iterative or CAD equals Part

[Robert Squire]

I have been trying explain to a coworker how and why to use iterative or CAD equals Part, when we have CAD files to work with, but I get stuck when I try to explain how the two work, when he's been able to get the CAD to align what appears to be properly using traditional 321 alignments and making sure the axis are the same as the CAD axis. It appears as if the CAD is perfectly aligned so my reasons for using iterative and or CAD equals part are not getting through. I tell him it "bridges" the CAD to the points measured, that it is actually ties measurements to the CAD data then, but I cannot explain any further because I can't find the right words to use, can anyone out there help me. I have 12 years experience with this and I have always used iterative or CAD equals Part but he's making my reasoning look bad. I need some help here.

[pcdmisexpert]

I don't understand , if he is not doing any of the CAD Alignments , how is he able to program off of the CAD at all ?

[Matthew D. Hoedeman]

ITERTAIVE is for the most part for parts without a fixture, without a clear-cut level / rotate / origin set-up. If your 3 (or 4 or 5 etc.) A-DATUM points are all at different heights, with different vectors, you will have onehell of a time getting it aligned with a 321 alignment (cad=part).

[tking]

Robert,

I can't help you explain it. I use 3-2-1 alignments as well. I program the setup on the cad model then run the program probing the part and after the alignment portion of the program has been run through I can continue to program from the model and the CMM probes the part no problem. I've had no training so I don't understand (trust) what ITERATIVE or CAD = PART does well enough to use either.

I'm looking forward to this thread, perhaps some of the folks can help my old grey matter get a handle on this. Thanks for the question,

TK

[Devaney]

For the most part I'll program to the CAD with the 3-2-1 like TK but we do some really funky skins for weapons systems and aircraft that are really tough to get set up without using an iterative alignment. I agree with Matt totally on the heights and vectors. My advice is get to know both.

[Jeffrey Lovely]

no need to use off set planes for datum -A-
DATUM -B- can also have off set points
no need to rotate - rotation can fluctuate
let pcdmis do it all for you
321 is old school
ok to measure a block with 321 !!!
iterative is quicker

[Matthew D. Hoedeman]

There's no school like the old school. AND, if you need to make an adjustment to an alignment, ITERATIVE is harderthanhell to adjust, 321 is a piece of pie.

[Jeffrey Lovely]

oh sorry matt
I know you like 321

[Matthew D. Hoedeman]

No big deal, but it is true, it is a pain to adjust an iterative alignment, a breeze to adjust the 321. Also, have you run into the problem with the dimensions NOT updating to the current check values in programs that contain an iterative alignment? I have only seen this in V3.7 MR3 (never saw it in V3.5 no MR). It can be 'fixed' by typing in the feature ID for the first dimension in the program, but that is a royal pain.

[Don Ruggieri]

Some overlap

Matt's explanations summarize it pretty well. Keep in mind though, there are 3 (or more) scenarios here. There are parts the are real easy to do with 3-2-1 alignments, parts that make more sense to do with iterative allignments, and then there are parts which can go either way.

Actually, any time you can do a 3-2-1 alignment, it could have also been done with iterative alignments. The same is not necessarily true in reverse though. There are parts that can be done with iterative alignments which would be near impossible to do with 3-2-1.

Sometimes, it just comes down to preference. . .

[Scott Appleyard]

I tend to use iterative alignments almost exclusively. When I'm training people, I'll usually explain that the CAD=PART alignment essentially merges the alignment created on the actual part with the alignment of the CAD. This can be very tricky when the trihedron doesn't reside on the part (i.e the CAD is in car-body position). CAD=PART works great for prismatic parts (usually machined) without a lot of free-form curvature.

As for Iterative alignments, I explain that PC-DMIS essentially creates a virtual fixture for the part. If you look at the Datum structure then visualize how it would be held in a fixture, that's pretty well what PC-DMIS is doing. As an example, think of a switch plate in a car for the power windows and locks. There really aren't any flat or straight edges for a 3-2-1 alignment to be viable. If a fixture isn't available to hold the part, an iterative alignment is the only way to go!

[Paul Sarrach]

Don,

Question. If you where to do an alignment, with CAD. Which way do you do it?

[ag1979]

I use Iterative when ever possible. Mostly I'll use 3-2-1 or the like when digitizing or when there is no CAD to work with.

[mchough1]

CAD=PART ?

Not to be redundant ( I posted this question on one of the other threads) I was taught to always use an iterative or 321 alignment. I see a lot of people saying they use CAD=PART in their alignments. I thought that CAD=PART was only to be used as a last resort or if there had been changes to the CAD after a program has been written. I never use CAD=PART, it always seems to screw up my program. Can someone explain the benefits of using this command.

Thanks

[Paul Sarrach]

OK, Here it goes.

Old school, iterative alignment, is like putting a part on a sine plate, and it fiqures out the trig and adjusts itself for you.

Cad=part well, that like the sine plate telling your height gage where its at, when it gets lost.

321 the just what it is.

And I use iterative exclusily, I do it with tooling balls, I even you it on gage bases using edge points, constucting a plane, line and the zero corner. And when I do that, I will take all my manual point within one inch of the corner, and let it go out to the ends during DCC alignments.

Also, I always align my Part to Machine, And never use CAD=Part. It works great, and the easiest I know, Because I am into Lean Inspection.....Translation. I am frichin LAZY.