V0.19: Top Down Design of a Folding Step Ladder

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ppemawm
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V0.19: Top Down Design of a Folding Step Ladder

Postby ppemawm » Sat Nov 23, 2019 6:21 pm

This is a project I used to further explore in-context modelling and top down design using a master sketch, carbon copies, fasteners workbench, and Assembly4 for the parametric rivets. The approach is a bit of a hybrid assembly which I would not recommend except for learning purposes. In this example, Assembly4 was conveniently used only for the rivets although all parts including ISO-standard fasteners could be included in the Model_Assembly with a bit more effort.

In this project, the model was created from dimensions scaled from photos of a commercial step ladder as shown in the following screen shot. All parts are in the same file, each of which were created from modified carbon copies of the top level master sketch. In this manner it is possible to articulate the moving parts of the model by changing one constraint defined in the master sketch. All of the moving parts follow the action of the master sketch.

Picture1.png
The model tree includes all the individual parts, standard fasteners, merged parts from a file defining a generic rivet with variable length, and Assembly4 model containing all of the rivets.

Standard fasteners from the Fasteners workbench were attached to appropriate edges in the completed model which is not always the best approach since their position can change if any of the model features are modified due to the topological naming problem. It is more robust to add circles at vertices in the master sketch which can then be used to locate the fasteners. Or, more recently, use the fastener tool in Assembly4 workbench similar to what was done in the below referenced second project.
Picture1.png (315.67 KiB) Viewed 1267 times
Picture2.png
This is the master sketch used to capture the design intent, main dimensions, and a constraint (opening) that can be changed to articulate the sketch. The sketch was created and constrained at an arbitrary opening. It is important at this point to exercise the sketch in small steps through the complete range of the variable to verify that the sketch solution is valid for all positions.

Note that the support brace on the left of the sketch was fixed by locating it at the origin and constraining it with a fixed angle in order to simplify the sketch as much as possible for the solver.

All bodies were created by starting with a Edit>Duplicate of a carbon copy of this master sketch toggled to construction geometry. Construction geometry is added and constrained to the appropriate edges and vertices of the carbon copy. It is important that each feature of the body is somehow related to the carbon copy construction geometry. No external references are required with this method.
Picture2.png (260.9 KiB) Viewed 1267 times
Picture3.png
There are four different lengths of rivets in the model. These were generated by merging a parametric model into a part container from an external file and changing the length as required for each.

Assembly4 workbench was used to attach LCS's for each rivet to the master sketch vertices. These were then offset to their proper position using the LCS Attachment Offset property.
Picture3.png (155.29 KiB) Viewed 1267 times
step_ladder2.gif
The model can be checked at all positions by changing the "opening" constraint of the master sketch in the model tree as shown. It takes about 18 seconds for the sketch solver to reposition all of the moving parts with a I7-2.5 GHz, 16 GB machine.
step_ladder2.gif (353.56 KiB) Viewed 1267 times
Picture5.png
This locking mechanism was probably the most difficult to model since it is attached to a moving part. It sometimes takes some ingenuity to reference the construction geometry of the master sketch carbon copy. The alternative is to put more detail in the master sketch in the beginning. The master sketch needs to be simple but not too simple. One must carefully think through the modelling of each body at the master sketch stage with the top down approach.
Picture5.png (417.84 KiB) Viewed 1267 times
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I can place the 4.8 MB file on Dropbox for anyone interested. Please contact me via Private Message.

Zolko has recently added several improvements to the Assembly4 workbench that facilitate top down design including fasteners which I used in another project https://forum.freecadweb.org/viewtopic. ... 30#p349238 that was built from detail drawings. This workbench is evolving nicely, at least for my purposes. Kudos to the developer

OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.18719 (Git)
Build type: Release
Branch: master
Hash: c021ff70debb106b27d03ed1707f4b05fcf385a6
Python version: 3.6.7
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)
"It is a poor workman who blames his tools..." ;)
paullee
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby paullee » Sun Nov 24, 2019 12:03 am

Impressive !

Personally, not designing anything about mechanical parts but interested very much in the way 'master sketch' is used.

Can have a simplified model for my slow 6G ram i3 machine ? :)
freedman
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby freedman » Sun Nov 24, 2019 12:53 am

Excellent work. I have read some your posts prior and the dup of a carbon copy is good stuff, that is next on my list of things to learn and get comfortable with.
triplus
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby triplus » Sun Nov 24, 2019 1:57 pm

Nicely done!

P.S. And i see you have started to introduce some assembly related FreeCAD capabilities in your workflow. That indicates, we are on the right track, when it comes to the ongoing development in this area.
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arturromarr
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby arturromarr » Wed Nov 27, 2019 12:58 pm

Well done work, congratulations on an interesting project.
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DeepSOIC
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby DeepSOIC » Wed Nov 27, 2019 1:13 pm

Impressive!
ppemawm wrote:
Sat Nov 23, 2019 6:21 pm
It takes about 18 seconds for the sketch solver to reposition all of the moving parts with a I7-2.5 GHz, 16 GB machine.
Hmm, that's slow as a dead cow. I wonder why... gotta check out Zolko's creation when I have time, to see why is it taking so long. I expected it to be pretty instantaneous.
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ppemawm
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby ppemawm » Wed Nov 27, 2019 3:42 pm

DeepSOIC wrote:
Wed Nov 27, 2019 1:13 pm
Hmm, that's slow as a dead cow. I wonder why... gotta check out Zolko's creation when I have time, to see why is it taking so long.
I suspect it may have something to do with using the master sketch solver to articulate the mechanism and multiple duplicated Sketch carbon copies rather than Assembly4 itself.

I am now creating a similar Assembly4 using only a master sketch and shapebinders of the sketch rather than the carbon copies to see if
that makes any difference.
"It is a poor workman who blames his tools..." ;)
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ppemawm
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby ppemawm » Wed Nov 27, 2019 3:46 pm

paullee wrote:
Sun Nov 24, 2019 12:03 am
Can have a simplified model for my slow 6G ram i3 machine ?
Please see similar file at https://www.dropbox.com/s/qlud995qng29f ... FCStd?dl=0 which is
discussed in https://forum.freecadweb.org/viewtopic. ... 30#p349238
"It is a poor workman who blames his tools..." ;)
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ppemawm
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby ppemawm » Wed Nov 27, 2019 3:52 pm

triplus wrote:
Sun Nov 24, 2019 1:57 pm
P.S. And i see you have started to introduce some assembly related FreeCAD capabilities in your workflow. That indicates, we are on the right track, when it comes to the ongoing development in this area.
Thank you for your comment.

Yes, indeed. I would like to be assured that the assembly developers take into account the need for a top-down design approach and requirement for in-context modeling.
"It is a poor workman who blames his tools..." ;)
paullee
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Re: V0.19: Top Down Design of a Folding Step Ladder

Postby paullee » Wed Nov 27, 2019 6:24 pm

ppemawm wrote:
Wed Nov 27, 2019 3:46 pm
Please see similar file at https://www.dropbox.com/s/qlud995qng29f ... FCStd?dl=0 which is
discussed in https://forum.freecadweb.org/viewtopic. ... 30#p349238
Thanks!