A Comprehensive Guide to Elimold’s Tolerance Standards for Precision CNC Machined Parts

A Comprehensive Guide to Elimold’s Tolerance Standards for Precision CNC Machined Parts

A key role of CNC machining in modern manufacturing is ensuring the precise machining and high repeatability of all parts. Achieving perfectly dimensional parts is virtually impossible in the real world. Therefore, tolerances are necessary regardless of the manufacturing process used. Simply put, tolerances are limits on the dimensions of a part within permissible variations. They define the acceptable range of deviations from the perfect shape of a geometrically perfect product. Understanding CNC machining tolerances is crucial for precision machining. This article provides some information on CNC machining tolerances, introduces tolerance capabilities in CNC machining at Elimold, defines some commonly used markings, and explores international industry standards for part tolerances.

What are CNC machining tolerances?

CNC machining tolerances refer to the permissible variations in the physical dimensions of a part. Essentially, they define the range of deviation between a specific measured value and the planned dimension. Understanding manufacturing tolerances is a complex topic; different components may require different types of tolerances, such as machining tolerance standards, standard tolerances, and typical machining tolerances. To delve deeper into the details, CNC machining technology involves a wide variety of equipment, such as CNC milling machines, CNC lathes, five-axis machine tools, EDM machines, and many more. These machine tools offer varying levels of manufacturing tolerance and control over the overall quality and fit of the machined parts. Simply put, high-precision machine tools can manufacture high-precision parts required in high-tech fields, high-tolerance machining ensures seamless connection and assembly of product components, while standard tolerances manufactured by standard machine tools ensure processes conform to industry standards.

Elimold’s CNC Machining Tolerance Guide

Please note that these are all two-sided tolerances. If expressed in a one-sided manner, the standard tolerance would be +0.000/-0.010 inches (or +0.010/-0.000 inches), while in our standard, the limit-based tolerance would be 1.005/0.995 inches. All of these are acceptable, and the units of measurement for tolerances can also be metric, provided they are explicitly specified in the design. To avoid confusion, please insist on displaying dimensions and tolerances to three decimal places, avoiding extra zeros such as 1.0000 inches or 0.2500 inches unless absolutely necessary. Consideration for machining tolerances also includes surface roughness; the standard provides a surface roughness of 63 microinches for flat and vertical surfaces, and 125 microinches or better for curved surfaces. This is sufficient for most applications, but for surfaces on metal parts where a high degree of appearance is required, a light shot peening treatment can often improve the appearance. Contact Elimold’s team for CNC surface treatment guidelines to get examples. If you need a smoother surface, please indicate this in your design, and we will do our best to meet your requirements.

Elimold offers tolerance capabilities for various CNC machining processes.

Our CNC machining services cover a variety of machining processes, with grinding, drilling, boring, turning, milling, and cutting being the main categories. However, the machining tolerances for each process are not the same because each process has its own process and machining bed deviations. Therefore, the tolerance allocation methods for each process are also different. This is why tolerance measurement data shows that CNC drilling processes have lower tolerance values than CNC grinding processes, because the surface roughness and machining of cantilever mechanisms in CNC drilling processes are less accurate.

Furthermore, tolerances can be influenced by a variety of factors, such as machine type, cutting type, the material being machined, and the complexity of the part. These are general guidelines, and tolerances may vary depending on the specific requirements of the job and the manufacturing process used. Additional precision techniques, such as ultra-precision machining, can be used for applications with very tight tolerances to achieve even tighter tolerances. Typical machining tolerances from Elimold are:

Turning	±0.005 inches to ±0.015 inches
Milling	±0.005 inches to ±0.015 inches
drilling	±0.005 inches to ±0.015 inches
Grinding	±0.001 inches to ±0.005 inches
Wire cutting	±0.001 inches to ±0.002 inches

Elimold’s formula for calculating tolerances of CNC machined parts

The concepts of part interchangeability and dimensional tolerances have since become standard practice in manufacturing. Unfortunately, misuse of tolerances can lead to a range of problems. For example, overly tight tolerances may require parts to undergo secondary grinding or electrical discharge machining (EDM) to complete, unnecessarily increasing costs and delivery times. “Too loose” tolerances or inconsistencies with the tolerances of mating parts can make assembly impossible, requiring rework, or, in the worst case, rendering the finished product unusable.

Therefore, to determine CNC machining tolerances, several fundamental aspects must be considered. These aspects include the function of the part, the required fit, manufacturability, and the material to be used. Let’s outline how to calculate your tolerances:

Nominal size definition: Determines the expected measurement value of a part.

