Answer:
Invest in a higher-quality compass
Step-by-step explanation:
You want a compass that will maintain its setting.
Compass
The attached images show a couple of the kinds of compass products available.
The first one (color) is typical of the kind one might use in elementary school. It is possible that it could be bent, wedged, or clamped in such a way that its tendency to change radius would be substantially reduced. There are versions of this first type that have a screw clamp to hold the parts in place.
The second one is an engineering/drafting quality compass that has its radius set by a screw. It is much less likely to change radius "uncommanded." Unfortunately, the cost of such a precision instrument can be somewhat higher than for the dime-store type compass.
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Additional comment
In general, it is a good idea to invest in the best tools you can afford. It is possible that these will show up at a second-hand store or flea market for less than their original cost.
