【转帖】full or partial gd

yang686526

full or partial gd&t
a long time ago, in a thread started before i joined eng-tips, someone initiated the debate over whether a drawing needs to be completely gd&t'd or not.  clearly each supporter of the rival factions has their own reasons for their assertions, but i'd like to see them listed, defended & debated in a logical & hopefully standards-based manner.  i'm not naive enough to believe that this will launch everyone over to one side, but it should give us all some perspective as to what the other side of the wall is thinking and why.
so here goes ... i'm for an essentially complete  product definition using geometric controls tied back to datums where possible.  the exceptions where conventional tolerances would be used as listed in y14.41 table 8-1 are:
1) fillets, rounds & chamfers
2) reliefs, step surfaces (i don't particularly support this one; surface profile is more concise)
3) countersinks
4) oblique surfaces
5) entry depth and spotface
6) remaining thickness
7) notches, flats, and pin heights.
now, if you look at the note in section 8.2 of y14.41, it indicates that the list is not exhaustive, but rather indicative to similar and other valid applications.  while it doesn't explicitly preclude the applicability to position dimensions, such application would be contrary to the indicated application methods of such controls which are restricted to attachment to size callouts, directed leader to the feature (surface), or on an extension line from the feature.
also, from an engineering perspective, i want to remove ambiguity from my documentation, so that many featues such as weight-reduction holes, non-functional outer geometries, etc. are at least related loosely back to my drf.  ambiguous documentation cannot be defended in court, and that is a growing reality in industry today as margins & survival dwindle.
et tu?
jim sykes, p.eng, gdtp-s
i'm with jim on this. i think applying gd&t to some features and not to others only perpetuates the myth that gd&t is only for critical features which in turn strengthens the belief of the uneducated that if gd&t is on a print then the part must be really critical and thus needs to cost more.
there will come a day when gd&t will be fully accepted as the "old schoolers" and "fence straddlers" retire and the new generation is fully on the gd&t side of the fence. this will be when people that hold the asme gdtp certification will be more the rule than the exception as it is today. but that day will not come as long as there are the those out there that preach "tolerance"...
   
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
the more i learn the more i tilt toward the viewpoint you give above.
for features of size asme y14.5m-1994 arguably dictates the use of position or profile in paragraph 2.1.1.1.  it doesn't differentiate between "critical functional features" and "non critical" features or similar.
however, i do think that +- dims can be usefully used for overall/outline dimensions in many, even most, cases.  this is fully compliant with my understanding of 14.5.  in fact many illustrations in the standard show this although one must bear in mind what paragraph 1.1.4 says about the figures in the standard.
problems come when you have a supply chain, or even your own staff, that don't adequately understand the gd&t.
the widespread perception(s) that "you only use gd&t when it's important" or "they used gd&t it must be tight so we'll charge more" and similar is one of the biggest obstacles i see to more widespread acceptance/use.
at the end of the day we/our employers are in business to make money.  how far do you take having a fully standard compliant drawing that follows all the best practices etc., just in case it ends up in court, despite the fact that in reality it causes parts to cost more.  not everyone has the ability to force their vendors to get trained etc. and it can be time consuming to shop around and change vendors just to find someone standards compliant etc.  it's a case of balancing risk/reward/consequence etc.
sorry if that's not quite as standards based as you intended mech north.
  
kenat, probably the least qualified checker you'll ever meet...
mechnorth,
   i do not think this discussion is meaningful.  the really important thing a standard like asme y14.5m-1994 does is provide a consistent interpretation of whatever is on your drawing.  even if you apply no gd&t, your drawing is unambiguous.  
   there is no guarantee of course that it means what you think it does.  
   gd&t is a tool, and i use it a lot because it expresses what i want to tell my fabricators.  i want to make my drawings as simple and clear as possible, and gd&t provides resources for this.
   most of the time, i apply a profile tolerance to my outlines.  this is simple and clear, and it gets me around the angle tolerance on my nominal 90° angles.
   most of the time, i apply sloppy ± tolerances to my chamfers.  this is because the chamfer usually is just a modification to an edge, and i cannot be bothered to relate it to a datum anywhere.  if it matters, then i think through what i am doing.
   i love positional tolerances because the tolerance is part of the hole specification.  applying the tolerance to the x and the y coordinate is not very logical, especially if there is more than one hole at that position, and the holes do not have the same requirement.
   i still apply ± tolerances to a lot of linear dimensions.  a positional tolerance would work, but often the drawing is more cluttered as a result.  a lot of linear dimensions do not rely on implied perpendularity.
                            jhg
   
