datum shif
 

datum shift ?
i have one dwg which has 3 holes on the plate.
in a dwg, they defined a
datum point a1, a2,a3 as the centre point of the hole.
datum b is defined as centre plane of one of the hole
in a feature controlled frame for datum b
block 1 has 0.5 positional tolerance with 0.5 mmc
block 2 has mentioned datum a
datum c defined as centre plane of one of the hole & there is attached featured controlled frame
blcok 1 (in feature control frame) has 0.5 mmc of positional tolerance
in block 2 there is only mentioned datum a
block 3 has datume b with mmc condition
now my question is
1, there is nothing like datum a defined but only the datum points a1, a2, a3 so how could i define the datum b& datum c using reference from datum a?
2. while defining the datum c ; what does the mean of block 3 in feature controlled frame " datume b with mmc condition" ( i think it is a datum shift - but not sure about the understanding or how to calculate tolerance stack)
could you explain me in detail.
thank you very much in advance,
datums b and c do not appear to be correctly defined. what exactly is "the center plane" of a hole. how is it oriented? how is it inspected? the part may be acceptably dimensioned if it weren't refering to those center planes. datums b and c should be the hole features, not some arbitrary planes along their centerlines.
if your part consisted of irregular surfaces (as opposed to a flat plate), then datums b and c need to reflect flat planes. datum a is correct in as it defines the primary datum plane consisting of three points. datum b should be the secondary datum consisting of two points, and should be perpendiculat to datum a. datum c should be the tertiary datum, consisting of one point and mutually perpendicular to datums a and b.
as to your second question, datum c is being defined as having 0.5 mmc relative to datum a (regardless of the material condition of datum a) and datum b (at mmc of datum b). this allows additional tolerance to be added to the positional tolerance of datum c as datum b approaches lmc.
here is how i would approach it (others may have better suggestions). if your part is a flat plate, and you are only trying to control the hole pattern and aren't concerned with its location relative to the part edges, i would define datum a as one of the surfaces. make one hole datum b and control its perpendicularity to datum a. make the second hole datum c, true position relative to datums a and b. the third hole would be true position to datums a, b and c.
a couple of additional comments...
datum a would have no modifiers as it is a plane.
datums b and c can be defined using the points that established datum a because those points in and of themselves are not datums. datums b and c are relative to the plane established by those points, not the points themselves.
having datums a1, a2, a3 is confusing. i have never seen that before. which one is the primary datum?
chris
sr. mechanical designer, cad
solidworks 2005 sp0.1
pts a1, a2, and a3 are not datums. they define datum plane a.
thanks for clarifying
chris
sr. mechanical designer, cad
solidworks 2005 sp0.1
thank you very much for your post,
let me define datum again as i did little bit mistake last time:
datum points a1, a2 & a3 are the centre point of hole
but, all the holes are at different level (means at different height)
datum b is attached the hole dia of one of the hole
block 1 has perpendicular toelrance
block 2 has diametric symbol, 0.5 with mmc
block 3 has mentioned datum a
datum c is attched to the one of the hold dia
block 1 has positional tolerance symbol
blcok 1 (in feature control frame) has 0.5 mmc
block 3 there is only mentioned datum a
block 3 has datume b with mmc condition
1. now, my question is as you mentioned that datum b is perpendicular to the datum a which is not the case b'coz all datum points a1, a2 & a3 are at different height ?
2. generally, i didn't see modifier with primary datum plane - does any reason for that?
3. also, if you able to understand the datum b & c - could u explain & also let me know if they are appropriate?
4. i understand primary, secondary & tertiary datum plane, but when i see the product its difficult to say anything so if you could give info about how to recognize datum & how to define datums while designing?
thank you,
that person has used their imagination of how the gd&t works instead of being actually knowledgable and fluent. i see this silly stuff all the time. found the best approach is ask them to point out the relevant sections of the asme y14.5-1994 book that justifies their technique of application, which they can't do. usually i find if they even have a copy of the asme book it's sitting on a bookshelf collecting dust but otherwise in pristine condition from never having been opened. unfortunately applying it poorly is apt to be more confusing then not using the gd&t system at all.
-keith
i have to agree with type26owner that this tolerancing scheme is apt to add confusion.
if datum hole b is to be perpendicular to datum a, and the points defining datum a are at differing heights on your part, then it cannot be normal (perpendicular) to the part surface. this means that your holes are angled relative to the part surface.
primary datum planes do not require a modifier. modifiers are limited to features of size, such as your datums b and c. your datum a is not a feature of size, but a perfect plane through 3 points.
if the intent is to have the holes skewed, then datums b and c are called out properly.
the best approach to defining datums while designing is to use the primary, secondary and tertiary scheme, while considering how the part will be made and inspected. it is also very important to recognize the function of the part itself. if you establish a, b and c using features that are easily made and inspected, you can always add additional datums to control features which are critical to the parts function (such as controlling runout on a drive shaft or the position of mounting holes relative to a pattern center).
i hope this helps you to understand gd&t a little better. the best way to understand it is to use it, either with hands-on machining experience or inspection experience. design experience is also a good way to learn it, as we don't all have the opportunity to get our hands dirty.
i don't know if you folks are aware of the appendix e section of the asme book. there is an extremely handy condensed tool there that's called the 'decision diagrams for geometric control'. it should help here somewhat. suggest you peruse fig e-7 for the essence of datum selections.
i've copied those seven pages and have them hung up above my monitor because i got tired of opening up the book.
-keith
par1,
let's see if i can interpret this datum scheme properly and work out an inspection fixture.
datum a is a plane defined by the three datum targets. a1, a2 and a3 are called datum targets. these would be modeled by three pins, slightly bigger than your holes, sticking up out of a base, each with its own elevation apparently. your part would sit on top of them, oriented to the same orthogonal coordinates as your drawing.
datum b is the hole. since it is inclined with respect to datum a, the actual datum point is the intersection of the hole's cylinder with the datum plane. this is actually not an issue, since a vertical pin models it correctly. this locates your part in x and y.
datum c is the other hole, which keeps your part from rotating. this would be modeled by a vertical diamond pin.
the only deficiency of your drawing is that the datum targets ought to be specified with a diameter. the actual contact point, the centre of your hole, does not exist, at least, not in the sense of possessing material. your drafter needs to define the diamter of the a1, a2 and a3 pins.
jhg