CNC machining has revolutionized the manufacturing of complex parts, making the process significantly easier and more efficient. It can reduce production time by a factor of hundreds compared to methods used decades ago.
CNC machining is a computer-controlled, automated process used to manufacture components that demand stringent tolerances, extreme precision, and adherence to strict specifications. It is essential in industries where there is no room for malfunction or even the slightest error, such as medical, aerospace, automotive, and more.
In this article, you’ll gain an in-depth understanding of CNC machining tolerances. We’ll cover everything from the basics of CNC machining and tolerance calculations to the different types of tolerances and how to determine the right one for your application. Read on to gain valuable insights.
What is CNC Machining?
CNC stands for Computer Numerical Control; it is an automated machining process that uses numerical control technology to shape objects. A computerized software system controls the movement and operation of the machine.
A digital model is created and loaded into the software, which then directs the machine’s actions during part manufacturing. The machine removes material from a workpiece to achieve the desired shape. It can be used on various materials such as metal, wood, stone, metal alloys, and more.
CNC machining is efficient, versatile, and delivers precise results, making it a preferred method across many industries and applications.
Calculation and Expression of Machining Tolerances
To fully understand this topic, it’s important to become familiar with some specific terms commonly used in CNC machining. A clear understanding of these terms is essential, especially when calculating machining tolerances. Let’s take a look at a few key definitions:
Basic Size
The basic size is the theoretical dimension of a workpiece as specified in the blueprint. Since a certain level of deviation is expected during the manufacturing process, designers use the basic size as a reference, anticipating the presence of tolerances in the final product.
Actual Size
The actual size refers to the real, measured size of the finished part after machining. It is nearly impossible to match the basic size exactly due to practical limitations, so the goal is to achieve a size as close as possible within acceptable tolerance limits.
Limits
Limits define the allowable dimensional range of a finished part. The upper limit is the maximum acceptable size, while the lower limit is the minimum. If the actual size falls outside this range, the part is considered defective and is typically rejected.
Deviation
Deviation is the difference between the basic size and the permissible limits. There are two types of deviation:
upper deviation and lower deviation.
These can be calculated using the following formulas:
- Upper deviation = Upper limit − Basic size
- Lower deviation = Lower limit − Basic size
Datum
A datum is a reference point, line, or plane used for measurements during the machining and inspection processes.
It is often an imaginary feature selected arbitrarily but consistently, and it serves as a starting point for geometric dimensioning and tolerancing, as well as in various applications in physics and engineering.
Geometric Dimensioning For CNC Machining
It strictly governs the relationship between part features, as well as form and fit accuracy. Here are some commonly used geometric dimensioning and tolerancing terms:
True Position:
True position is a reference used to measure deviation. It represents the theoretical, ideal location of a feature’s axis in 3D space. It indicates how far the actual part deviates from this ideal point. Commonly applied to holes, threads, and features where precise placement is crucial for function and assembly.
Flatness:
Although milled surfaces are intended to be flat, clamping forces and internal stresses during machining can cause warping. This is common after part removal or with thin-walled materials like plastic. GD&T flatness tolerance defines two parallel planes between which the surface must lie.
Cylindricity:
Cylindricity tolerance controls how closely a cylindrical part conforms to an ideal cylinder. It governs both roundness and straightness along the axis without requiring a datum. The tolerance is verified by assessing whether the part fits between two concentric cylinders. It’s commonly used in shafts, pins, and other round parts.
Concentricity:
Concentricity tolerance measures how well the center of a feature’s outer and inner diameters align. It ensures that both share the same central axis, verifying uniform wall thickness and axial symmetry.
Perpendicularity:
This defines the maximum deviation of a surface from a perfect 90° angle relative to a datum. Perpendicularity is measured by linear variation, not angular degrees, and is crucial for precise fit and alignment in assemblies.
Types Of CNC Machining Tolerancing
Standard Tolerance
Standard tolerance is used in the manufacturing of commonly produced parts where a moderate level of precision is sufficient. Machinists apply this type of tolerance to items like pipes, pins, threads, and other standard components.
A commonly accepted default is ±0.1 mm, especially when customers do not specify custom tolerance levels. These standards are defined by well-known organizations such as the American Society of Mechanical Engineers (ASME), the International Organization for Standardization (ISO), and the American National Standards Institute (ANSI).
