huangyhg

more about bolt circles
the center of a bolt circle has a nominal (basic) location and a tolerance zone.
is the location of each hole measured wrt to basic location, or the actual location?
if gd&t is not used, is there are tolerance on the diameter of the bolt circle? if so, and there is an even number of holes in the pattern, must the holes lie within d+/-t, or d+/-2t?
if gd&t is not used, what prevents the tolerance zone of each hole from being computed as the sum of the positional tolerance of the bolt circle center + the tolerance of the bolt circle diameter?
i think you've been hanging around btrueblood too long.
believe it if you need it or leave it if you dare.fff"> - robert hunter

mintjulep,
basic dimensions and gd&t:
the centre of the bolt circle is a figment of the drafter's imagination. it has no tolerance. each hole is located from the specified datums, which are required by asme y14.5m-1994 to be real features.
± dimensions:
i can define a pitch circle using the centres of three holes. this gives me a diameter and location to which we can verify tolerances. even the angle tolerances are verifiable. if there are more than three holes, all of this becomes ambiguous. this is an awful lot of work to avoid gd&t.
jhg
is there a difference between bolt circles and screw circles? just wondering.
perhaps 'politically correct' should be 'fastener locational circles' or such.

i've seen bc defined as both bolt circle and between centers, depending on the company standard. off topic again... time to go home.
believe it if you need it or leave it if you dare.fff"> - robert hunter

"if gd&t is not used, is there are tolerance on the diameter of the bolt circle? if so, and there is an even number of holes in the pattern, must the holes lie within d+/-t, or d+/-2t?
if gd&t is not used, what prevents the tolerance zone of each hole from being computed as the sum of the positional tolerance of the bolt circle center + the tolerance of the bolt circle diameter? "
well, what drawoh said.
yes, w/o gd&t, you could have a set of dimensions, each with their own tolerances, locating the position of the (center of) the bolt circle, relative to some other feature.
but, within the circle, the bolt holes are presumed to lie on the defined bolt circle +/- the circle's tolerance, and +/- any angle variation (usually confounded or double-dimensioned, as pointed out exhaustively elsewhere). in other words, the pattern as a whole (and thus the circle center) can shift from the controlling features, but the bolt holes relative to each other can only drift within the bolt circle tolerance.
now apply gd&t, and hold the pattern center fixed from the same features (now made "datums") using basic dimensions. how you tolerance the bolt holes relative to those datums affects how much tolerance you can allow within the pattern...something i tried to bring up long ago...and i believe ewh had the answer then, describing how you would control only one fastener hole's posisition (or perhaps the bolt circle's position? but this is a centerline, so you can't specify it as a datum in the 1994 version) relative to the datums, and reference the fastener hole (or bolts circle - pre 1994) as a new datum, and apply the within-the-pattern tolerance seperately to the remaining holes from that datum only. then spend a week explaining to various machinists what the %$$! your intent is, only to finish up with some 57-year old machinist telling you "the old way worked perfectly fine, why'd you go and mess it up".
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