Best Fit Alignment vs. Hard Datums

M

Maxebolo

#1
hello guyz,
need help please,
B/P calls out for datum's( profile) on a composite part(fiberglass), with a surface variation of .030" all over the surface.
when i run it thru verisurface (faro arm) using the datums it shows a bad report, but when i do best fit all points are now within tolerance? how could this be? should i use hard datums instead of best fit?if so, why, any mathematical explanation? Help Help...
 
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S

smkeilman

#3
Maxebolo,

This is my first post here, so I won't be offended if you take what I have to say with a grain of salt.

I use FARO's hardware and software, but don't use Verisurface, but I think I can help you. Draw a symmetrical trapezoid. Imagine you are measuring the profile around the trapezoid, and the print calls for a rectangle. If you measure the profile all around the trapezoid, it could be possible to have a good profile using best fit all points. The reason for this is that the two bases will be fit well, and any squareness errors are going to be split between the two sides. If you force an alignment to CAD using the left side, you will see a good fit at that side, squareness errors at both bases, and a parallelism error at the right side roughly equal to the sum of the squareness errors from the best fit scenario.

The reason you are getting different numbers is because the datums are changing the way the computer is solving the alignment to the CAD data. Using a best fit all points algorithm, the errors across ALL surfaces will be minimized, regardless of any other orientation constraints brought about by datums. Using the datum alignment algorithm, the datums will be matched and minimized prior to any other profile calculation. You should notice a smaller profile reading on the datums using the datum alignment versus the best fit.

I think it is very likely that your datums result was calculated correctly and that your part is not to print, unfortunately.
 
R

richw26

#4
Does anyone have a way to calculate profile (gage points, x-y-z) in excel? The CMM program that one of plants is using does not have the option to calculate it. Any help or direction here will be much appreciated.
 
S

smkeilman

#5
It depends quite a lot on the type of features you are trying to measure. First of all, it's not impossible, just very difficult. The biggest problem you will have is writing an equation for the nominal surface as a function of x, y, and z. For a plane, this is not exceptionally difficult, but for features such as a cone or torus, this will get prohibitively hairy. It will likely be cheaper (in terms of man-hours) to simply upgrade the measurement system, especially in the long run.
 
Z

zancky

#6
IMHO

All the algorithms try to reduce the sum of the squares of the differences between the actual data and the calculated surface.
For most of the features there isn't any exact solution.
If you wont to solve by yourself you should

1) write the mathematical formula of the surface
2) write the formula for the sum of the squares
3) identify the parameters (xyz origin, direction etc)
4) write an algorithm to find the minimum (usually Newton Raphson)
5) write a program/macro
6) validate the program by some emulation.

if point 4 it is too hard you can try to scan each parameters within a probable range (nested "for " cycles).

I have apply point 4 for a torus in a CMM running in windows NT4 machine but there was only 2 free parameters.
I have tried also the alternative method (nested "for" cycles) for a strange features, but with 6 parameters I need a C++ program with multi-thread (8!)
running on a I-7 in order to get the results within 3 minutes
 
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