Booga
New member
- Aug 2, 2006
- 5
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?
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?
