MadSci Network: Engineering

Re: How does the ball float in those car compasses?

Area: Engineering
Posted By: David Ellis, Researcher, NASA Lewis Research Center
Date: Mon Jan 27 10:57:25 1997
Message ID: 853894849.Eg

While 1 mm may seem like a small gap, in the case of a manufactured product it is relatively large. A survey of a local car parts supplier showed that most of these compasses appear to use a two part plastic ball for the compass itself. I assume they injection mold the plastic halves, insert the magnet in the bottom half, and seal the two halves together with a glue. For an injection molding process using a commercial plastic, tolerances of 0.05 mm are typically maintained. The outer sphere the compasses go into also appear to be made by injection molding two halves, inserting the compass and the fluid, and sealing with a glue. Some showed indications of having the fluid addded through a small hole after the halves were joined. The hole was sealed after filling using a plastic plug. The fluid is probably a water-based solution or a light oil so the compass can rotate freely and quickly as the car turns and tilts. The compasses are probably set to a neutral boyancy so they neither float nor sink. Knowing the weight and volume of the compass, it is relatively easy to calculate the density of the fluid required to achieve neutral boyancy. From that point on it becomes a manufacturing and quality control problem to maintain the design dimensions of the unit.

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