huangyhg

reultant condition boundary
hi,
i am reading an article that describes the resultant boundary condition. it says for an internal feature of size controlled at mmc:
resultant condition = lmc size limit + geometric tolerance + size tolerance.
for example for a .511 +/- .003 hole with the following feature control frame:
position symbol - phi .005 m - a- b m
(to show the feature control frame in text, i used m for mmc symbol, and used - to separate each box.)
the resultant condition is calculated as
.517+.005+.006 = .528
to me, it seems that since we accounted for size tolerance of .006 when choosing lmc, we should not use it again in the formula, that is, the resultant condition should be .517 + .005 = .522
i appreciate it if somebody can point out to me what am i missing. why should we include the size tolerance in the formula when we are using lmc to calculate the resultant condition.
thanks.
i figured out the problem. it is not very hard to figure it out once you make a sketch of the situation.
i am struggling with resultant condition myself and i don't see how they calculated it either. it seems to me that lmc would be .514, not .517. could you please explain?
alithepro and ktlasers,
the calculation of resultant boundary condition makes complete sense to me. your positional tolerance applies at mmc. when your hole is larger than minimum, the positional tolerance increases. at lmc, your positional tolerance is .005 plus the total tolerance, which is 2x.003". the resultant boundary is the area that may be occupied by your hole.
i like to specify tolerances this way, except for one thing. if i am going to call up mmc, i do not see the point of specifying a positional tolerance of anything other than zero.
phi .517/.503
position symbol - phi 0 m - a- b m
note how i have reduced the minimum diameter. i have defined a hole which must provide a clearance of .503" at the nominal position. presumably, this is what i actually want. my resultant boundary condition is still .528" as noted above.
jhg
a positional tolerance of zero is very handy, but i've had to change it several times because it seems that few really understand it. "how can you have a tolerance of zero? we can't afford that!"
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