Reporting Composite Position

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Racinslack20
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Reporting Composite Position

Post by Racinslack20 » Tue Sep 24, 2019 9:03 pm

Hello,
I am having some trouble trying to report some features as composite position. I know this has been discussed many of times, but still not sure how to set up my particular part.

To start, my company builds prototype automotive sub frames. The datum structure is a little different than most i have seen so looking for some help.

The datum structure is as below;

Datum A= 4 datum target points to establish z datum. 2 points offset from other 2. Still not sure why the over constraint.

Datum B= a circle parallel to datum A to establish x,y datum.

Datum C= a slot opposite side to datum B and still parallel to datum A, used for final clock in x.

We are currently welding a LH and RH bracket on to these parts and both have 2 threaded studs on them (4 Total). I have been measuring those as cylinders.

My position call outs are so;
2-10x1.5
(3.0m)(A_Bm_Cm)
(1.0m)(A_Bm)

When i pull up composite position window and input tolerances, mmc, and datum a,b,c into pltzf. I also uncheck "seperate feature contro frames". I then try to input datum A, and datum B into frtzf I get and error saying second datum must be a plane, line, or cylinder. Well my datum b is a circle. I tried one thing (which i know is not right) and constructed line from datum b to datum c and tried using that as my second datum in frtzf. I know this not the right way and am looking for some help on this. I am quite new to composite position on the CMM. I have been doing some reading and trying to understand this setup more. Thanks in advance!

medupriest
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Re: Reporting Composite Position

Post by medupriest » Tue Oct 01, 2019 11:44 am

In regards to Datum -A- being over constrained, it is just because of the 4th point. It is practically impossible that those 4 points will be exactly co-planar. This meant that CMM-Manager is going to have some element of Best-Fitting, and so provides the warning.

RPS is the way to go. If your Composite Position datum call is ABC, you can just use that RPS alignment and instead of building a new alignment, just select the use current reference frame. You can still add the datums to the call, but it doesn't change the calculation, it just shown the label.

You can also construct a line between two points and use the line as you were wanting.

Racinslack20
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Re: Reporting Composite Position

Post by Racinslack20 » Tue Oct 01, 2019 12:49 pm

I understand the constrained datum A. I guess in my post i meant not sure why the engineers made the drawing this way. I am currently using RPS Alignment. All is good there. Main question is on my composite position callout out it has a 3.0 mm call out to ABC which I have. The lower portion of that feature control frame has a call out of 1.0 to A-B. It does not allow me to enter my datum B, says needs to be plane,line or cylinder. As i was saying i constructed a line from datum B to datum C but wouldn't that be measuring the same as call out to ABC. When i measure it this way i get the same results for 1 cyl to the next. for example

CYL-1 gives results as such
position to ABC = 1.11
Position to AB = .787

CYL-2
Position to ABC = 1.28
position to AB = .787

medupriest
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Re: Reporting Composite Position

Post by medupriest » Wed Oct 02, 2019 9:44 am

It is not a function I use very often. Any chance you could zip and send me the project file so I can give it a look?

I do know that CMM-Manager is a bit picky when it comes to custom reference frames, but I suspect we can find a solution.

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Ryan Christopher
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Re: Reporting Composite Position

Post by Ryan Christopher » Wed Oct 02, 2019 10:17 am

If you are going to use auto-alignment fill-in tools it may be a good idea for you to test them for your application first to make sure they do what you are expecting. I have learned not to trust them (not in any software, yet) and tend to think of them as unicorns.

For your part:
If your feature control frame only has one "target symbol" then the datums referenced in the lower portion of the feature control frame only constrain the orientation of the resulting coordinate system. The remaining degrees of freedom can then be controlled by the features being reported (best-fit).

Because there are only 2 cylinders, the best-fit coordinate system should end up right in the middle of the them.

It is the datum B at MMB and datum C at MMB that you should really be careful of. These don't allow for "bonus tolerance", they allow for mobility of the coordinate system. That means that for a situation like |A|Bm|Cm| there is more than 1 possible resultant coordinate system.
Ryan Tackes
Applications Manager, VMS Inc.
http://www.vantagemeasure.com/

Racinslack20
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Re: Reporting Composite Position

Post by Racinslack20 » Wed Oct 02, 2019 11:57 am

Medupriest,
says file too large to attach. Any other way i can send to you?

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