What are the tolerances for machined parts?

In the realm of CNC mechanical processing, the tolerance of machined parts stands as a critical indicator that directly impacts the performance, assembly accuracy, and service life of the final product. Whether you’re a manufacturer striving for consistent production, an engineer designing precision components, or a customer seeking custom-machined parts, a clear grasp of part tolerances is indispensable to the success of your project. This article delves into all key aspects of machined part tolerances, equipping you with the knowledge to master this essential technical parameter.

Tolerance refers to the allowable range of variation between the actual size, shape, and position of a part and its designed ideal size, shape, and position. In real-world CNC machining, producing a part that perfectly matches the ideal design is nearly impossible. Factors like machine tool accuracy, cutting tool wear, material properties, and the processing environment all introduce slight deviations. Thus, setting a reasonable tolerance range is not just a choice but a necessity.

A well-designed tolerance system offers three core benefits:

▶ 1.  Ensures part interchangeability: When multiple parts need to be assembled, consistent tolerance standards enable components from different batches or manufacturers to fit seamlessly. This eliminates the need for time-consuming manual adjustments and reduces the risk of part scrapping.

▶ 2. Controls production costs: Higher precision (i.e., a smaller tolerance range) demands more advanced equipment, highly skilled operators, and longer processing times—all of which drive up production costs. By aligning tolerances with the product’s actual functional needs, businesses can balance quality and cost-effectiveness.

▶ 3. Guarantees product performance: Tolerances directly influence how well a product works. For example, in the transmission system of mechanical equipment, the tolerance of the shaft-and-bearing fit affects the system’s rotational accuracy and noise levels. In sealing structures, the tolerance of the sealing surface determines the product’s sealing effect and lifespan.

Several factors influence the tolerances of machined parts, and understanding them is crucial for optimizing production processes. Below are the primary categories:

▶ 1. Machine Tool Accuracy

The machine tool is the backbone of CNC machining, and its accuracy sets the baseline for the minimum achievable tolerance range of parts. Machine tool accuracy encompasses three key aspects:

●  Geometric accuracy: Refers to the precision of the relative position and shape of the machine’s components (e.g., guide rails, spindle). Examples include the straightness of guide rails and the coaxiality of the spindle.

●  Positioning accuracy: Measures how accurately the machine’s moving parts (e.g., workbench, tool post) reach a specified position.

●  Repeatability accuracy: Evaluates the consistency of the machine’s moving parts when returning to the same position multiple times.

High-precision CNC machines (e.g., those with a positioning accuracy of ±0.001mm) can produce parts with narrow tolerance ranges, while low-precision machines are only suitable for parts requiring larger tolerances.

▶ 2. Cutting Tool Performance and Wear

Cutting tools come into direct contact with the workpiece during machining, so their performance and wear status significantly impact machining accuracy and part tolerances.

●  Tool material: Options like high-speed steel, cemented carbide, and ceramic determine the tool’s hardness, wear resistance, and heat resistance. For instance, cemented carbide tools offer higher hardness and wear resistance than high-speed steel tools, maintaining stable cutting performance longer during high-speed, heavy-load machining—thus ensuring consistent part tolerances.

●  Tool wear: As tools wear, their cutting edges become dull, increasing cutting force and heat. This leads to thermal deformation of both the workpiece and the tool, expanding tolerance deviations. To mitigate this, regularly inspect tool wear and replace tools promptly during production.

▶ 3. Workpiece Material Properties

The material properties of the workpiece—such as hardness, elasticity, and thermal conductivity—also affect part tolerances:

●  Hardness: Materials like stainless steel and titanium alloy have high hardness, requiring greater cutting force and accelerating tool wear. This makes tolerance control more challenging.

●  Elasticity: Elastic materials (e.g., aluminum alloy, copper alloy) undergo elastic recovery after the cutting tool is removed. This causes the part’s actual size to be larger than the designed size, compromising tolerance accuracy.

●  Thermal conductivity: Materials with poor thermal conductivity (e.g., titanium alloy) trap cutting heat during machining, leading to thermal expansion. When the workpiece cools, it shrinks, resulting in size deviations and tolerance issues.

▶ 4. Processing Technology and Parameters

The choice of processing technology and parameters in CNC machining plays a vital role in determining part tolerances:

●  Processing route: A logical sequence of rough machining, semi-finishing, and finishing is essential. Rough machining removes most of the material to boost efficiency, while semi-finishing and finishing ensure precision and tolerances. Skipping semi-finishing (e.g., moving directly from rough machining to finishing) can lead to excessive cutting force in finishing, causing workpiece deformation and failed tolerance requirements.

●  Processing parameters: Cutting speed, feed rate, and cutting depth must be carefully selected:

        Excessively high cutting speed increases cutting heat, causing thermal deformation of the workpiece and tool.

