Another Under-constrained sketch

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earthcare
Posts: 26
Joined: Tue May 31, 2016 7:46 am
Location: Australia

Another Under-constrained sketch

Post by earthcare »

Hi all,

I've seen so many of these in the board, but I have a quite simple sketch that I can't get to pad because it has two remaining degrees of freedom.

I've tried:
* Grabbing each vertex and trying to move it - nothing moves
* Grabbing each element and trying to move it - nothing moves
* Using the Macro Shake Sketch - nothing happens

Please, help!

My configuration:
OS: Linux Mint 17.3 Rosa
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6703 (Git)
Build type: None
Branch: releases/FreeCAD-0-16
Hash: 2ce5c8d2e3020d05005ed71f710e09e9aa561f40
Python version: 2.7.6
Qt version: 4.8.6
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
Attachments
HeaterClamo.fcstd
(17.49 KiB) Downloaded 25 times
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Willem
Posts: 1615
Joined: Fri Aug 12, 2016 3:27 pm
Location: Lisse, The Netherlands

Re: Another Under-constrained sketch

Post by Willem »

See attached sketch
Knipsel.PNG
Knipsel.PNG (32.09 KiB) Viewed 1567 times
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NormandC
Posts: 18534
Joined: Sat Feb 06, 2010 9:52 pm
Location: Québec, Canada

Re: Another Under-constrained sketch

Post by NormandC »

earthcare wrote:I have a quite simple sketch that I can't get to pad because it has two remaining degrees of freedom.
Although I am among those who stress the importance of fully constrained sketches, it is actually not mandatory to successfully pad. As Willem's screen capture indicates, the sketch would not pad because your profile was not perfectly closed. You only need zoom in on the identified regions to see that they are not connected.

I would also add that both symmetric constraints are unnecessary. You can replace them with equal length constraints between the vertical lines. It is best to limit use of symmetric constraints to a minimum, and to prefer other types of geometric cosntraints where possible. Symmetric constraints are more taxing on the solver, and I've noticed that when you add too many of them in a sketch, it can "lock it up" - meaning that all the geometry becomes fixed and no element can be dragged even if the sketch is not fully constrained.

You are also missing a tangent constraint at the right of the outer arc. When applying a tangent constraint, select the end points, not the arc and the line. This way a coincident (point-on-point) constraint is unnecessary, and it is actually the recommended method. By adding the tangent constraints between the inner vertical lines and the 16mm arc, Constraint21 and Constraint22 (point on object constraints fixed to the horizontal sketch axis) are redundant.

And finally, you used the Image distance constraint for Constraint8 (51mm). The length constraint should only be reserved for diagonal distances. It should be replaced with a Image vertical distance constraint.

Here's my version of your sketch for reference. It's fully constrained with 32 constraints, whereas your original sketch had 36 with two DOF left.
Attachments
HeaterClamo_normandc1.fcstd
(3.79 KiB) Downloaded 22 times
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Willem
Posts: 1615
Joined: Fri Aug 12, 2016 3:27 pm
Location: Lisse, The Netherlands

Re: Another Under-constrained sketch

Post by Willem »

Best NormanC,
It is good to constraint everything in the proper way but the double dimension is still in the drawing. I would prefer than to add another constraint as you can see in the attached sketch
Knipsel3.png
Knipsel3.png (32.67 KiB) Viewed 1552 times
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NormandC
Posts: 18534
Joined: Sat Feb 06, 2010 9:52 pm
Location: Québec, Canada

Re: Another Under-constrained sketch

Post by NormandC »

I missed that one Willem. :)

My preference is to delete the 16mm radius constraint and set an equal length constraint between the bottom horizontal line and the upper left one (or the right).
Attachments
HeaterClamo_sketch_01.png
HeaterClamo_sketch_01.png (26.46 KiB) Viewed 1528 times
earthcare
Posts: 26
Joined: Tue May 31, 2016 7:46 am
Location: Australia

Re: Another Under-constrained sketch

Post by earthcare »

NormanC & Willem
Thanks for your suggestions: I've learned a bit more about #D CAD. (Until 18 months ago, the last time I had drawn anything up was with a drawing board!
Edwin
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