CNC Machining Tolerances: The Practical Chart
In the world of manufacturing, precision is paramount. A single error can compromise a part’s function. This is why CNC machining tolerances are so important. They define the acceptable range of variation for a part's dimensions. Understanding them is a critical skill for any designer or engineer. This guide provides a comprehensive overview of machining tolerances. It explains how to interpret tolerance charts and apply them to your projects.
Our team has years of experience in this field. We have seen how small details in a design can affect the final product. The insights in this guide are drawn from this practical knowledge. They are meant to help you avoid common mistakes. This ensures your parts are manufactured correctly the first time.
What are CNC Machining Tolerances?
Tolerances are the permissible variations from a specified dimension. No machine can create a perfect part. The purpose of a tolerance is to give a manufacturer a margin of error. It ensures that a part will fit and function as intended. A tight tolerance means a small margin of error. A loose tolerance means a larger one. Choosing the right tolerance is a balance. It must meet the part's functional needs without adding unnecessary cost.
Tolerances are often shown on a drawing with a plus/minus symbol (±). For example, a dimension of 10.00 mm with a tolerance of ±0.05 mm means the final part can measure anywhere from 9.95 mm to 10.05 mm. If the part falls outside this range, it is considered a defect.
General vs. Specific Tolerances
General tolerances are often applied to a drawing as a blanket statement. They are used for dimensions without a specific tolerance callout. The ISO 2768 standard is widely used for this purpose. It has different classes, from "Fine" to "Very Coarse." A "medium" tolerance class is a common default. It is a good starting point for many general-purpose parts.
Specific tolerances are applied directly to a dimension. They are used for critical features. This could be a bore that needs to fit a bearing. Or it could be a key feature in a complex assembly. These tolerances require more precise machining. They are more expensive to achieve.
Types of Tolerances
There are several ways to specify tolerances on a technical drawing. Each method serves a different purpose.
- Bilateral Tolerances: This is the most common type. The tolerance is distributed on both sides of the nominal dimension. For example, 25.00 ± 0.02 mm. The part's size can be slightly larger or smaller.
- Unilateral Tolerances: The tolerance is on only one side of the nominal dimension. For example, 25.00 +0.02 / -0.00 mm. This is often used when a part must not exceed a certain size, such as a shaft fitting into a hole.
- Geometric Dimensioning & Tolerancing (GD&T): This is a more advanced system. It uses a symbolic language to control a part's form, orientation, and location. GD&T provides a clear, precise way to communicate design intent. It helps to ensure parts fit together in an assembly.
The Practical CNC Machining Tolerances Chart
This chart provides a practical reference for common tolerances. It is based on industry standards and typical capabilities. Please note that these are general guidelines. Actual tolerances may vary. Factors like material, geometry, and machine type can all affect the result.
| Feature Type | Standard Tolerance (mm) | Fine Tolerance (mm) |
| Milled Features | ± 0.13 mm | ± 0.05 mm |
| Turned Diameters | ± 0.13 mm | ± 0.05 mm |
| Drilled Holes | ± 0.15 mm | ± 0.08 mm |
| Bored Holes (Reamed) | ± 0.05 mm | ± 0.02 mm |
| Hole Locations | ± 0.10 mm | ± 0.05 mm |
| Flatness | 0.15 mm per 25 mm | 0.08 mm per 25 mm |
Factors That Affect Tolerances
Achieving a specific tolerance is not always simple. Several factors influence the outcome.
- Material: Some materials are easier to machine than others. Soft metals and plastics can warp or flex. Hard materials can cause tool wear. This affects precision.
- Part Geometry: Simple shapes are easier to hold to tight tolerances. Complex geometries with thin walls or deep pockets are more challenging.
- Machine Type: The type and condition of a CNC machine is a major factor. A well-maintained 5-axis mill can achieve tighter tolerances than a standard 3-axis machine.
- Tooling: Sharp, high-quality cutting tools are essential for precision. Worn tools can cause chatter marks and dimensional errors.
The Relationship Between Tolerance and Surface Finish
Surface roughness is a measure of a surface's texture. It is a key consideration in machining. A part's surface finish can impact its function. It also relates directly to the tolerance that can be achieved. Generally, a smoother surface finish requires more machining time. This increases cost.
Understanding the difference between Ra and Rz is important. Ra is the average roughness. Rz is the maximum height of the roughness profile. Each provides different information about the surface. To learn more about this, please read our guide on Surface Roughness: Ra vs Rz.
The Importance of Quality Control
Once a part is machined, it must be inspected. Quality control ensures that all tolerances have been met. A Coordinate Measuring Machine (CMM) is a key tool for this. It takes precise measurements of a part's geometry. The CMM produces a detailed report. This report is a crucial part of the process. For a deeper understanding of this step, we recommend our guide: CMM Inspection Report Checklist.
The Cost of Tight Tolerances
It is a common misconception that tighter tolerances are always better. In reality, they add significant cost. Tighter tolerances require more machine time. They demand more specialized tools. They also increase the need for inspection. This drives up the final price of a part. Always choose the loosest tolerance that meets your part's functional requirements. This helps to optimize both cost and quality.
Tolerance Stack-Up Analysis
In assemblies with multiple parts, tolerance stack-up is a critical issue. It occurs when the tolerances of individual parts combine. This can result in a final assembly that does not fit. A tolerance stack-up analysis is a method used to predict this. It helps engineers identify and manage the risk. Understanding this analysis is vital for complex designs. You can find a detailed guide on this topic here: Tolerance Stack-Up Analysis.
FAQ: Common Questions
Q1: What is a standard tolerance for CNC machining?
A common standard tolerance for many features is ±0.13 mm (±0.005 inches). This is a good default for non-critical dimensions.
Q2: Why are some tolerances more expensive than others?
Tighter tolerances require more precise machines, more machining time, and more rigorous inspection. These factors increase the cost of production.
Q3: What is the purpose of GD&T?
GD&T provides a precise and unambiguous way to communicate design intent. It ensures that a part's form, orientation, and location are controlled for proper fit and function in an assembly.
