Destructive Gage RR - Using Crossed - want your thoughts

S

speedworks

#1
Hi, I am analyzing a Gage RR result, in a thermoforming process (plastic sheet run through a mold), and chose to select my part variation by selecting different mold cavities. I chose to have 10 parts, 3 ops, 2 repeats. The 10 parts (cavities) were selected from the mold that provided the most part variation. I chose cavities that produced the highest, lowest and in the middle range. I then made an assumption that each consecutive shot would represent a common part - ie, each cavity 3 for six consecutive shots represented the same part. So my repeats are created by consecutive cavities. I didn't select my parts until the machine had been stabilized (but recognize there could be some material variations). So, here is my question, since I am making the assumption that the 6 consecutive shots represent the same part (by cavity), I should be able to analyze the gage using a crossed method. By the way, the product is PET, and the test involves instron machines. Any thoughts or feedback? Thanks in advance.
 
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S

speedworks

#3
Hi Miner, I saw your posts, but I am not sure I agree. I think you can use a Crossed method, when the assumption is that you have created 'like' products (to represent the same part). The minitab statguide shows that as well. I think Nested is used when you can't set it up like that or make that assumption. Help me understand your thoughts on it. I too have been heavily involved with gage rrs and noticed you prefer only a nested analysis, but other searched have come up to similar thoughts of what I am suggested. Just want more insight. I also think using the different cavities is giving me my part to part.
 

Miner

Forum Moderator
Staff member
Admin
#4
I reviewed Minitab's StatGuide and found the guidance to which you referred. While I agree with the guidance in principle, you are applying it is a different mode from which it was intended. It could still work, but the assumptions depend heavily on whether the consecutive runs on a single cavity are indeed similar enough to claim that they are the same part. Do you have data that supports this assumption?

If I were running the analysis, I would run it using both approaches, compare the results and explore any differences. Since you appear to have Minitab and the effort involved in performing the Nested R&R is no different from the Crossed study, why are you reluctant to use it? The nested study is actually more sensitive than the crossed study if you assumptions are incorrect.
 
S

speedworks

#5
I don't think I have it set up like a nested design, and am not sure it makes sense in my design. Nested would assume that 2 consecutive cavities are the same for one operator, and if that is the case, I should just assume that all 6 consecutive are the same. These are high speed machines, so the least amount of variation will come with that scenario.
 
S

speedworks

#6
Unfortunately, I don't think I have the data, because this org has never done a gage RR; I'm new to the company and am bringing in opex tools and programs. Can't trust the data to see if it is the same anyway.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#8
your assumption that sequential part from the same cavity of a high speed injection molding process will be almost identical is a good assumption.

as with any assumption, only the data will tell you the truth. I have used this very approach for similar - and exact - situations and have performed the study as a crossed study. (but I always check the graphical results and put less emphasis on the statistic that is calculated...). When my assumption is wrong it is clear in the graph and I redo the study - or sometimes jus the analysis as nested...

Miner's point is well taken: perform the study and the analysis both ways. then you will see...
 
S

speedworks

#9
Bev, thanks for more insight. The process is Thermoforming PET vs injection molding, but feel the assumption is valid. I did run it both ways, but I think I have it set up wrong in Mtb for the nested way, since I number each part the same (for each cavity). How would you do it otherwise?

Also, would like to hear your thoughts on what I should look at, at the results to determine if either is a better analysis.
 

Miner

Forum Moderator
Staff member
Admin
#10
That is the issue. In Minitab, select File > Open Worksheet > Look in Minitab Sample Data Folder > Select GAGENEST.MTW > Open then look at how the parts are numbered in the worksheet.

In your example, number parts for operator 1 as 1 - 10, for operator 2 as 11 - 20, and for operator 3 as 21 - 30.
 
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