【转帖】rule 1 and position

yang686526

rule #1 and position?
i have a question on form and the positional tolerance.  if you had a hole controlled by a positional tolerance, does the hole require perfect form at mmc?  i wondering whether or not rule #1 applies to features of size when they have a positional tolerance on them.  if this is true, this would mean that a .190 hole at mmc with any type of positional tolerance would be able to accept a .190 gage pin.  is this correct?  thank you in advance for your advice.
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rule #1 still applies. the tolerance only controls the position of the hole (relative to the three mutually perpendicular datum planes). the size of the hole is still controled by the ± tolerance on the size, and so, at mmc it still is at perfect form, since all the material is present.
jmwalden,
   from asme y14.5m-1994...
   "maximum material condition (mmc).  the condition in which a feature of size contains the maximum amount of material within the stated limits of size -- for example, minimum hole diameter, maximum shaft diameter."
   the mmc condition of your hole is a perfectly round hole of the smallest allowable diameter.  the centre of your hole is located somewhere inside the diameter specified by your positional tolerance.
   your .190" gauge pin should pass through the hole assuming that it is a round pin that you are holding in your hand.
   if your pin is located rigidly from a datum, there is no reason to assume that it is aimed at your hole accurately enough to pass through.
   you may be thinking of a hole located to a positional tolerances of zero, at mmc.  now, mmc is the .190" hole located exactly at the nominal position.  a .190" pin located exactly and rigidly from the datum will always pass through a conforming hole.
                          jhg
   
rule #1 applies to a feature of size and is still relevant here. positional tolerances apply to location of the hole(s) and is independent unless the positional tolerances are applied at mmc. without mmc, it would be the centers of the holes, with mmc, it means the virtual condition size and should be checked with a functional gauges. hope this helps.
ded
i think that rule #1 does not apply any more. this rule is for the situation when no control is explicitly defined. when you apply a control on the feature, then there is no need for rule #1 and the feature control block determines the virtual condition.