Capability of new CNC machines

Wes Bucey

Quite Involved in Discussions
#11
As a former owner of a high precision contract machining company, I understand and agree with bob's premise.

In most modern CNC machining centers (turning or multi-axis) the repeatability is so good that in most cases, the primary variable is tool wear (which can be affected by purity or impurity level of the material being machined as well as speeds and feeds and tool design.)

Unless there is a probe (visual [infra red, laser, etc.] or touch) checking the tool bit continuously for wear, breakage, etc., all the algorithms in the world cannot overcome the machinist's favorite phrase: S--- happens!

In the 90s, when I was still running our shop, our turning centers and multi-axis machining centers routinely ran Cpk of 2.0+

We did our SPC in a relatively straightforward way, with the exception that instrument readings were directly input into a computer from the measuring instrument and the calculations performed by software instead of a guy working a calculator from hand written data entries.

Our SPC findings triggered whether we would enter into a Design of Experiments to alter feeds, speeds, tool design, or material supplier. I recall writing here in the Cove some time back that we ultimately settled on Carpenter Steel as a sole supplier of some specialty alloys because other suppliers did not provide the consistency of grain which is an important factor in consistency of machining, despite the chemical analysis being the same - physical properties of the material determine chip size and production as much as tool design coupled with feeds and speeds.
 
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WKHANNA

#12
If you can turn off the adjusting as well you could perform a study just like you would on a traditional machine.
Would not you have to do that anyway to develop the algorithm in the first place?
And would not you need a unique algorithm for each material & type of tool you use?
 
T

True Position

#13
If you have a process that consumes it's tooling at a fairly standard rate (Ram EDM, production OD grinding, production hard turning are the three that first come to mind) if your machine is capable of automatically compensating for this wear why wouldn't you? Because you're 'overcorrecting' doesn't seem like a reason to run your parts on average further from nominal.
 
T

True Position

#14
Would not you have to do that anyway to develop the algorithm in the first place?
And would not you need a unique algorithm for each material & type of tool you use?
Not so much a new algorithm as much as new coefficients for it.
 
W

WKHANNA

#15
I recall writing here in the Cove some time back that we ultimately settled on Carpenter Steel as a sole supplier of some specialty alloys because other suppliers did not provide the consistency of grain which is an important factor in consistency of machining, despite the chemical analysis being the same - physical properties of the material determine chip size and production as much as tool design coupled with feeds and speeds.
OT slightly, but we have a customer who specifies the stainless steel material for their part be sourced from Carpenter. Our machinists love it due to its consistency & wish it was used on everything.
 

bobdoering

Stop X-bar/R Madness!!
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#16
If you have a process that consumes it's tooling at a fairly standard rate (Ram EDM, production OD grinding, production hard turning are the three that first come to mind) if your machine is capable of automatically compensating for this wear why wouldn't you? Because you're 'overcorrecting' doesn't seem like a reason to run your parts on average further from nominal.
I did not say that auto-compensation should not be done. If you chose to over-control, that is your choice. But, I said that if you do it, it beomes the process, and you need to understand its basis (algorithm) in order to assure it is correct (how do you know now - it's black box!). If you don't know the process, and understand the variation it produces, your capability analysis is even less valid than usual - especially Cpk (as described above by AIAG.) To base the purchase of a 500,000 on invalid or inadequate capability analysis seems like a real cr*p shoot. Not sure that is the statistics one should be satisfied with.
 
T

True Position

#17
Hasn't the machine and it's wear compensating scheme always been a major part of the process?

Switching to more active regular moves should keep processes more steady and replace sawtooth looking control with normal ones where CpK is more valid.
 

bobdoering

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#18
Hasn't the machine and it's wear compensating scheme always been a major part of the process?

Switching to more active regular moves should keep processes more steady and replace sawtooth looking control with normal ones where CpK is more valid.
It might be normal based on the results generated based on the remaining many variances it ignores, and you could calculate Cpk. Just because you can calculate it doesn't make it right.

It gives you no information on the source of variation so if anything is no longer capable the core data that would guide you to the problem is gone. It also masks valuable tool wear information (unless the programmer was clever enough to simultaneously collect cumulative hi/lo data and report it also - then you might actually have a valid control system.) So, unles you are intimately aware of what that algorithm is, you are not in an enviable position.

Interestingly, you are correct in that adjusting for tool wear has always been a part of the process. It is also interesting that stopping over-control had once been a key goal of SPC. (See TS16949 section 8.1.2) Does anyone recall why that was so?
 
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A

adamsjm

#19
Let me throw my two cents in.
CNC variation and tool wear are minor when compared to part, fixture, tool, and spindle temperature. Is the CNC manufacturer responsible for the collant temp control and flow volume and direction, or is that the responsibility of the buyer and tool manufacturer in the case being examined?

Ppk values of >14 is not required if the specification is correct!
(Data supplied by major Automotive company for my analysis.)
 

bobdoering

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#20
Let me throw my two cents in.
CNC variation and tool wear are minor when compared to part, fixture, tool, and spindle temperature. Is the CNC manufacturer responsible for the collant temp control and flow volume and direction, or is that the responsibility of the buyer and tool manufacturer in the case being examined?

Ppk values of >14 is not required if the specification is correct!
(Data supplied by major Automotive company for my analysis.)
Actually, tool wear should be the most significant variation. If part, fixture, tool, and spindle temperature are more significant than tool wear, then you can rest assured the process is not stable...and that poses its own set of troubles. But, those are good examples of variances that are ignored and masked by auto-compensation. No matter what, long term the buyer needs to keep that process capable - because they will be the ones changing materials, specifications, characteristics, etc. - not the machine tool supplier.

So, forcing the machine tool supplier to mask valuable information for the sake of meeting a single value - as if it describes long term variation very well to begin with - is disturbing.
 
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