Pole Data Trend Analysis Help - Monitoring the % of poles that we have to straighten

[Andy Nutt]

Looking for some guidance from our statistical analysis experts...

My situation is my company has been monitoring the % of poles that we have had to straighten and we have had an increase lately. 30-40 ft length steel poles are dipped in a hot metal bath to be galvanized at one of our suppliers. I had a suspicion that things were getting worse when the weather turns colder, but was told that poles are allowed to reach ambient indoors and outside temperature is negligible. Regardless of the cause, looking at the %'s below or attached, statistically can I say that something in the process has changed?

month #bent #received %bent
May-04 18 1358 1.33%
Jun-04 30 2282 1.31%
Jul-04 31 2200 1.41%
Aug-04 10 1570 0.64%
Sep-04 13 1131 1.15%

Nov-04 53 1363 3.89%

[Bill Pflanz]

Andy,

I am not a statistician but if you do a t test, you will find that the two periods of production are statistically different. You had already suspected that but I think you were wanting more than just a mathematical proof of the difference. Just looking at your data, I have the following comments.

Not only is the % bent different in the warm and cold months but the average quantity run is about 600 higher during the warm months. I also noticed that 73% of the bent poles are Type 1T. Are you making the same products in the summer as the winter?

The histogram does show the beginning of a two hump process but there is not enough data to verify it. If you had another set of data from the prior year you may be able to draw a better conclusion.

Even if there is a winter temperature effect, you noted that the poles "are allowed to reach ambient indoors and outside temperature is negligible". Does that mean that someone is actually verifying the indoor ambient temperature is constant year round and that the poles have reached that ambient temperature before the dipping process?

Are the Type 1T poles more susceptible to bending if the temperatures are not at ambient before dipping?


Bill Pflanz

[Steve Prevette]

Definitely a trend is happening, based upon the p-chart (http://www.hanford.gov/safety/vpp/pchart.htm)

[Tim Folkerts]

Besides Steve's p-charts and Bill's insightful comments about different size runs and potentially different types of poles in each month , I tried a scatter plot of % bent vs average monthly temps for central Iowa this year. The correlation is R-Squared = 60%, which would also support the supposition that there is some correlation. The equation is
% bent = 4.71% - (0.0483%) x (monthly average temperature)

I just got ambitious and modified Steve's P-chart spreadsheet (by the way, nice spreadsheet) to predict each month's % bend based on the regression line. If you take as given that the temperature really does affect the % bent, then you can recalculate the baseline every month based on the changing prediction. With this, then no months are out of control (although March and August are close). I don't know that I have seen control charts done this way, but it seems to make sense.


Of course, you have to be careful about assigning a causal relationship. It you look at enough variables, there is a good chance of finding a correlation when none is truly present. There are also any number of factors that could be confounded with the seasonal change. As Bill suggests, if you had similar data back over the course of several years it would be much more convincing.

Tim F

[Andy Nutt]

Thanks to all for the help.
I will take a crack at the t test tomorrow, but the P-chart helps show that something has changed.

I wish we did have more historical data, but that was my first task when I started last year, to get them to start collecting data. At that time all they could tell me was there seemed to be a bent pole problem.

As far as the 1T poles that Bill had asked about, yes 1T poles are the most suseptable. The are the thinnest poles. We also ship more 1T poles than any other size, so the Pareto wasn't too surprising. Summer is also our busiest time, it is a seasonal business so that is why I had to focus on % bent.

The supplier doesn't seem to think outdoor temperature is a factor. I'm not sure why, but they are the experts and so I can at least ask them to consider what other factors may have changed since the data is showing something has. The supplier seems to focus on stresses inherent to the steel when the poles are formed. The galvanization process then relieves those stresses, causing the poles to warp. Perhaps I'll ask them about the raw steel before the poles are formed, and maybe the colder raw steel will have more inherent stresses.

Thanks again.

[bpritts]

Originally Posted by Andy Nutt

The supplier seems to focus on stresses inherent to the steel when the poles are formed. The galvanization process then relieves those stresses, causing the poles to warp. Perhaps I'll ask them about the raw steel before the poles are formed, and maybe the colder raw steel will have more inherent stresses.

Thanks again.

Andy - two general areas of question.

1) What is the "form" of the pole? Is it asymmetrical? If so, then there may be some inherent distortions that happen from heating, even if the heating is very uniform.

2) what kind of fabrication processes are done in the manufacture of
the poles? It is true that you will get internal stresses from many processes; look in particular for operations that might heat the material asymmetrically,
e.g. welding. You won't be able to eliminate the internal stresses but
you can improve the situation by reducing the welding heat to the
minimum possible consistent with a good weld.

If you can explain more about the fabrication processes we may be
able to help specifically.

Regards,
Brad

[Bill Pflanz]

Originally Posted by Andy Nutt

The supplier doesn't seem to think outdoor temperature is a factor. I'm not sure why, but they are the experts and so I can at least ask them to consider what other factors may have changed since the data is showing something has. The supplier seems to focus on stresses inherent to the steel when the poles are formed. The galvanization process then relieves those stresses, causing the poles to warp. Perhaps I'll ask them about the raw steel before the poles are formed, and maybe the colder raw steel will have more inherent stresses.

The supplier may be the expert but thinking temperature is not a factor is not as good as knowing. It sounds like they could do some DOE work for you. I don't know what kind of analysis is done on your raw materials but you may want to check that data for the same time period to rule out any changes that may have caused the problem.

Based on your comments about T1 versus the other types, you may want to separate out the defect data and other analysis for T1. It may change the histogram and control chart analysis.

If you can be successful on your analysis on this problem, it will make it easier to get them to collect other data for you. The more data you can collect the more you can use it to identify problems that even they don't know exist by using control charting.

Good luck.

Bill Pflanz

[Wes Bucey]

I tend to vote with Brad. My "gut" tells me if nothing is changing in your operation, something is different in the manufacture, handling, or storage of the bars before they get to you.

In the machining trade, I have seen 12 foot random length hex bars (cold drawn) have a 180 degree to 360 degree torsion twist from one end to the other as delivered from the mill. Sometimes a round bar will look like a strung bow because of bending. All are due from internal stress introduced during the bar forming process.

Are you buying from a distributor or a mill? This is a situation which should be referred back to mill metallurgist.

If these "poles" are welded tubing, certainly the forming and welding process can be causing this. If they are extruded, there are myriad opportunities for introducing stress.

I can't think of anything in your operation which would account for this condition.