i personally try to avoid using profile unless necessary for linear dims.  in my mind, profile still needs some refinement before it can be generally applied.  for example, it takes more space and time to set up a profile tol than just a linear over all with +/- right now.  
i'm trying to use positional more often these days.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i would use gd&t more, but i much prefer +/- tolerances for the reason that they are typically easier and cheaper to inspect.  i can pull out a drop mic, calipers, or other desktop instrument to check most +/- tolerances.  positional tolerances require the above, plus some mathematics to determine tolerance especially when the tolerance is based on mmc or lmc of not only the feature being checked for position, but of datum references as well.
most certainly, feature control frames - when properly applied - can actually save money by relaxing tolerances, the mere sight of them just screams of increased inspection cost that often exceeds the manufacturing savings of loose tolerance.  and the screams are not just screams of inexperience.  they are often justified screams because the determination of acceptance/rejection to the tolerance requires more than a pair of calipers.
--scott
i have to agree with mechnorth on this one also.
the part is dimensioned by how it mates, assembles or functions in the assembly. the features in the part need to be dimensioned by the function / role that they play in the part. if the correct geometric characteristic is not selected to locate or orient that feature back to the correct datum features, then how do you ensure that the feature is going to serve its purpose? if it is a hole to shed weight maybe a profile tolerance or possibly size tolerance and position at lmc apply. my point is the designer needs to make sure that every feature does what it needs to do and does not adversely affect other features. just because a feature may be able to have a large tolerance does not imply that you use an incorrect method to tolerance it.
drstrole
thanks everyone for the replies so far.  as i said originally, i'm not naive enough to believe that this would sway anyone's thinking, but i do like to get people thinking about their beliefs & convictions.  
i'm a notorious devil's advocate, and was one even before i started calling myself such.  we all have developed instinctive "gut" feelings, biases, etc.  what i've long ago discovered and repeatedly confirmed is that people rarely challenge their own knowlege base that is the basis of their instinct and intuition, and therefore have difficulty considering other points of view even when presented with empirical and scientific support.
as a gd&t virgin, i had some bad experiences with a mish-mash of modified standards and didn't see any value in the technology; we were highly profitable and our scrap rate was reasonably low, and we had a strong base of tribal knowledge developed internally and with external suppliers.  i was forced (kicking & screaming) into leading a gd&t implementation in which i had no faith.  as my knowledge of the technology evolved, so too did my scope of knowledge for our internal & external fabrication practices.  i started to see the cost of developing & re-developing tribal knowledge, and that gd&t could allow greater flexibility in fabrication sources.  i saw, too, that engineering had to start challenging their basic assumptions and core beliefs.  we were doing many things for archaic and invalid reasons when evaluated based on modern design & manufacturing technologies.  so i started to believe, but not completely.
i still allowed a mixture of gd&t and conventional tolerances despite being coached not to by new trainers.  after all, what did they know about our product lines, right?  turned out that the conflicts that they anticipated between the two methods being applied on the same drawing came to be, and we had unanticipated problems as a result.
after that, i lead a functional rethink of our basic product right down to the component level.  by playing devil's advocate, we peeled away our founding assumptions to recognize the true functionality of each component, resulting in changes to our design that would reduce fabrication costs significantly and improve tool life.
of course, the immediate reaction from our purchasing department was that the cost would double, and that's generally what happened when the quotes came back in.  i questioned a few of those suppliers and found that they made an assumption that the requirements were tighter because of the gd&t application.  in other cases, however, suppliers took the initiative to study the prints and recognized that the requirements were significantly looser than before, and reflected that in their quotes.  some suppliers, however, understood a bit of gd&t and came in significantly below the others. the difference was just a couple hours of training.
we went through the exercise of offering non-mandatory training which most suppliers didn't bother with, but those that did were able to reduce their costs, our costs, and open themselves to a broader market.  those shops are the ones that now have a competetive advantage in the global industrial war that is waged between western nations and the cheap labor markets of the east.  
gd&t is a tool.  any tool is intended to make your work easier and more effective.  hopefully you use the tool to your greatest advantage and greatest value.
   
