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

[bobdoering]

In Reply to Parent Post by Stijloor

Yes, because UCL and LCL are dependent on the variation in the process.
The spec limits come only into play when process capability is estimated.

OR if you are precision machining, and you are properly controlling your process with the sawtooth curve and uniform distribution.

[bobdoering]

In Reply to Parent Post by chvrajus

Our process is low volume mfg assembly.

We only produce 30 parts per month.

What is the process you are measuring the results of? Assembly can be a lot of various processes - crimping, press-fitting, riveting - it helps are great deal to understand what is the feature and the associated process - as well as potential "adjustment" techniques when out of control.

[Bev D]

In Reply to Parent Post by chvrajus

Thanks for your quick replay.

I have few questions.

Our's process is low volume mfg assembly.

We only produce 30 parts per month.

Q1. how to calculate LCL&UCL for 30 parts (Some parts will have spce limits & some don't). can you give me some example and charts for parts which has spec limint and which don't have spec limit

Note - I seen couple of control charts online but it only shows ucl & lcl in the chart but not USL,LSL & UCL,LCL.

Q2. If production only produces 20 parts in one month and 30 parts in other month. So then LCL & UCL will it change???

Q3. Below is our sample data. Value field is our ploting data. can you please give some chart and the forumla how to calculate LCL&UCL

Part NumberSerial Number ValueUSLLSL2304545410235487406030102355454010235603421023566140102357194810235777401023583550102358935010235951411023600945102360674210236125401023618341
Thanks in advance

Let's start with some basics:
how much do you know about SPC? have you been trained or are you being asked to do this without training?
WHY are trying to do SPC?
Why do you want spec limits on the chart?

It is helpful to understand the process/characteristic being measured in order to select the correct chart type. For example knowing that you are low volume, I would look first to try a individuals, moving range chart (aka I, MR or X, MR) for continuous data.

Please attach your data in an excel spreadsheet as teh posting you have is one great big number...

[chvrajus]

Hi-

I have very little knowledge about SPC.
We want to implement this to predict failures ahead of time.

Yes ours production is very low volume.
As I asked in my previous post, if one month production is 20 parts and another month production is 30 parts. Will it be the same formula to calculate control limits?


Example – can we create charts if production produces (20-30) parts per month?
Is their any limit like (X number of parts should be produced per month to calculate the control limits)

I attached some sample data.

Please let me know if you need any more information.

Please excuse me if I had not explained you clearly.

Thanks in advance

In Reply to Parent Post by Bev D

Let's start with some basics:
how much do you know about SPC? have you been trained or are you being asked to do this without training?
WHY are trying to do SPC?
Why do you want spec limits on the chart?

It is helpful to understand the process/characteristic being measured in order to select the correct chart type. For example knowing that you are low volume, I would look first to try a individuals, moving range chart (aka I, MR or X, MR) for continuous data.

Please attach your data in an excel spreadsheet as teh posting you have is one great big number...

[chvrajus]

Hi -
Any help for my post.

Thanks

In Reply to Parent Post by chvrajus

Hi-

I have very little knowledge about SPC.
We want to implement this to predict failures ahead of time.

Yes ours production is very low volume.
As I asked in my previous post, if one month production is 20 parts and another month production is 30 parts. Will it be the same formula to calculate control limits?


Example – can we create charts if production produces (20-30) parts per month?
Is their any limit like (X number of parts should be produced per month to calculate the control limits)

I attached some sample data.

Please let me know if you need any more information.

Please excuse me if I had not explained you clearly.

Thanks in advance

[Bev D]

patience...I am workgin on a response and have looked at your data. This is fairly straightforward to answer, but as you have little knowledge of SPC there are a lot of things to say...

[Bev D]

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.

[chvrajus]

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

In Reply to Parent Post by Bev D

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.