Well, I would post this in the TIPS & DESCRIPTIONS section, but there isn't one (yet?). I have put this here in the CODE section since it will not 'disappear' from the list as fast as in the Pcdmis section, i just copied it over:
SNAP POINTS:
1) Not a great idea to use for ITERATIVE alignment points
2) Great idea for all other VECTOR points in the program
3) Can be used in SOME instances for constructing other features, sometimes not, however, all features can be constructed with NON-SNAP points.
Snap Points simulate a perfect machine measuring a vector point, the perfect machine being able to STAY exactly on the approach vector, not deviating by as much as a micron. Snap points are a short cut for something YOU could do yourself with about 6 more steps.
a) Vector point nominal IJK used to create a 3-D line going through the nominals of the vector point
b) Plane perpendicular to the 3-D line is constructed THROUGH the actual measured point
c) 3-D line and PERP. Plane are intersected giving the reported XYZ location of the point. This puts the point EXACTLY on the nominal vector line eminating from the nominal XYZ location of the point.
By using SNAP point, the formula SQRT(Xdev^2+Ydev^2+Zdev^2) will match exactly the "T" axis as reported by Pcdmis
By NOT using SNAP, you must use Xdev*I+Ydev*J+Zdev*K to get the matching "T" axis value.
Both formulas are correct for the RADIUS of a SPHERE, which is really what the "T" value will represent, the distance from the center (XYZ NOMS) to a point on the surface (XYZ ACTUALS). HOWEVER, if NOT using SNAP, the ACTUALS will not match due to the 'drift' of the CMM, thus requiring you to use the VECTORS (IJK) of the point in the calculations to determine the "T" value.
It sure would be NICE if there were a section set up for this kind of thing. If the TITLE described EXACTLY what the post was dealing with it would make it MUCH easier for newbies to find QUICK easy help (since the search don't work so well). This could be the first such post in a TIPS and DESCRIPTIONS sections