jim sykes, p.eng, gdtp-s
jim:
please share with us your experience where a drawing containing positional on all holes and profile of a surface on all surface(feature control frames shown in notes) assisted the designer with design intent.
thanks   
dave d.
interesting question dave.  if i'm understanding you correctly, included in the "unless otherwise specified" notes, you have a general surface profile tolerance and a general positional control which is labeled as exclusively applicable to holes.
if you are questioning which to use, consider the following:
first, a surface profile is not used to control the location of a feature of size (fos) (criteria: directly opposed points and applied size tolerance).  6.5 profile controlfff">  ... with profile tolerancing, the true profile may be defined by basic radii, basic angular dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings, or formulas.  so, in the case of a hole with a conventionally toleranced size dimension (a fos), the general positional control for holes is applicable.  
next, a position tolerance can only be used to locate the center planes/axes/points of features of size (5.2 positional tolerance, a positional tolerance defines: a. a zone within which the center, axis, or center plane of a feature of size ... or b. ...a boundary, defined ast he virtual condition...), or to locate the boundary of an irregular enclosed feature (6.5.5.1 boundary control for a non-cylindrical feature)
so, in the case where you have a non-toleranced size dimension for a hole, and there is no indication that untoleranced dimensions are basic, then it is not known whether or not the hole is to be considered a feature of size, and you have no resolution, and the designer needs to be flogged.
if the size dimension has a directly applied tolerance, the hole is a fos and position is applicable, profile is not.
if, on the other hand, you have a non-toleranced size dimension for a hole, and there is a note equivalent to untoleranced dimensions are basic, then the hole cannot be considered a feature of size which means that the position control is not applicable and the surface profile is applicable.
as for my experience with this, unfortunately i have made the error of releasing drawings (early on) that had both the general surface profile and general hole position controls in the notes, but without the note "untoleranced dimensions are basic".  at first it wasn't an issue because the shop had no idea what it meant either way, but as they evolved their understanding of gd&t, they started to recognize the conflict and addressed it with me.  by that time, i understood the difference (subtle as it may be), and added the note to our templates.
the reason that i followed that approach was that initially the majority of our holes were held to a standard positional tolerance, regardless of size or function, so we could reduce drafting time.  we removed the general positional tolerance on holes when we started to actually do fixed & floating fastener calculations.  i still include a general positional tolerance for tapped holes in my notes under "unless otherwise specified" to reduce detailing.  where a different positional tolerance is needed on individual features or patterns, whether more or less restrictive, i apply it directly to the appropriate features.
the design intent for the features is indicated by a combination of the size controls and location controls, and the relationship to the datum reference frame.  without datum references, i would be free to verify the dimension pretty much any way that i wanted to.  for example, if the center of a dia 5+/-2mm hole is dimensioned as 50+/-5mm from an edge of a plate, i could do a two-point check from the tangent point on the hole to the edge of the plate; if the value is in the range of 47.5 +/- 7mm, the part could legitimately pass the spec.  but what if the edge of the plate has a significant deformation at that location?  the use of conventional tolerances doesn't require the use of the entire plate edge to establish an origin of measurement, just the closest point to the opposed measurement point.  by invoking the edge face of the plate as a datum feature, and referencing the center of the hole back to that datum, i eliminate the bias of that localized deformity along the edge.  in that way, my design intent is communicated to the drawing users.
jim sykes, p.eng, gdtp-s
jim:
you gave an example where you had in the notes section a general positional tolerance and a general profile tolerance that applies to all holes and all surfaces. your error was to also place a tolerance on the location rather than have them in basic dimensions. you were new at this at that time and was a clear error.
i realize that design time is reduced if all dimensions are basic and the gd&t in notes covers everything. shouldn't a design give the user a clear understanding of the design intent? there are so many features that could be out of specification and the part will still function while others are important to its function.
here is the question again.
you have a drawing that states in notes "complies with asme y14.5m-94" and all three datums are shown on the drawing. please explain why having the default gd&t in notes assists design intent rather than isolating important features from a design perspective and applying gd&t on those features only.  
   
dave d.