How to reference points & geometry outside of sketch

[dalewheat]

I'm learning FreeCAD (and I have been since v0.14) and the more I learn, the more complex problems I encounter. I'm not even sure how to ask this question "properly", but I will try. First, I am using:

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6706 (Git)
Build type: Release
Branch: releases/FreeCAD-0-16
Hash: f86a4e411ff7848dea98d7242f43b7774bee8fa0
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17

I'm modeling some triangular truss to print in sections with a 3D printer. I have had some success with simpler models. My workflow consists of 1) draw 2D sketch in appropriate plane, 2) pad to desired 3rd dimension, 3) add or subtract additional features using sketches mapped to appropriate faces, 4) select all relevant objects, 5) export as STL and finally 6) print. Mostly successful, so far.

I'm trying to lay out a flat pattern that I can then fold into the triangular shape I want. With a single padded sketch, this is simple enough. Then I wanted to add a couple of flanges to the ends of the sections to give me something to capture at the connecting points. Here is where I run into trouble. The flanges must follow some of the geometry of the sketch used to create the main sections, but I don't know how to or understand how to reference points in the previous sketch or extant 3D feature in a new sketch.

What I am currently doing is to copy the first sketch, paste it as a new sketch, turn all the existing lines into constructions lines, then build on top of that geometry. This further required me to "toggle selectability" of the existing points because I tended to end up altering the original sketch, somehow.

This seems to me to be a bit cumbersome and error-prone. I assume I am missing some basic understanding of how to accomplish this task in FreeCAD. Any suggestions would be appreciated.

I have attached the current model for reference. What I'd like to do now is to punch out some triangular "lightening" holes in the faces of the flat parts to reduce mass (and print time). I'd like to select the edge of the existing face and use it as a reference point. How to do this? What am I missing?

[DeepSOIC]

I can suggest some change to approach in modeling this.
-The thing is symmetric, so it is enough to model one quadrant of it, and reconstruct the rest using Part Mirror + Part Union.
-There are a few places where you could have used subtractive modeling process instead of additive, to reduce the number of sketches.
-You could use Draft Array or PartDesign LinearPattern to avoid repeating the profiles three times...

In v0.17, you can use links to external geometry in sketches to almost anything, most importantly to other sketches.

[dalewheat]

Thank you for your prompt reply to my question.

The 3 panels are not exactly duplicates. While the outside edges taper down to zero, the two inner "valleys" must reach a minimum at 0.3mm, the thickness of a single layer of plastic on my 3D printer. This ties the three sides together and allows me to "fold" the resulting part into a triangular truss section.

Should I be using v0.17 for this project, do you think?

I have been trying to use "link to external geometry" with mixed success. It worked once, on one section, and I could use the top edges of the 3D panel to place holes, but most of the time it just shows me a red-circle-with-a-line-through-it icon to say "no".

[DeepSOIC]

In 0.16, linking to external geometry is restricted to only the object the sketch is mapped to. In 0.17, this restriction was lifted.

Here's an example of how you can use Draft Array:

[sgrogan]

I have been trying to use "link to external geometry" with mixed success. It worked once, on one section, and I could use the top edges of the 3D panel to place holes, but most of the time it just shows me a red-circle-with-a-line-through-it icon to say "no".

Hello dalewheat,
Please see http://www.freecadweb.org/wiki/index.ph ... dencyGraph
You have 3 independent solids and a "dangling" sketch referenced to a fourth independent solid. Part Design workflow should be linear or nearly so.

Should I be using v0.17 for this project, do you think?

0.17 brings much power but a change in the workflow. IMHO better to master 0.16 first. If I'm wrong that you want a single connected "object" to print and then fold please correct me.

EDIT: DeepSOIC posted while I was typing.

[dalewheat]

@DeepSOIC: Thank you for the example. I will study it in detail and try to unravel its mysteries It's always helpful to see how someone else (who knows what they are doing) would approach a design, given a specific set of design criteria.

@sgrogan: I have just installed the dependency graph option on my computer. The "dangling" sketch is another part of the overall design. I've just included it in the same file for housekeeping reasons. There's no telling what all I did in this design as I am pushing my FreeCAD boundaries as far as they will go.

I've attached a photo of the finished product, all folded up.

Also a photo showing the print as it was created laying flat on the 3D printer.

The current design file is also attached. Lots of hand-placed triangles!

So I think I see that only geometry from the selected support face can be used in v0.16. That by itself would, to me, be a good reason to look at v0.17. I have downloaded it and will install it presently. Wish me luck!

[jwatkins672003]

Where is version 0.17 (32 bit)? At the site I only see 0.16 and external referencing in 0.16 is extremely limited. The external referencing within sketches described for version 0.17 appear to be what will solve my issue, which is how to quickly reference case geometry to create a cross section of a thread that will be added to a cylindrical case. The sketcher in 0.16 is not allowing me to select the solid cylinders center point. Right now I'm doing a longer workaround, but better external referencing would really be helpful.

[DeepSOIC]

Where is version 0.17 (32 bit)?

See https://github.com/FreeCAD/FreeCAD/releases. Use files that have x86 in them.

PS. You should have posted this in install/compile forum.