loose, transition and interference fit in Assembly 4

Discussion about the development of the Assembly workbench.
kcleung
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loose, transition and interference fit in Assembly 4

Postby kcleung » Sat Apr 17, 2021 11:33 pm

I am glad to see that FreeCAD 0.19 has finally be released, and that Assembly 4 is becoming usable.

So in assembly 4, how can one implement loose fit, transition fit and interference fit? It would be great if the workbench and the assembly data structure has a way to specify loose fit and interference fit, in particular the amount of interference.

Wouldn't it be great if each fit has a radio box asking whether the fit will be loose, transition or interference fit?

I feel that this feature would be very important to make the assembly feature adequate for industrial use, and we better get it right before Freecad reaches 1.0

I have filed an new issue ticket #190:

https://github.com/Zolko-123/FreeCAD_As ... issues/190

P.S. Upon reading:

https://en.wikipedia.org/wiki/Engineering_fit

It appears that within each category, we also better specify the subtypes of the loose, transition and interference fit as well.

For assembly to be of practical use, the system also needs to automatically validate all fits within the assembly, and to spot out any errors
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Zolko
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Re: loose, transition and interference fit in Assembly 4

Postby Zolko » Sun Apr 18, 2021 12:13 pm

kcleung wrote: Sat Apr 17, 2021 11:33 pm So in assembly 4, how can one implement loose fit, transition fit and interference fit? It would be great if the workbench and the assembly data structure has a way to specify loose fit and interference fit, in particular the amount of interference.
This is an interesting question ... in today's implementation, all "fits" are nominal fits, without any specified tolerance. In all the projects that I have been doing, the tolerancing of fits is done at the end, for the final drawings and manufacturing process. We have never done such tolerancing in the assembly at the design process, it was always done at a second stage, after the prototyping and before the serial industrialization, because this depends on the manufacturing process.

For assembly to be of practical use, the system also needs to automatically validate all fits within the assembly, and to spot out any errors
This is clearly false.

Thank-you for your proposal, but I will close the issue.
try the Assembly4 workbench for FreCAD v0.19
install with Tools > Addon Manager > Assembly4 — tutorials here and here
kcleung
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Re: loose, transition and interference fit in Assembly 4

Postby kcleung » Sun Apr 18, 2021 7:28 pm

Zolko wrote: Sun Apr 18, 2021 12:13 pm
kcleung wrote: Sat Apr 17, 2021 11:33 pm So in assembly 4, how can one implement loose fit, transition fit and interference fit? It would be great if the workbench and the assembly data structure has a way to specify loose fit and interference fit, in particular the amount of interference.
This is an interesting question ... in today's implementation, all "fits" are nominal fits, without any specified tolerance. In all the projects that I have been doing, the tolerancing of fits is done at the end, for the final drawings and manufacturing process. We have never done such tolerancing in the assembly at the design process, it was always done at a second stage, after the prototyping and before the serial industrialization, because this depends on the manufacturing process.

For assembly to be of practical use, the system also needs to automatically validate all fits within the assembly, and to spot out any errors
This is clearly false.

Thank-you for your proposal, but I will close the issue.
For a CAD system to be complete, it is essential that apart from catering the prototyping stage, it should also cover the final stage up to the production - namely when we export each part to CNC, 3D printing and/or sheetmetal workbenches for production of G-code for manufacturing. So this is why I said it is essential to get the three types of fits sorted out in order to cater the serial industrialization stage.

The alternative of manually jigging each part for fit would be too labour-intensive and prone to failure.

The specification of fit will also allow validation of all assemblies before production.

Many CAD system including Solidworks, GrabCAD and AutoCAD have fits implemented.

This is what AutoCAD does:

https://knowledge.autodesk.com/support/ ... 6-htm.html

It allows the user to select one of the following fits:


Force and shrink fits (FN).
Clearance and location fits (LC).
Interference locational fits (LN).
Transition locational fits (LT).
Running and sliding fits (RC).

then the system will calculate deviations according to the Engineers Handbook.

we can use the AutoCAD approach as guidance when we implement it in the future

Please re-open the issue with a tag of enhancement or feature request to be implemented at some stage in the future.
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ppemawm
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Re: loose, transition and interference fit in Assembly 4

Postby ppemawm » Sun Apr 18, 2021 7:51 pm

kcleung wrote: Sat Apr 17, 2021 11:33 pm So in assembly 4, how can one implement loose fit, transition fit and interference fit?
In my experience, assembly fits are applied to individual models or drawing dimensions not the assembly. Perhaps this would be better implemented in the TechDraw workbench if at all.
"It is a poor workman who blames his tools..." ;)
kcleung
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Re: loose, transition and interference fit in Assembly 4

Postby kcleung » Mon Apr 19, 2021 2:06 am

ppemawm wrote: Sun Apr 18, 2021 7:51 pm
kcleung wrote: Sat Apr 17, 2021 11:33 pm So in assembly 4, how can one implement loose fit, transition fit and interference fit?
In my experience, assembly fits are applied to individual models or drawing dimensions not the assembly. Perhaps this would be better implemented in the TechDraw workbench if at all.
However, the crux of an assembly is precisely about how different parts are fitted together to form an assembly, and to test whether the fits will work, so specification of each fit in an assembly, including the purpose of fit (such as journal vs a installation that do not expect to be disassembled during its life e.g. fitting a railway wheel onto an axle), the materials of both parts and the process of fitting the two parts in a fit is an important decision during the design of that assembly.

I propose that a class called "fit" to be defined in the data structure (if not already done so) and that is to be instantiated for each fit in an assembly.
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Zolko
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Re: loose, transition and interference fit in Assembly 4

Postby Zolko » Mon Apr 19, 2021 2:31 pm

kcleung wrote: Mon Apr 19, 2021 2:06 am I propose that a class called "fit" to be defined in the data structure (if not already done so) and that is to be instantiated for each fit in an assembly.
I do not deny the usefulness of such a feature, but there are much more important things still missing (BOM, CoG...) so this will remain low on the priority list for some time.
try the Assembly4 workbench for FreCAD v0.19
install with Tools > Addon Manager > Assembly4 — tutorials here and here