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Can we add MMC modifier on a datum feature estabilished by datum target point?
- Thread starter SeasonLee
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If you want to talk about machining set up in terms of material boundaries then any type of clamping/gripping of a workpiece is "RMB", regardless of the datum feature defined with datum targets or just "conventional" datum feature symbol at RMB, MMB, or LMB.
Obviously, that doesn't affect the fact that the part should be inspected in a fixture according to the material boundary modifiers applied to the datum feature.
Obviously, that doesn't affect the fact that the part should be inspected in a fixture according to the material boundary modifiers applied to the datum feature.
I haven't replied for a while but here goes. The simple answer is yes but why isn't the 1.734/1.797 diameter shown as datum B?. The datum targets are great but datum B at a.797 is its maximum material size. Go for it but please show datum B as the diameter.
Dave D.
Dave D.
I was initially thinking of it from a functional perspective, as in I couldn't see how function would dictate such a requirement (MMB with datum targets) unless the mating/assembly condition mimicked the datum target setup. I guess I should have kept in mind your earlier post though.
If I'm to play devil's advocate I could see a potential use in manufacturing for a family of parts similar to Y14.5-2009 fig 4-53 but say with a larger diameter B and each similar part in the "family" of parts with increasing diameter B and hole position called out to |A|B(M)|. The part would be clamped to A at A1,A2,A3 and allow float to B with 3x fixed pins. These fixed pins would likely be less expensive than either a large precision fixed ring/bore of the size required for MMB simulation (note I said a larger diameter than the one shown for B - say something on the order of 300mm or larger) or an adjustable centering/clamping device for RMB simulation. Additionally they could be changed out on the same fixture and placed in different radial locations to accommodate different parts of the same family.
Note that of course the additional float due to MMB and the nature of datum targets (trefoil) would have to be accommodated for and perhaps require some additional calculation over a simple fully-formed fixed pin, but if the tolerances were loose enough it might be acceptable. I'm also not sure that this less predictable behavior would be worth it in most cases over the slight increase in cost/effort of utilizing some sort of centering device (say radially located spring-loaded pins) but perhaps the weight of a large part would either require extremely high spring rates or simply make such centering unreliable (including possible combined deflection of the part/fixture due to high spring rates).
All that in mind, I'm not saying its the best option - but I wouldn't consider it outside the realm of possibility, at least from a manufacturing standpoint.
I'd argue those aren't targets, which need to contact the item being targeted, but a special consideration which should be handled by a note, which is also what I did in my example as the tolerance zone I was establishing was a cone with an elliptical section at a particular major axis angle to the nominally horizontal plane for the part.
If it's for manufacturing purposes then specify the type of contact, noting that there needs to be some clearance at MMC/MMB to allow the part to be installed.
Not sure why, if a crappy partly controlled method is acceptable why a precision ring gauge would be required. Just flame cut a ring. As long as it is better than the acceptable unknown error it should be good enough. Even centering is easy - weld nuts to the ring and thread in bolts. Install the part and give each bolt a quarter turn until all are in contact. No need for precision as this is the basic argument for accepting MMB on targets, a desire for poor precision at low cost. Confirm the centering with a caliper and know the precision.
If it's for manufacturing purposes then specify the type of contact, noting that there needs to be some clearance at MMC/MMB to allow the part to be installed.
Not sure why, if a crappy partly controlled method is acceptable why a precision ring gauge would be required. Just flame cut a ring. As long as it is better than the acceptable unknown error it should be good enough. Even centering is easy - weld nuts to the ring and thread in bolts. Install the part and give each bolt a quarter turn until all are in contact. No need for precision as this is the basic argument for accepting MMB on targets, a desire for poor precision at low cost. Confirm the centering with a caliper and know the precision.
If those aren't targets because they don't contact the feature then what other simulator geometry/behavior is meant by Y14.5-2009 section 4.24.12 which specifically refers to 4-53 that "For MMB, the centering method used to establish the datum axis has a set of three equally spaced features set at a fixed radial distance based on the MMB" ?
A fair point, as I said I don't think its the optimal method in most circumstances it just occurred to me that I could see a case made for it. An advantage could be a modular and easy setup with 3x pins in different locations covering a large range of sizes, as well as not requiring any additional input from the operator to center the part (adjusting bolts and verifying with calipers) which would be more difficult if the feature was an internal blind bore.
A fair point, as I said I don't think its the optimal method in most circumstances it just occurred to me that I could see a case made for it. An advantage could be a modular and easy setup with 3x pins in different locations covering a large range of sizes, as well as not requiring any additional input from the operator to center the part (adjusting bolts and verifying with calipers) which would be more difficult if the feature was an internal blind bore.
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