[Solved] Cant constrain centre of circle with origin of datum plane

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tonyaimer
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Joined: Mon Sep 30, 2019 4:17 pm

[Solved] Cant constrain centre of circle with origin of datum plane

Post by tonyaimer »

OS: Linux Mint 20 (X-Cinnamon/cinnamon)
Word size of FreeCAD: 64-bit
Version: 0.20.25997 (Git) AppImage
Build type: Release
Branch: master
Hash: 77b198048a63f1e9ca15eef64c8042d599a14cf3
Python version: 3.9.7
Qt version: 5.12.9
Coin version: 4.0.0
OCC version: 7.5.2
Locale: English/South Africa (en_ZA)

Greetings

For those following the book "Learning by Doing" on p102.

I have built the attached model but cannot constrain the centre of the circle with the origin of the datum plane.

I created the datum plane at the end of the sketch along which I did an additive pipe.

I then selected the new datum plane and drew the circle.

I am unable to constrain the centre of the circle with the origin of the datum plane which should be the end of the polyline
used for the additive pipe. I have even tried selecting "external geometry" by clicking on one of the circles forming the end
of the pipe.

The exercise is for FreeCAD 0.18 and I am using 0.20, could that be the problem?

Any ideas about this?

Thanks in advance

Tony Aimer
Tutorial 6.1.FCStd
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Last edited by tonyaimer on Sat Oct 30, 2021 3:11 pm, edited 1 time in total.
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Shalmeneser
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Re: Cant constrain centre of circle with origin of datum plane

Post by Shalmeneser »

You can attach the sketch like the DatumPlane.
You can set your sketch at the other end of the path.
You can use the Path with 'external construction'.

But you have well-centered your circle !!
tonyaimer
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Joined: Mon Sep 30, 2019 4:17 pm

Re: Cant constrain centre of circle with origin of datum plane

Post by tonyaimer »

I did not fully understand your answer.

You will see in the model an extra circle which is supposed to be centred on the origin
of the datum plane and that is where my problem is.

Tony
drmacro
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Re: Cant constrain centre of circle with origin of datum plane

Post by drmacro »

I think there is a bit of misunderstanding of the datum plane.

There is no concept of constraining geometry to a datum plane in the way you describe, a sketch is attached to the datum and the geometry in the sketch is constrained with reference to the origin of the sketch.

By default auto sizing of the datum plane is on. The 0,0 of the datum is (if you check the support property in the properties view) is Sketch [Edge9, Vertex10]. Which is the end vertex of the sweep line defined in Sketch.

The support property of Sketch001 is the datum plane. And, since Sketch001 has no attachment offset, the origin of Sketch001 and the origin of DatumPlane are identical.

If you change the datum plane property Resize Mode = Manual and set the size 100 x 100 then you can see they are as you wish.
Attachments
Snip macro screenshot-fb2957.png
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Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
tonyaimer
Posts: 241
Joined: Mon Sep 30, 2019 4:17 pm

Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by tonyaimer »

I think I found the answer to my own problem.

I needed to select "flip sides" when creating the datum plane.

It is the last option in the create datum plane dialogue and you have
to scroll down to see it.

I found this accidently by drawing my circle on the so called "back face" of the datum plane
and I was able to constrain the circle centre to the end of the polyline which was also
the origin for the datum plane.

Perhaps "flip side" should be called "reverse normal' which is more mathematically meaningful
in vector analysis and perhaps we could show the normal to the plane as created. We also need
to be able to reverse the normal if the datum plane was not created with the correct option selected.

At present we have to delete the datum plane and recreate it with the correct normal. ( Am I correct in this? )

I will leave these suggestions to the FreeCAD gurus who have more than enough on their plate at present.

Thanks to all who have helped with this problem.

Regards

Tony Aimer
drmacro
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Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by drmacro »

I don't bother with datum plaines most of the time. Sketches can be positioned with all the same operations available to a datum. So, unless there is going to be multiple sketches on a datum plane it is just redundant.

There is also the Select origin Image tool in sketcher that selects the Root point anytime.
PickRootPoint.gif
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Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
tonyaimer
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Joined: Mon Sep 30, 2019 4:17 pm

Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by tonyaimer »

But does that not lead to the naming problem that we have been warned about?

Regards

Tony
drmacro
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Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by drmacro »

tonyaimer wrote: Sat Oct 30, 2021 4:21 pm But does that not lead to the naming problem that we have been warned about?
...
If you attach a Datum to generated geometry it is the same issue.

It is the attachment to other than base planes that is the biggest issue.

Attach a sketch (or datum) to a base plane and move it with attachment offset, no generated geometry involved.
Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
tonyaimer
Posts: 241
Joined: Mon Sep 30, 2019 4:17 pm

Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by tonyaimer »

I still don't get why the normal of the datum plane in my case pointed back towards the origin.

I had clicked on the last segment of the polyline and the last point of the polyline to establish the datum plane.

Unless this last point chosen caused the normal to the plane to face back along the polyline.

I hear you about offsetting a sketch from the base planes as an alternative.

Regards


Tony Aimer
drmacro
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Re: [Solved] Cant constrain centre of circle with origin of datum plane

Post by drmacro »

tonyaimer wrote: Sat Oct 30, 2021 5:24 pm I still don't get why the normal of the datum plane in my case pointed back towards the origin.

I had clicked on the last segment of the polyline and the last point of the polyline to establish the datum plane.

Unless this last point chosen caused the normal to the plane to face back along the polyline.

I hear you about offsetting a sketch from the base planes as an alternative.
By polyline do mean a Draft Polyline?
Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
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