Determine the tolerance type: Generally, for most applications, two-sided tolerances can be used; however, depending on the product’s design and function, one-sided tolerances may be more preferable.

Calculate tolerance range: Define the maximum and minimum possible size range of the track.

Calculation example: If the nominal size of the part is 50 mm, the bilateral tolerance is +/- 0.2 mm.

Upper limit calculation: Nameplate aperture: Nominal size + positive tolerance 50 mm + 0.2 mm = 50.2 mm

Lower limit calculation: Nominal size – negative tolerance 50 mm – 0.2 mm = 49.8 mm

Tolerance calculation: The tolerance range is 49.8 mm to 50.2 mm. Tolerance (t) = Upper limit – Lower limit t = 50.2 mm – 49.8 mm = 0.4 mm

Elimold adheres to the CNC machining tolerance industry standard

Currently, commonly used international CNC machining tolerance standards include ISO 2768, DIN 7168, and JIS B0419, which provide general tolerance grades and numerical ranges. Among them, ISO 2768 is the most common, divided into three categories: precision (f), medium (m), and rough (c). The appropriate tolerance grade should be selected according to the actual application and design requirements of the part.

ISO 2768 standard

ISO 2768 is a general machining tolerance standard developed by the International Organization for Standardization (ISO), primarily applicable to metal parts with dimensions ranging from 0.5 to 3000 mm. Different precision grades are suitable for different machining processes; for example, “precision” is used for high-precision parts, “medium” for conventional parts, and “rough” for larger parts with lower precision requirements. The ISO 2768 standard is mainly applied to general mechanical parts, divided into two categories: dimensional tolerances and geometric tolerances, each further divided into different tolerance grades.

ASME Y14.5 Standard

The Y14.5 standard published by the American Society of Mechanical Engineers (ASME) is primarily used to control geometrical tolerances (GPTs), and includes a series of GPT symbols and their permissible ranges. This standard is mainly applicable to the control of the position, shape, and orientation of parts, such as parallelism, perpendicularity, and positional tolerances. ASME standards are widely used in parts and equipment requiring high precision, especially in aerospace, automotive manufacturing, and other fields.

GB/T 1184-1996 Standard

In China, CNC machining tolerances also refer to the national standard GB/T 1184-1996, which specifies general machining dimensional tolerances and geometric tolerances to ensure that domestically produced parts conform to a unified standard. This standard is somewhat similar to the ISO standard and is also divided into several typical precision grades.

Tolerance types in CNC machining

There are many types of tolerances used in CNC machining, each with a specific purpose depending on the design and function of the part, including:

One-sided tolerance

One-sided tolerance plays a crucial role in CNC manufacturing. Unlike other tolerances, one-sided tolerance defines the permissible variation of a part’s nominal size in only one direction. It allows deviations that are either higher or lower than the specified size, but not both simultaneously. For example, if a component must be 10 mm in size and the tolerance is +0.05 mm, the acceptable size range is 10 mm to 10.05 mm, but not lower than 10 mm.

Bilateral tolerance

Unlike one-sided tolerances, which allow deviations in only one direction, two-sided tolerances allow dimensions to be larger or smaller than the specified size. For example, if a part has a nominal size of 20 mm and a two-sided tolerance of ±0.05 mm, it means that the acceptable size range for the part is 19.95 mm to 20.05 mm. This type of machining tolerance is typically used when the shrinkage and expansion of the part are acceptable within a specific range.

Limit tolerance

Limit tolerances define the upper and lower limits of a dimension, specifying the range within which the dimension must fall. Unlike unilateral or bilateral tolerances, limit tolerances specify two distinct values: the maximum permissible size and the minimum permissible size. Limit tolerances are typically used in situations where explicitly defined upper and lower limits are required. For example, the limit tolerances for a shaft might be 50.05 mm and 49.95 mm, meaning the part must be manufactured within these ranges to be usable.

Geometric dimensions and tolerances (GD&T)

Geometry and Tolerances (GD&T) is a standard used to define and communicate machining tolerances in manufacturing. It uses a symbolic language to specify the form, orientation, and location of part features. GD&T is a CNC machining tolerance standard that provides a standardized method for defining complex geometries and ensuring proper fit and function. Key aspects of GD&T include:

Form tolerances: These control the shape of various features, such as flatness, straightness, and roundness.
Orientation tolerance: Orientation tolerance controls the alignment of features, including parallelism, perpendicularity, and angle.
Positional tolerances: These tolerances define the position of features relative to each other, such as concentricity and symmetry.

