Posted December 8, 2020

Why Are Tolerances in Manufacturing Important and How Do They Work?

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Customers who use CNC machining services often request parts that have complex geometries. Parts with curved sections must fit closely to ensure proper operation. Machining vendors must carefully analyze the part geometry to achieve tolerances with confidence. Compromising precision for speed may result in parts manufactured outside of tolerance, thereby unusable in a final assembly.

Manufacturing methods, temperature, different types of materials, changing atmospheric pressures, and other factors can cause the fit between parts to shift slightly. Adhering to tolerances in manufacturing reduces failure caused by degradation and increases performance predictability.

Tolerances in Manufacturing Align With a Standardized Approach

Tolerance represents the amount of allowed variance in the dimensional accuracy of a part. The amount of allowed variance occurs inside maximum and minimum dimensional limits. Within those limits, a range of dimensions becomes the tolerance band. A wide tolerance band shows that the machining has a loose tolerance. Tight tolerances have a narrow tolerance band.

Tolerances are normal and expected, so engineers design parts with a tacit understanding that not every geometry can always be made exactly as designed. The application of a standardized approach for tolerances allows design, engineering, and manufacturing teams—no matter the location—to interpret information accurately. 

Tolerances establish uniformity in specifications and reduce error opportunities, so projects can successfully transition from design to manufacturing. By applying standard tolerances to CNC machined-parts, manufacturers decrease costs, achieve higher quality deliverables, and reduce the time-to-market.

Good communication between teams also depends on the use of a common language for applying tolerances. Table 1 defines common tolerance terms.

Table 1: Common Tolerance Terms

Terms

Definition

Basic size

The diameter of the object or feature

Fundamental deviation

The minimum size difference between the component or hole and the basic size

Upper deviation

Difference between the maximum possible size and the basic size

Lower deviation

Difference between the minimum possible size and the basic size

Total tolerance

Value of the maximum amount of variation

International tolerance grade

The maximum size difference between the component or hole and the basic size.

Allowance

The minimum amount of clearance and maximum amount of interference

Maximum material condition

Condition of the component with the most material within tolerance (volume or size)

Least material condition

Condition of the component with the least material within tolerance (volume or size)

Quality Manufacturing Depends on Uniform Tolerances

Technical drawings rely on an understanding of the different methods for defining tolerance to ensure uniformity. Along with referring to the bilateral tolerance, unilateral tolerance, and limit tolerance values described in Table 2, technical drawings also use decimal places to express a tolerance’s tightness. Table 3 exhibits how a larger number of decimal places represents a tighter tolerance.

Table 2: Drawing Tolerances

Tolerance Type for Drawings

Tolerance Definition

Tolerance Symbol

Bilateral

Allows variation above and below the basic size with equal or unequal variance in either direction.

Plus represents the upper limit. Minus represents a lower limit.

Unilateral

Allows variation above or below the basic size but only allows variation in one direction

Plus represents the upper limit. Minus represents a lower limit.

Limit

Upper and lower limits of the dimension

Neither plus nor minus

 

Table 3: Tolerances Expressed by Decimal Places

Number of Decimal Places

Degree of Tolerance

Four (+/-0.000x)

Very tight

Three (+/-0.00x)

Tight

Two (+/-0.0x)

Medium

One (+/-0.x)

Loose


ASME Y14.5 

Geometric dimensioning and tolerance (GD&T) refers to the geometry of parts that interlock and directs to form, orientation, location, profile, and run-out tolerances (as defined in Table 4). For operations in the United States, the American Society of Mechanical Engineering (ASME) applies the ASME Y14.5 standard as the authoritative guideline for geometric tolerances. It uses symbols and rules for a standardized approach to communicating dimensions. 

The “Y14” designation for the standard identifies the governance as the Engineering Drawing and Related Documentation Practices Standards Committee, while the “.5” designation identifies the Dimensioning and Tolerancing Subcommittee. The ASME Y14.5 standard symbols describe different geometric characteristics within technical drawings and serve as tools for precisely communicating about tolerances between engineers and manufacturers. 

