Composite TP question

[d.evans]

I have a part whose C and B datums are long skinny edges of the overall part. The edges are radiused near the top surface, and have only a small 'flat' surface left to get hits for these datums. (Datum A is a huge, relatively flat surface compared to the B and C datums.)
I created a 3d line for each of these datums using about 4-5 hits on each line.
The problem is that the print has a CAB callout for one set of holes and a BAC callout for some others. These holes are coming thru the part normal to -x, +x, -y and +y, so I see why the engineer wants these holes TPd to the datum reference frame in this way, but in order to do it I have two choices that I know of...
1)
create a new alignment where I simulate the datum callout ( but how do I level to a line?)
2)
"USE DATUMS" checkbox, which in my experience doesn't work very well in my version (3.7MR3).

What would you do?

[#2#]

Hmm, I'm not sure that this is correct but I would do the following.

CAB - rotate C to an axis - level A appropriately - transfer orgin to C, A and B

BAC - rotate B to an axis - level A appropriately - transfer orgin to B, A and C

I guess I would rather see ABC and ACB - essentially the same thing as I did right?

[bw_bob]

I'll be honest with you. That is a stupid callout. I would level to -A-, rotate to -B- & set -C- as my X or Y orgin. Then try to explain to whoever did it & how you did it.

[Jan d.]

This may not be of much help, but this is exactly the call where the V4.1 XactMeasure GD&T shines! This is so incredibly easy to program using V4.1. And it outputs all kinds of very useful process information too!

I have never used anything before V4.0 so I have no idea on how to do this in an earlier version.

Since you have no Virtual Conditions on your datums, I am convinced that PC-DMIS will calculate the TP correctly.

Little piece of advise: although most of the blatant bugs for TP have been fixed, if you can, wait for V4.2 (there are still a few left in V4.1; deep down in the report world). You'll upgrade only once.


Jan.

[justncredible]

You can level a line, just be sure of the working plane, since it will level it in that plane.

[d.evans]

I'll be honest with you. That is a stupid callout. I would level to -A-, rotate to -B- & set -C- as my X or Y orgin. Then try to explain to whoever did it & how you did it.

That is how I did it. I told the engineer about the difficulties trying to 'level' to his b and/or c datums. I stayed in the general ABC alignment, and TP'd to two axes for each set of 'thru-the-edge' holes. For example on the set of 7 holes normal to -y, I did TP, selected X and Z axes only, typed in the +/- for the hole dia, typed in the TP tol (.012), unchecked 'use datums', then 'create'. This eliminates the y-axis from the TP calculation. I got OK results, but still have a lot of TP out-tol conditions.

[bw_bob]

You know your parts better then me but i had a hard time thinking that was what they wanted. Level to a small edge is a little funky, but we are taking about design engineers......

[d.evans]

You know your parts better then me but i had a hard time thinking that was what they wanted. Level to a small edge is a little funky, but we are taking about design engineers......

Maybe I don't understand GD&T well enough, but isn't the first datum to appear in a datum reference frame the 'level-to' datum?

That is the way I understand it, please correct me if I'm out in left field...

[Jan d.]

Yes, your first is the level-to datum.

It will be VERY difficult to properly level on a small ledge. But it is not impossible.

Typically, you want to make the first datum the one with the largest span. But sometimes that is not jive with the intend. And then you get poor call-outs, like what you get.

Although maybe intellectually correct, I agree with Bob. It is still a poor call.


Jan.

[d.evans]

You can level a line, just be sure of the working plane, since it will level it in that plane.

I thought it would level to the vector of the line.

[d.evans]

This may not be of much help, but this is exactly the call where the V4.1 XactMeasure GD&T shines! This is so incredibly easy to program using V4.1. And it outputs all kinds of very useful process information too!

I have never used anything before V4.0 so I have no idea on how to do this in an earlier version.

Since you have no Virtual Conditions on your datums, I am convinced that PC-DMIS will calculate the TP correctly.

Little piece of advise: although most of the blatant bugs for TP have been fixed, if you can, wait for V4.2 (there are still a few left in V4.1; deep down in the report world). You'll upgrade only once.


Jan.

I've downloaded 4.1, just haven't installed it yet. I wanted to find out first if it would screw things up even worse, or if it is better than what I'm using (3.7mr3). I like the idea that it will display you tp callouts sort of the way they are done on the drawing. That seems like a big plus to me.

[layout tech]

Here's my opinion and it is worth what it cost you.

I would use the ABC alignment for all dimensions.
But for BAC callouts I would change work plane to b datum workplane and for Cab callouts i would change workplane to c datum workplane.

I believe that this would meet the intend of callouts.

Hope this helps

[#2#]

Here's my opinion and it is worth what it cost you.

I would use the ABC alignment for all dimensions.
But for BAC callouts I would change work plane to b datum workplane and for Cab callouts i would change workplane to c datum workplane.

I believe that this would meet the intend of callouts.

Hope this helps

Changing the workplane will do nothing. The whole idea of datums is that the part is to be set up according to the datums. Let's say you have three planes as datums A, B and C are three sides of a skewed cube.

Alignment ABC means that you fix as many degrees of freedom as possible with A, then as many as possible with B, and then the remainder (hopefully) with C. You wouldn't level A then level B then level C because leveling B and C would throw off the level of A. You would level A, transfer axis orgin to A (3 degrees of freedom fixed 2 rotation and one translation). Then, rotate and transfer an axis orgin to B (2 degrees of freedom fixed - 1 rotation and one translation). Then, the final degree of translation is fixed by C.

CAB would mean that you fix as many degrees of freedom as possible with C then A then B. If the sides of the cube are not perfectly set at 90º The two alignments could yeild different measurements.

[layout tech]

I realize what the various datum callouts dictate. but because of the shape of the part the callout (as others have stated) appears bogus. Because Datum A is the largest and most likely stable datum on the part I would use it to give reading that relate to one another. If Datum B or C have the least amount of inconsistance and you use them as the primary datum. and change for the different callouts you will be giving data that is technically correct but
not with any relationship to the rest of the part. Datum A is Datum A because it most likely have the primary effect on the function of the part.

My opinion and worth what it cost you!

[#2#]

I realize what the various datum callouts dictate. but because of the shape of the part the callout (as others have stated) appears bogus. Because Datum A is the largest and most likely stable datum on the part I would use it to give reading that relate to one another. If Datum B or C have the least amount of inconsistance and you use them as the primary datum. and change for the different callouts you will be giving data that is technically correct but
not with any relationship to the rest of the part. Datum A is Datum A because it most likely have the primary effect on the function of the part.

My opinion and worth what it cost you!

I agree that his datum structure is ill concieved. I just wanted to point out that changing the workplane will not affect the reported true position. I believe he shoule be leveling to A and then rotating to B or C depending on which comes first in the callout. Rotate to B for BAC and rotate to C for CAB.