How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Since you are low volume and you measure each device, it makes sense to use a chart known as and Individuals, Moving Range chart. I have attached the formulas for you.

With a I, MR chart you can include spec limits. (Please note that it makes no sense to include spec limits on charts with ‘subgrouped’ data where you are plotting the average and range or standard deviation of the subgroup or sample.) Remember that your process can be “out of control” but still within specifications. It can also be “in control” and out of specifications.

Things to know:

You do NOT recalculate the limits based on months. You will be setting ‘baseline’ limits from 20-30 units and then plotting your process against those limits. To repeat: do NOT recalculate the limits on an on-going basis.

You need to identify a ‘stable baseline period’. When selecting the first 20-25 subgroups of data to calculate the initial control limits, at least one of the subgroups will fall out of the statistical control limits. This is because real manufacturing processes are never in statistical control for 25 subgroups. Use good common sense to remove the biggest offender and recalculate the limits. Do not remove all out-of-control points, just the worst ones. This is an iterative process.

Each characteristic / part combination will require it’s own chart.


Out of Control Rules
These are some of the rules that define when an “out-of-control” condition exists…The process has changed and needs to be corrected.

• Any plotted point that falls outside a Control Limit
• Any 2 out of 3 points that fall ‘close’ (outer third) to the same Control Limit, even if non of the points exceed the Control Limit.
• Any 7 plotted points in a row that fall above (or below) the Average (or center line).
• Any 7 plotted points in a row that are each greater than (or less than) the plotted point before it
• Any other pattern that does not look “random”

You should recalculate new limits only when a known and sustained improvement has been made.

attached is a spreadsheet with your data in an I, MR chart

you should also invest in a good SPC book.

two authors I highly recommend are:
Donald Wheeler and Fair & Wise.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

First of all, thank you very much for your time and inputs and I will surely buy one of the books you suggested.

Is it possible to get your personal email Id?

Thanks again

Bev D said:

Since you are low volume and you measure each device, it makes sense to use a chart known as and Individuals, Moving Range chart. I have attached the formulas for you.

With a I, MR chart you can include spec limits. (Please note that it makes no sense to include spec limits on charts with ‘subgrouped’ data where you are plotting the average and range or standard deviation of the subgroup or sample.) Remember that your process can be “out of control” but still within specifications. It can also be “in control” and out of specifications.

Things to know:

You do NOT recalculate the limits based on months. You will be setting ‘baseline’ limits from 20-30 units and then plotting your process against those limits. To repeat: do NOT recalculate the limits on an on-going basis.

You need to identify a ‘stable baseline period’. When selecting the first 20-25 subgroups of data to calculate the initial control limits, at least one of the subgroups will fall out of the statistical control limits. This is because real manufacturing processes are never in statistical control for 25 subgroups. Use good common sense to remove the biggest offender and recalculate the limits. Do not remove all out-of-control points, just the worst ones. This is an iterative process.

Each characteristic / part combination will require it’s own chart.


Out of Control Rules
These are some of the rules that define when an “out-of-control” condition exists…The process has changed and needs to be corrected.

• Any plotted point that falls outside a Control Limit
• Any 2 out of 3 points that fall ‘close’ (outer third) to the same Control Limit, even if non of the points exceed the Control Limit.
• Any 7 plotted points in a row that fall above (or below) the Average (or center line).
• Any 7 plotted points in a row that are each greater than (or less than) the plotted point before it
• Any other pattern that does not look “random”

You should recalculate new limits only when a known and sustained improvement has been made.

attached is a spreadsheet with your data in an I, MR chart

you should also invest in a good SPC book.

two authors I highly recommend are:
Donald Wheeler and Fair & Wise.

Click to expand...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Hi Bev-

• Reply to my previous post you given some control rules. Are the rules same for the low volume and high volume or it varies?
• You said not to recalculate the limits based on months. Set baseline limits for 20-30 units and use for rest.

But what if I want to see last 3 month data or 6 months data, so on….

• Mainly we want to do control chart and histogram chart.

Is it possible to do histogram chart for my data? Can you please give the formula and sample chart with LCL and UCL limits.


