CHECKING FIXTURE

A checking fixture is similar to a gauge except that it can hold the part it measures. It can accommodate any number of gauging devices while holding the part to be inspected.

What is a checking fixture?
A checking fixture is a quality assurance tool used in industries to check for the quality of components with complex shapes. They aren’t used in making components but to accept or reject already-made components according to the dimensional accuracy. Manufacturers seek help in checking fixtures to inspect the dimensional accuracy of their products. They also inspect components for distortion, and scratches and check whether the parts are properly aligned. In short, a checking fixture provides a certification that the product satisfies all the requirements for a safe operation.

In contrast to a checking gauge, a checking fixture can hold the component or the workpiece being examined securely in a highly predictable orientation. In addition, it may also accommodate any number of gauging and other workholding tools, such as locating pins, jigs, and clamps.

Types of Checking Fixtures:
1. Stationary Checking Fixtures
As the name suggests, it is a stationary fixture that consists of a frame that is present on a floor or a table and the part to be inspected is brought and mounted on it. Once the part is positioned properly on a fixture frame, the other gauging devices are employed. These gauging components may or may not be attached to the frame. Sometimes the frame may have the structure to hold the part and the series of checking surfaces are placed at some distance from the part. A feeler gauge is inserted between the part and the check surface to confirm the correct shape of the part.

2. Apply-Type Checking Fixtures
The Apply-type checking Fixture operates exactly opposite to the standard checking ones. These fixtures are loaded onto the larger parts or assemblies and inspected for the dimensional and profile accuracy of the parts. Some heavier parts, automobile components, in particular, can’t be placed on a fixture. But the companies cannot afford the poor accuracy of any component which can fail the product and cost them thousands of dollars. The apply-type fixture does the work in such cases. They are carried to desired locations and loaded onto the parts that are to be measured for accuracy.

3. Coordinate Measuring Machine (CMM) Fixtures
A Coordinate Measuring Machine (CMM) is a measuring device to inspects the physical geometry of an object by sensing discrete points on the surface of the object or a part using a sensing probe. When the probe touches a surface, the transducer records the input and sends it as an electric impulse to the computer.

The computer then interprets the signal and registers its location in space. The points are recorded in terms of (X, Y, Z) with X, Y, and Z being distances of each point in X, Y, and Z directions from the origin (0,0,0) using a cartesian coordinate system. The CMM fixture is made to hold the work part in the exact position as in actual operation. A known point in space is located on the fixture or on the part to be inspected and the CMM is located relative to that point.

It starts to inspect other points on the workpiece to determine their locations. In this way, the dimensional and positional accuracy of the complex and intricate shaped parts or sub-assemblies can be measured using the CMM fixture.

4. Progressive Inspection Metal Match(PIMM) Fixtures
While we have other fixtures to inspect all the parts of an automobile, they are not suitable for checking a newly designed car or its prototype. Any single error can cost millions of dollars and takes a lot of time and effort to spot the error after assembling the vehicle. To avoid such a blunder, the PIMM fixtures are used. These fixtures confirm whether all the sheet metal parts of the assembly fit together in the prototype stage itself. The PIMM fixture comes with a huge frame, series of locators to mount the panels and a clamp to hold the panels in position.

Each prototype panel is placed on its locators and positioned using clamps. This is repeated until the entire side of the vehicle is made up virtually. All of them should fit on the fixture accurately. Inspect the fits and clearances with the feeler gauge and check for any misfits or errors. If everything sits well, then the parts are within acceptable limits.

5. Coordinate Metal Match Inspection Fixtures
The Coordinate Metal Match Inspection Fixture is a combination of CMM fixture and PIMM fixture. The body parts can be in a particular shape but after being welded together, they may get twisted. The CIMM fixtures detect this and inform whether the twist is under acceptable limits or not.

All these fixtures work in coordination with one another to inspect a full assembly of a product. A set of fixtures can be incorporated into a single fixture which can save time and building costs of additional fixtures. A combination of one or more fixtures can be used to inspect specific parts. In the case of automobile manufacture, there are several operations involved to make the individual parts that are assembled later to form a full automobile.

Each of these operations is to be checked for accuracy to avoid distortion and failure. The welded assemblies can be checked using a stationary checking fixture. While the formed sheet metal parts are checked with a CMM fixture. A partially assembled automobile can be inspected for trunk opening accuracy using the apply-type fixture whereas a newly designed automobile can be inspected using the Progressive Inspection Metal Match (PIMM) fixture while the part is still in the prototype stage.

The Difference Between Jigs and Fixtures
Jigs and fixtures are both work holding devices that are used to guide different types of cutting tools. The main purpose of a jig is to increase the interchangeability and repeatability of the machining process while maintaining high levels of accuracy. A fixture, on the other hand, is used to support the workplace in the correct position, maintaining its orientation concerning the machine and the cutting tools. It, in itself, doesn’t guide a cutting tool.

How is it different from a BIW fixture?
While both of them are intensively used in the automotive industry, there is a slight difference between them. A BIW (Body in White) fixture is a manufacturing tool that holds 3-dimensional parts together which are to be welded. In the automotive industry, a checking fixture is used to examine the final components of the sheet metal body parts such that when all parts of the vehicle chassis are placed in a BIW fixture, they are accurately positioned with one another.

GPT Precision offers assembly and checking fixtures for vehicle forms and body sub-assemblies, single component fixtures for all metal and plastic parts, flush and feeler gap checking, mating parts simulation, and three-axis true-position probing. Our professional staff carefully designs and engineers fixtures for repeatable reliability.

We are using the latest technology and sophisticated equipment to design and manufacture a host of customer driven requests. We customize to your particular needs and specifications with excellence and quick turn around time to keep your production thriving.

Conclusion
Checking fixtures are essential tools in ensuring the quality and dimensional accuracy of manufactured components. They play a crucial role in various industries, especially automotive manufacturing, by verifying that parts meet strict specifications and are free from defects. By utilizing different types of checking fixtures, manufacturers can maintain high standards of quality and avoid costly errors in the production process.