Standard tolerance

Standard tolerances are predefined values that are generally accepted within the industry for use in common manufacturing processes. These tolerances are typically used when no specific custom tolerances are provided and follow recognized machining tolerance standards. The following are the types of standard tolerances:

General tolerances: These are typical values used for most parts and are usually defined by international standards such as ISO 2768. They apply to standard machining tolerances as well as typical tolerances for machining.
Material-specific tolerances: Some materials, such as CNC machined aluminum, may have specific standard tolerance definitions.
Tolerances specific to a particular process: Different manufacturing processes, such as CNC milling tolerances, milling tolerances and CNC machine tool tolerances, may have unique standard tolerances depending on their capabilities.

Dimensional tolerances

Dimensional tolerances are fundamental to manufacturing and CNC machining, defining the permissible deviations in the linear and angular dimensions of a part. Dimensional tolerances are crucial for ensuring that parts fit and function as intended. In CNC machining, dimensional tolerances are used to control critical features such as holes, slots, and surfaces. They are standard tolerances in machined parts and are applied in various forms, including CNC machining tolerances and standard machining tolerances.

Surface finish tolerance

Surface roughness tolerance controls the texture and appearance of a part’s surface. Surface roughness tolerance includes:

roughness

Surface roughness is a standard for measuring the degree of minor irregularities on the surface of a part. It significantly affects the wear resistance, friction, and sealing ability of the part.

Waviness refers to the wider, more spaced irregular shape of a surface. Compared to roughness, its variation is smaller and may be caused by machine vibration or deflection.
Lamination refers to the orientation of the main surface pattern, which is usually determined by the manufacturing method. Lamination orientation affects how lubricant is retained and how parts slide against each other.

Tolerance

Fit tolerances ensure that parts fit as expected, neither too tight nor too loose. Fit tolerances are generally divided into three categories:

Clearance fit refers to a fit that provides space or clearance between mating parts. This type of fit is typically used for parts that must move or rotate relatively freely.
An interference fit, also known as a press fit, refers to a deliberately created interference fit between parts. This interference fit ensures a tight connection and is suitable for situations where relative movement between parts is not allowed.
A transition fit is a fit that provides either clearance or interference, depending on the precise tolerances of the mating parts. This type of fit is used when an assembly requires a degree of flexibility.

Dimensional characteristic tolerances

Characteristic dimension tolerance refers to the deviation between a characteristic dimension and its nominal size. It ensures that the characteristic dimension can still function properly even if it varies within the specified tolerance range.

Maximum Material Condition (MMC) refers to the condition where a feature contains the maximum amount of material that meets its size limitations.
The minimum material condition (LMC) is the opposite of the MMC, where the feature has the minimum amount of material but still meets its size constraints.
Regardless of Feature Size (RFS), it indicates that the actual size of the feature is not considered when applying tolerances.

Relationship and standards between geometric dimensions and tolerances

In CNC machining precision parts projects, another factor to consider is, as mentioned earlier, whether we can accept GD&T tolerances. This provides a deeper level of quality control, including the relationships between various part features and the constraints on shape and fit. Here are some common tolerances:

True position

We specify the hole location by specifying the X and Y distances and their allowable deviations from a pair of perpendicular part edges. In CNC machining tolerance control, the hole location is specified by its true position relative to a reference datum, accompanied by limiting conditions of MMC (Maximum Material Condition) or LMC (Minimum Material Condition).

Flatness

Milled surfaces are generally quite flat, but some deformation can occur due to internal material stress or clamping forces during machining, especially on thin-walled and plastic parts, where some warping can occur once the part is removed from the machine. The flatness tolerance of precision parts is controlled by defining the two parallel planes in which a milled surface must lie.

Cylindricity

Because most milled surfaces are fairly flat, and most holes and rotating surfaces are fairly round, a tolerance of ±0.005 inches (0.127 mm) is used. To put it simply, a 0.25-inch (6.35 mm) hole in a precision part might be elliptical in one direction, measuring 0.245 inches (6.223 mm), and in another direction, 0.255 inches (6.477 mm). By using roundness, which specifies that a machined hole must lie within two concentric cylinders, the manufacturer can eliminate this unlikely situation.

Concentricity

The rings on the bullseye are coaxial, just like the wheels on your car are coaxial. If a drilled or reamed hole must perfectly align with a coaxial flange hole or circular flange gasket, then coaxiality is the best way to ensure this.

Perpendicularity

As the name suggests, perpendicularity determines the maximum deviation of a horizontally machined surface from a nearby vertical surface. It can also be used to control the perpendicularity of a turntable shoulder to adjacent diameters or the central axis of a part.

