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Machining Tolerances

Also referred to as dimensional accuracy, machining tolerances is the amount of deviation in a specific dimension of a part caused by the manufacturing process.

Assigned as boundaries for acceptable build, dimensional tolerances are assigned to parts, with acceptable degrees of variation set simply because no machines are capable of precisely holding dimensions. When it comes to tolerances, the main concern is how wide the tolerance is able to be without ultimately affecting other factors or the outcomes of the process.

Once a part has been manufactured with dimensions that are out of tolerance, it becomes unusable as the main characteristics and functions of that part is not according to design intent. Tolerances are able to be applied to any dimension, with the following terms commonly used:

 

Lower Deviation

This is the difference between the component's minimum possible size and the component's basic size.

Basic Size

This is the nominal diameter of the bolt (or the shaft) and the hole. This will generally be the same for both the components.

Upper Deviation

This is the difference between the component's maximum possible size and the component's basic size.

Fundamental Deviation

A form of allowance, fundamental deviation is the minimum size difference between the component and the basic size mentioned above. When the fundamental deviation is larger than 0, the bolt will be smaller than the basic size and the hole wider.

International Tolerance (IT) Grade

This is the maximum size difference between the component and the basic size.

In cases where no other tolerances are provided, the following standard tolerances are used:

Standard Tolerances

 

Because CNC milling can offer such high tolerances and surfaces finishes, it is more commonly used for adding precision features to parts which have already been formed (in its basic shape) in a previous manufacturing process. Typically used to create parts which are not axially symmetric and that has features such as slots, holes, pockets and 3D surface curves.

Components manufactured completely using CNC milling are more often the type of components that are used in limited quantities, such as custom-made brackets and fasteners, or components for prototyping purposes.

Milling is also used to fabricate tooling that is used in other manufacturing processes.

CNC turning a process used for adding precise, rotational features to a component that has already been shaped in a previous manufacturing process. It is able to deliver high tolerances and good surface finishes, making it a choice for the adding of features or the refining of a component.

It is commonly used to yield parts that are rotational and axisymmetric, which have features such as tapers, threads, grooves and contoured surfaces. As with CNC milling, components that are fabricated using on this process tend to be components used in limited quantities, such as custom-made fasteners, or components made for prototyping purposes.

Machining tolerance chart for standard process tolerances in CNC Milling

In Millimeters:

Note: You can click on the table below to view a larger image.

Machining tolerance chart for standard process tolerances in CNC Milling in Millimeters

NOTE: Numbers rounded off to nearest decimal.

*Note that further accuracy can be achieved as per customer's request.

In Inches:

Note: You can click on the table below to view a larger image.

Machining tolerance chart for standard process tolerances in CNC Milling in Inches

NOTE: Numbers rounded off to nearest decimal.

CNC milling can be used on a wide variety of materials (both metals and plastics), with the following materials commonly CNC milled:

 

Commonly Milled CNC Materials

Used to create machine components and engine components, CNC milling typically range from solid cubic shapes to complex solid shapes. Other possible shapes include solid cylindrical shapes, flat shapes, as well as thin-walled cubic, complex and cylindrical shapes.

Part sizes here range from 0.04 inches to 72 inches in length, and from 0.04 inches to 72 inches in width. In terms of capabilities, CNC milling has the following Realistic/Feasible and Typical parameters:

Realistic and Typical Parameters of CNC Milling

NOTE: Numbers rounded off to nearest decimal.

Machining tolerance chart for standard process tolerances in CNC Turning

In Millimeters:

Note: You can click on the table below to view a larger image.

Machining tolerance chart for standard process tolerances in CNC Turning in Millimeters

NOTE: Numbers rounded off to nearest decimal.

In Inches:

Note: You can click on the table below to view a larger image.

Machining tolerance chart for standard process tolerances in CNC Turning in Inches

NOTE: Numbers rounded off to nearest decimal.

Turning is used on a wide variety of metals and plastics, more commonly:

Commonly Turned CNC Materials

Shapes in CNC turning range from thin-walled cylindrical shapes to solid cylindrical shapes, with a part size with a diameter of 0.02 inches to 80 inches

In terms of capabilities, CNC Turning has the following Typical and Realistic/Feasible parameters:

Realistic and Typical Parameters of CNC Turning

NOTE: Numbers rounded off to nearest decimal.

 

Back to Main Page: CNC Machining

Further Suggested Reading:

 

 

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