Problem extruding / padding existing STL

[PeterFinderup]

Hi,

My son likes to play with BRIO trains and have just started to show interest in LEGO. Therefore, I went online and found a free STL which combined the two.

My son asked me to do a small correction on the STL. I have not used much time modifying STLs, but it was a minor change. With a few tutorials, I should be able to do it. Oh, boy was I wrong. I have spent around 15 hours trying to understand freeCAD with the help of tutorials from youtube, and I have still not succeeded.

So - here I am hoping one of you can point me in the right direction.

My problem is two-fold.

1) I like to add 1 extra row of buttons in each end. I will just extend the layout, while keeping the length. It should be possible by decreasing the length of the magnet holder.

2) The magnet holder is too small for my magnets, so I need to increase the inside diameter to 13mm in each end.


What have I done:

Imported STL -> mesh -> refined -> solid. I have tried to extrude/pad, but didn't work out as I hoped.
I have tried to create my own sketches from scratch but the constrains always explode after 1 hour of drawing.

If any of you had to do these modifications - which workbench would you use? (I have only used parts, partdesign and sketches).
Which tools would you use? Maybe I missed the obvious?
Do you have a link for a youtube / tutorial that could give me the insight I need? Maybe just the buzzword so I can continue my search.

I have linked the STL and a picture of the changes for better understanding.

Any input is welcome!

/Peter

[kwahoo]

If I understood correctly, you need something like this:

phpBB [video]

As you can see editing a solid created from mesh is bit expensive. It's better to recreate that shape from scratch if this is more than a single time job.
In the video I used LinkStage3 FreeCAD branch, but that should work very similar in the vanilla FreeCAD.

I have tried to create my own sketches from scratch but the constrains always explode after 1 hour of drawing.

Sketches should be as simple as possible. Use Part Design patterns, mirrors etc. where is possible.

[heda]

as pointed out, if doing larger modifications, remodelling is likely to be the best choice.
here is a file with the original piece remodelled, should make it easy enough to modify to get to your final goal.

stlremod.png (9.58 KiB) Viewed 248 times

it is done using "mesh remodel" add-on wb, it is one of the many ways to be able to pick up geometry from the stl.

I have just recreated some of the basic geometry like circles, positions etc, either through 3 point circles or sections,
then those are either used directly or converted to sketches, that when needed is tuned a bit to make them usable for extrusions.
used wb's are: mesh remodel / draft / sketcher / part