"Design of Fixtures for Mass Properties Measurement"

By

Boynton, R. and Wiener, K. (1994)
53rd Annual Conference of the Society of Allied Weight Engineers, Inc.
Long Beach, CA, 23-25 May 1994.

Abstract:

The number one source of measurement error in most mass properties measurements is the inability to accurately position the object being measured relative to the measurement axis of the instrument. Traditionally this has been accomplished using a precision fixture which supports and locates the object. For horizontal measurements of cylinders, Vee blocks are commonly used. For vertical cylinders, an adjustable fixture is often used and the object is centered using dial indicators. Rectangular objects are usually fixtured using a grid plate fixture that has "fence" type end stops which interface with a precise pattern of holes on the fixture. For unusual shapes, custom fixtures are designed for the specific object.

Although the limiting factor in the accuracy of mass properties measurement is usually the accuracy of fixturing rather than the accuracy of the balancing machine, surprisingly little information is available to improve fixturing accuracy. Other than Space Electronics, very few balancing machine companies offer precision fixtures as standard products. (All the fixtures, accessories, and software described in this paper are available from Space Electronics). Furthermore, mass properties fixturing is often handled as an afterthought and the available funding for the design woefully inadequate. The most important single goal of this paper is to emphasize the importance of fixturing for accurate mass properties measurements.

In addition to a discussion of standard and custom fixtures, this paper presents another basic concept of fixturing. Instead of positioning the object at a precise location relative to the instrument, the object can be supported at approximately the right location, and with automatic gauging and software compensation the true location can be determined. A new automated gauging system and position error compensation methods are described. This approach allows payload placement to be made in a fraction of the time that is typically required to move the object to a precise location and introduces no loss of accuracy. This method is particularly useful when measuring large heavy objects that are difficult to reposition.

This paper includes drawings and descriptions of a number of specific fixtures, case studies, tips for good fixture design, and recommendations on how to get the best measurement accuracy. This paper also reviews three essential concepts which are related to fixturing:

1. The payload must have precisely defined measurement axes. Both the calculated and measured data are only as good as the definition of the reference axes of the payload.

2. Symbols and polarities must be clearly defined, i.e. which axis is +X, which axis is -Y, etc. These problems can be minimized by following the Recommended Practice for Mass Properties Reporting which we anticipate will be released in 1994 by the SAWE. This recommended practice defines the symbols used for the axes.

3. Methods must be developed to discriminate between true unbalance and apparent unbalance due to fixturing errors.


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