【转帖】position tolerance of the opening which is in distance from

yang686526

position tolerance of the opening which is in distance from
hello,
in my daily work i deal with drawings where position tolerance is marked in
the way like on the attached example 1 - please take a look on doc file where i combined all examples.
in my opinion, this way of dimensioning is incompleted. according to
example1 i understand this tolerance like on the pic no. 1
the shape of the tolerance zone is a cylinder which is perpendicular to
datum a, lies on axis between opposite walls (datum b) and is 40mm away from
datum c. but we don't control a 'height' from datum a. so we cannot mark
dimension 25 as a basic.
i spoke about this with other guys and i received a tip that you cannot
treat a tolerance zone, in this case, as a cylindrical zone but you have to think
about tolerance zone as a sphere which is connected to the upper, or lower,
side of the axis of mentioned opening.then we control the 'height' and we
can mark dimension as a basic dim. i'm not fully convinced that it's right.
please take a look on attached pic no. 2
there is also third theory, that the tolerance zone is a cylinder like on
the attached pic no.3

mentioned cylinder is on specified position in 3d and you can control
everything. i don't agree with this theory.
finally, i can show you my way of dimensioning stuff like this. please take
a look on attached pic no. 4 and example 2.
the shape tolerance controls mentioned 'height' so 25 can be marked as
basic, the rest remains the same.
i know that it's complex post but i wanted to avoid creating a lot of posts
which wouldn't be connected to each other.
i hope that i was clear for you and many thanks for your any remarks.so ..... what do you think about it ? which way is correct ? or maybe there is also another way how to solve this problem....
best regards
michal77

hi michal77
i agree you can't put a basic dimension on the height without giving a tolerance.
however i also disagree with the profile symbol you have attached to try and control the height, why can't you just
give a toleranced dimension on the height ie:- 25.5/25.0.
or am i missing something?
desertfox
my picture is not good at at all but i would suggest that you apply a projected tolerance from the datum plane.  i think that should work.  it would also be helpful in determining the best method to know the mating part.   
oh i forgot to ask, but what is the standard you are using for dimensioning and tolerancing?
michal77
  the 25 dimension is completely unrelated to the tolerance of position. whether you tolerance the 25 with a profile of a surface or just a plus/minus tolerance, it has no effect on the orientation of the hole. the hole is oriented to datum a and the axis of the hole must be within the 2mm cylinder but only across the thickness of that lip, the axis of the hole does not have to be within the cylinder, outside the thickness of the lip.
  the theory that has to do with the sphere is just wrong. the tolerance zone in the case that you are stating is absolutely a cylinder and nothing else. pic 3 is the correct representation of the tolerance zone but that 25mm height is totally irrelevant.
  example 1 is incorrect as it has a basic dimension of 25 with no feature control frame to control the surface. as i previously mentioned this dimension has nothing to do with the positional tolerance as you have it specified. example 2 is correct but i'm worried that you may think the addition of the profile of a surface somehow makes a difference in what the positional tolerance means. it doesn't. don't apply a projected tolerance from datum a either. if you need the perpendicularity to datum a to be controlled more tightly than the position of the hole to b and c, use a composite tolerance block.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
i basically agree with powerhound on this.  the position tolerance defines a cylindrical zone that is oriented to datum a and located to datums b and c.  the segment of the actual axis that is between the top and bottom surfaces must be within this zone, wherever the top and bottom surfaces happen to fall.  unfortunately the locations of the top and bottom surfaces are not completely controlled.
simply putting a basic dimension between datum a and the top surface (as in example 1) does not control the location of the surface.  specifying a surface profile tolerance (as in example 2) does control the location - now we're getting somewhere.  just omit the b and c datum references, because the top surface is nominally parallel to datum a.
but the drawing still needs something to control the location of the bottom surface.  this could be a surface profile referencing datum a, or a linear distance from the top surface (i.e. thickness).
evan janeshewski
axymetrix quality engineering inc.
first as ringman asks, what spec are you dimensioning to?
if asme series then i'd agree with powerhound.
kenat, probably the least qualified checker you'll ever meet...
the reason i didn't ask about whether he was using something other than asme is because in this case, and relevant to the question asked, it likely doesn't matter. iso defines position the same way as asme, with the exception of iso using position the same way asme uses profile of a surface, but that is not relevant to the question asked. any other standards will probably do the same.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
you're probably right powerhound, haven't looked at the iso for a while and then probably not closely enough.  i was thinking about the postition for a surface in case some how it affected it but i couldn't see how it would, but i've had iso specs confound me before so i'm loathed to rule it out.
kenat, probably the least qualified checker you'll ever meet...
hi,
many thanks for your quick reaction and very instructive replies
daily i work with drawings acc. to asme but also i'm in touch with iso.
powerhound - my main goal was to control everything (control position of the opening in 3 directions). i know that profile tolerance doesn't have an influence for the position tolerance. so, if i uderstood correctly, tolerance position controls opening in directions which are perpendicular to the opening axis (of course it's my quick simplification), and if we want to control position of the opening in direction along the axis, we can use profile tolerance or just simply give tolerance to the dim 25.
axym - ok, you are right about erasing datums b,c from profile tolerance. i will have to re  
michal77
the location of the surface on which the hole is placed is more or less independant of the actual callout/tolerance for the hole itself.  trying to tie the 2 together is confusing.
you need the create/dimension/tolerance the feature that the hole is through separately.  you could use profile on the top & bottom surface but +- dimensions may just as good a choice, maybe better for this application.
you are then creating a hole through this feature.
this can get more complicated if your hole is at an angle to the face rather than normal but even then the dimension/tolerance of the feature that the hole is through is separate.
powerhound is suggesting that if perpendicularity to -a- is required tighter than that caused by the position tolerance, that rather than using projection (p in a circle in the fcf) you add a perpendicularity control in a composite block.
kenat, probably the least qualified checker you'll ever meet...