Alignment by Coordinate System v.s. Alignment by Geometric Elements

[[Ry...]]

Hello All,

I am new to the software, went through the training about 2 months ago. Wondering what the difference between alignment by coordinate system and alignment by geometric elements is?

Using both methods on the same part, I am noticing the part doing some significant walking around on it's butt end when alignment by coordinate system is applied. When I apply the alignment by geometric elements, the part is much more stable & consistent. 

Looking at the few projects that our company had contracted by Zeiss, we noticed that they are always using the geometric elements alignment. So we are assuming this is the correct way to do it, but we want to understand the difference. I would think that if you are selecting the same elements in the coordinate system as the alignment by geometric elements, you would not have a difference.

Thanks in advance to anyone who responds.

 

Ryan

[[DW...]]

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 Just curious, what training did you attend?

[[Ry...]]

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 Our company just acquired a Metrotom 130Kv & I took the on site training that came with it. 

Do you know what it could be? If there is a difference, I did not catch what it was.

[[DW...]]

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 From the help files - 

• With the By Geometric Elements alignment, you align two data sets with the help of constructed geometric elements (e.g., points, planes, lines, cylinders, circles).
• The By Coordinate Systems alignment transforms a chosen actual coordinate system into a chosen target coordinate system.

The By Geometric Elements alignment is typically used as a main alignment. Using function By Geometric Elements, you can align two data sets with the help of constructed geometric elements (e.g., points, planes, lines, cylinders, circles). You can use up to three actual elements with their corresponding target elements for the alignment. Depending on the geometric element, different degrees of freedom are locked, see Tab. 1 . For example, you can use three planes or two cylinders to lock the six degrees of freedom.

 

Alignment By Coordinate Systems is typically used as a main alignment. This alignment transforms a chosen actual coordinate system into a chosen target coordinate system. With the coordinate system, the GOM software as well transforms the data on which the actual coordinate system is created. The GOM software fits the two coordinate systems perfectly together so that they have the exact same position and orientation. As target coordinate systems, you can choose the global coordinate system or a local coordinate system.

 

 

[[Ry...]]

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 Thanks, but this does not necessarily answer my question. I understand what is written in the help files, but in my mind they read as being the same thing. What I don't understand is when put into practice in my program, I see differences in my results, even when using the same elements for both alignment methods.

[[Na...]]

Hello Ryan,

the used coordinate system might use a GD&T standard like ISO1101, which will calculate the elements according to the norm. A plane constructed with Gaussian fitting method will be calculated according to the norm as defined in the standard in the background. 

With a coordinate system you are also able to create it with different geometries and geometry combinations like a datum element created from two cylinders (A-B callout on a drawing for example), or even a surface profile. 

If in the alignment by geometric elements and alignment by coordinate system and the resulting alignments differ, there is probably used a GD&T standard used in one of the alignments. 

Nanno

[[Ti...]]

Ryan,

At the end of the day both methods are very similar.  I use one geometric alignment after the prealignment usually.  I like 3 stable features to lock the part in, in case the prealignment is poor.  Then I program....if I want to see the blend of the mesh and CAD model for a surface profile I create a coordinate alignment system for viewing purposes.  

 

Now is either method going to change the quality of a cylinder or plane you create, answer is highly unlikely...and if it did perhaps you then need a local alignment.

 

Now do you understand the difference in related actual mating envelope and unrelated actual mating envelope?  Knowing the difference in how a secondary datum must be related to the primary is one glaring distinction bw using a coordinate(datum) system vs geometric elements from using both.

 

Let me know if you know the difference in the UAME and RAME and then do you understand what I am trying to say.  Those are more advanced concepts for most GDT novices.

 

Edited September 26