Can some explain strategies for applying a datum system at a global level e.g. an entire car, plane?

[Tetragrammaton]

Hi All,

I am curious to understand the bigger picture as when we talk about applying GD&T, it always see to be at a micro/part level.
It seems like we are, understandably, always focused on the this fine detail at the part and assembly level.

What i would really like to know if anyone has anyone worked on seriously large project e.g. an entire car / truck / small-large plane / helicopter / boat / ship?
I would be really interested and appreciate hearing how such an undertaking was managed.
I am trying to imagine and understand how the global datum strategy was defined and what drove it.

Then how was this broken down to the further sub-systems etc? How is it pieced together and connected or is it not?
For connected-ness I can image both scenarios.
Meaning where there is a clear relationship between 2 or more systems then I expect depending on the function and relationship there would be a strategy that links them.

Can anyone with this sort of knowledge or experience step me through it, even it is initially top-level?
I would like to understand how the different sub-systems / module of the entire project where linked to each other and back to the top level, global system.

I am sure there must be something like this being done in industry but it seems hard to find knowledgable folk who can impart some insight into this.

Also if anyone knows of any online courses that teach about this higher level strategy planning approach I would be grateful to know more.
Even if it just 1 less/PDU that's part of another different course but covers this specific question, I would really like to hear about it.

So many thanks in advance for any insight and replies.

Best Regards

Tetragrammaton

 

[3DDave]

More later, but for right now, you establish a set of datum points/features that will be critical to positioning other major components for the body/frame. All these points will be where fixtures will hold the parts when the final assembly/welding/riveting is done. Then those interfaces are analyzed very carefully for acceptable variations such that loads induced by the assembly process won't significantly displace the datum points/features when the fixture is let loose.

This top levels set of datums establishes a framework for the entire finished assembly.

You will need to understand the process capability you are willing to pay for in order to design interfaces that don't exceed that ability.

 

[Tetragrammaton]

Many thanks 3DDave,

I did imagine something of that nature, that it would come from what can be achieved in production.
I have vague recollections about doing SPC (Statistical Process Control) studies and determining things like Cp, Cpk
But that was many decades ago since I was on the production line as a Manufacturing Eng.

I spoke with a few of the Structural Engineers and found out that the is datum system strategy that ties back to the Production tooling / jigging, which makes sense.
Obviously (for Aero, where I am) this is heavily tied into the the STA, WL and BL positions as reference as well to determine key locations / references for the main modules/systems.

I am trying to get some more info as well but if you have further feedback then that would be interesting to hear about as well.
Many thanks, I appreciate you taking the time to reply share your insight and experience.

Kind regards + thanks again.

Best Regards

Tetragrammaton

 

[3DDave]

There are design datums - these are theoretical controls for the geometry rather than a basis for controlling variation; they don't appear on the parts themselves. I'm a bit unsure how references to assembly fixtures are toleranced for these. I expect that is where understanding production tolerances for fixtures will come in handy.

An interesting approach was used on the Pontiac Fiero - after the entire frame was welding up and doors installed it was placed onto a full-body fixture, mounting pads on the frame were machined in a single setup for all the plastic/molded body panels to attach to.

https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20160323-2-116809.html

 

[Wuzhee]

This makes me want to buy a Fiero just to admire the machining work.

 

[3DDave]

I expect that the tolerance analysis for these situations includes the variation expected in the fixturing as if it was a permanently assembled part.

For really big items the use of laser theodolites would allow a optical gauging/fixturing. Adjust the supports to meet the location requirements instead of building precision positioning tooling. I expect that is the case for shipbuilding. In the Fiero case they claimed 60 bodies an hour; for a ship the pace is slower.

 

[jassco]

Hi, Tetragrammaton:

Datums and feature control frames in GD&T have nothing to do with size or complexity of an item. It can be a watch or a press. Datum selection is based on design intent of the subject part/assembly.

Best regards,

Alex

 

[Eric Gushiken]

jassco,

Yes but sometimes the top level assembly is huge or the finished product has no easily accessible datum surfaces. On one contract I worked for they used laser tracking targets like these.

https://www.faro.com/en/Resource-Library/Whitepaper/understanding-laser-tracker-targets

It was pretty amazing to see how the laser head automatically moves and tracks the ball when you move it around.

I have no experience or exposure to this other method but it uses laser radar.

https://youtu.be/weunlu_dkRg?si=mW9bT81lf1gw42Hv

 

[greenimi]

There are design datums - these are theoretical controls for the geometry rather than a basis for controlling variation; they don't appear on the parts themselves. I'm a bit unsure how references to assembly fixtures are toleranced for these. I expect that is where understanding production tolerances for fixtures will come in handy.

Per your statement above, I found this from General Motors

http://www.delta-mold.com

Google seatch search for "General Motors gage fixture standards"

1. All fixture details including fixture bases, datums, and inspection
details shall be accurately manufactured in order to ensure the
accuracy required for product inspection.

2. General guidelines are as follows:
a. All datums used to position the part in the gage are to be
located in the gage within +/- 0.10 mm.
b. All fixture details such as check pins and bushings, details
used for electronic measuring devices, etc. which check part
features are to be located within +/- 0.10 mm.
c. Surface contour features for in-line/feeler checks are to be
within +/- 0.15 mm.
d. Trim line features for in-line/feeler checks are to be within
+/- 0.15 mm.
e. Templates are to be within +/- 0.25 mm.
f. Sight checks are to be within +/- 0.50 mm.

3. When certain part features drive deviations from the above
specifications, the 1/10th rule can be utilized for fixture tolerancing.
Ten percent of the tolerance specification indicated on the part
drawing for the particular part feature can be used for build
tolerances.