【转帖】olerance zone of r is ambiguous

yang686526

tolerance zone of r is ambiguous
asme y14.5m, 1994 section 2.15.1 radius tolerance states:
"a radius symbol r creates a zone defined by two arcs (the minimum and maximum radii).  the part surface must lie within this zone...."
section 2.15.2 controlled radius tolerance states:
"a controlled radius symbol cr creates a tolerance zone defined by two arcs ( the minimum and maximum radii ) that are tangent to the adjacent surfaces..."
how do you connect the two arcs? because no matter how you do it, there's a zone that is zero wide.  wouldn't you need to connect the two arcs with straight edges?  and if you did, wouldn't that be the tolerance zone of cr?
what does the tolerance zone look like for r2+-2?
same definition, but:
r; flats and reversals allowed
cr; flats and reversals not allowed
chris
solidworks/pdmworks 08 2.0
autocad 06/08
the r2 +/-2 is a silly callout because you don't have to have a radius at all and the part will be to print. it also allows all kinds of crazy geometry on that corner and still you would have a good part. i wouldn't preoccupy myself with this issue because the dimension, as specified, doesn't really make sense. if you give your customer a goofy looking corner that is still to print they are just going to reject it and you'll make the part again to keep him happy. slap a decent looking radius on that corner and send him on his way. if you want to push the envelope, leave a sharp corner and see what he says...haha.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
ctopher,
how could the tol zones be the same when cr requires tangency and r does not?  the tol zones in the figures in asme for r and cr appear the same but are they really the same?  try drawing it for the example of r2.4 +/- .3 from asme.  because the arcs aren't require to be tangent, the zone can move anywhere between the tangency point of the minimum radius and the sharp and you still satisfy the definition of r.  (see attached pdf)
powerhound,
this may seem silly but it is fundamental in understanding what r means.  so, is it possible to draw the tol zone of r2 +/-2 using the asme definition?
r does have less restrictions than cr.  r used to mean some of what cr now means, but cr was created (i believe) to address the issue of tangency and fairness of curve.  so, yes, the start of a r can have a variety of possibilities, where cr has only one.  a minimum, r can be used to break a sharp edge.  a minimum, cr can be used to establish an actual radius form from the two surfaces.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
tgwow,
  regarding your drawing; the arcs are required to be tangent per 1.8.5 and 1.8.6 of the standard but not tangent to each other, they must be tangent to the corner that they are supposed to fillet. you would draw the minimum arc tangent to the edges of the corner and then the maximum arc tangent to the edges of the corner. the bottom view of your drawing is correct.
regarding the "silliness" comment; what i meant was that if a sharp corner is acceptable then why even put a radius there? it's just one more thing that qc will have to inspect. the tolerance zone for a 2 +/-2 will look like a triangle with one of the legs as a 4 radius.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
powerhound,
section 1.8.5 and 1.8.6 requires that the actual part geometry be tangent to the sides but says nothing about the tolerance zone.  
therefore, why would you draw the min and max arcs tangent for r?  additionally, the definition of the r tolerance zone does not state inclusion of the tangent sides as boundaries.
by definition, wouldn't the tol zone for r2 +/-2 be just the max arc (since the min arc of zero just disappears) located somewhere in the corner (not required to be tangent)?
if i had it on the drawing i'd have thought "r4 max" would be a more logical callout.
depending on the application the 'silliness' point about why even break the edge may have merit.
kenat, probably the least qualified checker you'll ever meet...
kenat,
we're kind of going on a tangent but for parts which are molded i've seen requests from suppliers who would want the cad model modeled at nominal which would mean in order to "show" the dimension from the model on the drawing for "r4 max" and keep it parametric, "r2 +/-2" would be prefered over "r4 max".
the alternative is to model at r2 then fudge a dimension "r4 max" on the drawing.
yeah a bit off tangent, more related to
i mentioned this in the unilateral tolerancing thread but it seemed to go over everyones head.  with pro/e & i assume other cad programs you can have an r4 max. dimension (fully parametric model driving dimension) which is set to regenerate the geometry at the middle value (in this case 2) so the drawing calls out r4 max. and the model measures r2.  the drafters, design engineers, manufacturing engineers, cam jockies & mold makers are all happy.  this is not a fudged in dimension.  
the same technique can be used for asymmetric tolerances like shaft & clearance holes where the tolerances are both positive or negative.  the dimension can read 1.000+.005/+.001 and the model will measure 1.003.  you can also set the dimensions to regenerate at the maximum or minimum value to do tolerance stackups. the state of the dimension (max, nominal, middle or min) is indicated by color.