Problems with pocket/substract while designing a plastic part

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liana
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Location: Germany

Problems with pocket/substract while designing a plastic part

Postby liana » Fri May 14, 2021 7:53 am

Hi there,

as a former user of commercial/'professional' CAD programs i am now trying to migrate to freecad for my private use and i have to admit i'm really impressed by the high functionality and workbenches available. But after trying to create a plastic part with drafts according to the workflow i would do in other CAD programs i am now having an issue:

at first i've created the outer shape with 'pad', after that, i formed the rounded face by 'pocket', then i applied the draft to the outer faces and added some fillets to the edges, finally i converted the part to a shell form with 'thick solid'

after that, i tried to create two cutouts on the sides and that's where i am having an issue:

at first, i created a sketch on the midplane of the part and tried 'pocket', 'through all', 'symmetric' but that didn't work

at secound, i tried with the same sketch, then 'pocket', 'through all'. now the cutout was created as it was supposed, but then i tried to mirror the cutout on the midplane an there it failed me again

after that, i played around a bit with all possible options in the 'pocket'-command that could have been working from my point of view but they didn't

so finally i created a secound body and tried to substract it from my first part by the 'slice' command from the part-workbench and the boolean command from the part design-workbench.

in the enclosure you can find the part i created so far.

could someone please give me an advice if i'm doing something wrong with my workflow or maybe show me an alternative workflow that might be better for freecad?

thank you so far!

best regards

liana
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plastic_part.PNG
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chrisb
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Re: Problems with pocket/substract while designing a plastic part

Postby chrisb » Fri May 14, 2021 9:44 am

Hi and welcome to the forum!

You stumbled right away over one of the most picky functions in FreeCAD: Thickness. When you do a Part->CheckGeometry it shows errors. Thickness is great if it works, and in this case you can fumble with it by making it inwards, do it before the fillets, and more, but usually it is not worth investing too much time into fixing broken thickness.

In this case I would construct the first pad already from a sketch in XZ-planeand then make pocket with a smaller variant of the sketch to remove the inner stuff. Fillets must then be added inside and outside.
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liana
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Location: Germany

Re: Problems with pocket/substract while designing a plastic part

Postby liana » Fri May 14, 2021 11:10 am

Chris,

thanks for your reply!

So then i am going to use 'thickness' very carefully.
drmacro
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Re: Problems with pocket/substract while designing a plastic part

Postby drmacro » Fri May 14, 2021 12:12 pm

liana wrote: Fri May 14, 2021 11:10 am ...

So then i am going to use 'thickness' very carefully.
Most long time user, just avoid it all together. :mrgreen:
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papyblaise
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Re: Problems with pocket/substract while designing a plastic part

Postby papyblaise » Fri May 14, 2021 12:22 pm

sometimes Partdesign is still cheeky like a bad kid
I would have drawn the starting pad in relation to the face, it would have avoided the Pocket and the Draft
then the rest goes by itself
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chrisb
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Re: Problems with pocket/substract while designing a plastic part

Postby chrisb » Fri May 14, 2021 2:05 pm

papyblaise wrote: Fri May 14, 2021 12:22 pm I would have drawn the starting pad in relation to the face, it would have avoided the Pocket and the Draft
You didn't look properly. That's not what should be modeled.
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liana
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Re: Problems with pocket/substract while designing a plastic part

Postby liana » Sat May 15, 2021 9:24 am

Today i tried again to create this part and i think a found a way to do it. I don't know if there's a better way doing it in freecad or if i've been using some features that are better avoided using (as with 'thickness' ;) ) as i am pretty new to freecad but maybe that's a way to create a plastic part for injection molding:

1. the inner shape of the plastic part was created as solid in part design WB including outer draft and fillets
2. the outer faces were put into a shape binder
3. an offset of the shape binder was created in part WB
4. the spaces between the faces of the shape binder and the offset were connected/closed with the fill command in surface WB
5. the surface model was converted into a solid in curves WB
6. the cutouts on the sides were created using a hexahedron shape an the boolean command from part WB
7. the remaining draft and fillets were added in part design WB

Would be great if someone could leave a short reply if that's a reliable solution of creating this part or if there are things that should be better made another way.
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papyblaise
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Re: Problems with pocket/substract while designing a plastic part

Postby papyblaise » Sat May 15, 2021 11:23 am

I let chrisb say "it's not this it's not that, without ever proposing anything
With Part this is done in 3 or 4 steps:
an extrusion with an angle, a cut on the curved side, a thickness (which works better than some would suggest) a pad subtraction for the hole, and radii, that's all
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chrisb
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Re: Problems with pocket/substract while designing a plastic part

Postby chrisb » Sat May 15, 2021 12:28 pm

papyblaise wrote: Sat May 15, 2021 11:23 am I let chrisb say "it's not this it's not that, without ever proposing anything
Seems a bit overstated, if you look at the posts I ever made.
With Part this is done in 3 or 4 steps:
Seems a bit understated (I count 3 sketches plus 9 operations).
So overall it's a well balanced contribution :lol:
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Willem
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Re: Problems with pocket/substract while designing a plastic part

Postby Willem » Sat May 15, 2021 1:01 pm

Here is my solution for this design. It can be done very simple if the first sketch is not top view bus side view
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