Axis of Transposition

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Vagulus
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Location: Perth, Western Australia

Axis of Transposition

Postby Vagulus » Tue May 04, 2021 11:16 pm

I am continually confused as to the axis on which I am working.
This Sketch (the green circle) was drawn on the YZ-plane
210505 Which Axis.png
210505 Which Axis.png (50.91 KiB) Viewed 466 times
and I wish to transpose it along the X-axis (to my right).
To do so I had to make the transposition on the Z-axis.

Porquoi? :?
"It is much harder to simplify than to complicate."
Joseph Kimble
Bance
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Re: Axis of Transposition

Postby Bance » Tue May 04, 2021 11:28 pm

Ummmm because its on the YZ plane?
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Vagulus
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Re: Axis of Transposition

Postby Vagulus » Tue May 04, 2021 11:59 pm

Bance wrote: Tue May 04, 2021 11:28 pm Ummmm because its on the YZ plane?
Mornin' Bance. My Mummy always said that it was naughty to answer a question with a question ;) but I didn't listen to Mummy anyway! 8-)

Orrite! 8-)

First assumption - My green circle sketch in the image above is on the YZ-plane so any transposition of that sketch is according to its own (Local ) Coordinate System (LCS) not the Global Coordinate System (GCS) as specified by the three-arrow thingy the name of which escapes me.
Question 1: Is the first assumption correct?

Second assumption - The Z-axis in the LCS is normal to the XY-plane of the LCS (in the image above a positive transposition moves the Sketch to my right).
Question 2: Is the second assumption correct?

Third assumption - The X-axis in the LCS is horizontal (corresponding, in this case, to the Y-axis in the GCS) with a positive transposition moving the Sketch away from me. This image shows a negative transposition on the X-axis of the LCS.
210505 Which Axes.png
210505 Which Axes.png (3.25 KiB) Viewed 453 times
Question 3: is the third assumption correct?

Question 4: Where in the Wiki is this explained? It is not here PartDesign_CoordinateSystem.

Question 5: I there a way to display the LCS for a given Sketch?

(See - I didn't pay much attention to Mummy! :shock:
"It is much harder to simplify than to complicate."
Joseph Kimble
jmaustpc
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Re: Axis of Transposition

Postby jmaustpc » Wed May 05, 2021 12:18 am

Vagulus wrote: Tue May 04, 2021 11:16 pm and I wish to transpose it along the X-axis (to my right).
To do so I had to make the transposition on the Z-axis.
Yes, that is exactly how it is expected to function. I suggest you go and read the wiki for a detailed explanation of how attachment works.

Your confusion is because you are thinking as though there is only one axis system in a FreeCAD model but that is not the case, there are in effect numerous axis systems nested inside each other, that is attached to each other, some times they align such that their XYZ are the same but often they don't and shouldn't.

Attachment properties are within the axis system of the object being attached (in your example in the XYZ of the sketch being attached) and not the axis system of the object you are attaching it to (in your case the YZ axis plane of the Body).

It makes perfect sense when you think about it. :) That way when you move an object that is higher up in the hierarchy, all the items below will move as well but stay in the same place relative to whatever they are attached to.
jmaustpc
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Re: Axis of Transposition

Postby jmaustpc » Wed May 05, 2021 12:23 am

You replied while I was posting my last post.

Here is the attachment page in the wiki, offsets is down near the bottom of the page.

https://wiki.freecadweb.org/Part_Attachment
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Vagulus
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Re: Axis of Transposition

Postby Vagulus » Wed May 05, 2021 1:19 am

jmaustpc wrote: Wed May 05, 2021 12:23 am Here is the attachment page in the wiki, offsets is down near the bottom of the page.
Thanks, but two words have come to mind whenever I have looked at that page - daunting and bewildering. Once again I am gazing at terminology which must be familiar to the writer but which is foreign, even alien, to the unenlightened like myself. For example, Usage, Clause 4 :
"Select a topology element on the object to attach to: vertex, edge or face/plane. Datum geometry from Part containers are also selectable."
Forgive me for saying that terms like 'topology element', 'datum geometry' and 'part containers' are rare in workshop conversation. I really have to work to get the gist of that sentence.

