【转帖】clocking question

yang686526

clocking question
a drawing shows a cylinder with a flange on each end and each flange has a 8 hole pattern of holes in it. the cylinder o.d. is datum a. the flange face at one end is datum b. the flange face at the other end is datum c. the holes at the datum b end are controlled with a positional callout to b/a. the holes in the datum c end are controlled with a positional callout to c/a. the holes are shown in line. does the drawing require the holes to be in line?  
don't think so.  to align them you should assign a datum identifier to either one of the holes or the pattern on one flange and relate the other set of holes to it.
asme y14.5m-1994
section 4.4.2 parts with cylindrical datum features - a cylindrical datum feature is always associated with two theorectical planes intersecting at right angles on the datum axis.
section 4.4.3 rotational orientation - to establish rotational orientation of two planes about a datum axis, a third or tertiary datum feature is referenced in the feature control frame.
so with that said, just like ringman stated above you need a tertiary datum to clock both sets of holes to or make one hole a datum and clock the other set to that datum feature.
imo, your datum [-c-] is redundant since you have datum [-b-] in the same plane.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 5.0 & pro/e 2001
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never argue with an idiot. they'll bring you down to their level and beat you with experience every time.
datum c is not in the same plane as datum b, though it is in the same orientation.  if it is critical that all holes be perpendicular to datum b, then omit datum c.  if it is more critical that the holes be perpendicular to their respective flange face, then leave datum c as you have it, or control that surface relative to datum b.
as far as clocking, the holes will only be misaligned by the angular block tolerance allowed (provided it is obvious on the drawing what their orientation is)(see para 2.1.1.2).  if they need to be more tightly controlled, then the addition of making one of the holes a datum and clocking the rest relative to it is a good idea.
my previous post should read
"if it is critical that all holes be perpendicular to datum b, then omit datum c, and control that surface relative to datum b.  if it is more critical that the holes be perpendicular to their respective flange face, then leave datum c as you have it."
i've had this discussion with someone before and no conclusion was ever reached. this seems like the perfect time and place to bring it up again. does the implied 90 rule apply if you are locating features? the rule itself refers to right angles and surfaces of features but says nothing about aligning centers. in regards to the case that caseynick proposed; the part is still not fully constrained as all degrees of freedom have not been restricted. i had always understood standard practice to be to use a, b, and c datums to restrict the degrees of freedom then use additional datums to further define your part if necesssary. if the holes have to be aligned, i would have called datum c out as one of the holes to stop rotation of the cylinder and then position the rest of the pattern to it. a composite tolerance block could be in order here depending on the application. as ewh said, you can control the end opposite datum b with a profile of a surface or something like that if it has to be constrained with respect to the datum b surface. as you have it, the only thing controlling the orientation of those end surfaces is the implied 90 rule.
powerhound
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while i agree that it is good practice to clock the holes as suggested, i don't feel that it is an actual drawing requirement.  it is more of a fool-proofing exercise.
perhaps it would be well to know the mating relationship at assembly??
if orientation of the 2 patterns to each other is necessary, i don't believe that the title block tolerance can be applied.  that would be mixing basic and plus minus.
the hole orientation on the drawing is not optional unless specifically stated.  as long as one of the holes on each hub lies on the hub(s) centerline (12, 3 6 or 9 o'clock) and the spacing is noted (eq sp for example), then the block tolerance would apply.  if the holes are located with basic dimension(s), then the geometric control applies and the centerlines are assumed to be a basic 90 degrees.  it really is better to be safe and specify the clocking.  
heckler,
have ansi memorized eh?
namdac
no, but like a good engineer it's one of the tools in my toolbelt that gets used a lot next to roaks formulas for stress & strain & the machinery's hdbk.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 5.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
xp pro sp2.0
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
never argue with an idiot. they'll bring you down to their level and beat you with experience every time.