PR# 263893 Calculation of True Position involving Composite

huangyhg

Calculation of True Position involving Composite
<< JuangSheng Koo  --  08/26/09  18:59:16>>
May i know how true position with composite is calculated?
Any formulas for this?
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<< Changes made by Don Turcotte (Review Notes) -- 11/11/09  08:12:08>>
Explanation provided.  There are no specific formulas with this.
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<< Don Turcotte (Development Notes)  --  11/11/09  08:10:48>>
The true position analysis performs a virtual hard-gauge bestfit of the considered feature and datums at material condition.  This algorithm works based on allowed degrees of freedom (DOF) as determined by the datums that are selected.  There are six possible degrees of freedom for the bestfit -- translation in x, y, or z, and rotation about the x-axis, y-axis, or z-axis.  For example, a primary plane datum would be used to control rotation about the x-axis and y-axis, and translation in z.  A secondary cylinder datum would control translation in x and y and a tertiary cylinder datum would control rotation about the z-axis.  When primary, secondary, and tertiary datums are specified with all at RFS, the true position is fully constrained and no bestfit is performed.  When one or more of the datums is at material condition (MMC or LMC), the corresponding degrees of freedom normally constrained by that datum can be used by the bestfit algorithm up to the limit defined by the bonus on that datum of size.  The bestfit algorithm is attempting to minimize the deviation on the considered feature(s) and will use all the available datum bonus if necessary to do this. This is in accordance with ASME Y14.5.1.
With a composite callout, the upper segment true position is the "attern Locating Tolerance Zone Frame" (PLTZF) and the lower segment is the "Feature Relating Tolerance Zone Frame" (FRTZF).  PC-DMIS first bestfits the upper PLTZF segment.  Then using this bestfit transform, relaxes those degrees of freedom not constrained by the lower FRTZF segment to find the tighter fit of the lower segment (the lower segment will typically have a tighter tolerance than the upper segment),  For a composite callout that has a single true position symbol next to the upper and lower segment tolerances, the lower segment will always unconstrain translation regardless of the datums used.  For a composite callout that has a true position symbol next to each of the upper and lower segment tolerances, the lower segment datums control which degrees of freedom shall be unconstrained.
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