Dear Sirs,

We are basically automotive battery manufacturers. For making battery we use grids.
For the process of grid manufacturing, casters with gravity casting are used.

For gravity cast grids, mould cleaning (the mold is brushed with a wire until the old cork is removed from the surface of both halves of the mold) and corking (Cork solution is wood fibre based & used to coat the mold surface) is periodically done for maintaining Grid weights.

Mold cleaning & corking adjustment is an integral part of the process & these adjustments are inherent to the Grid Casting process.

In between 2 mold cleanings, we apply cork spraying about 15 minutes once.

We were measuring grid weights at a frequency of once in 1 hour.

An increasing trend in grid weight is observed up to about 4 hrs. At this instant, mould cleaning and corking brings the weight of Grid to lower warning limit .The weight of the grid is controlled by the size of the cast grid wires. The size of the cast grid weight is controlled by the cork sprayed into the wires on the mold.

At present we are using Run charts (i.e., Pre-control chart with warning limits) for monitoring of the grid weight.

Hence we conclude that Grid casting process is inherently unstable.

From the above, don’t we say that the Grid casting process is unstable?

1. If this is unstable, don’t we need to calculate Capability indices?
2. If we observe the grid wt data between mold cleanings, the data follows non normal pattern. Are we using transformations for this?

And suggest us which chart is suitable for our process and applicability of the SPC 2nd edition with respect to said process.

Thanks in advance

Jyothiswar.M

1. If this is unstable, don’t we need to calculate Capability indices?



Capability studies should be used on processes found to be "in control".

Wayne

I would be a little concerned that you are using "pre-control" limits. If those are based upon the specifications, you process may indeed be "in control", but not meeting specifications. It sounds like you have enough data to make a regular control chart. Go ahead and do so - then answer the question if you are in control. At that point, you would want to correct for the special causes of any "out of control" conditions, and gain "control" of the process. After 25 points "in control", go ahead and do the capability calculations.

Dear Sirs,

We are basically automotive battery manufacturers. For making battery we use grids.
For the process of grid manufacturing, casters with gravity casting are used.

For gravity cast grids, mould cleaning (the mold is brushed with a wire until the old cork is removed from the surface of both halves of the mold) and corking (Cork solution is wood fibre based & used to coat the mold surface) is periodically done for maintaining Grid weights.

Mold cleaning & corking adjustment is an integral part of the process & these adjustments are inherent to the Grid Casting process.

In between 2 mold cleanings, we apply cork spraying about 15 minutes once.

We were measuring grid weights at a frequency of once in 1 hour.

An increasing trend in grid weight is observed up to about 4 hrs. At this instant, mould cleaning and corking brings the weight of Grid to lower warning limit .The weight of the grid is controlled by the size of the cast grid wires. The size of the cast grid weight is controlled by the cork sprayed into the wires on the mold.

At present we are using Run charts (i.e., Pre-control chart with warning limits) for monitoring of the grid weight.

Hence we conclude that Grid casting process is inherently unstable.

From the above, don’t we say that the Grid casting process is unstable?

1. If this is unstable, don’t we need to calculate Capability indices?
2. If we observe the grid wt data between mold cleanings, the data follows non normal pattern. Are we using transformations for this?

And suggest us which chart is suitable for our process and applicability of the SPC 2nd edition with respect to said process.

Thanks in advance

Jyothiswar.M


A few comments - First, I don't think you should be using "Pre-control" charts in this process. Having said that, you may be using the term for something other than the normal use. "Pre-control" charts should only be used on a stable process where the normal variation is well within the tolerance limits. There are some good threads on the subject - here is one - New kind of control Chart? Pre-Control Chart (http://elsmar.com/Forums/showthread.php?t=18591)

You won't be able to determine capability until you stabilize your process. Short run charting will help you determine causes of variation in the process however it sounds like you already know a major cause. It seems to me the length of time between cleanings is a major cause of variation.

I would look at the following questions. How many grids are made each hour? How long does the cleaning process take and is it possible to reduce the time between cleaning? Is it possible to study the effects of cleaning/not cleaning by weighing every grid?

