【转帖】pos tol applied to pattern 9how to0

yang686526

pos tol applied to pattern (how to?)
i have a x,y grid pattern of holes laid out flat plate.  datum a is on the facing surface.  datum b runs through the part in the x direction.  datum c runs through the center of the part in the y direction.  (see the attached drawing.)
i wish to apply positional tolerance to the holes of the pattern.
what's needed, what's basic, what's reference?
matt lorono
cad engineer/ecn analyst
silicon valley, ca
the equally spaced issue comes up frequently on here (you must have seen it) and while not forbidden in asme y14.5m-1994 i'd strongly suggest using 49 x .500 basic instead.
arguably the 1.750 dimensions are not required and if given should be either basic or reference (depending on if you really think you need them).
kenat,
lawyers only ask questions in court that they already know the answer to. fcsuper, you indeed are quite experienced in this stuff... why don't you tell us what you think is the answer to your question?
paul   
personally, i'd need to know more about the function and assembly before making any judgements as to correctness or what is needed vs ref dimensions.
"equally spaced" is entirely redundant and should not be used. i would likely question the use of the mid planes as datum features for this application, without further insight. tol on hole location vs hole tol? seems tight.
kenat,
you might want to rethink the 49x.  i believe 7x in a vert and horiz direction to be more appropriate.
it is complicated to dim off of the theoretical cl.
the tol needs to be added for overall length of part, cl to 1st hole, each space between holes, then position of each hole.
to add these up and then equal 3.500 could be confusing for the machinist and inspector.
i agree that the equally spaced is redundent. i would use only the 7x. the 1.75 dim should be basic.
which dimensions are needed is hard to tell without knowing the design.
i'm looking for input because a scenario similar to this one is coming up for me, and discussion regarding this has been ...well...  anyway, i am looking for some input.
i cannot get too much into function.  the holes line up with an array that hovers over one whole row at a time.  i will say that we are concerned with how the holes are located to each other and don't much care about the edges, per se.  there are two alignment pins that locate the plate (i didn't model those for the example).  one pin is the upper left, one in the lower right.  the part is  designed to be symmetric about the c datum for handling.  
i don't see much issue with using the centers of the part as the datums, as this is not difficult for machinists to handle, though i may have to take additional action to allow the part to be more readily inspected.
futher input is welcome.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
you shouldn't have a dimension from the center of the part to the outer row of holes. when you locate the hole pattern symmetrically within the datum structure it is implied centered. the 1.75 can be reference at most.
if the hole pattern itself is more critical than where the pattern lies in the plate then consider composite position. be sure to consider position at mmc if applicable. the only other thing i would change goes along with kenat and that is making your .500 dimensions basic. using equally spaced can become a bear but if your "equally spaced" is applied to basic dimensions, then most of the problems associated with it go away. i'm still not a fan of it though.
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the primary needs a form control.
the secondary needs a size constraint and an orientation to the primary.
the tertiary needs a size constraint and an orientation to the primary and secondary.
the hole, countersink, and angle each need a tolerance.
the descriptions of the basics work ok as they are but could be improved by making them more universally interpretable by losing the english language and boxing the value. there are a number of ways to do that... try one and bounce it off this group and count the yeas and nays.
i don't see any problem with the opposing surfaces of the length and width being the secondary and tertiary datum features and referencing the basics from the center plane... if that mirrored function... but since you revealed that two alignment pins actually do the job of a pattern secondary (two round pins in two round holes) or a secondary and a tertiary (two round pins in a hole and a slot)... i would put the dimensional reference line at the center of the 2x pattern or the center of the 1x secondary... tolerance the pattern for position to a (which controls both orientation and spread) or the orientation of the secondary to a then the position of the tertiary to a|b. then i would tolerance the edge profile "all around" from a|b or a|b|c which ever reflects function. naturally the basic dimensions to the holes - countersinks would originate from that functional reference as well.
paul  

i'll post an updated image tomorrow with more detail.
i'm also been asked to completely disassociate the edges of the part from the pattern, without the use of locating pins or any other reference.  the suggestion was to use "witness holes" to control the location of the datum.  am i completely off base by seeing this as trying to create a datum by using the very features created to mark the datum (chicken-and-the-egg self referencing).   
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i horsed around with your picture... this is using the locating pins as a secondary 2x pattern.
paul  
or... if there were no locating pins.
paul