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Within object space, the model world is characterized by the magnitude of the numbers on the entities being modeled; e.g., the smallest and largest coordinates existing on the model and the smallest and largest difference between any two coordinates. All object space numbers are represented in
ACIS as double precision floating point numbers which contain roughly 14-16 significant digits.
ACIS considers four of the least significant digits to represent numeric round-off errors that occur during calculations. Thus, there are roughly 10-12 digits to represent the
dynamic range of numbers (smallest and largest numbers) within object space.
ACIS has used the more conservative estimate of ten digits as a rough guideline to set the tolerances used internally to the modeler.
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As described in the section
Tolerance Variables,
SPAresabs is the smallest quantity representable in
ACIS, and
SPAresnor represents the ratio of the smallest quantity to the largest quantity. The default values are 10-6 for
SPAresabs and 10-10 for
SPAresnor. The largest quantity which can be represented with the ten digits of precision is therefore 104 (SPAresabs/SPAresnor = 10-6/10-10).
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The default values were chosen assuming:
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The units of the parts being modeled were approximately millimeters.
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Machining operations on parts would require 10-3 accuracy.
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Modeling operations would require several orders of magnitude more accuracy than machining.
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At least an order of magnitude guard band around
SPAresabs is required. For practical reasons, there can be ambiguity about distances that are close to
SPAresabs, so this guard band is a safety margin to improve reliability.
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Practically speaking, this means that the dynamic range of models is 10-5 to 104. The dynamic range can be shifted by scaling
SPAresabs, but its magnitude cannot be expanded.
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Theoretically, the magnitude of the dynamic range could be increased by altering
SPAresnor. However, this is
strongly discouraged because the default value of
SPAresnor reflects limitations in double precision arithmetic, consideration of round-off errors, and the limit of 10 significant digits.
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Note
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If SPAresnor is altered, the behavior of the modeling algorithms is unpredictable.
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