Machining Tolerances: Understanding Different Manufacturing Process Tolerance


The permissible variation in dimensions and specifications that is thought to be acceptable during the manufacturing or machining of a part is referred to as standard machining tolerances. These machining tolerances specify the allowable range of deviation from the ideal or nominal design parameters for the finished product. They are an essential element of the machining process because they guarantee that the created parts satisfy the necessary functional and quality standards while enabling practical and affordable manufacturing.

Sungplastic is a professional custom parts manufacturer, has made great efforts at machining tolerances.

standard machining tolerances for CNC machined parts

Types of Machining Tolerances

Machining tolerances encompass several key aspects:

Dimensional Tolerances
These tolerances specify the acceptable range of variation in the size of a part. For example, a hole’s diameter might have a dimensional tolerance of ±0.1 mm, indicating that the actual hole size can be up to 0.1 mm larger or smaller than the nominal dimension.

Geometric Tolerances
Geometric tolerances relate to the shape, orientation, and position of features on a part. They include tolerances for characteristics like straightness, flatness, parallelism, perpendicularity, and circularity.

Positional Tolerances
Positional tolerances control the allowable deviation of specific features’ locations relative to a reference point or datum. These tolerances are particularly important for ensuring the correct assembly and functionality of components.

Concentricity and Runout Tolerances
Concentricity tolerances specify how much a feature’s axis or center can deviate from the center of a reference feature. Runout tolerances address the circularity or eccentricity of features, such as shafts and rotational components.

Surface Finish Tolerances
Surface finish tolerances dictate the permissible range of roughness or smoothness for the surface of a part, typically expressed in terms of parameters like Ra (average roughness).

CNC Machining

Our CNC machined parts are produced to ISO 2768 medium linear tolerance.

Part Dimensions – Tolerance
0.5-3.0 mm: +0.10 mm
3.0-6.0 mm: +0.10 mm
6.0-30 mm: +0.20 mm
30-120 mm: +0.30 mm
120-400 mm: +0.50 mm
400-1000 mm: +0.80 mm
Angular Dimensions: + 0.5
Surface Roughness: 1.6μm Ra(64.3 RMS,micro-inches)

Part Dimensions – Tolerance
0.020-0.118 in: +0.004 in
0.110-0.230 in: +0.004 in
0.230-1.180 in: +0.008 in
1.180-4.720 in: +0.012 in
4.700-15.740 in: +0.020 in
15.700-39.370 in: +0.031 in
Angular Dimensions:+ 0.5
Surface Roughness: 1.6μm Ra (64.3 RMS, micro-inches)

In addition, it’s feasible for us to achieve even more stringent tolerances, reaching ±0.025mm or ±0.001″ when an engineering drawing specifies crucial features.

Our inspection and measurement process involves the use of calipers. As a result, any features specified in the drawing must be measurable using this method.

3D Printing

In the realm of 3D printing, where resin is solidified or materials are fused to create objects, the precision standards differ from those of higher-precision manufacturing techniques.

As a general guideline, one can typically expect machining tolerances of approximately ±0.2mm for PolyJet parts, ±0.3mm for SLS and MJF parts, ±0.1mm for SLA parts, and ±0.5mm for FDM (specifically ABS and PETG) parts. It’s worth noting that FDM PLA, typically printed under open-air conditions, might exhibit machining tolerances closer to ±1.0mm. Furthermore, for all these 3D printing technologies, larger parts may experience a broader range of tolerance.

The figures provided above offer a rough framework and are intended for reference purposes. For a more comprehensive understanding of design parameters, it’s advisable to undertake a tolerance analysis. When designing parts, it’s crucial to always consider machining tolerances.

If precision and dimensionality hold paramount importance, it might be prudent to explore manufacturing methods like CNC (Computer Numerical Control).

Plastic Injection Molding

Injection molding is a manufacturing process where molten material is injected into a mold cavity and then cooled to form a solid object. The tolerances in injection molding depend on various factors, including the material used, the design of the part, and the capabilities of the molding equipment. Here are some general guidelines regarding injection molding tolerances:

Tolerance Range: Injection molding tolerances are typically specified as a range rather than a single value. For example, a tolerance might be specified as ±0.1mm, meaning the actual dimensions of the part can vary within a range of 0.1mm from the intended dimensions.

Part Size: Larger parts generally have looser tolerances compared to smaller parts. The size of the part affects factors such as material shrinkage and warping, which can impact dimensional accuracy.

Material: Different materials have different shrinkage characteristics during the cooling process. High-performance engineering plastics, for example, may have tighter tolerances due to their lower shrinkage rates compared to standard thermoplastics.

Design Considerations: Factors such as wall thickness, draft angles, and surface finish requirements can also influence the achievable tolerances in injection molding. Design features that are difficult to mold or demold may require looser tolerances.

Manufacturing Capability: The precision of the injection molding equipment and the expertise of the manufacturer can also impact the achievable tolerances. Experienced plastic molders with advanced equipment can often achieve tighter tolerances.

It is worth noting that the actual custom injection molding manufacturing process should vary based on specific requirements and restrictions. For critical applications or when tight tolerances are required, it is recommended to consult an experienced injection molding manufacturer. If you have questions about tolerances, please contact us today to determine which tolerances are appropriate for your specific project.

Efforts for Standard Machining Tolerances

By following these steps and paying strict attention to part design, machining processes and quality control, we can effectively control machining tolerances and produce parts that meet required specifications and performance standards.

Consider the intended function of the part and the specific role of each dimension or feature to help you prioritize critical tolerances.

Choose a machining process that meets your tolerance requirements. While high-precision processes such as CNC machining or grinding are suitable for achieving tight machining tolerances, less precise methods such as casting or forging may require wider tolerances.

Choose a material that meets your tolerance requirements.

Processing equipment is regularly calibrated and well maintained.

Implement robust quality control and inspection processes through the use of precise measurement tools such as calipers, micrometers, and coordinate measuring machines (CMM).

Optimize machining parameters and tool selection, fine-tuning cutting speeds, feeds and tool geometry to better control tolerances.

Make sure the workpiece is securely held in place during machining.

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