Once again thank you very much for your time and inputs

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

chvrajus said:

Hi Bev-

• Reply to my previous post you given some control rules. Are the rules same for the low volume and high volume or it varies?
• You said not to recalculate the limits based on months. Set baseline limits for 20-30 units and use for rest.

But what if I want to see last 3 month data or 6 months data, so on….

• Mainly we want to do control chart and histogram chart.

Is it possible to do histogram chart for my data? Can you please give the formula and sample chart with LCL and UCL limits.


Once again thank you very much for your time and inputs

Click to expand...

first I suggest not getting fancy with font sizes and types it makes things more difficult when responding as your text gets lost.

the only difference between high and low volume production is that with high volume production you would most likely want to use subgrouping and plot on an Xbar, R or Xbar, S chart.

It makes absolutely no difference how many months of data you want to look at. plot each data point on the chart, determine your current stable period, calculate the limits then continue to plot future results against those limits.

I don't understand what you are asking for when you say: "Is it possible to do histogram chart for my data? Can you please give the formula and sample chart with LCL and UCL limits" Of course you can plot a histogram of your data, why do you think you can't? or do you not know how to? Also, since I put the formula for the I, MR UCL and LCL and included a sample chart in my attachments I don't understand why you are asking for them again? what sample chart are you asking for?

I strongly recommend that you purchase a good book on SPC as it will be much easier to get the basic questions answered....no one should be expected to implement SPC without getting a solid grounding in the techniques. Otherwise it's like doing dental surgery on yourself without training or anesthesia...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Thanks for your inputs Bev.

Bev D said:

first I suggest not getting fancy with font sizes and types it makes things more difficult when responding as your text gets lost.

the only difference between high and low volume production is that with high volume production you would most likely want to use subgrouping and plot on an Xbar, R or Xbar, S chart.

It makes absolutely no difference how many months of data you want to look at. plot each data point on the chart, determin your current stabel period, calculate the limits then continue to plot future resutls against those limits.

I don't understand what you are asking for when you say: "Is it possible to do histogram chart for my data? Can you please give the formula and sample chart with LCL and UCL limits" Of course you can plot a histogram of your data, why do you think you can't? or do you not know how to? Also, since I put the formula for the I, MR UCL and LCL and included a sample chart in my attachments I don't understand why you are asking for them again? what sample chart are you asking for?

I strongly recommend that you purchase a good book on SPC as it will be much easier to get the basic questions answered....no one should be expected to implemetn SPC without gettgin a solid grounding in the techniques. Otherwise it's like doing dental surgery on yourself without training or aneshesia...

Click to expand...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

When or how I can say my process is stable and with in my limits ??. Is their any calculation or ratio behind it or we should define by ourselfs.

I heard their is some way to define by calculation Cp/Cpk values.

Please let me know.

Thanks

Bev D said:

The formulas I described in my original response ARE the formulas you are looking for.

Click to expand...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Vens59 said:

When or how I can say my process is stable and with in my limits ??. Is their any calculation or ratio behind it or we should define by ourselves.

Click to expand...

Capability calculations are how you determine if your can generate product within the specification. That is done with a carefully prepared capability study. It should also support any theory of the expected distribution you have developed for your process. A properly prepared control chart determines if your process is stable and within limits. Stable means you have eliminated conditions described in the Western Electric rules, etc., although the applicability of any or all of those rules are conditioned depending on the distribution. They do not apply to all distributions. Many, but not all. That is why for more specific answers we always ask what kind of process you are trying to analyze.

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Hi Bev-

Thank you very much for your inputs. I presented the charts you provided to my team and it was good.
But they came up with some questions.

1. They are looking for Histogram (Can U please provide me a Histogram chart with the same sample data ??) with limits and Cp&Cpk values
2. The UCL,LCL & CL only tells you that the readings you have got are in statistically under control or Not,
you have to calculate the Process capability & index then only you will find out that the obtained values are good enough to satisfy the customer specification.CP & CPk value tells that the process is capable of producing the parts per the customer specification ( How can I tell my process is capable and enough to satisfy the customer specification??)