If you currently weigh every hour but clean every 4 hours, your recorded weights should reflect an upward trend in weight. At what point does that trend start? If it starts immediately, you may have to clean the mold after each grid.

Knowing more about the process would probably be helpful. Good luck on your project.

Dave

On the other hand - if I'm reading this correctly - you may have a perfectly stable process that exhibits a stable and predictable DRIFT. This is analogous to tool wear. You won't be able to effectively use a standard Control chart - however there are several lesser known charts that will do quite nicely.

Can you simply post the data in time sequence?

The distribution will also not be Normal; it will be closer to a Uniform distribution. (calculating capability indexes at this point woudl be frustrating becasue the traditional formulas are based on a Normal distribution with thin tails - but processes with systemic drift will have very heavy tails and other formulas must be used to calcluate the capability.

On the other hand - if I'm reading this correctly - you may have a perfectly stable process that exhibits a stable and predictable DRIFT. This is analogous to tool wear.

Good point Bev.

Dave

Hi Mr. Dave,

sorry for the late reply.

thanx for ur feedback and I herewith posing the answers to ur questions.

How many grids are made each hour?
-Approximately 500 grids per hour

How long does the cleaning process take and is it possible to reduce the time between cleaning?
-Cleaning process may take 15 to 18 minutes and it is not possible to reduce the time between cleaning(it depends upon the opeartor skills).

Is it possible to study the effects of cleaning/not cleaning by weighing every grid?
-not possible(economically not feasible).

If you currently weigh every hour but clean every 4 hours, your recorded weights should reflect an upward trend in weight. At what point does that trend start? If it starts immediately, you may have to clean the mold after each grid.

-yes u r right, we got an upward trend in weight about 4 hrs.
In between mold cleanings we touch up the mold every 15 to 20 minutes(this is one of the special cause) for reducing the quality defects (like wire missing, cracks etc.) and touchup time depends on the profile which we are running.(we are having more than 50 profiles right now)

Mr.Bev

Can you simply post the data in time sequence?
-I like to share here with the data in the attachment(here we take the data once in 2 min for ur ref.(actually we are taken data hourly once)

The distribution will also not be Normal; it will be closer to a Uniform distribution. (calculating capability indexes at this point woudl be frustrating becasue the traditional formulas are based on a Normal distribution with thin tails - but processes with systemic drift will have very heavy tails and other formulas must be used to calcluate the capability.
-the dist. which we get is close to uniform and some times bi model dist.
can u suggest us or ref. which fomulas can i calculate.

Thanks for all ur support and may suggest us on implementation of spc is possible or not.

Jyothiswar

I added the control chart I see in the data.

Don Wheeler wrote an article about trends and SPC.

The idea is to obtain the trend, then obtain the difference against the original data, the difference is charted.

That data looks like it has some ups and downs, the Capability index is more right but still a little instable, but as I readed somewhere "if you don't meassure, you don't administrate".

there is a systemic trend wiht some 'ups and downs'. This isn't unusual - otehr causes than the one causeing the systemic trend will occur bu overall your primary causal sytem is the systemic trend. I have attached a couple of formulas for you to use...including one for a control chart for the trend. Wheeler's approach works too - but I like to keep to the view of the trend so I dont' to translate from the differences to the actual trend...

Sir,

above posted data is between two mold scrapping data(i.e., with in 4 hours).

here with i'm posting ongoing grid weight data with trends.

Pl. suggest us is this method correct? if not suggest us which method is suitable for us for control the grid weight in the on going process.

Based on SPC 2nd edition, Regression control charts is suitable for us.Pl. suggest us is this apllicable or not.

Thanx in advance

Jyothiswar

I was just reading thru, and I am not the brightest kid in the class :biglaugh:

But my guess is also that temperature might also be a factor in this case.
High temperature may expand the tool and when you stop after 4h production the tool drops in temperature. This may cause some weight difference from 1st hour to the 4th hour.

Do you use spc to control how much you actually poor into the mold?

I would do some tests just to check some stuff:

What happens if you turn of cooling in the tool will this give you a higher acceleration in weight increase?

What if you change from water cooling (just a guess) to oil?