huangyhg

quiz on straightness of line elements
i ran into a quiz about the straightness control. i can not understand one of the answers. this is a simple quiz. i illustrate it below (with question and answers separated in case you want to test your selves) the quiz shows a bar with a diameter of .495 and tolerance of +/- .005 (so mmc is .500 and lmc is .490) and a straightness control of .004 on the surface. of course, as you would expect, there is no material condition or datum references in the feature control frame, since they would not be appropriate. the quiz then asks about the available tolerance if the actual size is .500, .492, .501, .498, .490, and .495
if you want to try this quiz first yourself stop reading here!
now, the answer which i don't understand is when the actual size in .498. to me, it seems that the form tolerance can be from .500 to .496, this is within the size tolerance and correspond to maximum form tolerance of .004 but this is not the answer. i appreciate your help in understanding this.
the answers given for the quiz are:
.000, .004, rework, .002, .004, .004
thanks
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you can apply straightness as rfs or mmc. this condition was applied at rfs so anything over .500 is rework. the collective effect of size and form variation can produce a virtual condition equal to the mmc size plus straightness tolerance. when applied at rfs the maximum straightness is the specified tolerance plus the amount the feature departs from its mmc size.
if this example was taken with straighness at mmc then as the feature departs mmc towards lmc then the diameter tolerance zone increases.
i hope this helps
best regards,
heckler
sr. mechanical engineer
sw2005 sp 5.0 & pro/e 2001
dell precision 370
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heckler,
thank you very much for your response. i don't agree with some of the points you made. below, i quote from you message(the text in quotaion mark) and add my comment below them:
1. "you can apply straightness as rfs or mmc"
this is true only if the staightness is applied to the feature of size, if applied to feature (surface) you can not use in material condition modifier ( so it's ffs)
2."when applied at rfs the maximum straightness is the specified tolerance plus the amount the feature departs from its mmc size."
when applied at rfs the maximum straightness is always the specified tolerance.
i still don't know how you can calculate an available tolerance of .002 when the actual size is .498 can you show the steps of calculations?
yes, you're correct. i assumed you applied it to the feature not the surface. do you have a copy of the spec? they show you have to manage the tolerances.
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
_`\(,_
(_)/ (_)
"ns/fs" does not actually mean anything in any actual drafting standards. it's only widely accepted drafting folklore. - thetick

you indicate the straightness is on line elements. that being the case, should not the straightness be .004 for all cases?
is it a fair question to ask whose test this was?
ringman,
thank you for responding. the test is from a text book by e. max raisor. i thought about this and .002 is the right answer. the reason is that the standards, rather arbitrary, distinguishes between when the control is applied to the feature (that is the surface) or feature of size (i.e. to the diameter). compare figures 6.1 and 6.2 in asme y14.5m-1994. in the first case the first rule is not overridden, while the second case overrides rule number. therefore, in the first case the virtual condition is the mmc so when the actual size is .498 you cannot have more than .002 variation. otherwise your virtual condition will be more than mmc which overrides rule number 1. i am pretty convinced that i came to the right conclusion. please let me know if you don't agree with me (and definitely check those figures in the standards).
i agree with your last post because you are right that you have to distinguish whether or not the straightness tolerance is being applied to the surface elements or the centerline of the part. that in turn will tell you whether or not you can have a virtual condition greater than the mmc boundary.
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