Position INDIVIDUALLY w.r.t. Datum Targets INDIVIDUALLY? 2

[SSO Contractor]

Hi,

Is there an issue Positioning FoS INDIVIDUALLY to Datum Targets INDIVIDUALLY? Similar to Figure 10-27 from the 2018 standard?

 

[3DDave]

For the vague question, vaguely, yes. How they are used affects whether it's a good use.

 

[CheckerHater]

I don't see anything looking like "Datum targets", but otherwise, it's legitimate way to make sure bolt/screw fits thru the counter-bore.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[SSO Contractor]

The image I posted is just the example from the standard I'm researching. I've used this before, but never with Datum Targets. For example I have critically positioned v-grooves that are oriented perpendicular to the center of optical alignment on a reflective surface. Each v-groove is accompanied with flexures, also perpendicular to this point of interest to make the alignment athermal (or unaffected by thermal cycling) and there's a tapped hole at the bottom of each v-groove. I'd like to use Spherical Datum Targets e.g., A1, A2, and A3 for the v-grooves themselves, add the note INDIVIDUALLY next to each and also adjacent to each Position FCF for the flexure width and the tapped hole on the Induvial basis, since it's is most critical that the flexure and tapped hole are always aligned with each v-groove independently. I can't why this wouldn't work, but it is unusual.

 

[Burunduk]

If one spherical datum target is all you are going to use in each v-groove, it won't constrain a sufficient amount of degrees of freedom for the location and orientation of each corresponding tapped hole. A sphere constrains zero rotational degrees of freedom.

 

[SSO Contractor]

Good point about the spherical degrees of freedom. I could establish orientation with a second FCF, Perpendicular to Datum A collectively. I could also make the tapped holes the Datum Features and control other features relative those and an additional datum feature perpendicular to the tapped hole pattern. I didn't do that initially because I wanted to be sure to use the v-grooves as a Datum Feature, but using a set of v-groove already controls all 6 DoF which would make me choose something other than the v-groove for the additional Datum Feature. Thanks for the feedback.

 

[Double-Edged]

It's legitimate however I suspect an over specification given that the counterbore is specified with a very generous tolerance and MMC. Every hole - counterbore requires a new setup to measure and verify which is time consuming.

If the clearance between whatever and the counterbore hole is sufficient then a single datum reference is likely adequate.

 

[SSO Contractor]

Thanks for the input Double-Edge.

The image I provided might be causing some confusion and respecting the fact that I can't show my actual work, I simply grabbed this from the Y14.5-2018 standard as an example. But if you can imaging the thru hole being my tapped hole (1 of 3) and the Counter-bore being a v-groove (1 of 3) and those are both machined through a flexure (1 of 3) 120 degrees from one another around a fold mirror axis. The v-grooves need to be tightly toleranced in location and orientation. The tapped hole and flexure just need to be on center of each v-groove and both features are unrelated to the other 2 sets. My thought was to actually simplify the inspection by allowing a Gauge 'Plug' (Go No-Go) that can simply pickup the 3 features that are in-line to each other, and that can be done individually 3 times. If you take the image I posted as an example, you don't necessarily need to move the part to inspect the counterbores, put rather you could have a Gauge Plug that essentially looks like a featureless screw (shaft to pickup the MMC of the thru hole, and a larger diameter head that picks up the coaxial relationship and FoS of the of the counterbore at MMC. My functional gauge plug would be similar, but it would have a sphere to pickup the v-groove, a shaft protruding out to pick-up the Minor Dia of the tapped hole, and then 2 parallel plates to pickup the width and centering of the flexure. I think this is an interesting method of dimensioning because, if done right, could potentially ease the inspection process, loosen tolerances, and it also tells a story about the parts functional requirements by practically saying this feature is only related to this feature and does so very specifically.

All of that said, I'm also not sure this is what I will do. I could take advantage of default Simultaneous Requirements and simply reference the same datums, in the same order, with the same Material Boundary with individual FCF callouts for each feature. That will require a tightening of tolerances, makes tolerance stack-ups a bit more tricky, and it does not identify the functional relationships as clearly, but it is generally more understood and that is worth consideration. So far, everyone I've talked to about using the Individually method has never done it and isn't crazy about it. I will likely respect that even if I think this method makes sense and is under utilized, which makes this feedback meaningful. Thanks again.