SeasonLee
Mechanical
- Sep 15, 2008
- 918
Please advise if it is correct on the FRTZF tolerance of position calculating, without any reference datums on the lower segment.
Thanks
Season
Thanks
Season
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3DDave 10 Dec 18 17:02 said:then it's up to the QA guys to figure out if the way the chip is described on the data sheet is compatible with the callout on the drawing.
3DDave 9 Dec 18 15:51 said:if the whole pattern of sensors shifts as a block it will increase the difference in pickup location as the entire group drifts off axis, so I fail to see the benefit to a composite tolerance.
That is still not clear to me. Separate with no datums or separate with reference to A|B(M) or separate with reference to A only?3DDave said:pmarc - separate, as in it's own FCF.
Really? That would be the only way to accept such a part? Besides... would you really like to use a physical gage with pins to check the FRTZF requirement - the requirement with a tolerance value defined at RFS or LMC?3DDave said:I suggested that the ONLY way to accept such a part might require a large rotation for evaluating the two segments of the one composite frame. That is, if one made a physical gage, the datum feature A could sit at a significant angle to the axes of the pins, which means that the pins would have to be long enough to accommodate that orientation across the width of the part. While an academic curiosity, it would be an inspection nightmare.
Too bad designers don't use this kind of callouts more often (when functionally reasonable). Too bad suppliers often don't realize that this kind of callouts gives them more freedom in making the part. Too bad inspectors often don't try to make their life easier in the first place by checking this kind of callouts as if A was referenced in the FRTZF (because if it passes the requirement with A added, it will definitely mean that it passes the datumless requirement). Too bad words such as 'stupid' are used in a conversation between two MVP members of the forum in a hope that this will help to win a debate. Really too bad.3DDave said:OTOH this is exactly the stupid callout that should result in suppliers making conformant parts that force the buyers to spend a ton of money to inspect. Too bad suppliers never do and so this bad callout gets onto drawings unchallenged or even supported.
Does "all by itself" mean that this would be the only position callout applied to the pattern of lightening holes?3DDave said:pmarc [pos|dia.25(L)|A] on a line all by itself, if wall thickness is so important.
It would be also much easier from inspection point of view if, instead of using secondary datum pattern at MMB, the side faces were defined as datum features B and C. It would be even better if the holes (as relatively irrelevant for function of the part in assembly) were located with +/- dimensions. This would make the part the simplest and cheapest to inspect. Should I change the drawing accordingly?3DDave said:Good point - it's going to take hours to check RFS and LMC on a CMM for holes that are only there for decreasing the weight of the part. Makes it a perfect choice to allow parts through that cannot be handled with a simple optical comparator, but will require a skilled CMM operator to see if it can be accepted.
I would say even smarter thing is to make drawings that in addition to accepting what will work, will not to reject what might work. Your proposal of adding A to the callout does not work that way.3DDave said:The smarter activity is to make drawings such that part are simple and cheap to inspect while also accepting what will work.
pmarc said:It would be also much easier from inspection point of view if, instead of using secondary datum pattern at MMB, the side faces were defined as datum features B and C. It would be even better if the holes (as relatively irrelevant for function of the part in assembly) were located with +/- dimensions. This would make the part the simplest and cheapest to inspect.
pmarc said:Too bad words such as 'stupid' are used in a conversation between two MVP members of the forum in a hope that this will help to win a debate. Really too bad.
3DDave said:mkcski - what is important is the location of the hall-effect element buried in the epoxy shell. The most likely indicator of it's position is the lead frame which is what it is attached to before the epoxy is injected. The makers of the chips sometimes refer to the center of the chip as the location of the hall-effect element - so it's one electrical industry standard way to refer to it.
3DDAVE 10 Dec 18 19:47 said:The pattern can be defined with separate radial and angular position tolerances, allowing a significant rotation about the circuit board axis of the chips as a group, with smaller angular differences between them, while limiting radial travel to a narrow band. At least with separate FCFs for radial and angular, one could use faces or widths of any number of features on the chips.
3DDave 10 Dec 18 19:47 said:If free rotation of the package about the invisible hall-effect element is to be allowed, and estimating the center isn't, then an electro-magnetic test is required as no FCF will be suitable to describe the required performance.
3DDave said:At this point I go with what I have hinted at before, that there is an electrical performance specification without any FCFs at all.
chez311 said:Another could be the center of the circumscribed circle/cylinder around the points of the rectangular chip - assuming the sensing element is directly in the middle.