[To...] Posted June 2, 2021 Share Posted June 2, 2021 At the risk of starting another discussion as large as a "war and peace" novel, I'd like to get your opinion on the following datum reference frame. In my opinion, (and that of Calypso) Datum Feature A will control 4 degrees of freedom and Datum Feature C will only control rotation about A. However, I'm guessing some, or one, of you is going to present an argument that Datum Feature C is also take away some responsibilities from Datum Feature A.Screenshot 2021-06-02 111608.jpg Link to comment Share on other sites More sharing options...
[Cl...] Posted June 2, 2021 Share Posted June 2, 2021 I will say this, in my opinion Datum -C- looks to be more substantive or robust than -A- does. I think -A- makes for a weak datum. Link to comment Share on other sites More sharing options...
[Th...] Posted June 2, 2021 Share Posted June 2, 2021 Well, unless the C datum feature is used at a higher precedence than the A datum feature, it can't "take away" responsibility, can it? Unless there's something I'm not aware of, the primary datum feature will always constrain every degree of freedom it can, irrespective of the datum features that follow it. Please sign in to view this quote. I would agree with this, though. Link to comment Share on other sites More sharing options...
[SH...] Posted June 2, 2021 Share Posted June 2, 2021 What is the order?? ACB?. Link to comment Share on other sites More sharing options...
[Th...] Posted June 2, 2021 Share Posted June 2, 2021 Please sign in to view this quote. Without any other info, I would *assume* the order is A|B|C since that tends to be how I've seen datum feature letters ordered. Makes sense intuitively as well since it follows their order in the English alphabet. Although, in this case I'm not sure the order of the secondary and tertiary actually makes a difference. A|B|C and A|C|B should be equivalent. Link to comment Share on other sites More sharing options...
[An...] Posted June 2, 2021 Share Posted June 2, 2021 _ Link to comment Share on other sites More sharing options...
[SH...] Posted June 2, 2021 Share Posted June 2, 2021 Please sign in to view this quote. Alphabetical order is not a mandatory requirement, it is upto the designer.... Link to comment Share on other sites More sharing options...
[To...] Posted June 2, 2021 Author Share Posted June 2, 2021 A-B-C Sorry, I left that out. Link to comment Share on other sites More sharing options...
[Ow...] Posted June 2, 2021 Share Posted June 2, 2021 There are no stupid questions. However, KISS, keep it simple stupid 🤣 . https://www.youtube.com/watch?v=diu8i5UKTHc Link to comment Share on other sites More sharing options...
[To...] Posted June 2, 2021 Author Share Posted June 2, 2021 I was singing that in my head when I wrote it. Curses you Owen. Link to comment Share on other sites More sharing options...
[An...] Posted June 3, 2021 Share Posted June 3, 2021 _ Link to comment Share on other sites More sharing options...
[An...] Posted June 4, 2021 Share Posted June 4, 2021 _ Link to comment Share on other sites More sharing options...
[Ma...] Posted June 4, 2021 Share Posted June 4, 2021 I would preffer A|C|B or A(M)|B|C to make sence. I see datum A setting rotational axis, datum C for locking rotation and datum B to lock last axis. Datum B possibly could lock rotation, but in real world it would be meaningless in usage. Link to comment Share on other sites More sharing options...
[Da...] Posted June 4, 2021 Share Posted June 4, 2021 Please sign in to view this quote. Andreas, Y14.5.1 is clear about the effects of rocker conditions, or other datum feature form states that create multiple candidate datums. This explicitly includes tapered, coned, and barreled cylinder datums. (Y14.5.1-2019, Section 4.7). Datums lower in the precedence order are not permitted to constrain DoF that are assigned to higher order datums. The datum simulators are to be basically oriented and located to the candidate datum of the higher order datum, and they are applied in the order of precedence. For cylinders at RMB, the candidate datum set is the set of all axes of datum feature simulators that satisfy the requirements of the expanding datum feature simulator. That means that, where there are multiple axes where the DFS maxes maximal contact with the datum feature, then all of those axes are valid candidate datums. For each higher order candidate datum, the next datum feature simulator is applied, potentially itself generating multiple candidate datums. The same process is performed for the tertiary datum. This process generates a set of candidate datum reference frames. The "actual" datum reference frame is the one in the set of candidate datum reference frames that generates the lowest actual value for the feature, or set of features, in question. Link to comment Share on other sites More sharing options...
