Constraint Confusion

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Constraint Confusion

Postby Gandalf_Sr » Mon Jun 06, 2016 2:20 pm

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6704 (Git)
Build type: Release
Branch: releases/FreeCAD-0-16
Hash: 0c449d7e8f9b2b1fb93e3f8d1865e2f59d7ed253
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17

I am an EE trying to learn FreeCAD. I watched a few tutorial videos and started my first design which is a '+' shaped object with 4 counterbored holes. I padded my first sketch and then drew a second sketch of 4 circles on the top surface to create the counterbores which I was able to do (see picture) but FreeCAD is telling me that I have still 2 degrees of freedom. I can take each pair of opposite circles and move them in the x or y axis but how do I constrain them? Is there a way to lock to an axis?

You may note from the purple circles in the picture that I have created an edge linked to an external geometry for 2 of the original holes but I can't see how to constrain the new circles to be concentric to the original ones.

Please help.
Constraints.PNG (58.3 KiB) Viewed 983 times
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Re: Constraint Confusion

Postby Gandalf_Sr » Mon Jun 06, 2016 4:40 pm

I figured it out, I needed to set constraints for zero distance between my circle centers and the origin. That got rid of the last 2 constraints.
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Re: Constraint Confusion

Postby r-frank » Mon Jun 06, 2016 5:06 pm


Please don't use zero length constraints.
They will make the solver slow and maybe lead to a crash ...

The better idea is to set a coincident (or PointonPoint) constraint between the center points of your circles.

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Re: Constraint Confusion

Postby quick61 » Mon Jun 06, 2016 5:15 pm

Yes, r-frank is correct, one should not use 0.0 mm distance constraints. Following the general rule that geometrical constraints should be used whenever possible, there is no instance where a 0.0 mm constraint can not be replaced by a geometrical constraint. While in a simple Sketch like you have in your example, using such a constraint would be no big deal, it is just a very bad practice to get into and can lead to real troubles on down the line when your sketches get more complexity to them. ;)

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Re: Constraint Confusion

Postby Gandalf_Sr » Wed Jun 08, 2016 1:59 pm

Thanks guys :D