【转帖】diameters shown in line not necessarily coaxial

yang686526

diameters shown in line not necessarily coaxial
can someone please remind me, in asme y14.5 is it basically just 2.7.3 that says that just because diameters are drawn coaxial there is no coaxiality requirement implied.  to get coaxiality you need to add controls such as position, runout or concentric (i know, concentric is rarely if ever the right one depending which 'expert' you're talking too).
i get a lot of drawings of parts with 'coaxial' diameters without any controls on the coaxiality.
do other checkers/peer reviewers etc find the same thing?
has anyone seen any really good articles or explanations on this i could pass around.
thanks, ken
kenat, probably the least qualified checker you'll ever meet...
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i find this to be pretty common.  if it is imperative that they be coaxial, i will suggest a runout or true position control.
how are the holes located? if there is a basic dimension to the c/l, then some geometrical constraint becomes necessary.
a positional tolerance is a good start, of course depending on the functionality of the features involved.
the largest, longest, or functionally most important coaxial feature needs to be made a datum--perhaps a 4th datum tied to the 3 main datums--and other coaxial features tied to it by position or runout.
  usually coaxial features are located using a composite tolerance block as in fig 5-51 and 5-52 in the standard. i suppose there are a few different means to achieve the same end in this case but i always stick to whatever callout is specifically for the circumstance. start at para. 5.11 and see if any of those example gives you what you are looking for. the answer to your question is that just because features are shown coaxial, if there are no controls that tie them together, any relationship between them should not be assumed. keep in mind that if each of the holes are located relative to identical datum reference frames and the datum features are not subject to size tolerance, then they are considered a single pattern. see para. 5.3.6.1 for a more detailed explanation.
powerhound, gdtp t-0419
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inventor 2008
mastercam x2
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thanks all, i'll have to do a little more reading from the suggestions that powerhound gives.
just to clarify i'm talking all coaxial features both male & female.
in fact it's most common on shafts and the like with a number of nominally coaxial diameters but no relation between the diameters.
kenat, probably the least qualified checker you'll ever meet...
in most operations i've seen, it would be very hard to have multi-diameters on the same axis (and side of the part) not be co-axial simply because these normally use one op on a lathe.  in those cases, i wouldn't call out any co-axail specifications beyond what is immediately functionally required.  to call it out is a bit like saying that your mother's mother is one of two of your grandmothers.  however, there's nothing wrong with spelling it out.
matt
cad engineer/ecn analyst
silicon valley, ca
if circles are drawn to look coaxial, and have center-lines drawn through the center at 90 deg, they are usually implied to be coaxial. similar to two line drawn perpendicular, they are implied to be 90 deg.
chris
solidworks 07 4.0/pdmworks 07
autocad 06
chris,
  yes, two lines shown perpendicular on a drawing can be implied 90 degrees but that's because it's a fundamental rule in the asme standard. there is no such rule to imply coaxiality. the biggest isue you have to contend with is tolerance. even if it was a safe practice to imply coaxiality, what would the tolerance be? with implied 90 degree rule, it's the tolerance in the tolerance block. showing two diameters coaxial using only a centerline doesn't give a dimension so the tolerance block cannot be used.
matt,
  you can't assume that just because features are on the same axis and on the same side of the part, that they will be done at the same time. it definitely makes more sense to do it that way but that would be like putting a tolerance of +/-.005 on a feature simply because you knew it would be made on a machine that could hold it even though +/-.5 would have been just fine. you can't assume a shop will do anything you expect that they will do. all you should count on is getting your parts made to the dimensions specified on your print.
kenat,
  i see uncontrolled coaxial diameters all the time. it's frustrating when our qc department rejects a part in which an injector tube is welded 1mm off center yet the print says nothing about how close the injector really needed to be to the center of the burn chamber. it just shows a centerline. rework of that sort is unnecessary when the rejection is unjustified.
  the standard says each dimension must have a tolerance except max, min, or stock dimensions. coaxiality is no exception to this fundamental rule. go back to what kenat asked in the op; 2.7.3 specifically states that features shown coaxial must be controlled for location or orientation. this means that you can't imply anything in this regard.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
i agree with chris on this one and i would use the standard tolerance shown on the drawing if there was no a feature control frame.
dave d.
quote:
asme y14.5m-1994
2.7.3 relationships between individual features.  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.
fcsuper, ctopher & dingy, i don’t see any ambiguity in what the standard says.  showing them coaxial doesn’t imply anything and while for parts created in a single turning or boring op you’d expect pretty good coaxiality the drawing doesn’t require it from just showing them coaxial.  as such if they somehow create parts that aren’t close to coaxial, such that they aren’t fit for use, you have no comeback.
14.5 has a number of suggestions for general notes etc if perfect form at mmc is required but for a lot of the ones i see this is probably tighter than warranted by function.
i just looked at some training notes from gary whitmire and he has some explanation.  if anyone has any other sources that would be great.
kenat, probably the least qualified checker you'll ever meet...