The lean inspection process does not allow for disassembling/re-assembling holding fixtures for medium- long running jobs. The development of new more economical modular fixture systems dramatically reduces the cost of dedicated holding fixtures, and reduces lead times. A typical FixLogix semi-dedicated fixture on an 18” x 18” plate will cost $450 - $700, with a delivery of one week.

The new approach to modular fixtures mimics the thinking that drove down the original high prices of Coordinate Measuring Machines. The original CMMs were built with an extremely high degree of mechanical accuracy. Developments in computers and software led to the introduction of 3D error mapping of measuring machines. This shifted the emphasis for CMM construction to rigid and stable machine structures. A CMM structure could have some absolute error, as long as it was repeatable such that the 3D error map could compensate for the error. This thinking can be applied to part-holding fixtures also.

Key Concept : The CMM is the gage, the holding fixture is just a stage

For the vast majority of CMM inspection applications, the CMM interrogates the actual part to confirm the orientation of the part on the machine, and subsequently establish the part datums. This is best practice as it greatly reduces the influence of part miss-loading in the holding fixture as a source of error. The holding fixture rarely needs to be built to gage tolerances, as it serves merely to present the part to the measuring machine.

A part holding fixture often simplifies the inspection task, even for manual inspection systems, simply by holding the part still, and in a position that facilitates checking it. For automated inspection holding fixtures are indispensable, as they allow the part to be presented in a repeatable position, allowing the CMM to execute an automated inspection sequence without collision into the part. The more automated the inspection process, the more critical the role of the holding fixture.

It follows that the true role of the holding fixture is to accomplish three tasks:

- Present the part to the CMM in a repeatable position that gives the CMM access to the features of interest on the part

- Prevent the part from moving during the inspection program

- Hold the part without distortion due to with improper clamping forces

The CMM fixtures shown below cost $500 - $700, and were delivered in 4 days. The manufacturer greatly reduced inspection times by implementing semi-dedicated fixtures.

As CMMs are combined with non-contact sensors such as laser scanners and vision sensors, similar concepts can be applied. Minimal fixture structures are critical to give the scanner maximum access to the part surface, especially for reverse engineering applications.

The photo above shows fixtures for sand casting forms for automotive engine components. The automotive manufacturer is scanning the forms with a non-contact sensor to improve dimensional control of the casting process. The semi-dedicated fixtures streamline the inspection process, reduce the inspection cycle time, and the production setup process down-stream.

Simple is best for CMM holding fixture construction. The goal is to give the CMM full access to the part features hence a simple structure minimizes interference. Similar to the concept of a 3-2-1 part alignment in the CMM software, a 3-2-1 (six point nest) locating scheme is a very common structure for a CMM fixture.

Shown here is a simple 3-2-1 “pinch” fixture with a single overhead clamp. The top surface of the (3) standoffs establish the primary plane datum, the (2) pins in the rear standoffs are the secondary, and the clamp rod is the tertiary locator. The single opposing pin standoff doubles as a pinch clamp, pressing the part against the two opposing pin standoffs in the rear. The “pinch” function is an advantage of the t-slot style fixture plate system. The top clamp may be optional.

For more information www.fixlogix.com 

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