Table 4: Geometric Dimensioning and Tolerance Types

Type

Definition

Form

Determines form of a part

Profile

Establishes a boundary around a surface. Elements of a component must lie within the boundary.

Orientation

Defines the orientation of the form according to a reference.

Location

Indicates the location of a component feature according to a reference.

Run-out

Establishes the run-out fluctuation of a feature of a component when the component rotates on an axis.

 

ISO Standards

The International Organization for Standardization (ISO) issues globally recognized standards for overall geometrical products within the ISO 1101:2017 standard and geometrical tolerances in machining within the ISO 2768 standard. 

While ISO 2768-1 defines linear dimensions and angular dimensions, ISO 2768-2 describes geometrical tolerances according to form and position. According to the limit deviations over nominal dimension ranges, the ISO 2768-1 standard also classifies linear and angular dimensions, such as the radius of a part’s curvature and chamfer. In contrast, ISO 2768-2 classifies form and position according to straightness, flatness, perpendicularity, symmetry, and run.

Datums and datum systems represent an exact plane, line, axis, or point location for geometric dimensioning and tolerancing. For CNC machining, a datum translates to machine offset or Work Coordinate Systems that provide position and orientation references for the machine and the workpiece. 

The ISO 5459:2011 standard addresses the terminology, rules, and methods used for datums and datum systems. CNC programming includes G code blocks that command the interpreter to move to real or absolute axis positions defined within the datum or datum systems.

Aerospace and Military Applications Require Precise Tolerances

Despite the lack of standard machining tolerances, different industries offer guidelines for tolerances. In most instances, those guidelines depend on the type of material required for an application, the machining method, and any plating or finishes applied to a workpiece. Precise tolerances also increase the cost of CNC machining so that project budgets can impact tolerance guidelines.

Depending on the type of application, aerospace and military projects may require precise tolerances and complex geometries. Yet, differences exist. For example, the components used to form a commercial aircraft wing’s outer skin will not require the same precision as the composite materials used to construct the wing surface of a fighter that employs stealth technologies. The reasons for the different approaches to tolerances included the type of materials used, the material stresses caused by operating conditions, and the difference in missions.

Compared to the aluminum used for a commercial aircraft, the composite materials used for fighter aircraft outer surfaces withstand extreme temperatures and speeds. Additionally, the fit of the components used to assemble the outer surface requires precise tolerances. 

Any mismatch in fit can create a radar image that discloses the location of the aircraft. While the geometric tolerance specified for brackets used in the transport aircraft can range around 0.05 inches, the geometric tolerances for fighter aircraft components range as precise as 0.00007 inches. Guidelines for the fighter jet surface flatness may specify tolerances of ±0.005 inches.

Achieving the Correct Tolerances in Manufacturing Depends on the Right Equipment

The geometries specified for a particular component often impact decisions about using three, four, or five-axis CNC machines and lathes. CNC machining companies that produce components requiring precise tolerances or complex geometries for the aerospace and defense industries rely on five-axis machine tools equipped with lasers that track movements and triangulate positions. Those machine tools also feature controllers that can rapidly process data to produce smooth, uniform motions and software that provides the correct motion’s exact code.

CNC machining companies that produce components requiring precise tolerances or complex geometries for the aerospace and defense industries rely on five-axis machine tools equipped with lasers that track movements and triangulate positions.

If your part designs require specific tolerances, be sure to work with your CNC vendor ahead of time to understand their capabilities thoroughly. 

Plethora is a precision CNC machine shop that uses state-of-the-art software and equipment to adhere to tight tolerances in manufacturing. Learn more about the precise tolerances provided by CNC machines by contacting our experts today at 415-726-2256. Or, get started with your next project by uploading your design files to Quote My Part.

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