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3D Model hole sizes

3D Model hole sizes

3D Model hole sizes

(OP)
This may sound like a very simple question but I wanted to get an idea of what other Aerospace companies do with regard to modelling holes.
The company I work for has adopted a methodology that, to me, seems a little strange particularly when you consider the increasing use of models as definition of the part.
We currently have not adopted model based definition (MBD) so our drawing is master, but I wondered whether our current practice of hole size determination would be valid for a MBD approach.
We have a spreadsheet based on the IT11 & IT12 tolerances that defines the Model to, bi-lateral tolerance and resulting upper and lower drawing limits.
For example:
For a IT11 tol. 4.1mm hole.
Drill Model to Tol. Limits Limits
(mm) (ins) (ins) (ins) (mm)
4.100 0.1626 0.0017 0.1609 4.09
0.1644 4.17

There are some rounding erros and fudge factors in the sheet (which the originator insists are acceptable) which I personally don't understand.
Historically we would have modelled to the drill absolute size and the drawing would have shown the metric size with the imperial conversion bracketed afterwards. There would also be a general drill tolerance applied via a border note that would be dependant on a range of drill sizes. Holes requiring tolerances outside of those defined in the boarder notes would have been defined by upper and lower limits, which may not have been based on an equal bi-lateral tolerance around the modelled (drill) size.

There are various things to consider when defining the hole, clash detetion, manufacture and inspection being a few. Some of these may be in conflict with each other, so what should we designers do?

http://www.martin-baker.co.uk/

RE: 3D Model hole sizes

Booga - if you don't get enough answers here you could post over in http://www.eng-tips.com/threadminder.cfm?pid=1103 and put a link back to this thread & ask them to come take a look.

We do a tiny bit of MBD, and even for parts with a full drawing routinely supply the model in step or similar format for reference in writing the CAM program.

We still spec the nominal drill size with unequal bilateral tolerances e.g. .125+.004-.001 (inch). As far as I know it's never caused a problem but I'm not sure our inspection would routinely catch it even if it did.

This is in the USA for reference, and we have low volume so don't really have an integrated supply chain as such.

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RE: 3D Model hole sizes

(OP)
Thanks Kenat for the reply.
I will do as you suggest and post a link back to this in the thread suggested.
Once again thank you.
Paul

http://www.martin-baker.co.uk/

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