        Too high a feed rate raises surface roughness, affecting dimensional accuracy.

        Overly deep cutting depth increases cutting force, triggering vibration in the machine and workpiece and reducing machining accuracy.

Always adjust parameters based on the workpiece material, tool type, and required tolerance range.

▶ 5. Operator Skills and Experience

While CNC machining is highly automated, operator skills and experience still contribute to maintaining part tolerances:

●  Machine and tool familiarity: Operators must understand the CNC machine’s operation, tool performance, and part processing technology.

●  Real-time monitoring: During machining, operators should closely observe the process—checking chip shape, listening for abnormal cutting sounds, and inspecting workpiece surface quality. If issues arise (e.g., uneven chips, strange noises), stop the machine immediately to adjust and prevent tolerance deviations.

●  Machine debugging: Skills in calibrating tool length and setting the workpiece coordinate system directly affect accuracy. Experienced operators can complete debugging quickly and accurately, ensuring parts stay within the specified tolerance range.

In CNC mechanical processing, machined part tolerances are primarily categorized into three types based on the characteristics they control: size tolerance, shape tolerance, and position tolerance.

▶ 1. Size Tolerance

Size tolerance is the allowable variation range of a part’s actual size relative to its basic size (the size specified in the design drawing, which serves as the basis for determining the tolerance range). It is typically defined by upper deviation and lower deviation:

●  Upper deviation: The difference between the maximum limit size and the basic size.

●  Lower deviation: The difference between the minimum limit size and the basic size.

●  Tolerance value: The absolute value of the difference between the upper and lower deviations.

Example:

If a shaft has a basic size of 50mm, an upper deviation of +0.02mm, and a lower deviation of -0.01mm:

●  Maximum limit size = 50 + 0.02 = 50.02mm

●  Minimum limit size = 50 - 0.01 = 49.99mm

●  Size tolerance = 0.02 - (-0.01) = 0.03mm

This means the shaft’s actual size after machining must fall between 49.99mm and 50.02mm to meet size tolerance requirements.

Size tolerance is further divided into two subtypes:

●  Unilateral tolerance: The tolerance zone lies on only one side of the basic size (only upper or lower deviation is specified). For example, a hole with a basic size of 100mm, upper deviation of +0.05mm, and lower deviation of 0mm—its actual size must be between 100mm and 100.05mm.

●  Bilateral tolerance: The tolerance zone is distributed on both sides of the basic size (both upper and lower deviations are specified). For example, a rod with a basic size of 80mm, upper deviation of +0.03mm, and lower deviation of -0.02mm—its actual size must be between 79.98mm and 80.03mm.

▶ 2. Shape Tolerance

Shape tolerance controls the allowable variation range of a part’s actual shape relative to its ideal shape. It addresses shape errors in the part’s surfaces, lines, and points, with common types including:

●  Flatness

Flatness refers to the allowable variation of a part’s actual flat surface relative to an ideal flat surface. It is used to control surface unevenness.      

Example: A rectangular plate (200mm × 150mm) with a flatness tolerance of 0.02mm—all points on the plate’s actual surface must lie between two parallel ideal planes 0.02mm apart.

●  Straightness

Straightness is the allowable variation of a part’s actual line (e.g., a cylinder’s generatrix, a plate’s edge) relative to an ideal straight line. It can be measured on a plane or in space.

Example: A machine tool guide rail with a straightness tolerance of 0.01mm/m—within every meter of the guide rail, the actual line’s maximum deviation from the ideal straight line must not exceed 0.01mm.

●  Circularity

Circularity controls the allowable variation of a part’s actual cross-section (e.g., a shaft’s or hole’s cross-section) relative to an ideal circle. It addresses out-of-roundness.

Example: A shaft with a diameter of 30mm and a circularity tolerance of 0.005mm—its actual cross-section must lie between two concentric circles with a radius difference of 0.005mm.

●  Cylindricity

Cylindricity is a comprehensive tolerance that controls the allowable variation of a part’s actual cylindrical surface relative to an ideal cylindrical surface. It accounts for circularity, generatrix straightness, and cross-section coaxiality.

Example: A cylinder (50mm diameter, 100mm length) with a cylindricity tolerance of 0.01mm—all points on its actual cylindrical surface must lie between two coaxial ideal cylinders with a radius difference of 0.01mm.

▶ 3. Position Tolerance

Position tolerance regulates the allowable variation range of a part’s feature (e.g., hole, boss, groove) relative to its ideal position (defined by a reference feature). It controls position errors between features or between a feature and a reference, with common types including:

●  Coaxiality

Coaxiality ensures the actual axis of a part’s feature (e.g., shaft, hole) aligns with an ideal reference axis. It is used to keep the axes of multiple cylindrical features coincident.

