Position determination of workpieces in positioning jigs

The automation of many processes in the industry, production and assembly requires precise knowledge of the framework conditions and required movement sequences. Positioning Jigs are often used for this purpose. The position determination of a component must be clearly defined in the device. The workpiece holder defines the position in order to carry out a repeatable inspection or to receive materials such that they can be produced relative to a home position. Various Positioning Jigs from tool & die making, such as test devices or assembly devices, are used for this purpose.

Why is determining the position of workpieces important?

During assembly, the components and assemblies to be assembled must be stored, moved and positioned correctly in parts storage. The position of the components, semi-finished products or workpieces must be clearly determined and reproducible in the Positioning Jigs in order to produce in accordance with quality and to carry out tests in mass production.

Various test and assembly jigs are used for joining and separating, which restrict as many degrees of freedom as possible with mechanical positioning elements or guides.

Sensors such as cameras, beam sensors, end switches or pneumatic control elements can also be used to determine the position, and to check and release the correct positioning. A component inspection is also possible with this.

Position determination is of crucial importance, for example, in the following processes of precision production and assembly:

• Welding: Locating pins and Positioning Jigs are used to produce high-quality weld seams for the precise positioning of the workpieces.
• CNC processing: Workpieces are precisely positioned to perform milling, drilling and turning work. Position determination systems, such as workpiece clamping systems and zero-point clamping systems, are used to keep the workpieces in the correct position.
• Prototype construction: When building prototypes and for research and development tasks, determining the position of tools is important in order to test and review new designs.
• Robotics: In robotics, tools and end-effectors must be accurately positioned to handle tasks such as picking and placing workpieces or performing welding and assembly operations.

Principles of Position Determination

Position determination is based on the alignment against fixed geometries and has the goal of being reproducible with regard to the coordinate axes. A position determination therefore always requires a reference system or a reference point. This reference can then be used to reliably measure the position and orientation of an object. The reference can be a coordinate system, a reference point, or another predetermined datum system.

Approaches for positioning

Positioning is carried out by Positioning Jigs, which generally have fixed reference surfaces and additionally suitable insertion aids in relation to the tool coordinate system. This facilitates automatic assembly and separation as well as manual assembly.

Use of Positioning Jigs

Positioning Jigs are a subset of production tools For example, there are:

• Assembly jigs: They hold workpieces in the correct position during the assembly process, often with the aid of face plates. They ensure the correct composition of assemblies from individual components.
• Inspection Fixtures: They check the quality, tolerances or performance of a workpiece, e.g. measuring and testing equipment.
• Clamping devices / Clamping Fixtures: They hold workpieces in position during the machining process, e.g. during milling or joining. Examples of clamping devices are, for example, magnetic clamping devices in machine tools or chucks or turning chucks for milling machines.
• Drilling jigs: They are used for drilling, tapping or assembly. Examples of clamping devices include chucks on drilling machines.

Mechanical Positioning Jigs are particularly important for determining the position. They are used to position or secure workpieces for the purpose of processing and are particularly useful for repetitive industrial processes. The goal is to prevent movement of the workpiece in all degrees of freedom or to make it adjustable in a defined manner (adjustment degrees of freedom):

• Translatory degrees of freedom (axes: X, Y, Z)
• Rotary degrees of freedom (rotation around axes: X, Y, Z)

Positioning Jigs often have the same basic components:

• Base plate with stops: It prevents downward, tilting or rolling movement.
• Locating pins: They limit linear motion and distortion.
• Toggle clamps: They limit upward movement.

Centering of workpieces

Centering is the process or technique of positioning a workpiece such that it is precisely aligned in relation to an axis or a plane. Centering is achieved by means of special fixture elements and fixture mechanisms.

The workpieces that are to be placed in the fixture are provided with precisely fitting bores or recesses that exactly match the centering elements of the fixture. Subsequently, the workpieces are placed on the centering elements, which usually occurs automatically due to the precisely fitting bores holes. After the workpieces are centered, they are fixed in this position to ensure that they do not slip or move during the manufacturing process. This can be done by clamping, screwing, clamping or other fixing methods.

Locating pins also play a major role in the centering process.

Use of locating pins

Positioning Jigs often contain special centering elements, such as locating pins. They are usually made of hard material such as steel. Locating pins are used in inspection fixtures to ensure that manufactured parts or assemblies meet the specified tolerances and specifications. If a workpiece is not correctly positioned, it does not match the locating pins, which indicates a defect.

Depending on the application, it may make sense to use locating pins with different heads:

• Conical: Locating pins with conical heads are recommended for radial positioning. They can minimize jamming.
• Diamond-shaped: Compared to the round head, the diamond-shaped head offers additional clearance to the side. This can minimize tolerances.
• Spherical: Locating pins with a spherical head minimize the probability of damaging the workpiece.
• Flat: A flat head is recommended for lateral guidance or when using straight edges on the side of the pin to position a surface at a certain height. However, positioning downwards is not recommended in this case as this can damage the workpiece.

Securing workpieces in the workpiece holder

When transporting or inspecting workpieces, it is necessary that the workpieces are secured in their defined position in the workpiece holder. The support surface and the alignment surface of the fastening elements can have a flat or structured surface for each area of use and material. The selection of materials also depends on the operating environment and the expected load on the components. Various metals and plastics or ceramics and wood can be used here.

The surfaces can already have bore holes or fastening elements in order to enable a secure placement during workpiece transport, the machining process and the inspection. Locating pins are used to hold the workpieces in the correct position by engaging in corresponding bores or holders in the workpiece. This ensures that the workpiece meets the required tolerances and specifications after processing. Depending on the workpiece transport concept, a plurality of workpiece holders can be provided on the workpiece carrier, which then also pass through the individual process and test stations over a plurality of tracks.

Inspection of components and workpieces

In order to ensure that the workpieces in the workpiece carrier meet the required inspection criteria and specifications, it is necessary to record and evaluate them with Inspection Units. In order to maintain repeatability and process reliability, it is necessary to perform a measurement system analysis for the measurement and testing process. A measurement system analysis checks whether the measurement results are correct and reproducible and how other process parameters influence the result.