Understanding Precision CNC Machining Tolerances

The concept of component interchangeability is the very basis of mass production. Yet, in essence, it’s simply about achieving sufficiently tight CNC machining tolerances between different parts without significantly raising manufacturing costs. Not correctly applying this concept can result in numerous unforeseen consequences during CNC machining. For example, tolerances that are too tight may require a secondary procedure to grind them down, which adds to lead time and costs. Alternatively, tolerances that aren’t tight enough may make piecing parts together during assembly impossible, resulting in the need to rework or even make the end product inoperative. When it comes to precision, CNC turning and milling of components must balance desire with price when it comes to achieving tight tolerances.

Determining CNC Machining Tolerances

Though CNC machining tolerances can be produced to very tight levels of accuracy, no production method is perfect. Every part will differ slightly from another made via the same manufacturing process, even those produced by the same CNC machine. Tolerances are necessary because no two parts have the exact dimensions, regardless of their efforts to make them identical. This difference is accounted for in CNC machining, with tolerances given a standardized limit.

For CNC machinists, tolerances are measured as numerical values with a “±” symbol typically preceding them, meaning “plus or minus” the given amount. For example, a part that measures 5.110 inches (129.794 mm) long that requires a tolerance of ±0.002 inch (mm) would need to measure between 5.108 and 5.112 inches (129.7432 and 129.8448 mm) to pass quality control inspections. Specifying tolerances shows the precision needed in CNC turning and milling when manufacturing a component. The smaller the range of acceptable measurements, the tighter the tolerance and the more precise the part needs to be. 

By limiting CNC machining tolerances, manufacturers can achieve a practical range to allow specific differences in dimensions between components. With CNC machining, tolerances are generally set at a standard limit of ±0.005 inch (±0.127 mm); for reference, the average human hair is around .0028 inch (.071 mm). This standard limit is acceptable and usually makes no difference in the final product’s performance.

For certain applications, even these small dimensional variations in tolerance can affect a product’s ability to function properly. Using CNC machining, tolerances can be made even more accurate, with some machines capable of achieving ±0.0000984 inch (about ±0.0025 mm). To achieve this level of precision, CNC turning and milling operations often must undertake more time-consuming and costly processes. For this reason, CNC machining tolerances should only be used to fabricate parts requiring high accuracy. As these tighter tolerances can make the final product prohibitively expensive, it’s normally more economical for product and component designers to specify acceptable tolerances.

Commonly Used CNC Machining Tolerances

Standard Tolerances

The most widely manufactured components like pins, pipes, and threads have typical CNC machining tolerances around ±0.0039 inch (+0.1 mm). With precision CNC turning and milling, machinists normally apply these tolerances when there isn’t a specified tolerance given. The standard ranges in CNC machining tolerances are set by organizations that include the American National Standards Institute (ANSI), the American Society of Mechanical Engineers (ASME), and the International Organization for Standardization (ISO).

Standard CNC machining tolerances depend on their linear measurements: 

• Fine (f) ranges from ±0.0019685 to ±0.019685 inch (±0.05 to ±0.5 mm)
• Medium (m) ranges from ±0.00393701 to ±0.0787402 inch (±0.1 to ±2.0 mm)
• Coarse (c) ranges from ±0.00787402 to ±0.15748 inch (±0.2 to ±4.0 mm)
• Very coarse (v) ranges from± 0.019685 to ±0.314961 inch (±0.5 to ±8.0 mm)

These generalized tolerances can be defined to measure angular or linear dimensions, along with chamfers and other grooved components.

Bilateral Tolerances

Used mainly for measuring exterior components, bilateral CNC machining tolerances can vary on either side of the value given. This means it can only be a smidgen smaller or larger. For example, bilateral tolerances of ±0.002 inch (about ±0.051 mm) for a 1.1 inch (27.94 mm) part would be between 1.102 and 1.098 inches (27.8892 and 27.9908 mm).

Geometric Dimensioning and Tolerancing

As a type of CNC machining tolerance, geometric dimensioning and tolerancing (GD&T) offer a more detailed assessment. It emphasizes both acceptable deviations and dimensions of components, outlining specific geometries of the machined part, including its concentricity, flatness, and positioning. GD&T uses extremely precise measurements for a part’s dimensions.

GD&T looks at the following when considering CNC machining tolerances: 

• Concentricity: Like rings in a bullseye, CNC machining tolerances of drilled or reamed holes must run true along bore toolpaths like circular bosses or counterbores, with concentricity measurements ensuring this.
• Cylindricity: Milled holes should be as perfectly cylindric as possible, with the machined hole’s internal measurements lying within two concentric cylinders so as not to be oblong; precision CNC turning ensures tight tolerances with the parts that mate with them.
• Flatness: Even though milled surfaces are largely flat, various forces can cause them to warp once a component is finished and removed from the CNC machine; tolerances controlled via GD&T flatness measurements define the parallel planes where the milled surface should lie.
• Perpendicularity: This determines the maximum horizontal deviation regarding CNC machining tolerances for surfaces when compared to a nearby vertical surface; this measurement can also be used to control precision CNC turning of a shoulder to a part’s center axis or adjacent diameter.
• True position: Using qualifying data points, GD&T looks at the locations of a part’s specific features to control variation on its actual to the desired positioning; this measurement is critical to ensure seamless fabrication when joining components together.  

GD&T is especially applicable to CNC machining tolerances as it has stringent requirements that help ensure various features’ dimensional accuracy.

