【转帖】positional tolerance

yang686526

positional tolerance
we have a discrepancy between options about the positional tolerance of a drilled hole in a cast part.  i would appreciate anothers opinion.  the drawing calls out the following tolerance (as closely as i can imitate the actual call outs, i wish i could attach a drawing):
4 holes (diameter symbol)9.0 +-0.2
(control frame)|(position symbol)|(diameter symbol)0.3|a|b|e (end of control frame)
chamfer (diameter)10.2 +-0.2 * 45
the horizontal and vertical linear dimensions (for one hole for example) are marked as 29 (horizontal from center) and 50.23 (vertical from center).  there is no specific tolerance called out on these dimensions.  the title block also has a note about unspecified tolerance on linear dimensions equal +-.2.
what linear tolerances apply to this 9mm hole?
thanks in advance,
steve
what good is a positional tolerance on a +/- located feature?  the positional dimensioning should somehow relate back to your datums, and should be basic.
the locating dimensions must be basic.  you can calculate what the linear tolerances would be if you were not using the true position callout....re  
i'm sorry i did not make myself clear ... these are not my drawings or can i change them in any way.  i'm simply trying to interpret what is already there.  i'm just not sure how to.  well, i'm pretty sure i get what the drawing is getting at, but them someone says your wrong, doubts creep in and you post on message boards .  the main discrepency is the linear tolerancing of the hole position.  is it a priority issue (.3 in the control block or .2 in the title block)?
you could play it safe and assume the locating dimensions are basic, or you could assume that the geometric control is in error.  i would play it safe and consider the dimensions basic (provided they do link back to the referred datums).
since the true position is taken at rfs (regardless of feature size) you get no bonus tolerance from the feature tolerance.  the title block tolerances should only apply is no other tolerances are specified.  their should be a note somewhere in the title block that states "unless otherwise specified"
have you had any formal training in the area of gd&t?
i actually don't think this one is that confusing.  before the days of cad, a lot of effort went into ensuring that drawings were as simple as possible, with the minimum of data required.  it took a long time to add dimensions, notes, callouts, etc.  i know times have changed, and it is now quite easy to overdimension or put notes in everywhere.  if i had a drawing like seco1, i would assume:
1) title block doesn't apply whenever someone goes to the trouble of specifying a tolerance, and
2) no one 'accidentally' adds a positinoal tolerance callout.
i know, never say never, but this would be my starting point.
things that supoprt it are heckler's points about rfs and the usual notes in a tolerance block that qualify when the block is applied, and the fact that the positional callout is tighter than the tolerance block.
thank you for your replies.  personally, no i wouldn't say i have formal training in gd&t, but many years of practical experience.  in a nutshell, we are a manufacturing plant and i disagree with the customers interpretation of the tolerancing on this old drawing.  i too have been assuming basic location tolerancing.  of course the customer is always right, but if i could show the purchasing agent (who does know anything about gd&t) something concrete it would make my life considerably less complicated.  i spent sometime checking asme today, but have not yet found anything.  i was hoping to be able to craft (or find one) a simple explanation of standards, not that they necessarily have to follow standards.
is the chamfer dia suppose to be part of the fcf?
i would make it a separate callout.
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
sounds like they just forgot to put the box around the basic dimension.  i'd treat the 29 and 50.23 as basic.
i told you an explaination, according to you original post it's true position regardless of feature size of dia 0.3 mm.....so in practical terms regardless of what size the feature comes in between 8.8 - 9.2 mm it's location must be within a 0.3 mm diameter tolerance zone.  this translates into +/- 0.21 mm linear tolerance if you do the math....re