【转帖】shaft datum as a-b set as primary control within a-b

yang686526

shaft datum as a-b set as primary control within a-b
if i have a shaft that has one end set at -a- and another feature set as -b-
i then define my primary for other features to reference a-b
can i set a control to define the position of b in reference to a?
or would this not be acceptable since a-b is the datum for other features.
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"can i set a control to define the position of b in reference to a?"
yes
i would suggest that if this is a relatively lone shaft, i probably would not use position but maybe circular runout of one end to datum a to create b if that is what you want.
if i saw a-b on a shaft, i would simply place datum a in a chuck but not too far in. i would then use a digital indicator on datum b and "true" it up to datum a. you now have an axis created by a-b.
dave d.
you can control datum feature-b relative to datum-a, or you can control both datum features relative to datum a-b.  this tends to cause a bit of a brain-implosion for many people, the idea of "referencing a datum back to itself" but you're not, you're controlling a feature relative to a compound datum.  this is actually a great way to relate two coaxial features back to the overall datum.
the reality of fixturing for a compound-datum axis may be different, but the principle is that you have two "perfect" chucks or whatever that will simultaneously close around the respective features while maintaining a coaxiality of the chucks; the resulting axis of the chucks will simulate the datum axis.  in practice, i would tend to use an inspection package that can probe/scan the two datum features to generate their individual axes, and resolve within the software the compound datum.
i have a concern, dave, that your methodology would result in datum-a dominating the compound-datum a-b, which effectively eliminates the contribution of -b.
jim sykes, p.eng, gdtp-s
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jim:
this certainly does get confusing but what you are saying conforms the asme y14.5m-94 as per page 193 in the standard.
but:
datums are either designated or they must meet some criteria (i call this qualifying a datum). when they are designated, what you see is what you get as long as the feature meets rule #1 if it applies. one would set up as if the feature was perfect.
when one qualifies a datum, it must meet a certain criteria to become that datum. if the feature does not meet the criteria, you do not have the datum. once it meets the criteria, one must then assume then assume it is perfect.
datums are like building a house. one cannot place a roof on it first. we must prgressively build up the datum set up. we cannot or should not reference a datum that we do not have yet.
page 193 is not consistant to this concept. one either designates a datum or qualifies the feature but on page 193  (and in your example)it does not follow either. is the datum designated? or does it have to been the circular runout tolerance to become the datum?
i hope that asme deletes this page to be consistant but you are correct as far as the standard is concerned.
please designers, do not place this on a drawing. yes, the shop floor people would implode but worst, we would like to ask you to explain it.
as far the measuring method on the shaft, both datums a & b have equal importance a-b so that i would chuck on one (don't care if it is a or b) a small distance in from the end and true up the other end the same distance in on the feature.
hope this helps and not confuses.
dave d.
dingy,
i believe your point is well made. the example on 193, effectively creates 2 axes, c and d, rather than points, which theoretically never would align.
ringman:
the asme example on page 193 reflects a single axis created by datums c and d but, as i stated before, are they designated or qualified in some manner?
dave d.
dingy,
i understand the intent of the illustration.  however the datum features are cylinders, providing axes as the datums.  it becomes difficult or impossible to average them out into a single axis does it not?
i believe there is a similar problem that occurs when we attempt to orient planes to a slotted feature.
ringman:
an axis is developed with 2 points or sections on a single cylinder and i would use a pair of v-blocks towards each end but not exactly on the end.
when one has 2 different coaxial diameters to develop the axis, we would not average the sizes but select a segment or cross section location on each datum for set up purposes.
the example on page 193 of asme, i would set up using a chuck on the end diameter with very little material in the chuck. i would then select a segment or cross section location of the other diameter and, using a dial/digital indicator on a base, true this up to the end.
thus we have c-d.
my original question is still valid. in 6-51 page 193 of the standard, is datum c designated or qualified using circular runout and cyclindricity??
dave d.
dingy,
i believe that by the definitions it is designated.
im not clear as to the implications of qualified, but would suppose that the modifiers would be considered as qualifiers. perhaps the size tolerance too.