![]() As CH mentioned, the origin of the DRF can be at the center of the pattern of simulators, or at some other location defined by basic dimensions. The important thing is that the datum reference frame is a coordinate system that is defined in the pattern of simulators. It should be an axis and centerplane, as shown in Figure 4-3 f) in Y14.5-2009.Īt the end of the day, it doesn't really matter what the datums are. But if the datum feature is a pattern of holes that constrains 3 degrees of freedom, the datum can't be a single axis. If the datum feature was a single cylindrical hole, the datum would be a single axis and would constrain 2 translational degrees of freedom. The misleading part, that I believe is the root cause of a lot of the debates, is that the figure shows "Datum Axis B" as the datum for the 4-hole pattern. This is very similar to your original example, except with 4 holes instead of 2. Section 4.12.3 in Y14.5-2009 describes a datum feature comprised pattern of features of size at MMB. In particular, the step of defining datums becomes difficult and somewhat arbitrary. This concept is workable and intuitive for very simple datum feature types and combinations, but doesn't hold up well when applied to more complex configurations such as hole patterns. Part of the issue lies in Y14.5's basic datum reference frame concept of a three-plane coordinate system built on datums (theoretically exact plane/line/point geometry extracted from datum feature simulators). The explanation is better in Y14.5-2009, but there are still some misleading things. If you look to the standards, there were some very misleading explanations in ASME Y14.5M-1994 that (IMHO) caused a lot of confusion in industry. The debate at your workplace is just the latest in endless debates on the question of "where is the datum for the pattern". RE: Common Datums CheckerHater (Mechanical) 14 May 14 09:40 It certainly isn't clear how to calculate the effect of variations in parts based on the given FCFs. It isn't clear that there is any value to adding the reference to the compound of the two datums. The parallel holes do not share this characteristic. Obviously (or not) one has to accept that part of the surfaces interfaced to the chucks will not be available for inspection, but just as not every single atom is checked on a surface anyway, that the parts are sufficiently uniform that the missed areas can be judged adequate. Once they are aligned one can check runout to the common axis in a way that mimics hundreds of millions of electric motor and gear shaft installations. ![]() Clamping on them both orients the part in a way that using either one by itself would not and simulates a commonly used bearing support. A practical application is to have two nominally aligned jawed chucks, one clamping on C and the other on D. To measure runout one needs an axis to rotate about. Thank you RE: Common Datums greenimi (Mechanical) 13 May 14 13:27 Could anyone shed some light on this, or perhaps show me a published example? Unfortunately, I cannot find an example in either the ASME GD+T standard or the ISO standard which can confirm or deny that either way is factual - almost every example I've seen only relates to 2 planar surfaces or 2 nominally coaxial surfaces, not 2 holes at some distance apart. Previously, we've been operating under the assumption that it only defines one plane, however reccently I had a conversation with an outside source that claimed that because the datum simulator would basically be 2 pins, it in fact defines 2 datum planes. The question we're debating is if B-C defines 1 datum wherein the axis of both B and C must lie (call it a horizontal datum), 1 datum between the axes of B and C at an implied center(call it vertical), or actually 2 datums orthagonal to eachother (controls both the horizontal and vertical datums). I have another feature with a positional tolerance with respect to A and a common datum defined by B and C (B-C). Each hole is specified as a datum (in this case, datums B and C, where datum A is the large flat face of the plate). Suppose I had a piece of plate with 2 holes in it. We're having a bit of a debate here at work.
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