Drilling Close Tolerance Holes - Advice needed on Sampling

M

Megaman

#1
Folks
I appeal to your collective brainpower.
We are currently completing testing of a new machine, drilling close tolerance holes (+.0015", -.0005") for special fasteners.
I have currently Cpk of 2.5 on entry, & 1.50 on exit in carbon, and even better in al alloy - 3.70 & 2.30.
Given those Cpk figures, how do I calculate sample size?
We wil be drilling lots of about 300 holes per set, and our AQL has got to be as close to 0 as possible
 
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bobdoering

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#2
No way of knowing accurately from this data. The true data that will provide the direction is a time-sequence chart that shows the variation over time, as well as an understanding that the gaging is adequate to detect the tool wear variation.
 

Bev D

Heretical Statistician
Staff member
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#3
in addition to Bob's comments I would add:
what was your sample size for those Cpk values?
remember that you probaly don't want an AQL (Acceptable Quality Level) but an RQL (Rejectable Quality Level). An AQL plan will ACCPET the AQL defect rate 95% of the time and an RQL plan will reject theRQL rate 95% of the time.

Do you wan tot use categorical (pass/fail) approach (VERY large sample sizes or such small defect rates) or will you use the continuous data approach (this will provide smaller sample sizes than the categorical approach (can be dangerous if a defect has a high severity as it won't catch categorical type failures (such as an undrilled hole or a 'messed up' hole due to a broken bit)
 
M

Megaman

#4
Thanks for the quick responses:
Some more details for you, then.
Sample size is 568 holes, nominal dia .3290" There are other diameters, but this is fairly typical.
Carbon entry Cp 3.11, Cpk 2.51, Xb .32919", R .0015"
Carbon exit Cp 2.28, Cpk 1.50, Xb .32936", R .0012"
Al alloy entry Cp 4.44, Cpk 3.70, Xb .32890", R .0012"
Al alloy exit Cp 4.36, Cpk 3.55, Xb .32869", R .0012"

You can see it's a very good process (should be at the price!!!) It's a cnc machine, so it also self checks every 10 holes, and is constantly monitored.
We need to work out a reasonable sample rate, given that it takes a couple of minutes per hole to gauge (we measure max/min), so we don't want to hold up production, nor do we want any quality escapes, if we can avoid it.
 

bobdoering

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#5
You can see it's a very good process (should be at the price!!!) It's a cnc machine, so it also self checks every 10 holes, and is constantly monitored.
We need to work out a reasonable sample rate, given that it takes a couple of minutes per hole to gauge (we measure max/min), so we don't want to hold up production, nor do we want any quality escapes, if we can avoid it.
Again, if this is considered precision machining, than the only way - but an extremely effective way - to determine the sample frequency is based on time-ordered data. Cpk provides little useful data to make that decision . A back yard rule of thumb, if the time-ordered frequency data is not available, is 7 data points between tool changes. But, if your drills break without warning (and you will not have much warning if you do not chart correctly , then you may need more frequent check to catch the "special cause" of tool breakage to minimize scrap.

What is the self-checking feature? If it actually makes adjustments automatically, then you have another issue altogether.
 
M

Megaman

#6
We can get the time data from the machine output - basically it will drill up to several hundred holes of each diameter at a time. We have access to the programming and can extract hole posiiton, dia, time, tool no etc. There are somewhat over 2500 holes per unit. As far as I know, it will complete all holes of each size before tool change - they tend to be grouped in sequence anyway, reducing in dia as the component tapers. The machine carries a hole probe (3 point touch probe) which checks every 10th hole. If within 80% of tolerance, it will stop.
 

bobdoering

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#7
We can get the time data from the machine output - basically it will drill up to several hundred holes of each diameter at a time. We have access to the programming and can extract hole posiiton, dia, time, tool no etc. There are somewhat over 2500 holes per unit. As far as I know, it will complete all holes of each size before tool change - they tend to be grouped in sequence anyway, reducing in dia as the component tapers. The machine carries a hole probe (3 point touch probe) which checks every 10th hole. If within 80% of tolerance, it will stop.
To clarify, my term "tool change" meant tool replacement, not swap to next tool.

I guess the question that arises form this is if the machine is checking the holes, and the machine gaging is calibrated and shows adequate gage R&R, why perform any additional measurement? Its sampling plan seems to be more than adequate.

Also, do you typically wait until the machine hits 80% of tolerance before replacing the tool with a new tool, or is that a fail-safe?
 
Last edited:
M

Megaman

#8
Yes, I know it seems like overkill, but we are required by our Engineering spec to check hole ovality (max/min) which the Bowers gauge does as it's a two point gauge which sweeps the hole & the machine gauge doesn't (3 point touch only).
I've had quite a fight to prevent the "measure 100% of everything for First Article" mindset already
The 80% rule is a machine fail safe. We will change each tool well within the predicted wear rate, based on our current trials. Hopefully, also within the 80% rule as well.
This is why our Quality measurement is important, as we wil be monitoring tool wear from our sample. Our software produces run charts as soon as we upload the data, so we can keep up with production, and the gradually increased rate as we go forward.
 

bobdoering

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#9
This is why our Quality measurement is important, as we wil be monitoring tool wear from our sample. Our software produces run charts as soon as we upload the data, so we can keep up with production, and the gradually increased rate as we go forward.
Sounds like you are doing the things you need to do - and I commend you on that. I suggest doing a X hi/lo-R chart and watch both the tool wear (much more accurate than X bar-R), and when you get a good feel for the tool wear rate, you will get a real indication of a check frequency. You need to do a more frequent check until the rate can be verified. Maybe 30 checks between tool replacement.

Also, keep a close eye on the R chart of the X hi/lo-R chart, as that roundness is a good indicator of when the tool is starting to dull and needs replacement before it breaks.

You may find from that chart that you only need to check one a day - or much more frequently - but it will be a real, understandable, and justified frequency.
 
M

Megaman

#10
We'll look at that.
Thank you for your input, w've just got another batch of test drilling completed, with about 800+ holes, so there's plenty of data
 
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