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Tolerances

This article covers CNC machining tolerances, types, and sources of variation to ensure precise, high-quality parts that fit and function correctly in your projects.

Table of Contents

Overview   |   Types of Tolerances   |   Sources of Variation 

Overview

Tolerances refer to the permissible range of variation in a dimension that ensures a part fits and functions correctly. According to ASME, engineering tolerance is the total allowable deviation from a dimension's basic value.

Tolerances are checked during the quality inspection after CNC machining. They guide the choice of manufacturing methods needed for a part. Tighter tolerances might need advanced equipment or extra processing to meet the required dimensions.

For CNC machined parts, various types of tolerances can be defined and used.

Types of CNC Machining Tolerances

In CNC machining, "tolerance" refers to how precisely a machine can shape a part. CNC machines are incredibly precise, with some achieving an accuracy of ± 0.0025mm, about a quarter of a human hair's width. However, tolerances differ among machines and are typically set by the manufacturer, with 0.02mm being a common standard. CNC service providers also inform customers about their machines' tolerances.

In design and manufacturing, tolerance refers to the allowable range of size variations for a part that still ensures it works properly. Designers set these tolerances based on the part's function, fit, and form. Tolerances are vital for parts that connect or interact with others. For instance, parts of an electric engine require tighter tolerances than a door handle because they have more features that need to fit together precisely. Tolerances are indicated by a number next to the relevant dimension.

Standard Tolerances

Standard tolerances are widely used to define how much a CNC machined part's dimensions can vary. These tolerances might represent what a machine shop can produce without extra cost or serve as a general variation applied to an entire drawing, as noted in a title block or note, unless specified otherwise in the drawing or on certain features.

Limit Tolerances

A limit tolerance is the specified minimum and maximum dimension allowed on a drawing. Also known as limits of size, these provide the upper and lower size limits for a feature. This makes it easy to quickly check if a measured dimension falls within the tolerance zone, without needing to calculate boundary dimensions from tolerance values.

Unilateral Tolerances

Unilateral refers to something related to one side. In terms of tolerances, it means the tolerance is applied in only one direction, either plus or minus. In GD&T, this is called an unequally disposed tolerance and is marked with a U symbol. Typically, unilateral tolerances are used at maximum material condition (MMC).

Bilateral Tolerances

Bilateral refers to something involving two sides. In terms of tolerances, it means that both the positive and negative limits are considered for a specific measurement. This provides the smallest and largest acceptable sizes for that measurement. The tolerance can be equal on both sides or vary between them.

Sources of Variation in Machined Components

CNC-machined parts are not flawless. Variations among these parts are expected and result from several factors.

Type of Raw Material

Harder materials allow for tighter tolerances because they maintain their shape and resist deflection during machining. They offer greater dimensional stability compared to softer materials. Additionally, materials with a lower coefficient of thermal expansion (CTE) are less affected by temperature changes, enabling even tighter tolerances.

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Condition of the Workpiece

Using uniform and precise stock materials significantly minimizes variations. The quality of your raw materials directly affects the quality of your final product.

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CNC Machine Used

While more expensive machinery doesn't always guarantee higher quality parts, it often does. Machines that maintain tighter tolerances are typically built with superior raw materials and advanced programming, ensuring precise tooling alignment during milling.

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Tooling

Without precise and accurate tooling, achieving tight tolerances is impossible, regardless of how well you prepare your material.

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