Question about composite position
Question about composite position
(OP)
I am inspecting a part on a CMM that has holes toleranced with composite true positioning. The PLTZF has an MMC tolerance of .030 and the FRTZF has a MMC tolerance of .015 with no datums referenced.
Reviewing data from the CMM report I am seeing that the PLTZF positions are good with deviations from .004 to .008. What confuses me is the FRTZF positions are reported as not meeting specification with deviations from .018 to .025. Is this possible? Or am I right to believe that there is an error in my CMM program.
Reviewing data from the CMM report I am seeing that the PLTZF positions are good with deviations from .004 to .008. What confuses me is the FRTZF positions are reported as not meeting specification with deviations from .018 to .025. Is this possible? Or am I right to believe that there is an error in my CMM program.





RE: Question about composite position
You really can't perform the FRTZF on a CMM since it requires one of the holes as a datum. It then can calculate the positional tolerances of the other holes relative to the selected hole but that is not the requirement. If one changes the selected hole, then the result would also change.
The best way to check the FRTZF at MMC is with a checking fixture made at its virtual condition size. It truly checks the relationship inside the pattern. I would not have any confidence in the FRTZF result from your CMM.
Dave D.
www.qmsi.ca
RE: Question about composite position
There may be something wrong with your program or there may not! If your CMM software has the ability to do a pattern "best fit" to accomplish the FRTZF portion of the analysis and you have not constrained it to datum features then it may be working correctly.
You said that the tolerance for the FRTZF is 0.015(M)... Just curious what is the tolerance for the PLTZF ???.?(M).
I disagree with Dave's statement
Paul
RE: Question about composite position
Paul
RE: Question about composite position
I input the tolerance of .030 for the PLTZF and .015 for the FRTZF both with MMC modifiers.
The only other option is a box for the FRTZF, which is titled 'Alone'. According to the CMM software help, "When checked, the alone checkbox indicates that the same geometric characteristic will be applied to both the PLTZF and the FRTZF, the software will use the FRTZF datum features to report on the relationship of features within the pattern. Clear the alone checkbox to verify the feature position relative to the datums, as well as relationship between the features"
I will not state how I have the alone box to get unbiased opinions. Also I can state that like most problems in life, this was simpler that I expected and I just found two holes from another pattern dragged into my composite position matrix thus throwing off the FRTZF calculations.
RE: Question about composite position
Like you said, however, when feature location or orientation deviations are not uniform and there are outlier pattern deviations the algorithms can sometimes yield surprising results.
Paul
RE: Question about composite position
I've had the same problem pop up, mcaruso ... hang in there.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Question about composite position
You have a problem with the cmm program. The positions among the holes, FRTZF, will always be equal to or better than the positions as related to the datums, PLTZF.
RE: Question about composite position
So lets say that my holes deviations are split between positive X and negative X values from true center. Wouldn't that calculate the FRTZF values bigger than the PLTZF values?
RE: Question about composite position
Let's say you have a pair of holes with a composite positional tolerance. One hole is .004 short of the basic dimension and one is .004 long of the basic dimension. All the deviation is in the x direction. No deviation in the y. The postion of each hole, for the top part, pattern locating portion, of the composite callout would be .008. Agreed? The postion for the bottom part, feature relating, portion of the callout would be the same. .008 for each hole, not .000 and .016. Nowhere does it say that you have to set up one hole as perfect and check the rest from there.
If you were to make a gage based on virtual condition it certainly would not be perfectly centering one hole and checking the other hole from there. The part with the holes would be rotated, slid sideways, etc. till it was determined if it did or did not fit on the gage regardless of the direction of deviation of the holes.