[Er...] Posted June 4, 2021 Share Posted June 4, 2021 Now the real question is, what exactly does the design engineer want, not what was drawn up on a piece of paper??? 😃 Link to comment Share on other sites More sharing options...
[An...] Posted June 5, 2021 Share Posted June 5, 2021 _ Link to comment Share on other sites More sharing options...
[Da...] Posted June 5, 2021 Share Posted June 5, 2021 Please sign in to view this quote. If I am interpreting your image correctly - For a cone-like shape of perfect circular form, you are correct. The axis of a RFS datum feature must be perpendicular to the section that has perfect circular form. As you deviate from that perpendicular axis, the actual mating envelope will get smaller. Angle alfa (the orientation of the axis) will in this case be well defined by datum feature A. Your psudocode seems correct in intent. However, there are actual geometries that do not have well defined axes orientations. For example, given a nominal cylinder with actual geometry that is elongated, ie: a slot, with otherwise perfect form. The actual mating size will be defined only by the width of the slot, leaving the orientation to be constrained to a range of values by the width of the slot. Link to comment Share on other sites More sharing options...
[An...] Posted June 7, 2021 Share Posted June 7, 2021 _ Link to comment Share on other sites More sharing options...
[Da...] Posted June 7, 2021 Share Posted June 7, 2021 Please sign in to view this quote. Andreas, you are going to need to provide a little more context for your image. There is a lot of stuff going on there, and I am not really sure what point you are trying to make. What do you mean by 100% ton and 50% ton? For the second image (sheet 1, 50% ton) - I am not totally sure I am interpreting the image correctly. It looks like you are implying that datum C is locking the "out of the screen" rotation axis of the pin. Which, as I mentioned before, is not compliant with the standard. Link to comment Share on other sites More sharing options...
[An...] Posted June 7, 2021 Share Posted June 7, 2021 _ Link to comment Share on other sites More sharing options...
[An...] Posted June 8, 2021 Share Posted June 8, 2021 _ Link to comment Share on other sites More sharing options...
[Da...] Posted June 8, 2021 Share Posted June 8, 2021 Andreas, if you are going to post an 18 page document, you really should be more clear about your point. I am not sure if you are disagreeing with me or not. Sheet 1: if D1>D2>D3, then only the D1 axis is a valid datum. Sheet 2: Least squares axis has nothing to do with it. Sheet 3: Yes, the standard is clear. If you have questions, I am happy to elaborate. I can't post the actual text, for copyright reasons. Sheets 4-17: This is not what the standard requires. Datum C ONLY controls rotation around the axis of A. I don't deny that this is often what actually happens in hard gauging. But that is part of why hard gauges should have reduced acceptance zones - to account for uncertainty in gauging. Datum C can not rotate the part in any axis other than the axis of A. To do so would mean the mating size of A is reduced, and therefore the axis is no longer valid. Sheet 18: The right side is not A|B|C. It is C|B|A. If the gauging uncertainty allows you to align the part that way, and still claim compliance to the drawing, than that is a risk that may be a valid business choice. But it is not compliant to the standard. Link to comment Share on other sites More sharing options...
[Re...] Posted June 8, 2021 Share Posted June 8, 2021 I'm having a hard time comprehending most of this discussion. I could probably get there, but not without my head hurting. I'm glad that I only have a couple of years until retirement. Link to comment Share on other sites More sharing options...
[An...] Posted June 9, 2021 Share Posted June 9, 2021 _ Link to comment Share on other sites More sharing options...
[Me...] Posted June 9, 2021 Share Posted June 9, 2021 Please sign in to view this quote. What if the -A- was a cone? It would still restrict 4 degrees of freedom, no? Link to comment Share on other sites More sharing options...
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