olerancing

huangyhg

tolerancing
hello everyone, i'm a newbie here. i can't say i'm a diploma'd engineer, i am more of a hands on kind of guy. anyway i have been in aerospace for 15 years now, almost half my life, mostly spent in turbine engine repair. i have recently picked up employment in a high speed testing faility. we test everything from vcr bits and pieces to turbine disks. we have many vendors who machine test arbors and other articles for us, and the issue i'm having is the tolerancing that we are holding them to. in some cases we are holding them to .0002". now a tolerance like this can be affected by pretty much anything and if we find an item that is .0001" over than we find it non-conforming. my question i quess is this when is to much, just to much.  
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your test requirements should be consistent with your specs
ttfn
and your specs are based on your experience, small imbalances in high speed equipment can quickly become a problem.  another clue is how readily people fill your contracts ... if your competitors allow larger tolerances then people will prefer to make their bits.  how much does your procurement people whine about how much your parts cost (relative to your competitors) ?  and re  
thanks guys, i understand the need for the tight tolerance, we do however perform balancing on the rig prior to it making speed. i quess my worry is if we are holding a-1 machine company to a dia of say .551"-.553" and we measure a dia of .5531" there is definately room for error there, who is to say that temp. is playing a role, or that our inspector didn't handle the part to long, or that our measuring equipment or practices did not cause the deviation. for the most part in my existence we have not gone into the tenths of an inch so i'm just trying to get a feel for what should be done. our inspection facility is far from being top of the line, we have a zlyvac z cal to perform our measurements with, this isn't a top of the line cmm here. growth or shrinkage  of .0001" cannot be that uncommon, can it? maybe i'm wrong
strictly speaking, the dia you've measured is out of tolerance.
but you should control which is the error tolerance of your measurement machine; say, if it measure with a precision of .0001" or less. that could make your measure re-enter in the tolerance required
everything should be made as easy as possible, but not easier
albert einstein
if you hold most parts in your hand, you'll probably make them grow .0001. we tend to hold balance arbors very tight as well. it makes sense to have the arbor as close as possible and worry about the part separately. sure you can do arbor compensation, but on r&d hardware, you try to eliminate as many of the variables as possible.
when you're qa/qc critical components like that you really need to make sure the area is clean and temperature controled.  also the equipment used to measure should be clean and calibrated.  otherwise half of your parts will not pass first article.  are you doing 100% inspection?
best regards,
heckler
sr. mechanical engineer
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never argue with an idiot. they'll bring you down to their level and beat you with experience every time.
hmxsgt...
never hurts to have a few references on the subject...
asme y14.5m dimensioning and tolerancing
asme y14.5.1m mathematical definition of dimensioning and tolerancing principles
asme y14.5.2 certification of geometric dimensioning and tolerancing professionals
asme y14.43 dimensioning and tolerancing principles for gages and fixtures
mil-hdbk-682 valid notice 1 tolerancing, dimensioning, and gaging techniques for the design of inspection equipment
sae arp 587b balancing machines - description and evaluation horizontal, two-plane, soft-bearing type for gas turbine rotors
sae arp 588b balancing machines - description and evaluation vertical, single-plane, soft-bearing type for gas turbine rotors
sae arp 1202a balancing machines, dynamic, ball type slave bearings for rotor support
sae arp 1382 balancing machine tooling, design criteria for
sae arp 4048 balancing machines - description and evaluation horizontal, two-plane, hard-bearing type for gas turbine rotors
sae arp 4050 balancing machines - description and evaluation vertical, two-plane, hard-bearing type for gas turbine rotors
sae arp 4162a balancing machine proving rotors
sae arp 4163 balancing machines: tooling design criteria
sae arp 5323 balancing machines - description and evaluation vertical, single-plane, hard-bearing type for gas turbine rotors
regards, wil taylor