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...]]

Please sign in to view this username.

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.

 

 

 

[[Ma...]]

Hello team , 

I think that pre-aligment is first step after that i construct main aligment according to drw. 

You can use RPS , by geometric elements , plane/line/point. 

The drawing must have references or at least axes

For example i use RPS and a lot of best-fit (based on RPS) - but it depents on the evaluation of the elements.

One of the solution is Dewarp the parts.

What do you think ?

Nice day.

 

[[Ma...]]

Please sign in to view this username.

Well without pre-alignment you can not use CAD model to define elements and it's measuring principle.

Although you can do definition of element on CAD and manually select on mesh to create same element on actuals.
Or measure only on mesh.

Many older drawings are showing only axis of a few elements, like for cylinders or dividing axis.
But missing how to achieve questionable dimensions on deformed part ( like in our case of forged parts with places of missing material due material flow )

About Dewarping part - that goes to specific industry and material.
I can not tell to a customer that after straightning that part he will achieve a part according to drawing. That can be on bendable parts like plastics and thin metal sheets.

Also dewarping is not for beginners.

From your screenshot i can not tell what you are doing, but that seems to me like an overkill.
Basic RPS should be sufficient with correct measuring principles - like projected distance, selecting axis in distance.

But i would use that while on measuring single parts on an assembly. There it would be usefull.

[[Ma...]]

Hello Martine , 

In general , if you have large complex plastic parts , which are deformed ,  you cannot avoid "best fit" for the easy program function.

Of course , it depents on the drawing (dimensioning).😉

 

 

 

 

[[Jo...]]

Please sign in to view this username.

"From your screenshot i can not tell what you are doing, but that seems to me like an overkill."

At the moment I am working on a project where i need  around 40 local best-fit alignments. I love radii on partial circles with way too tight tolerances and no real function beside not being an edge on a part that is so far off the CAD, that you need to local best-fit everything.

Edited 50 minutes ago

[[Ma...]]

Please sign in to view this quote.

Ups, sorry - i have overlooked that those are bestfits on mesh - i thought it was a next RPS - My apologies 👍