datums in drawings and aligenement

[Mranaskw]

hello

how do the datums serve in drawing if we can chose our own aligenement features , because when aligening a part i hv never been based on the drawing on where to pick the plane , line and point , so for what does it operate

tnx

[Schrocknroll]

Datums are for GD&T callouts. If you use legacy dimensions you will need to align to the datums. If you are using XactMeasure it's not necessary but doesn't hurt.

[Mranaskw]

excuse me sir , but im awfully a beginner can u please explain to me the terms legacy and XactMeasure , because i have no idea about what they are , thank you

[KIRBSTER269]

When you create a manual alignment, all you're doing is helping your CMM locate the part, nothing more, nothing less. When you bring in a CAD, that doesn't mean the CMM see's it. Even when you bring it on the screen, What do you see behind the CAD model? nothing but space, that part is out in space nothing controlling it. Once you help your CMM find X, Y, and Z, that still doesn't mean it can see the part, just means you have established the 0's in relation to the part. That's when you start taking more and more hits identifying Datums and Features, Now don't get confused by the CAD model on screen, when you create a surface based off of that CAD model, maybe Datum A? you are creating a surface plate, next you create a Cylinder inside a hole, maybe Datum B. You just created a Gage Pin, and so on with other datums, and when you plug these Datums in FCF's, It locks these features in the perfect place according to your CAD, this just replaces you having to lay these parts out on a surface plate. When you have created your program, then what you have created is the perfect MATING part. At this point you could actually toss the CAD model as it becomes useless. When you have more Datums maybe D, E, and F. Now you have a second mating part that attaches to this part that you are making. The Datums are important, because these features are where your part and the other part, maybe made by another company, will come together with the tolerance given, when they put them together.

[Mranaskw]

thank you sir for the well explained answer , regarding what have you said , i have a couple of inquiries i wish if you can take time to answer them as well ,

-When we de an iterative alignment the machine tries to assemble the cad and real part into one object (Theoretically speaking ) so that where ever u click on the cad the machine will locate is this correct

-our programs are created with jigs and using the 3-2-1 utilities box , the inquiry is that each feature is aligned at a time , for exemple the plane is aligned as z and level , then OK (A0 added) , then line as Y origin then OK (A1 added) etc , why have they have not aligned all the features/datums in one alignment

-our programs are created to begin after a first manual hit then DCC begins to aligne automatically the other features that were aligned with a custom offset as an origin for each axe , my inquiry is why the first hit was aligned as origin for all axes ( first point was aligned as an origin for X Y Z)

-the custom origin i talked about is taken for cad surface of each Axe why is that for exemple if we want to align the z we select the plane as level then as Z origin but with a specific number (that we took from the cad) and then in the dialog box mentioned as an offset , why does it mean an offset

it will mean a lot to me if you could answer i laterally know nobody to fulfill my inquiries at work , all programs are pre-made , and i'm just tutoring my self by myself

[Douglas]

Mranaskw it sounds like you want to set z origin to a feature but that feature is not at Z0...? The offset will need to simply be the nominal Z value of that feature on the CAD model for your alignment to match the model

Offset just means the feature you probed is offset that much from the coordinate system origin

[KIRBSTER269]

After you create your program, the CAD is useless, you have created the mating part, it's not very visible, because PC-DMIS is not a CAD building program like Solidworks or other similar programs. When you bring in an actual part, if your program is built correctly, basically it is taking hits from your actual part and virtually lays it on the Mating part and sits it on top of it (your program) does it fit? Well is your report Red or Green. Your first hit is that you speak of, forgive my French, but shi_tty alignment, <<< that does not mean it's bad, just somewhat a crappy 0, 0, 0, in the X, Y, and Z. This says from this point created is somewhat from your part and you put that offset number saying your part is a certain distance from this point. This is to have less error for an operator,(less hits, less error from operator) that does not know PC-DMIS. Then it goes into DCC Mode and it SHOULD establish a better DCC Alignment. <<<which is basically a Manual Alignment to establish a better 0, 0 , 0 in X, Y, Z and this alignment means nothing but helping the CMM establish the 0's in a location that you placed the part on the table.