How to ensure the accuracy of tooling and fixtures

Shenjia Hydraulic

Published time:

2025-06-19

Unified and simplified positioning datum: Following the principle of "datum coincidence", the positioning datum of the fixture is consistent with the design datum and machining datum of the workpiece to reduce datum conversion errors.

How to ensure the accuracy of tooling and fixtures

Precise Design of Locating and Clamping Schemes

Unified and Simplified Locating Datum: Adhering to the "Datum Coincidence" principle, ensuring the locating datum of the fixture is consistent with the design datum and machining datum of the workpiece, reducing datum conversion errors. For example, when machining box-type parts, if the design datum is the bottom surface and side surface, the locating elements (such as support plates and locating pins) of the fixture need to be arranged based on these two surfaces.

Avoid Over-location and Under-location: Over-location will cause interference between the workpiece and the locating elements, leading to deformation or unstable location; under-location cannot constrain all degrees of freedom of the workpiece, resulting in positional deviation. It is necessary to calculate the degrees of freedom of the workpiece (such as the 6-point locating principle) to ensure the reasonable number and position of the locating elements. For example, when using two short V-blocks to locate shaft-type workpieces, the distance between the V-blocks needs to be controlled to avoid bending of the shaft due to over-location.

Clamping Force Optimization: Simulate the distribution of clamping force through tools such as finite element analysis to ensure that the clamping force is uniform, sufficient, and does not cause plastic deformation of the workpiece/fixture. For example, for thin-walled parts, multi-point flexible clamping (such as pneumatic diaphragm clamping) is used to avoid workpiece deformation caused by excessive clamping force at a single point.

Rigid Design of Materials and Structures

Material Selection: Select materials with good rigidity, high wear resistance, and strong stability according to the stress conditions and accuracy requirements of the fixture. For example:

Cast iron (such as HT300) is commonly used for fixture bodies. It has high compressive strength, good shock absorption, and strong dimensional stability after aging treatment;

Locating elements (such as locating pins and V-blocks) commonly use alloy tool steel (such as Cr12MoV), which is quenched (HRC58-62) to improve hardness and wear resistance;

For high-precision fixtures (such as precision boring jigs), granite (small thermal expansion coefficient, excellent stability) can be used as the substrate.

Structural Rigidity Enhancement: Improve the anti-deformation ability of the fixture body through design such as reinforcing ribs and closed sections (such as box structures). For example, a grid-like reinforcing rib is added to the bottom of the fixture body of a large milling machine to reduce deflection under the action of machining force.

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