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  1. Nescius

    Breaking apart a Cots part

    Apologies if my spitballing ends up as only a distraction; MRP issues are one of my least favorite things about engineering and I devote about as much time to their study as you'd expect. That said, I might try to squint my eyes and imagine the purchased item as a sort of raw material. A...
  2. Nescius

    Quality Control Process for China

    Depending on the nature of your specific product, my experience may or may not be applicable to your situation. As Prometheus21 essentially says above, there is no prescribed formula to ensure good parts. At best, even in US production of tightly regulated products, a mathematically airtight...
  3. Nescius

    Size, Rule #1, and parallelism

    It's worth the few minutes to sketch out the situation. Some trends shine through if you play with it. Roughly, this quasi-parallelism error varies from 1x to 2x the size tolerance as you move the crease from the end toward the middle. If you "go past the middle" so to speak, your quasi-datum...
  4. Nescius

    Size, Rule #1, and parallelism

    This quasi-parallelism error between two nominally parallel faces can exceed the size tolerance, even while local size requirements and rule #1 are satisfied. Imagine if the local size was near the smallest allowed, everywhere. Also, imagine if the part was "maximum banana" and barely...
  5. Nescius

    New to GD&T. Critique my drawing

    If you're talking about your thread over on...um...that social media site, you should be careful. I'm not referring to any specific advice you received when I say this, but that site is not a good source of engineering or machining knowledge. I don't want to mention the site by name. Getting...
  6. Nescius

    Numerical comparison of 14.5 between 2009 vs 2018

    3DDave, I generally agree with the sentiment of your post. In my opinion, the folks in charge of the standard deserve hard criticism; the standard is simply not very good. I will temper my rebuke a bit, though; it is not in the nature of a committee to produce art. Note, 3DDave, I'm only...
  7. Nescius

    Positional Tolerances

    That's another, equally depressing, "Triad of How Engineering Often Actually Works". 1. Mystery design, nobody knows why. 2. Tolerances tight to make it function. 3. Money, manpower, and sanity expended for the next 20 years. then... 4A. It was all a waste of time, or 4B. It was legit, design...
  8. Nescius

    Positional Tolerances

    I agree. Absent any other information, that number is unremarkable, not crazy at all. That said, your application may still satisfy the very depressing "Triad of How Engineering Often Actually Works": 1. We don't actually need such a tight tolerance. 2. The vendor has never actually met this...
  9. Nescius

    Customized DRF instead of a Translation Modifier

    A relevant thread, maybe: https://www.eng-tips.com/viewthread.cfm?qid=363878 In that thread, pmarc gives a link to another thread: https://www.eng-tips.com/viewthread.cfm?qid=215333 Finally, here's the Voelcker paper...
  10. Nescius

    Profile with all over and all around

    OK, you are buying bars of material. This is important. You are making a drawing for the extrusion supplier, and you probably do not care about the exact length of the bars. So, tolerances like all-over profile probably don't make sense here. Please clarify for me. How is your supplier using...
  11. Nescius

    Profile with all over and all around

    Hi Sa-Ro, In a way, 3DDave is asking: Are you buying a "part", or are you buying bars of extruded material? If you're buying material, talk to the extruder to see how they prefer tolerances to be specified. You might use notes to specify straightness per unit length and twist per unit length...
  12. Nescius

    Fixture design for inspecting part with datum targets

    pmarc and J-P, I'm referring to paragraph 7.24.9, not 7.20.2. Does the 2018 standard indicate "UOS" in both instances? Burunduk points out that similar language was actually in 2009 all along, although it references a depiction of square contact patches on a car hood in a section named...
  13. Nescius

    Fixture design for inspecting part with datum targets

    I wish I could dig deeper, but I don't have/use the 2018 standard. Of course, it sometimes would be reasonable to match the tooling to the basic shape of the part in the area of contact. However, mandating this is very limiting. A primitive thing can serve perfectly well as a locating stop...
  14. Nescius

    Fixture design for inspecting part with datum targets

    Many of us live in the land of gauge balls and gauge pins, where a CMM is simply not realistic. Likewise, a fixture with elements that mimic a complex surface may be orders of magnitude more expensive than an old-fashioned setup of precision primitive shapes. Bluntly, that language from the...
  15. Nescius

    Fixture design for inspecting part with datum targets

    Hi Belanger, I don't have the 2018 standard. Does it allow for deviation from the language you cited? I would be very disappointed if the most recent standard mandates that a counterpart MUST mimic the surface it touches. See Figure 4-47 in the 2009 standard for an example where straight...
  16. Nescius

    Fixture design for inspecting part with datum targets

    ...the issue of checking the part in a restrained state, as that's not something I have experience with. The nature of the thing that touches the part* (I'm assuming a target area in your case) can be whatever you want it to be, including a complex surface. Depending on your application, the...
  17. Nescius

    TP without a basic dimension?

    Loosely speaking, pmarc wants a (datum) point to anchor infinite lines, each line containing one of the infinite pairs of opposed points on the nominally spherical surface. Loosely speaking again, Burunduk says "no problem", there are an infinite number of candidate anchor points along the...
  18. Nescius

    GD&T symbols themselves - is there any official guidance about their shape and proportions?

    Interesting, 3DDave. That tool may be useful for some quick and dirty image generation. It could be a reasonable alternative to building FCFs in a CAD program and exporting as an image. As you point out, though, using the font outside of that application can turn into a massive kluge. That's...
  19. Nescius

    GD&T symbols themselves - is there any official guidance about their shape and proportions?

    I want complete control over the aesthetics and very few constraints on how I make use of the symbols. The sky is the limit. Go wild. Any conceivable special diagram is possible because you're literally describing an image with typed code. Again, though, it's also for fun.
  20. Nescius

    GD&T symbols themselves - is there any official guidance about their shape and proportions?

    It's a very low priority project for me, but I'd like to be able to seamlessly prepare written documents with symbols, feature control frames, etc. (inline with text, in figures, tables, wherever) So, I'm building a LaTeX package of shortcuts to render GD&T symbols with the TikZ package. This...

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