3. If my data always plot with in the limits, Can I still tighten the limits?? if so how?

I really need this answers to get back to them.

Thanks in advance.

Bev D said:

Since you are low volume and you measure each device, it makes sense to use a chart known as and Individuals, Moving Range chart. I have attached the formulas for you.

With a I, MR chart you can include spec limits. (Please note that it makes no sense to include spec limits on charts with ‘subgrouped’ data where you are plotting the average and range or standard deviation of the subgroup or sample.) Remember that your process can be “out of control” but still within specifications. It can also be “in control” and out of specifications.

Things to know:

You do NOT recalculate the limits based on months. You will be setting ‘baseline’ limits from 20-30 units and then plotting your process against those limits. To repeat: do NOT recalculate the limits on an on-going basis.

You need to identify a ‘stable baseline period’. When selecting the first 20-25 subgroups of data to calculate the initial control limits, at least one of the subgroups will fall out of the statistical control limits. This is because real manufacturing processes are never in statistical control for 25 subgroups. Use good common sense to remove the biggest offender and recalculate the limits. Do not remove all out-of-control points, just the worst ones. This is an iterative process.

Each characteristic / part combination will require it’s own chart.


Out of Control Rules

These are some of the rules that define when an “out-of-control” condition exists…The process has changed and needs to be corrected.

• Any plotted point that falls outside a Control Limit
• Any 2 out of 3 points that fall ‘close’ (outer third) to the same Control Limit, even if non of the points exceed the Control Limit.
• Any 7 plotted points in a row that fall above (or below) the Average (or center line).
• Any 7 plotted points in a row that are each greater than (or less than) the plotted point before it
• Any other pattern that does not look “random”

You should recalculate new limits only when a known and sustained improvement has been made.

attached is a spreadsheet with your data in an I, MR chart

you should also invest in a good SPC book.

two authors I highly recommend are:
Donald Wheeler and Fair & Wise.

Click to expand...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

Can anyone help me here??
Thanks

chvrajus said:

Hi Bev-

Thank you very much for your inputs. I presented the charts you provided to my team and it was good.
But they came up with some questions.

1. They are looking for Histogram (Can U please provide me a Histogram chart with the same sample data ??) with limits and Cp&Cpk values
2. The UCL,LCL & CL only tells you that the readings you have got are in statistically under control or Not,
you have to calculate the Process capability & index then only you will find out that the obtained values are good enough to satisfy the customer specification.CP & CPk value tells that the process is capable of producing the parts per the customer specification ( How can I tell my process is capable and enough to satisfy the customer specification??)

3. If my data always plot with in the limits, Can I still tighten the limits?? if so how?

I really need this answers to get back to them.

Thanks in advance.

Click to expand...

Re: How to Calculate UCL (Upper Control Limit) & LCL (Lower Control Limit) & CL?

chvrajus said:

1. They are looking for Histogram (Can U please provide me a Histogram chart with the same sample data ??) with limits and Cp&Cpk values

Click to expand...

See attached.

chvrajus said:

2. The UCL,LCL & CL only tells you that the readings you have got are in statistically under control or Not,you have to calculate the Process capability & index then only you will find out that the obtained values are good enough to satisfy the customer specification.CP & CPk value tells that the process is capable of producing the parts per the customer specification ( How can I tell my process is capable and enough to satisfy the customer specification??)

Click to expand...

It does not look like there is enough data to fill out your distribution - which may be one reason why the Cpk appears to be a tad low (1.2 - 1.33 would have been more comforting) Because you have so much data just below the mean, it skews the Cpk value low. Cp at 1.4 is not too bad.

chvrajus said:

3. If my data always plot with in the limits, Can I still tighten the limits?? if so how?

Click to expand...

Was the current data created without adjustment? Was it affected by setups between data points?
You can 'run tighter', but it goes back to what really controls the process. Do you dial it in? If so, with what parameter? If you compress the limits, you can meet them by adjusting the process more. Is that cost effective? Do you want a person (the adjuster) to "become" the process? You can...but at a cost.