Capability of new CNC machines

T

True Position

#21
What valuable information is being hidden? Any process deviations other then what the process is automatically compensating (cutter wear, etc) will still be in the finished parts.
 
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bobdoering

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#22
What valuable information is being hidden? Any process deviations other then what the process is automatically compensating (cutter wear, etc) will still be in the finished parts.
Exactly. But it is masked in the data from constant adjusting - so yes, it is in the parts (output) but your data can never reveal how it is changing due to the masking. And that is the fundamental problem.
 
T

True Position

#23
Exactly. But it is masked in the data from constant adjusting - so yes, it is in the parts (output) but your data can never reveal how it is changing due to the masking. And that is the fundamental problem.
You determine how the part is changing due to the masking by measuring it. If the parts are shifting over time your comp is set wrong.
 

bobdoering

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#24
You determine how the part is changing due to the masking by measuring it. If the parts are shifting over time your comp is set wrong.
Is it?

Or is the algorithm inadequate or incorrect? How would you ever know? Or, have other variables changed - many of which have been mentioned in previous posts - more than the algorithm expected? That would cause some of those variation distributions to "peek through" the noise of the constant adjusting. How would you react to that? Are there temperature changes that are not being compensated by the measuring system? How is the dimension being measured? Continuously? One point? what is its gage R&R? Does it have adequate ndc for the small variation it should be seeing? And what about the condition of the cutting tool? Where is it at on its continuum from cutting to rubbing? On the X hi/lo-R charting methodology, we can see a significant shift that is a leading indicator of the need for tool change before it breaks. It does not rely on guessing that any one cutting tool might work the same as the original tool used for setting tool count. Auto-compensation wipes out that data.

Have a fishbone diagram, CNX analysis and PFMEA even been done before whipping out a capability study? What are the reaction plans? How would you know when "the process" is essentially "black box"? Assume there are no failure modes? Seems pretty unreasonable and oversimplified.

To look at the issue as simply at "comp set wrong" is a vast oversimplification - especially when you have absolutely no idea how the comp is making decisions. None. ....And you are willing to bet half a million dollars on that? Seems pretty weak.
 
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T

True Position

#25
It would depend on the level of complexity being discussed. This could be as simple as a grinder adjusting constantly to as complicated as a robot fed completely automated cell with 100% inline gaging.

If your complaint is 'the gaging system could be inadequate' a non compensating setup wouldn't work if the gaging was inadequate as well.

The process isn't a black box, you know the data you're feeding it and how it reacts to those changes. I know how a CMM compensates for temperature, that I don't know the exact formulas used doesn't really change how it functions as the end user. (Same with how it calculates circles or other geometric shapes)

Is '(dimension) is now 0.0005mm above nominal, adjust associated tool 0.0005mm' a black box nobody can understand? You could even log these changes if somebody wanted to go back.
 
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bobdoering

Stop X-bar/R Madness!!
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#26
It would depend on the level of complexity being discussed. This could be as simple as a grinder adjusting constantly to as complicated as a robot fed completely automated cell with 100% inline gaging.

If your complaint is 'the gaging system could be inadequate' a non compensating setup wouldn't work if the gaging was inadequate as well.
That's not true, because by constantly adjusting, you use less of the tolerance, and have a reduce PV causing your ndc to drop. Using a gage to make a constant adjustment requires more resolution that one that uses an acceptable amount of tolerance to adjust the process.

Even auto-compensating a grinder can be more complicated if one has tight tolerances and you are trying to meet tighter tolerances by an forcing your distribution to be "normal" and meet Cpk. A lot of unnecessary dabbling.

The process isn't a black box, you know the data you're feeding it and how it reacts to those changes.
If you don't know how it reacts to the changes, it is black box. You might accept the black box output as acceptable, but that does not make it any less black box. You have absolutely no idea what its logic is.

Is '(dimension) is now 0.0005mm above nominal, adjust associated tool 0.0005mm' a black box nobody can understand? You could even log these changes if somebody wanted to go back.
Even that logic is not a correct adjustment. You would adjust to 0.00025 below. Besides, do you adjust it based on its largest diameter, average diameter or minimum diameter? Or do you throw all caution to the wind and ignore roundness altogether? That would be pretty darn sloppy for precision machining.

Logging the adjustment is part of my point stated above...although you would actually need to know what to do with the logged data to extract back out of it its value.

In essence, it sounds like to me as if you are supporting forcing an expensive, automated system to go back to the garbage statistics of X-bar/R and Cpk that have been shown to be incorrect and were corrected by the X hi/lo-R charting methodology and the correct capability calculation for the continuous uniform distribution. Doesn't seem to be a good idea to me in the long run. Hard to see the justification.
 
T

True Position

#27
That's not true, because by constantly adjusting, you use less of the tolerance, and have a reduce PV causing your ndc to drop. Using a gage to make a constant adjustment requires more resolution that one that uses an acceptable amount of tolerance to adjust the process.
Sure, you must complete your MSA and pick something capable for the process.

Even auto-compensating a grinder can be more complicated if one has tight tolerances and you are trying to meet tighter tolerances by an forcing your distribution to be "normal" and meet Cpk. A lot of unnecessary dabbling.
Except the average part is closer to nominal.

If you don't know how it reacts to the changes, it is black box. You might accept the black box output as acceptable, but that does not make it any less black box. You have absolutely no idea what its logic is.
A black box is a black box but I've never seen a comp/offset scheme that was one.

Even that logic is not a correct adjustment. You would adjust to 0.00025 below. Besides, do you adjust it based on its largest diameter, average diameter or minimum diameter? Or do you throw all caution to the wind and ignore roundness altogether? That would be pretty darn sloppy for precision machining.
As to your math correction, the offset would be 0.00025 or 0.0005 based on the type of machine.

Deciding which diameter to work to and how much attention to pay to roundness are questions you'd be asking yourself automated compensating system or not.

In essence, it sounds like to me as if you are supporting forcing an expensive, automated system to go back to the garbage statistics of X-bar/R and Cpk that have been shown to be incorrect and were corrected by the X hi/lo-R charting methodology and the correct capability calculation for the continuous uniform distribution. Doesn't seem to be a good idea to me in the long run. Hard to see the justification.
Except you make parts closer to nominal.
 
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