Sketch is offset by small amount

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dcschooley
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Joined: Sat Nov 21, 2020 10:20 pm

Sketch is offset by small amount

Post by dcschooley »

In the attached design, the sketch labeled "Mount Holes Left" and its associated pad end up being raised in the y-axis direction by a small amount when the part is exported to STL. It's probably always like that but it's not noticeable within FreeCAD. Cura lets me know because it wants to put supports under that section of the design. Any suggestions how how to fix and prevent this problem are welcome.

This part also suffers from not always exporting correctly. The hole and slot on the rounded end don't always end up in the exported STL.

This design has been victimized by the topological naming problem multiple times, so that might have something to do with it. I need to work on datum planes I think. This design is something I've done as a learning exercise from a design I did in OpenSCAD. I like the FreeCAD part better.

Thank you.

OS: macOS 10.16
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24366 (Git)
Build type: Release
Python version: 3.9.7
Qt version: 5.12.9
Coin version: 4.0.0
OCC version: 7.5.3
Locale: C/Default (C)
Attachments
servo_end_block.FCStd
(375.42 KiB) Downloaded 21 times
Syres
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Re: Sketch is offset by small amount

Post by Syres »

The whole model looks like it's symmetrical, can I ask why you don't just model half and then Pattern>Mirror at the end (possibly without the Fillets)?
openBrain
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Re: Sketch is offset by small amount

Post by openBrain »

Your model has errors.
Also where is that offset ? What is the reference point for measurement ? What is the expected value ?
Better if you can minimalize your file with only needed objects in it. ;)
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adrianinsaval
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Re: Sketch is offset by small amount

Post by adrianinsaval »

dcschooley wrote: Tue Jan 18, 2022 5:09 pm It's probably always like that but it's not noticeable within FreeCAD
it is and it's noticeable, it's like that because you constrained it like that and positioned your sketch like that. Edit either the sketch placement or the constraints in your sketch to move it to the desired position.
There seems to be a lot of things wrong in your model, you have a random plane outside of any body, no idea were that came from, a fillet is broken, you have a lot of things visible that I don't think are practical to have visible and I don't understand how you placed your sketches and why many don't have a support (not attached). And as syres points out, it's probably better to use a mirror instead of modelling the same thing twice.
dcschooley
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Re: Sketch is offset by small amount

Post by dcschooley »

Mirroring would have been useful on the two "Mount Holes" parts because those two sketches are essentially identical. The rest does well without needing to mirror anything. I guess a better way to do it would have been to create a mount-holes part as a single 3D thing and then place twice in the main part. The clamp section is something that gets used elsewhere, so eventually it needs to be its own drawing that gets reused.
dcschooley
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Joined: Sat Nov 21, 2020 10:20 pm

Re: Sketch is offset by small amount

Post by dcschooley »

adrianinsaval wrote: Tue Jan 18, 2022 5:49 pm
dcschooley wrote: Tue Jan 18, 2022 5:09 pm It's probably always like that but it's not noticeable within FreeCAD
it is and it's noticeable, it's like that because you constrained it like that and positioned your sketch like that. Edit either the sketch placement or the constraints in your sketch to move it to the desired position.
There seems to be a lot of things wrong in your model, you have a random plane outside of any body, no idea were that came from, a fillet is broken, you have a lot of things visible that I don't think are practical to have visible and I don't understand how you placed your sketches and why many don't have a support (not attached). And as syres points out, it's probably better to use a mirror instead of modelling the same thing twice.
I don't doubt that it's messed up. This drawing has gone through several iterations of deleting pads, fillets, and pockets, and on more than one occasion sketches to end up in strange places. This has been one of my frustrations with FreeCAD and why I keep going back to OpenSCAD. I'll double check the two mount-holes sketches. They are supposed to be identical.
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Shalmeneser
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Re: Sketch is offset by small amount

Post by Shalmeneser »

Mount Holes R sketch and Mount Holes L sketch are not the same. (and are too complex to understand why)

Use PartDesign_Mirrored or Sketcher_CarbonCopy or Sketcher_External.
Attachments
Capture du 2022-01-18 19-15-54.png
Capture du 2022-01-18 19-15-54.png (40.48 KiB) Viewed 1044 times
Capture du 2022-01-18 19-16-33.png
Capture du 2022-01-18 19-16-33.png (40.63 KiB) Viewed 1044 times
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adrianinsaval
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Re: Sketch is offset by small amount

Post by adrianinsaval »

They are mostly identical but they aren't positioned in the same way, the placement of the left sketch is not the same as the right and also the constraint that set's the distance to the axis is not the same. If you change the constraint that is 19.83 to 20 the geometry is moved to be aligned with the other sketch (quick visual judgment, did not check). Check whatever method you used to calculate that constraint.
dcschooley
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Re: Sketch is offset by small amount

Post by dcschooley »

Shalmeneser wrote: Tue Jan 18, 2022 6:15 pm Mount Holes R sketch and Mount Holes L sketch are not the same. (and are too complex to understand why)

Use PartDesign_Mirrored or Sketcher_CarbonCopy or Sketcher_External.
The rotation was caused when I changed a pad or something and it rotated the sketch. I tried to fix it by rotating it somewhere else. I tend to tweak my designs a lot, so that topological naming problem causes all sorts of havoc. I am going to redo this drawing from scratch using datum planes to see if that helps.
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