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Guidelines for Tolerance Control of CNC Machined Precision Parts

December 2, 2022

The process of CNC machining depends on the precision to a large extent. However, no CNC machine has absolute accuracy. Differences due to material factors or processing techniques used. Therefore, at Weimeite, we assign specific part tolerances to all CNC machining processes during the design process. Machining tolerance is the allowable deviation of part size. It is also called dimensional accuracy. It has minimum and maximum size limits. Any part size falling within these limits shall meet the tolerance requirements.

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Importance of CNC machining tolerance
Most manufacturers refuse to start manufacturing parts until all features have well-defined tolerances. This is because it is the reference point for understanding how a part interacts with other parts. Lack of information limits our understanding of the final design. Therefore, the chance of obtaining inaccuracy is higher.
For example, consider a shaft that fits the part design. The part has a hole of a specific diameter for the exact installation of the shaft in it. If the hole is smaller than the shaft size, it does not fit the shaft. The following are possibilities for providing or not providing tolerances.
Provide tolerances. We will start the project immediately. This is because we know the required size limits. As a result, it shortens delivery times and minimizes costs.

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No tolerance provided. At Wilmet y, we decided to use standard tolerances. For example, ± 0.01mm for parts. Indicates that the diameter of the part will increase or decrease. If it reaches a tolerance lower than this limit, it needs to be CNC machined again. This only increases turnaround time and costs.
Factors Affecting CNC Machining Tolerance
Material Science
Materials behave differently under stress. In addition, some materials are easier to use than others. Details of material properties shall be taken into account when determining tolerances. These characteristics affect the processing ability of materials. Here are some material properties:

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Thermal stability: non-metallic materials, such as plastics, will warp when heated. This limits the range of processing processes to be used because we have to take heat into account. Taking this into account will keep tolerances within acceptable limits.
Abrasivity: It is difficult to process highly abrasive materials. Examples of such materials include phenolic resins and glass laminates. They can cause tool wear, which can lead to errors in machining.
Stiffness and hardness: Flexible and soft materials can vary in size. This makes it more difficult to process them. Examples of such materials include: foam, polyisocyanurate and polyurethane

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Processing type
CNC machining methods significantly affect the possible tolerances of finished parts. Some processes may be more accurate than others.
Rail cutting: This process involves manual operation of the rail saw. Because of its manual characteristics, it requires a larger tolerance zone. This will result in better accuracy.
Shear cutting: This process involves applying sufficient force to cause material failure. We often use a set of blades or punches and dies. Therefore, it is not applicable to brittle or soft materials. Because they are easy to warp and break under the action of great force. Therefore, it cannot maintain a tighter tolerance of less than ± 0.015 ".
CNC screw processing: This process uses a disc cam to feed the workpiece. Since the part moves rather than the tool moves, there is less vibration and deflection of the part. This allows higher achievable accuracy. We recommend using it to process phenolic, foam, plastic parts and other materials.
Steel rule die-cutting: This process uses custom die punches to create specific shapes. However, it does not apply to fragile or soft materials.