Example: A stepped shaft with two sections (40mm and 30mm diameter) and a coaxiality tolerance of 0.008mm (relative to the 40mm section’s axis)—the 30mm section’s actual axis must lie within a cylinder (0.008mm diameter) coaxial with the reference axis.

●  Perpendicularity

Perpendicularity controls the allowable variation of a part’s feature (e.g., plane, line) relative to an ideal direction perpendicular to a reference feature. It applies to plane-plane, plane-line, and line-line relationships.

Example: A vertical plane with a perpendicularity tolerance of 0.01mm/m (relative to a horizontal reference plane)—within every meter of height, the actual vertical plane’s maximum deviation from the ideal perpendicular plane must not exceed 0.01mm.

●  Parallelism

Parallelism ensures the actual direction of a part’s feature (e.g., plane, line) aligns with an ideal direction parallel to a reference feature. Like perpendicularity, it applies to plane-plane, plane-line, and line-line relationships.

Example: A guide rail surface with a parallelism tolerance of 0.005mm/m (relative to another reference guide rail surface)—within every meter of length, the actual guide rail surface’s maximum deviation from the ideal parallel surface must not exceed 0.005mm.

●  Position

Position tolerance defines the allowable variation of a part’s feature (e.g., hole, boss) relative to its ideal position (set by a coordinate system from reference features). It is widely used for parts with multiple holes, bosses, or similar features.

Example: A 10mm-diameter hole with a position tolerance of φ0.02mm (relative to two perpendicular reference planes, forming an X-Y coordinate system)—the hole’s actual center must lie within a circle (0.02mm diameter) centered at the hole’s ideal position in the X-Y system.

●  Symmetry

Symmetry ensures a part’s feature (e.g., groove, boss) is symmetrically positioned relative to an ideal reference symmetric plane.

Example: An 8mm-wide groove with a symmetry tolerance of 0.01mm (relative to a reference symmetric plane)—the groove’s actual centerline must lie between two parallel planes (0.01mm apart) that are symmetric about the reference plane.

After CNC machining, detecting part tolerances is essential to verify if they meet design requirements. Below are the most common detection methods:

▶ 1. Manual Detection with General Measuring Tools

General measuring tools are widely used for manual tolerance checks, as they are accessible and easy to operate. Common tools include:

●  Calipers

Calipers (vernier, digital, or dial) measure length, width, diameter, and depth.

          Vernier calipers: Typical accuracy of 0.02mm or 0.05mm.

          Digital/dial calipers: Accuracy up to 0.01mm.

           Suitable for parts with larger tolerance ranges (e.g., size tolerance > 0.01mm).

●  Micrometers

Micrometers specialize in high-precision measurements of outer diameter, inner diameter, and thickness. Their accuracy is usually 0.001mm—higher than calipers. They are ideal for parts with small tolerance ranges (e.g., size tolerance < 0.01mm), such as precision shafts or washers.

●  Depth Gauges

Depth gauges measure the depth of holes, grooves, and similar features. Their accuracy matches calipers (0.01mm or 0.02mm).

●  Dial Indicators

Dial indicators detect shape and position errors (e.g., flatness, straightness, perpendicularity, runout). They are used with auxiliary tools like measuring stands and V-blocks for precise measurements, with an accuracy of 0.001mm or 0.002mm.

●  Feeler Gauges

Feeler gauges measure gaps between two surfaces (e.g., guide rail-slider gaps, valve-valve seat gaps). They consist of a set of thin steel sheets with marked thicknesses, offering an accuracy of 0.001mm.

▶ 2. Advanced Detection with Precision Equipment

For parts requiring ultra-high precision (e.g., aerospace components, medical devices), advanced equipment is necessary:

●  Coordinate Measuring Machine (CMM): Uses a probe to measure 3D coordinates of part features, enabling accurate detection of size, shape, and position tolerances. It offers an accuracy of up to 0.0001mm and is suitable for complex, high-precision parts.

●  Optical Comparators: Project a magnified image of the part onto a screen, allowing operators to compare the part’s actual shape with a reference template. They are ideal for detecting shape tolerances (e.g., circularity, straightness) of small parts.

●  Laser Scanners: Capture 3D data of the part’s surface quickly, enabling comprehensive tolerance analysis. They are efficient for large or complex parts and can detect both shape and position errors.

Tolerances are a cornerstone of CNC machining, influencing every stage from design to production and quality control. By understanding their significance, the factors that affect them, the different types, and how to detect them, you can optimize your machining processes, reduce costs, and deliver high-quality parts that meet or exceed customer expectations. Whether you’re new to CNC machining or an experienced professional, mastering part tolerances is key to staying competitive in the precision manufacturing industry.