Unilateral Tolerances

Unilateral CNC machining tolerances only go in one direction. They’re either positive or negative, so a tolerance might be described as +0.00/-0.001 inch (+0.00/-0.0254 mm) if it can only be smaller or +0.001/-0.00 inch (+0.0254/-0.00 mm) if it can only be larger than the specified tolerance. These unilateral tolerances are used when one part goes into another or when the part covers another. Unilateral CNC machining tolerances are easier to inspect, as dimensions should only vary on one side.

Limit Tolerances

Expressed as an array of values, these CNC machining tolerances show the component as acceptable if a measurement falls within the accepted range. For example, a limit tolerance might be expressed as between 2 to 2.1 inches (50.8 to 53.34 mm). The part’s dimensions, thus, must fall between these upper and lower limits.

Determining the Best CNC Machining Tolerances

Choosing the best tolerance for a component often involves not choosing the tightest one possible. For designers of parts and products, it’s imperative to understand how much leeway they have, as it affects both turnaround time and the cost of the components. A customer might ask for the tightest CNC machining tolerances possible to achieve, which may, in turn, double the cost. Not every part requires such precision. CNC turning and milling processes shouldn’t always seek the tightest tolerance.

Depending on the application, sometimes looser tolerances are more advantageous. For example, a component that doesn’t connect to another part doesn’t need the same degree of precision. CNC turning and milling operations should depend more on how the look at how the component performs when its size or shape varies during prototyping to determine the best tolerance. Specific critical components may require extremely tight tolerances, for which even a small variation might have horrific consequences. Yet other parts might not need this same precision, and a more significant deviation would bring down the cost.

When to Choose Tighter CNC Machining Tolerances

Sometimes certain features of a component require tighter tolerances. Typically, the most precise CNC machining tolerances should be at points in the component’s structure where it fits into or joins with another part. For this reason, features like holes should be machined with tighter tolerances than other less essential features. In addition to the longer time taken to machine workpieces to higher accuracy, the need to replace CNC machine tools can also raise the cost. While a new tool may not have any trouble achieving tolerances of ±0.001 inch (± 0.0254 mm), if it’s being used to fabricate a thousand units at this tolerance, it may either require an adjustment of the machining speed or even replacement at some point during production.

Higher Costs & Tight Tolerances

There are a few reasons why it usually costs more to fabricate a part with tighter CNC machining tolerances.

These include: 

• CNC tools able to achieve tighter tolerances tend to cost more.
• Components need to be machined more slowly.
• Failure rates for parts are often higher.
• More extensive quality inspections are required since the margin of error is significantly smaller.

Higher tolerance CNC machining not only drives up the cost of production but also makes quality inspections more expensive.

Materials

The material used to fabricate a component will affect CNC machining tolerances as material properties differ.

A few material properties that should be considered include:

• Abrasiveness: When cutting abrasive material, cutting tools wear more quickly, making it more difficult to achieve precise tolerances and often requiring the operator to change the tooling multiple times throughout a production run.
• Hardness: Cutting tools are more likely to alter a part’s dimensions when they come into contact with softer material, meaning machine operators need more patience to achieve tight tolerances when less hard machining materials.
• Heat stability: This mainly affects non-metallic components, as the machining process causes many polymers (and even softer metals) to lose their shape, restricting the fabrication techniques a machinist can use.

Choosing materials that can achieve the desired tolerance without significantly raising the price per piece is essential.

Manufacturing Methods

Not all CNC machines are built the same, and not every precision machining company has the experience to achieve tight tolerances. When using a third-party vendor for CNC machining, the tolerances required should be achievable by the manufacturer.

Finding a Vendor: Staub’s CNC Machining Capabilities

It’s important for those looking to attain sufficient precision in CNC turning and milling operations without breaking the bank to engage a trustworthy vendor. Not every CNC machining company can achieve the tight tolerances a customer may need. Perhaps even more importantly, not every company will know when tolerances can be loosened without affecting the operational aspects of the final product.

Engaging with an experienced partner will ensure components are made with sufficient precision. Staub Inc. has the tools and expertise to achieve the tightest tolerances for CNC turning and milling, along with other precision machining operations. We offer CNC machining services, including 5-axis, electrical discharge, hybrid mill/turn, rotary transfer machining, laser-cutting, waterjet-cutting, and finishing services. Our precision CNC turning and milling services can save you time and money.

Staub offers the following precision CNC turning capabilities: 

• CAD and CAM software that optimizes machining processes.
• Fabrication of parts with tight tolerances and complex geometries.
• Finishing capabilities done in-house that include assembly and deburring.
• Lathes that feature chuck loading robotics for slugs and larger diameter workpieces.
• Machines that accept bar stock with diameters from 0.1 to 3.0 inches (2.54 to 76.2 mm)
• Trusted partners for painting and plating components.

Staub specializes in mid-level and high-volume projects that require recurring production runs. Our precision turning centers are automated and can complete production overnight without oversight. We also offer both fixed head and Swiss turning, so we have flexibility in the processes available to customers. Our engineers and machine operators carefully consider a component’s complexity, choosing the best lathe.  

To increase efficiency, Staub uses the following: 

• Precision CNC turning centers are equipped to augment flexibility and increase productivity.
• Production of complex parts without the need for secondary operations.
• Technologies that include articulating and collaborative automation, along with automated unloading technology.

Contact our expert technical staff today to learn more about Staub’s precision CNC turning and milling services.