Bilateral Tolerance
As the name suggests, bilateral tolerance allows variations in both positive and negative directions from the basic size. The final dimension of a component can fall between an upper and a lower limit. If the variation is equal in both directions, it is written as ±0.x mm. If the variation is unequal, it is expressed separately, for example, +0.x mm / −0.y mm.
Example:
Consider a pipe with a 10 mm nominal diameter and a bilateral tolerance of ±1 mm.
- Basic size: 10 mm
- Upper limit: 11 mm
- Lower limit: 9 mm
Any pipe manufactured within the range of 9 mm to 11 mm is acceptable, even if the final size is slightly larger or smaller than the nominal size.
Limit Tolerances
Unlike standard and bilateral tolerances, limit tolerances do not use plus-minus notation. Instead, they directly state the upper and lower dimensional limits. The only requirement is that the actual size of the part must fall within these limits—no specific “basic size” must be targeted.
This method is easier for blueprint readers because it eliminates the need for calculations.
Example:
instead of expressing the tolerance as 10 ± 1 mm, a limit tolerance would simply be written as:
9 mm – 11 mm.
As long as the part falls within this range, it is acceptable.
Profile Tolerances
Profile tolerance is distinct from other types of tolerances. While most tolerances deal with variations in dimensional accuracy, profile tolerance governs the curvature and contour of a feature across the cross-section of a workpiece. It ensures that the shape of a curved surface or line remains within a specified tolerance zone. The symbol used is a semicircle lying on its diameter. Units are typically expressed in millimeters (mm) or inches.
To understand this tolerance, it’s important to define the profile line—a line that follows the cross-sectional contour of the workpiece. The profile tolerance specifies how much this contour can deviate from its true theoretical shape. The actual profile must fall within the allowed variance from this ideal line.
Orientation Tolerance
Orientation tolerance refers to the allowable variation in the angle or alignment of a feature in relation to a reference, known as a datum. Even when dealing with angular differences, the units used are typically millimeters (mm) or inches, not degrees.
Orientation tolerances include perpendicularity, parallelism, and angularity. These are critical in ensuring that features are properly aligned in assemblies. Measurements are taken with respect to the datum to verify that the orientation is within the specified limits.
Location Tolerance
Location tolerance is similar to orientation tolerance, but it specifically refers to the allowable variation in the position of specific features on a workpiece. A datum (reference line or point) is used to measure this shift. The true position is the exact, intended location of the feature as per the design.
Form Tolerances
Form tolerances relate to the shape and geometry of individual features of the workpiece, such as roundness, flatness, and straightness. These tolerances are measured using tools like height gauges, micrometers, and calipers. Units of measurement are typically in millimeters (mm) or inches.
Runout Tolerance
Runout tolerance is used to measure the variation of a feature as the part is rotated 360 degrees around its central axis relative to a datum. It applies to individual or combined features and helps control circularity and concentricity. The symbol for runout is a circular arrow inside a rectangular frame pointing to the top right corner.
Unequally Disposed Tolerances
Unequally disposed tolerances are used when a feature requires unequal unilateral tolerance—meaning more tolerance in one direction than the other. This type is often referred to as the U modifier, and its symbol is a capital “U” inside a circle.
How to Find the Right Tolerance?
1- Consider the need for tighter tolerances:
Some industries—such as medical, automotive, and aerospace—demand extremely tight tolerances to ensure parts fit and function flawlessly. Evaluate whether your application truly requires such precision.
2- Keep budget and time in mind:
Achieving tighter tolerances increases production costs and time. Meeting strict standards often involves multiple tool adjustments and frequent tool changes, which can significantly affect your project’s budget and schedule.
3- Understand the material:
Different materials respond differently to CNC machining. A thorough understanding of your chosen material’s properties will help you determine the achievable tolerances and avoid unnecessary errors.
4- Choose the right machining method:
Various CNC machines offer different capabilities. In precision manufacturing, selecting the appropriate machine based on its strengths and limitations is essential for achieving accurate results.
5- Account for inspection time:
Tighter tolerances require more frequent and detailed inspections. Compared to looser tolerances, achieving high precision demands greater time and attention to quality control.