【转帖】symmetrical tolerancing

yang686526

symmetrical tolerancing
hello world,
the part shown in the link is a hinge block. it is to be sent out for fabrication. i’d like to dimension the pin hole in such a way that variance occurring to the width of the bar stock will not cause the pin hole to be off center due to the fact the pin hole is dimensioned from one of the two vertical edges. this block is used multiple times in a weldment and both sides are used as positioning surfaces which is why i’d like to convey the importance of the pin hole lying mid-plane rather than just dimensioning off an edge.
our drawing checker promoted a symmetry callout referencing the mid plane, but symmetry is only appropriate for non-cylindrical features and can be expensive to inspect (geometrics iii). the only dimensioning scheme i’ve come up with is positional tolerance relative to a datum, but then again the hole would still be dimensioned off 1 edge.
  then again it is bar stock which shouldn't have much variation. what do you think?
failure is a prerequisite of successful design
how about make .563 a 'datum' (asme y14.5m 3.3.2b, this invokes the center line as the datum).
use positional tolerance on the hole, can't find an example with a hole in the standard but 5.60 is similar idea.
i'm not sure you properly define your end radius/overal length either but my brains aching, we just had lay-offs so someone check what i'm saying.
kenat, probably the least qualified checker you'll ever meet...
always sorry to hear about lay-offs. *fig 5.60 to avoid confusion. the picture provided is just a snippet of the actual drawing. kenat, you're right in that it should state full r.
from fig 5.60 refers to a center plane and a datum plane of the actual mating envelope. if i were to insert a diameter cylinder into the feature control frame to represent a cylindrical axis, then perhaps the same logic would apply, but to cylindrical tolerance zones. is this sound logic?
failure is a prerequisite of successful design
look at my attatchment, is this in the ball park?  i've made some assumptions as i don't have all the info.
it assumes a is a surface the hole is perpendicular to.
i think per 14.5 para 1.8.4 it's r not full r.
anyone want to pick my sketch apart
kenat, probably the least qualified checker you'll ever meet...
other than making the 1.000 a basic dimension, i think it looks good.
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... and the .281 basic.
chris
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i'm not sure the 1.000 and r should be basic. kenat said that datum a is the surface perpendicular to the hole. that said, the 1.000 and r are not measure from any datums, are not the basis of any fcf's, and therefore shouldn't be basic.
is it too early? or am i right?
v
mechanical engineer
"when i am working on a problem, i do not think of beauty, but when i've finished, if the solution is not beautiful, i know it is wrong."
- r. buckminster fuller
i agree with vcastro66 about not making the 1.000 and the r dimensions basic.  kenat's sketch seems to address the op well.
oh yeah!  congrats on tipmaster of the week, kenat!  that should look good on your resume ;)
yup, you're right vcastro. i looked at it too quickly. the bottom edge needs to be datum c and then the 1.000 made basic, that'll fix it. as it is, it's not a complete print.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
quote (tomfin):
this block is used multiple times in a weldment and both sidesfff"> are used as positioning surfaces which is why i’d like to convey the importance of the pin hole lying mid-plane rather than just dimensioning off an edge.
doesn't the width .563 (datum b) need to be basic as well?  seems if both sides are critical, the width must be controlled as well.
"art without engineering is dreaming; engineering without art is calculating."