GD&T, you manage the shape, orientation and location of part features with precision. Using GD&T, you convey precise design needs to machinists, keep extreme accuracy, and enable interchangeability and decrease ambiguity in complex assemblies, aiding uniform quality in important parts across production batches.
Factors Affecting Tolerance in CNC Machining
| CNC Machining Operation | Acceptable Tolerance |
| CNC milling tolerance | · ±0.005 inches (±0.13 mm) |
| CNC turning tolerance | · ±0.003–0.005 inches (±0.08–0.13 mm) |
| Routing | · ±0.01 inches (±0.25 mm) |
| Reaming holes | · ±0.001–0.002 inches (±0.025–0.05 mm). |
Material Types
Customize tolerances depending on your machining material. Metals enable tighter tolerances as they are stable and may be ±0.002–0.005 inches. Plastics are shrink or swell, and you retain the looser tolerances, usually ±0.005–0.010 inches, to compensate thermal and mechanical deformation during machining.
Part Design & Geometry
When determining tolerances you take part complexity into account. Complex shapes, thin walls, or deep cavities demand less stringent tolerances to prevent deformation or tool deflection. Simple shapes enable tighter tolerances, which guarantee easier machining, superior fit, and less chance of errors.
Degree of Accuracy (Very Tight Tolerances vs. Standard Tolerance)
Tolerance levels are chosen depending on functional needs. Looser tolerances help reduce machining costs and shorten processing time.
Inspection and Measurement
You check tolerances with accurate inspection instruments such as calipers, micrometers, CMMs, or laser scanners. Regular measurement guarantees that parts comply with specifications, are of quality, preventable assembly problems and helps to save on rework or scrap.
Conclusion
During your CNC machining process, you must pay attention to tolerance. Remember, very tight tolerances are costly to machine. Therefore, you should only choose very tight tolerances when necessary.
FAQs
Q1. What is the average CNC machining tolerance in mm?
Average CNC tolerance is normally held at ±0.05 mm, and more critical parts may be held at ±0.01 mm or less, depending on the capabilities of the machine and material.
Q2. What is the general tolerance for CNC machines?
The normal tolerances is about ±0.1 mm of non critical features that provide functionality without making the production costly.
Q3. How accurate is a CNC machine?
Accuracy is +-0.01 mm to +-0.05 mm, which is dependent on machine type, set-up and material. Machines with high end are able to reach within +-0.005 mm.
Q4. Which is the most accurate CNC machining technique?
The best accuracy is obtained with precision turning or CNC milling using fine tooling, controlled feeds, and repeatable machine configurations.
Q5. Why is ISO 2768 tolerance standards critical in CNC machining?
Tolerances of general parts are standardized with ISO 2768 to ensure the consistency, interchangeability, and cost-effective manufacturing in various machines and operators.
Q6. What are the surface roughness considerations for CNC machining tolerance?
You take into account surface finish since rough surfaces may compromise dimensional accuracy. Finer finishes could be enabled by slower feeds and reduced tolerances to accommodate functional requirements.
Q7. Is high precision same as tight tolerance in CNC Machining?
High precision refers to repeatability of results, while tight tolerance defines acceptable dimensional variation. Both affect the quality of parts, yet they are not the same.
Q8. What is the difference between dimension tolerance and tolerance zone?
Dimension tolerance defines the maximum variation in a feature, and the 3D space a feature should be within to satisfy the design conditions, is the tolerance zone.
