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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 the help in checking fixtures to inspect the dimensional accuracy of their products. They also inspect components for distortion, 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.
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.
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.
The Stationary checking fixtures
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 inspect 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, 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.
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.