Then there's 'Align-O-N-Y' which, I assume, is better than a kissin' cuzzin to Align O-Y-N in the sidebar illustration.
210505 Align O-N-Y.png
210505 Align O-N-Y.png (44.77 KiB) Viewed 419 times
You might think that is nitpicking but it shouldn't take much to correct (and remove any confusion) but what does Align O-N-Y mean (or Align O-N-X for that matter)?

Hey! :shock: I am not complaining because I like to have a whinge. I am describing the way in which these things hit me when I look at them.

Basic_Attachment_Tutorial is good stuff although I have never used the techniques shown there. My models must be much simpler and more closely associated with the GCS - only the once removed. I'll face the confusion I will feel when I get more complex at the time the issues come up. For now I am concentrating of getting a firmer grip on the very basics.

What does Align O-N-X mean?

Also, if you wouldn't mind, what are the answers to the questions I asked in my previous post. Bance must have called it a day. :)
"It is much harder to simplify than to complicate."
Joseph Kimble
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Re: Axis of Transposition

Postby Modeler » Wed May 05, 2021 3:06 am

You need to get used to the idea of looking at the axis indicator in the lower right of the screen. You will get used to it. If you have a sketch and you need to rotate it you can do it by rotating the attachment point.

In the combo view pane,
Select the sketch in Tree View
This will open a property view pane below it.
Select the data tab
You will see a line that looks like > Attachment [(000,000,000)\

Click that line and you will be presented with the opportunity to change the placement including the rotation.

Play with that for a bit. You will get what you want.
chrisb
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Re: Axis of Transposition

Postby chrisb » Wed May 05, 2021 6:16 am

Vagulus wrote: Wed May 05, 2021 1:19 am What does Align O-N-X mean?

Also, if you wouldn't mind, what are the answers to the questions I asked in my previous post. Bance must have called it a day. :)
As we say here: don't expect to get carried to the hunt. O-N-X is explained in the Part Attachment page, it is Origin-Normal-Xdirection.
I think it would be a good idea to explain these shortcuts in general on the said page, so it wouldn't need to read the description of every single one (from which I derived my interpretation).
Question 1: Is the first assumption correct?
yes, Attachment Offset is always relative to the local coordinate system.
Question 2: Is the second assumption correct?
yes
Question 3: is the third assumption correct?
see answer to question 1
Question 5: I there a way to display the LCS for a given Sketch?
no
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Vagulus
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Re: Axis of Transposition

Postby Vagulus » Wed May 05, 2021 7:16 am

Thanks chrisb.
"It is much harder to simplify than to complicate."
Joseph Kimble
Bance
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Re: Axis of Transposition

Postby Bance » Wed May 05, 2021 7:34 am

Vagulus wrote: Wed May 05, 2021 1:19 am Bance must have called it a day.
Had my beauty sleep, let's see if we can prepare you to sleep soundly :D
Vagulus wrote: Wed May 05, 2021 1:19 am rare in workshop conversation
Yes, but if we document in terms of oojits,wasnames,thingamybobs et al, we'd all be in the dark (or not?)

I think that CAD sw is not really a workshop tool anyway, the output of it may be.

Chrisb answered all of your questions bar one, I concur with those.

Question 4. this can be found in the wiki page Part_Attachment under sub section attachment offset.
Vagulus wrote: Wed May 05, 2021 1:19 am Then there's 'Align-O-N-Y' which, I assume, is better than a kissin' cuzzin to Align O-Y-N in the sidebar illustration.
All of the align options are similar in nature, but the differences are subtle therefore not easy to explain in just a few words.

The image below shows two cubes attached at one corner.

How many other ways can you imagine them arranged, but still attached at the same corners?
Align2.png
Align2.png (14.22 KiB) Viewed 343 times
Well as a simplification those are the align options.
Vagulus wrote: Wed May 05, 2021 1:19 am Hey! :shock: I am not complaining because I like to have a whinge. I am describing the way in which these things hit me when I look at them.
I think you know, my patience wears thin sometimes :lol: :lol: But this is not easy to picture in ones minds eye. The documents could be improved for sure. Having to explain in the simplest of terms, is useful for everybody, including me :lol: