When is Main alignment needed after pre-alignment?

[[Pa...]]

Hi All,
 

I have a general question regarding alignments in ZEISS INSPECT.

In my workflow, I use Pre-Alignment to initially bring the CAD model and the mesh into the same coordinate system. After achieving a good pre-alignment, is it always necessary to create a Main Alignment as well?

I would like to understand:

• What is the specific purpose of a Main Alignment compared to a Pre-Alignment?
• In which situations is a Main Alignment mandatory or recommended?
• If pre-alignment works fine, could those evaluations be used with confidence?

I would appreciate any guidance or best practices regarding when to use Pre-Alignment only versus when a Main Alignment should be added.

Thanks in advance for your help!

[[Ma...]]

Hi,

you can use only pre-alignment if you have good quality of parts - bended and differentiating mesh against CAD model can lead to bad pre-alignment and your nominal elements may be not calculated.

Many drawings have RPS and other datum definitions, so you may need alignments or at least datum systems.

While using heat map of deviations you want them to be stable, so you would need to make main alignment for that.
Also you would like to know what those deviations are teling you.

If you have bended parts, then you would need for calculations of elements bestfitting - which will bring mesh closer to nominal element and allow calculation.

While you ask for this type of question i would ask with what measuring machine you have knowledge.
We are using here ScanMaxes, so to get comparable results we had to use geometry norm instead of ISO/ASME.

Also i would suggest training - it's not that simple how it looks - you may have wrong results at reports due overlooked method.

[[No...]]

A main alignment is always recommended, but may be mandatory in special cases. For example, I once had a part that was mostly cylindrical, but had a bulge at a specific location. I tried to use only pre-alignment with Gauss best fit, but it wasn't enough. You have to consider that Gauss is outlier-tolerant. That means if a large amount of the geometry fits well to the CAD model and only a small amount (the bulge in this case) does not, it is more or less ignored. With my part, this led to a situation where the bulge ended up "somewhere" and didn't match the model most of the time. The fitting algorithm doesn't "notice" that a few degrees more rotation could make an even better fit than it has already achieved. So in situations like this you absolutely need to make an additional main alignment to achieve a stable rotation.

[[Pa...]]

Hi Both,

Thanks for the insights.

We are using T-Scan hawk 2 for scanning our parts and we use it for quality inspection of variety of parts (metal and plastic).
We have been using the scanner and Zeiss inspect software for less than 6 months now. We had initial e-training from Zeiss on how to use the software and the scanner.
Earlier we are measuring manually where possible. The issue is we don't have RPS or datums on any of our drawings. It will be a huge effort to include datums or RPS on the drawings as we have thousands of legacy drawings.

In most cases, pre-alignment works fine with some minor deviations on the surface comparison. Sometimes the measurements that deviate from nominal, we are not confident if we are making the right evaluation.

So we wanted to understand what is the trade-off between relying primarily on pre-alignment versus modifying our drawings to include datums or RPS to implement main alignment.
 

[[Ma...]]

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Hi,

we have also many drawings without those specifications, so we are using the most stable elements allowing us to describe where are deviations comming from.
We are hot forging company, so what is one cylinder in drawing, we sometimes use only half of cylinder due to die tools.

You will need to figure out what will work at your situations.

[[Jo...]]

Prealignment is called "pre" because without positoning the mesh to the cad you can`t do anything.

If your drawings dont state specific reference systems, you can use the prealignment as it should function as an best-fit alignment, like any other best-fit-alignment you could do as main aligment or a local best-fit you might need if your parts deviation from the ideal geometric form is "larger" and the software can`t tranlate from CAD to mesh.

 

A drawing without a reference system is a flawed/ambigous drawing.

In case of new projects I would always contact the customer and ask for a drawing according to the appropriate standards.

With old projects this is of course not that easy 🙂

 

As a suggestion:

Try using simple measurements that correspond to your manuell measurement systems before (virtuell calipers for example). That way alignments don`t matter that much.