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CNC Machining Precision Parts Tolerance Control Guide
Date: 2022-09-14
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The process of CNC machining relies heavily on precision. However, no CNC machine exists with absolute precision. Variations arise due to material factors or processing techniques used. Therefore, at Concord, we assign specific part tolerances to all CNC machining processes during the design process. Machining tolerance is the allowable deviation from the dimensions of the part. It is also called dimensional accuracy. It has minimum and maximum size restrictions. Any part dimensions that fall within these limits meet the tolerance requirements.

The following is a further understanding of CNC machining tolerances.

Machining tolerance is the sum of specific dimensions that are allowed to differ from a given value. We express it in several ways, for example:

Below and above the allowable amount of a specific size. For example 0.14999 ±0.0001

per se allowed differences. eg ±0.0001

Lower and upper bounds. For example 0.1500, 0.1498

These defined areas of allowable tolerance range are called tolerance zones. A larger difference between the lower and upper limits results in a looser tolerance band. Smaller differences result in tighter tolerances.

Another way to express tolerance is to use decimal places in the numbering. The more decimal places, the tighter the tolerance value. The following are examples of decimal tolerance expressions:

One decimal place ±0.2”

Two decimal places ±0.01”

Three decimal places ± 0.003”

Four decimal places ±0.0005”

The importance of CNC machining tolerances

Most manufacturers refuse to blindly build parts until all features have well-defined tolerances, because this is the point of reference for understanding how the part interacts with other parts. The lack of information limits our understanding of the final design, and the chance of a part going wrong is higher.

For example, consider a shaft that fits the part design. The part has a hole of a specific diameter for the shaft to fit exactly in it. If the hole is smaller than the shaft size, it does not fit the shaft. Below are the possibilities with or without tolerance.

Provides a tolerance. We started this project right away because we knew the size constraints needed, it shortened lead times and minimized costs.

No tolerance is provided. At Concord y, we decided to use standard tolerances. For example, for parts ± 0.01mm. Indicates that the diameter of the part will increase or decrease. If it reaches a tolerance below this limit, it will need to be reworked. This only increases turnaround time and increases costs.

Factors Affecting CNC Machining Tolerance

Material

Materials behave differently under stress. Also, some materials are easier to work with than others. Details of material properties should be considered when determining tolerances. These properties affect the processability of the material. Here are some material properties:

Thermal stability: Non-metallic materials, such as plastics, warp when heated. This limits the range of machining processes to be used, as we have to account for heat. Taking this into account will keep the tolerance within an acceptable range.

Abrasive: Highly abrasive materials are difficult to machine. Examples of such materials include phenolic resins and glass laminates. They cause tool wear, which can lead to errors in machining.

Rigidity and stiffness: Flexible and soft materials variably change size. This makes processing them more difficult. Examples of such materials include: foam, polyisocyanurate, and polyurethane.

Processing type

The CNC machining method significantly affects the possible tolerances of the finished part. Some procedures may be more precise than others.

Orbital Cutting: This process involves manually operating the orbital saw. Due to its manual nature, it requires a larger tolerance zone. This will get better accuracy.

Shear cutting: This process involves applying a force large enough to cause the material to fail. We often use a set of blades or punches and dies. Therefore, it is not suitable for brittle or soft materials. Because they warp and break easily under enormous force. Therefore, it cannot hold tighter tolerances of less than ±0.015”.

CNC screw machining: This process uses a disc cam to feed the workpiece. There is less vibration and deflection of the part due to part movement rather than tool movement. This allows for higher achievable precision. We recommend it for processing materials such as phenolic, foam and plastic parts.

Steel Ruler Die Cutting: This process uses a custom die punch to create a specific shape. However, it is not suitable for brittle or soft materials.

Plating and Finishing

These two properties add a small amount of material to the surface of the part, which changes the dimensions of the final product and should be considered before design and production.

Cost

We recommend achieving precise tolerances. However, it will cost more to obtain tighter tolerances than required. If working on a tolerance of three decimal places, do not exceed four decimal places.

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