In Process Inspection Frequency and Documentation - CNC lathe/mill work

D

dtr18c

#1
We do CNC lathe/mill work. I know that one of the best control methods for this type of work is SPC. However, I would like to get your input on setting up sequential sampling frequencies. My definition of sequential sampling is: Operator inspects 1 out of every X pieces. (Is that the correct definition of sequential sampling?)
Our operators inspect parts by this methodology. The frequencies we've setup are determined today by evaluating the tolerance and what we think would be a good frequency would be to prevent too much scrap. I feel this method isn't the correct way to do things. I would like to move in a direction where these frequencies are more statistically driven. From what I've seen, inputs to deterermining this would be cpk and PFMEA; are there any other tools/methods to determine this? Any advice on how to use these tools to develop these frequencies with these tools?

From a documentation standpoint what should be recorded? I would think that the frequency we define on our control plans dictates how many times we should record dimensions. I saw this thread on sampling evidence requirements: Sampling Evidence Requirements - Pursuing AS9100. I was asked by management if we could provide multiple frequencies on our control plans (One frequency for how often operator should check the dimension and another frequency for how often to record the dimension). I feel that not recording dimensions every time the operator checks and just at specified document dimension frequency would not properly control the process. Any thoughts about this? Is it feasible to specify multiple frequencies as said above?

Your help is greatly appreciated.
 
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bobdoering

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#2
Re: In process Inspection Frequency and Documentation

You really need to read how to control your process using X hi/lo-R charting. Once you have found the slope of your tool wear, you divide the time it takes for your tool to wear from the lower control limit to the upper control limit by 5. That is your sampling frequency. If the calculation ends up being once per day, then you can increase it to balance the need for control with possible special causes (e.g. tool breakage) to minimize the period of time you would have to look back through your product.

Any other approach is dartboard.
 
D

dtr18c

#3
Re: In process Inspection Frequency and Documentation

Once you have found the slope of your tool wear, you divide the time it takes for your tool to wear from the lower control limit to the upper control limit by 5. That is your sampling frequency.
What is the significance of the dividing by 5?
 

bobdoering

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#4
Re: In process Inspection Frequency and Documentation

What is the significance of the dividing by 5?
Well, 7 is a "run"...but more importantly 5 is about the minimum data sampling resolution you should have to keep from overshooting your control limits.
 
A

alspread

#5
Re: In process Inspection Frequency and Documentation

Another piece of this is the control method required based upon the manufacturing method or manufacturing method varaibility.

For example:
An NC machine generates a pattern of features that is as good as the machines capability, which is usually pretty good when compared to the tolerances being asked of it. (Unless your machine is in tough shape).
So, if we can be assured that the pattern is correct and repeatable, then we only need to measure selected "controlling" features to make sure that the pattern has been properly placed on the part. This assumes, of course, that there is some control over the NC program to prevent changes and when changes occur they are re-validated. This concept can greatly reduce the number of features required to be inspected after the NC program has been proven and validated. You don't have to check all of the features of the program, only a select few Key Control Characteristics (KCC's)

Another example:
If you had a drill jig that made the same pattern, you would have to periodically validate the drill jig for wear and changes and then check only a few "control" features to be sure the part conforms.

You can come up with similar control plans for a variety of processing methods (NC program control, tool control, fixture control, etc.).

If, as part of your first article, you can code all of your features accordingly, you can have a good idea of the control methods to employ for each feature for the entire part.
 

bobdoering

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#6
Re: In process Inspection Frequency and Documentation

An NC machine generates a pattern of features that is as good as the machines capability, which is usually pretty good when compared to the tolerances being asked of it. (Unless your machine is in tough shape).
So, if we can be assured that the pattern is correct and repeatable, then we only need to measure selected "controlling" features to make sure that the pattern has been properly placed on the part. This assumes, of course, that there is some control over the NC program to prevent changes and when changes occur they are re-validated. This concept can greatly reduce the number of features required to be inspected after the NC program has been proven and validated. You don't have to check all of the features of the program, only a select few Key Control Characteristics (KCC's)
If you are only inspecting, that may be correct. But, if you are controlling precision machining (making decisions on offsets, tool changes, etc. based on statistical data), you need variable data, not attribute from a check pattern. And, if you are controlling you should be measuring the tightest tolerance feature of each finish tool - which you may refer to as the KCC if you wish.
 

Caster

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#7
Re: In process Inspection Frequency and Documentation

An NC machine generates a pattern of features that is as good as the machines capability..........So, if we can be assured that the pattern is correct and repeatable, then we only need to measure selected "controlling" features to make sure that the pattern has been properly placed on the part. This assumes, of course, that there is some control over the NC program to prevent changes and when changes occur they are re-validated. This concept can greatly reduce the number of features required to be inspected after the NC program has been proven and validated. You don't have to check all of the features of the program, only a select few Key Control Characteristics (KCC's)If you had a drill jig that made the same pattern, you would have to periodically validate the drill jig for wear.....
Glad you qualified this method with control and validation. This was the source of much heated debate here. The machinists called it "cutter path control". The argued that they needed to check only one feature on the entire path and because it was CNC controlled every other feature "just had to be" OK.

Except of course for insert wear, wrong insert, wrong tool holder, insert changes not validated, backlash, minor code changes that couldn't possibly matter, and a dozen other amazing things.

So use caution with this approach, I saw too many failures to believe in it. Not to say it wouldn't work with really good control and discipline.
 
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