Of course, to obtain precision parts, other aspects need to be considered, including parallelism, straightness, profile, and angles. However, as with any other non-standard tolerances, they must be specified when uploading the design.

How do the tolerances of CNC machined parts affect product quality?

Assembly designers always require that, as long as the work exists in physical form, it must have good machining tolerances. This is why every effort must be made to achieve good machining tolerances; otherwise, work exceeding tolerances will cause problems in mass production. Because if each of many products has different dimensions to fit its corresponding product, it is very likely that neither product can be assembled at the application stage. For example, if the machining tolerances of automotive repair parts are outside the acceptable range, all parts on the market may be rejected.

How to perform zero-tolerance machining

Zero tolerance is impossible to achieve in the real physical world. Zero tolerance primarily reflects the highest level of machining tolerance, not a value exactly zero, because a zero value cannot be achieved through any physical means. However, real-world publications in the field of machining show that 0.010 mm is close to a zero-tolerance machining value. Achieving such high tolerances involves controlling material composition, machining calibration, qualification, temperature, tool flatness, and workpiece heat treatment. If these factors are controlled, precise work and machining can be performed without any repairs or rework.

Why do engineers have difficulty determining appropriate tolerances?

Engineers often face challenges when selecting tolerances because each tolerance depends on multiple interacting factors—functional requirements, material properties, machining capabilities, and geometric complexity. Many designers tighten tolerances “for safety’s sake,” which increases costs and may exceed actual machining capabilities. Others set tolerances too large, leading to clearance issues or assembly inconsistencies. Another challenge is supplier variability. Not all CNC machining workshops maintain the same machining tolerances. Some workshops can repeatedly achieve accuracy of ±0.01 mm, while others may only achieve ±0.05 mm, depending on the age of the equipment, environmental stability, or operator experience. Without early communication, expected tolerances may not match actual machining capabilities.

Tolerance techniques for CNC machined parts

Tolerances are the process of determining part dimensions within an acceptable range of error. Understanding design techniques can help minimize tolerance errors. Let’s discuss some design techniques.

All the features in your design may not require tolerances. Therefore, define which features are critical and require tolerances. For example, tolerances for mating or interference may be more important than any other feature.
Avoid maintaining strict tolerances in unnecessary areas, as it’s not necessary to do so for all features. Unnecessary tolerances increase time and processing costs.
When designing parts, consider the tolerance capabilities of CNC machine tools. Sometimes you may need to design parts with tight tolerances that existing CNC machine tools cannot accommodate.
Consider the type of material you will be using the machine for. The type of material will also affect the acceptable tolerance range. For example, it is difficult to maintain tight tolerances for soft materials.

Elimold, your professional CNC machining service partner

Elimold’s standard prototypes and manufacturing tolerances conform to ISO 2768: metals follow ISO 2768-m, while plastics follow ISO 2768-c. Furthermore, we cater to specialized, high-precision needs. Simply specify your unique requirements on your drawings, and our advanced machining technology will ensure precise compliance. Our large manufacturing facility in Shenzhen, China, also boasts numerous 3-axis, 4-axis, and 6-axis CNC machine tools. We also offer various surface treatments for our CNC-machined parts. In addition to CNC machining, we provide diverse manufacturing capabilities to customers worldwide.

Our CNC machining services also encompass quality inspection, material certification, and full-size inspection with reports. The full-size CNC inspection process relies on cutting-edge metrology and measuring tools and is crucial for ensuring that prototypes or final parts meet the precise specifications and tolerances required by your client. In other words, we will repeatedly check to ensure your requirements are met, and if not, we will work with you to deliver parts that do. If you have any questions about CNC machined parts tolerances or would like to learn more about Elimold’s CNC machining services, please feel free to contact a member of our manufacturing team.

in conclusion

CNC machining tolerances are a critical aspect of precision manufacturing, representing how closely dimensions approximate design specifications. Therefore, correctly understanding and selecting the appropriate tolerance grade is essential for ensuring part functionality, maintaining quality, and reducing costs. Appropriate tolerances can be achieved by considering material properties, machining methods, and the necessity of part inspection. This approach optimizes efficiency while minimizing costs. Whether it’s stringent tolerances for critical workpieces or more lenient tolerances for general workpieces, good tolerance design is a hallmark of success in any CNC operation. Elimold, as one of the most reliable CNC machining companies, has been producing precision tolerance parts and prototypes for customers in virtually every industry. If this is exactly what you need, our professional team can certainly meet your requirements!

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