【转帖】coaxiality overboard

yang686526

coaxiality overboard?
i checked a drawing the other day that had 5-6 machined diameters all on the same centerline. there is no specification concerning coaxiality of the diameters on the drawing. without some sort of coaxiality control (either position or runout) on these diameters i feel that the drawing is not complete. so i sent it back to the engineer for changes indicating this problem.
i think some sort of control needs to be in place but the amount of resistance to this is intense - this still surprises me. this has set off a firestorm of epic proportion.
am i going over the top here?
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does the lack of controls effect the end function of the part, or will it work as dimensioned?
asme y14.5m-1994, paragraph 2.7.3 "relationship between individual features." states the following:
"the limits of size do not control the orientation or location relationship between individual features. features shown perpendicular, coaxial, or symmetrical to each other must be controlled for location or orientation to avoid incomplete drawing requirements."
i think your assessment that some type of control for eccentricity allowance needs to be stated is valid. you just need to decide which control works best for the desired design application. if you put nothing, then you'll get what you get.
gdt_guy
i agree with ewh - check the function and relationship of the diameters. will it work without controls or are controls required?
dave d.
i suspect you're right that it needs to be controlled, as gdtguy gives the reference.
however, the tolerance needs to be driven by function as ewh & dingy2 point out.
i have something similar that i'm looking at right now.  it's a modified version of an existing component.  the drawing for the existing componenent has a 'block tolerance' of .002 concentricity on all diameters shown concentric.  on the new drawing there is nothing.  i have nominal dimensions for one of the mating parts but not tolerances.  i'm meant to do a check on it with only this information and i know that if i add gd&t the engineer in question will almost certainly protest.
good times!
kenat, probably the least qualified checker you'll ever meet...
joebk,
  i agree that something needs to be specified. if all you have is a series of diameters shown on the same centerline, this officially says nothing. fundamental rule 1.4(a) says that all dimensions must have a tolerance. simply leaving a dimension off of the print doesn't mean that the rule doesn't apply. if you have coaxial holes, they must either be toleranced relative to each other or each hole relative to other datums but leaving tolerances off completely is just plain wrong. regardless of how perfectly the print shows the part, there will be some coaxiality error. the question is how much is acceptable. i think you're on the right track to try to get some controls on the print. i constantly hear about "implied centerlines" and how they are used to line up coaxial diameters, but i have yet to see it in the asme standard. the only thing that i've seen in the asme standard regarding implied centerlines is specific to 90 degree angles and nothing else and even those use the tolerance value in the standard tolerance block.
   
powerhound
production supervisor
inventor 11
mastercam x2
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just information, i never use concentricity.
read the note: on page 146 in asme y14.5m-1994 5.12.1 concentricity tolerancingfff"> regarding the concentricity.
quote:
鈥漣t is recommended that a control be specified in terms of a runout tolerance or a positional tolerance.鈥?/div>
bradley
solidworks premim 2007 x64 sp4.0
pdm works, dell xps intel(r) pentium(r) d cpu
3.00 ghz, 5 gb ram, virtual memory 12577 mb, nvidia 3400
section 1.4 fundamental rules b)dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each feature.
the engineer knows how this part is designed within it's next level assembly.  if he/she left off any diametrical control of these features it could be because lack of knowledge in that area.  you're not over the top but how you present this to the engineer could affect the outcome and all future correspondence.
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
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"first they ignore you, then they ridicule you, then they fight you, then you win." - mahatma gandhi
a star for heckler.
quote:
how you present this to the engineer could affect the outcome and all future correspondence
there is so much true in heckler鈥檚 statement.
bradley
solidworks premim 2007 x64 sp4.0
pdm works, dell xps intel(r) pentium(r) d cpu
3.00 ghz, 5 gb ram, virtual memory 12577 mb, nvidia 3400
tip, don't start with the word 'redraw'
kenat, probably the least qualified checker you'll ever meet...
p.c. doublespeak for "redraw": "chronologically reverse-iterate the descriptive manuscript in question from the commencing step, with the intent to establish specification based on core commonalities aligned with applied uniforming enterprises."  
matt
cad engineer/ecn analyst
silicon valley, ca