How to determine the correct Sample Frequency for SPC Features

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#51
Re: How do I determine a correct Sample frequency for SPC features?

So where do you stand on Bob Doering's approach. Although I understand his approach, I'm handcuffed with using QC-Calc (Xbar / R charts only). Please expound on how you would approach should be used for sub-grouping if your only option was Xbar and R charting (what would be a rational sub-group for precision machining?)
The problem is if you are precision machining, there is no sampling rational that can make the X-bar/R work. I have case studies that prove that not only is X bar-R OR I-MR wrong, they give the exact opposite signals to the true condition of the process - i.e. if the process is out of control on X bar/R it is actually in control and vice versa. Those are the absolute worst charts you could ever use for precision machining - bar none. You are better off using paper charts than confusing your operators with an automated system that is wrong. In fact, I did that, replaced a computerized SPC system with paper charts - in a union plant - and they actually appreciated it because finally the charting made sense.

You may want to approach QC-Calc and ask them is they have correct charting (X hi/lo-R) for machining, since they claim to offer SPC software. You get more traction as a customer than other people might. It's sad if they don't know about it - the technique has been published for over 10 years and implemented much longer than that. And....it is correct!
 
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ncwalker

#52
Re: How do I determine a correct Sample frequency for SPC features?

Got an example of Xhi Rlo?

I'm the guy who can sexy it up.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#55
Proud Liberal: Bob's approach is correct for 'slow' tool wear that occurs over many parts. (And there are other processes that will exhibit this behavior).

Technically this steady movement in results is an assignable cause. We know what's causing it. The Xbar, R chart and I, MR chart will correctly identify it. The issue is that this cause is not something that can be economically removed or controlled to the 'within subgroup' variation of a homogenous process.

The answer is not to try to find some other rational sub grouping scheme as we might do with processes that have other types of non-homogeneity (like multiple cavity plastic injection molds). The answer is to change the chart type. (Which can be argued to be a form of rational sub grouping). The chart that Bob proposes is not the only choice. Donald Wheeler proposes the use of 'sloping limits' that incorporates the within subgroup variation as well as the mean drift. Both work just fine.

You are not forced to handcuff yourself to a statistical program that doesn't include this type of chart. You chose to accept this constraint. You can choose to not use it. Remember that the point of SPC is to control and improve your processes as close to the process as possible. It is not to use SPC software.

There is no logical reason to not use another approach when the process calls for it. Have the operators use paper or Use EXCEL if they have laptops at their machines.

I suspect that your organization is using QC-Calc for purposes other than on line control processes by the operators?
 

Proud Liberal

Quite Involved in Discussions
#56
The use of QC-Calc's XBar charting is MANDATED by the owner and president. We even had Bob out to our facility presenting his approach (which fell on deaf ears). When the decision was made to move our SPC activities from PLEX (ERP software) to a dedicated statistical package, my lobbying for InfinityQS was overruled in favor of QC-Calc.

Now, I'm tasked with making this work (not possible in my opinion). In addition to charting offset-able dimensions (ones that the operator can make process adjustments to), we also chart dimensions they can't control (ie: roundness, runout, etc). The cherry on top is that the president wants to "control" the processes in the shop by the daily monitoring of Cpk.

My apologies if my frustration is showing but I don't like being setup to fail. At least here at the forum, rational minds prevail. There, now that I've let off some steam, I feel better even though I'm destined to fail.
 

Miner

Forum Moderator
Staff member
Admin
#59
Presidents do not have the competency to determine a rational subgroup any more than you have the competency to run a merger or acquisition. You need to educate them that correct SPC depends on the type of data and the process. These determine the type of chart and the basis for rational subgrouping. This cannot be done by edict.
 

Miner

Forum Moderator
Staff member
Admin
#60
Donald Wheeler proposes the use of 'sloping limits' that incorporates the within subgroup variation as well as the mean drift.
While I am a huge fan of Wheeler, this is one of those concepts that sound good in theory, but don't work in practice. First of all, it is overly complicated for use in a factory by machinists. Second, there are too many variables in machining that will affect the tool wear rate (i.e., the slope of the limits). Once you have accepted the fact that you can live with the quasi-uniform distribution of the output, detecting a signal that the tool is wearing faster or slower than the baseline is irrelevant. I agree with Bob D. about keeping it simple and easily understood.

As Shewhart (and Deming) always said SPC wasn't about probability, it was about economic control of the process. 3 sigma limits happened to be economical and struck the right balance between false alarms and missing a signal.
 
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